Fixing a problem inside one of the world's most sophisticated scientific observatories might seem like the kind of challenge that demands a multimillion-dollar upgrade or revolutionary new technology.

But for scientists working on the Laser Interferometer Gravitational-Wave Observatory (LIGO), which listens for ripples in spacetime generated by cosmic collisions like merging black holes, the solution to a mild but persistent engineering challenge turns out to be as simple as an off-the-shelf camera.

By pairing commercially available thermal imaging cameras with computer models, a team led by Jonathan Richardson at the University of California, Riverside, has developed a technique that corrects tiny, heat-induced distortions in the observatory's mirrors — an elusive flaw that scientists say currently limits how far into deep space the facility can look.

"It doesn't require any new technology development, which is almost unheard of for solving a LIGO instrumentation problem," Richardson said in a statement.

Once incorporated into LIGO's upcoming upgrade, Richardson and his team estimate the fix would extend the observatory's reach by roughly 33 million light-years.

That gain might sound like a drop in the ocean against the unimaginably vast scale of the universe, but because space expands in three dimensions, pushing a detector's reach even slightly opens up an exponentially larger window of space. Being able to look further into the universe will allow astronomers to "hear" many more cosmic ripples, in turn increasing the potential for discovering the universe’s most violent collisions that lie beyond LIGO's reach today.

LIGO detects these cosmic ripples, known as gravitational waves, using twin L-shaped facilities in the U.S. — in the states of Washington and Louisiana. Inside each detector, a laser beam shoots down two 2.5-mile-long (4-kilometer-long) tunnels, bouncing off pristine mirrors at each end. When a gravitational wave passes through Earth, it subtly stretches one tunnel and squeezes the other. That microscopic shift alters the laser beams ever so slightly, producing a tiny flicker of light that alerts scientists to a distant cosmic event.

Because these cosmic signals are inconceivably small, preserving every single photon is crucial. To accomplish this, LIGO relies on mirrors polished to reflect 99.9999%t of the laser light that strikes them, ranking them among the purest optical components ever built.

Yet even these near-perfect mirrors have had one unavoidable flaw. The mirrors still absorb a tiny fraction of that intense laser light. That energy turns into heat, warping the mirror's surface by just a few nanometers, enough to distort the laser beam and reduce the observatory's overall sensitivity.

Physicists already knew they could counteract these distortions by applying targeted heat to the back of the mirrors. The difficult part was measuring the distortions accurately enough such that the correcting heat could be applied with exact precision.

The new technique uses infrared thermal images and existing computer models to reconstruct a map of distortions across the mirror's surface.

"You can think of it like taking an infrared picture of a car engine," Richardson said in the statement. "An engineer can look at the temperature pattern on the outside and infer what's happening inside the engine. We're doing the same thing with LIGO's mirrors."

And the technique isn't just a fix for LIGO. It is also expected to become part of the foundational design for Cosmic Explorer, a proposed next-generation U.S. gravitational-wave observatory targeted for the mid-2030s.

With 25-mile-long (40-km-long) arms — 10 times larger than LIGO's — Cosmic Explorer is already designed to detect gravitational wave events far beyond the reach of today's observatories. This new technique will only supercharge its ultimate reach.

"The goal for the next generation of gravitational-wave detectors is to achieve about 10 times the sensitivity of today's instruments," Richardson said in the statement. "One of the key obstacles to achieving that is reducing the fundamental quantum mechanical noise that limits the precision of the measurements."

The technique is described in a paper published July 16 in Classical and Quantum Gravity.

The loudest crash of gravitational waves ever heard has offered us insight into event horizons, the boundaries beyond which nothing can escape the grips of black holes.

The gravitational wave signal GW250114 was picked up in January 2025 by LIGO (Laser Interferometer Gravitational-Wave Observatory), Virgo, and KAGRA ( Kamioka Gravitational Wave Detector). The signal was created when two black holes with around 32 times the mass of the sun collided and set the very fabric of space rippling.

Now, a team of researchers assessed this signal and found a feature in the gravitational waves represents the collective event horizon of the involved black holes at the very moment of that collision.

"We measured the last sound the black holes made when they crashed. Hidden within that signal is a small component, called direct waves, that had not previously been well understood," research co-leader Neil Lu, from the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav), said in a statement. "Our new analysis allows us to decipher this component and extract unique information from close to the event horizon."

The team's research presents the intriguing possibility that scientists could use gravitational waves to study these mysterious black hole boundaries.

Event horizons and the point of no return

The concept of an event horizon first emerged through solutions to the equations of Albert Einstein's 1915 theory of gravity, general relativity. These solutions were developed by Karl Schwarzschild while serving with the German army on the Eastern Front in the First World War.

Schwarzschild found a point around a body with mass at which the escape velocity, the speed needed to escape the gravitational grip of that body, exceeds the speed of light. Also known as the Schwarzschild radius, the size of that boundary depends on the mass of the body. So the Schwarzschild radius for the sun would be about 1.86 miles (3 kilometers) from its center of mass; for the Earth, it would be just 0.35 inches (9 millimeters) from our planet's center of mass. That's the case with all planets and stars; the Schwarzschild radius is well within the bodies of those objects.

However, for a black hole, the Schwarzschild radius is far from the center of mass, acting as a light-trapping outer boundary: the event horizon. To escape the gravitational grip of a black hole from this point, matter would have to accelerate to a speed faster than the speed of light, which Einstein's theory of special relativity tells us would require infinite energy. Nothing in the universe travels faster than light; thus, nothing escapes the event horizon.

To understand why that shrouds a black hole in mystery, consider how no signal can travel faster than light. That means the event horizon is a one-way barrier for information. A black hole can swallow it, but the event horizon prevents it from spitting information out. We can never observe the interior of a black hole.

It's little wonder scientists are so keen to study event horizons and what happens there. They don't only want to understand the physics of matter engaged on a one-way trip into the maw of a black hole, but the effect on the very fabric of space itself these cosmic titans have.

The immense gravitational influence of black holes means that, as they spin, they drag the very fabric of space along with them, a phenomenon called "frame-dragging" or the Lense-Thirring effect. This introduces another rule about event horizons — not only does nothing escape this boundary, nothing there sits still either. This research brings scientists one step closer to understanding those rules in greater detail than ever before.

"We studied GW250114, the loudest binary black hole signal observed to date, about three times louder than the first gravitational-wave signal detected a decade ago," team co-leader Ling Sun of OzGrav said. "Our analysis shows that this exceptionally loud signal can be used as a powerful probe of the remnant black hole's horizon, allowing us to measure its two fundamental properties: rotation frequency and surface gravity."

The results could also shed more light on the behavior of gravity in the most extreme environment in the universe, at the very edge of a black hole.

"These measurements mark a first step towards future tests of general relativity with direct waves," Lu said.

The research was published on Wednesday (June 24) in the journal Nature.

The most mysterious and yet ubiquitous stuff in the cosmos, dark matter is effectively invisible. This is simply because it doesn't interact with light. But what if instead of trying to see dark matter, scientists attempted to hear it instead?

New research suggests dark matter could leave a tiny but discernible imprint in the cacophony of ripples in spacetime called "gravitational waves" that ring through the cosmos when two black holes slam together and merge. However, this is only if spinning black holes can "churn" dark matter like cosmic butter. (We'll get to that shortly.)

The team behind this new research suggests that if two black holes merge in a region of space populated by dense dark matter clouds, then the gravitational waves emerging from the event could carry the imprint of dark matter across the universe. And it's possible, they say, that our detectors could find that imprint. This would be akin to someone coughing at a Metallica concert, and that cough being only discernible over the fury of "Seek and Destroy" or "Master of Puppets" with the most sensitive instruments.

Fortunately, when it comes to detecting gravitational waves from colliding black holes, humanity's instruments, such as LIGO (Laser Interferometer Gravitational-Wave Observatory), are getting more and more sensitive all the time. And in preparation for a time when such imprints could become even more easily logged in gravitational wave data, this team developed a method that predicts just what shape a gravitational wave should take when moving through dark matter, rather than empty space.

"Using black holes to look for dark matter would be fantastic," team member Rodrigo Vicente, a researcher at GRAPPA (Gravitation Astroparticle Physics Amsterdam), said in a statement. "We would be able to probe dark matter at scales much smaller than ever before."

I can't believe it's not butter

Dark matter represents such a puzzle because, despite being invisible to us, it still "outweighs" ordinary matter by a ratio of about five to one.

Its lack of interaction with light means it can't be composed of protons, neutrons and electrons — the particles that compose atoms. That's because atoms compose all the "ordinary matter" we see around us, from stars and planets to the device you're reading this article on and our own bodies. In other words, atoms do interact with light (more technically, electromagnetic radiation). In fact, the only way astronomers know dark matter exists is via its interaction with gravity and the way this interaction curves spacetime, indirectly influencing ordinary matter and light.

With this knowledge, scientists have been hunting for particles outside the Standard Model of particle physics that could account for dark matter. These particles have a wide range of potential masses and properties, with one hypothetical particle being the "light scalar" proposed to have a mass much smaller than that of an electron. One characteristic of the light scalar would be the fact that dark matter composed of these particles would act like coordinated waves around black holes.

Around a spinning black hole, rotational energy would be transferred to light scalar dark matter, amplifying its density, almost like a paddle churning cream into butter. If this dark matter "butter" gets dense enough, it could affect gravitational waves from merging black holes, leaving a telltale imprint.

After determining what this signature would look like, Vicente and colleagues searched through data gathered by LIGO and its fellow gravitational wave detectors, KAGRA (Kamioka Gravitational Wave Detector) and Virgo, focusing on 28 of the clearest signals from merging black holes. Of these, 27 appeared to have come from mergers that occurred in the relative vacuum of space. One signal, however, GW190728, first heard on July 19, 2019, and the result of merging binary black holes with a combined mass of 20 times that of the sun and located an estimated 8 billion light-years away, seemed to carry the telltale trace of this merger occurring in a region of dense, "buttery" dark matter.

The team behind this research is quick to point out that this can't be considered a positive detection of dark matter, but does say it gives us a hint at what to look for and thus where to direct follow-up investigations — something that could be increasingly useful as dark matter detectors on Earth continue into their fifth operating run with boosted sensitivity.

"We know that dark matter is around us. It just has to be dense enough for us to see its effects," said team leader Josu Aurrekoetxea, of the Massachusetts Institute of Technology (MIT) Department of Physics. "Black holes provide a mechanism to enhance this density, which we can now search for by analyzing the gravitational waves emitted when they merge."

The team's results were published on Tuesday (May 12) in the journal Physical Review Letters.

Ripples in the very fabric of space and time called "gravitational waves" may have provided the first tantalizing evidence of tiny black holes born during the Big Bang. These primordial black holes could, in turn, account for most if not all of the universe's most mysterious stuff, known as dark matter.

Unlike stellar mass black holes, primordial black holes weren't born when massive stars died, but instead from fluctuations in density that occurred immediately after the birth of the cosmos. That means they can be much smaller than stellar mass black holes, which have at least the same mass as several suns. These Big-Bang-born "non-astrophysical" black holes can have masses as small as that of an average asteroid or as large as a massive planet.

Yet, primordial black holes remain frustratingly hypothetical despite being first proposed by Stephen Hawking in the 1970s. But now, the first potential hint of their existence comes in the form of a gravitational wave signal "heard" last by LIGO (Laser Interferometer Gravitational-Wave Observatory), which indicated a collision between two black holes, at least one of which has a mass smaller than the mass of the sun.

"The most common black holes form as the result of a supernova, the death of a massive star. So, their masses can range from a few times the sun’s mass to billions of solar masses," University of Miami researcher Nico Cappelluti said in a statement. "We believe our study will aid in confirming that they [primordial black holes] actually do exist."

There remains the possibility that the gravitational wave signal mentioned above was a false alarm, the result of interference or "noise" in LIGO's massive interferometer laser arms. However, Cappelluti and his University of Miami colleague, Alberto Magaraggia, believe that the unusual signal couldn't be caused by anything but a primordial black hole.

And they intend to prove it.

"We attempted to estimate how many primordial black holes may exist in the universe and how many of them LIGO should be able to detect, and our results are encouraging," Magaraggia said. "We predict that subsolar black holes like the one LIGO may have observed should indeed be rare, consistent with how infrequently such events have been seen so far.

"The most plausible explanation for the LIGO signal, which lacks any conventional astrophysical explanation, is the detection of a primordial black hole. And our research indicates that these primordial black holes could account for a significant portion, if not all, of dark matter."

Connecting dark matter and primordial black holes

Dark matter is a pressing puzzle for physicists because, despite accounting for 85% of the universe's matter and thus outweighing the "everyday matter" comprising stars, planets, moons, asteroids, our bodies, and everything we see around us by a ratio of five to one, they have no idea what this stuff actually is. That is partially because, unlike the particles that account for that everyday matter, dark matter doesn't interact with electromagnetic radiation, light to you and me. That makes it effectively invisible, with scientists only able to infer the presence of dark matter due to its interaction with gravity and the knock-on effect this has on light and everyday matter.

In fact, the gravitational influence of dark matter is crucial as the gravity of the visible matter in galaxies alone isn't sufficient to hold them together.

The unusual characteristics of dark matter have prompted scientists to search beyond the standard model of particle physics for particles that could comprise it. Thus far, this search has turned up empty-handed. That has led some scientists to postulate that dark matter could be partially or wholly accounted for by primordial black holes. Like all black holes, primordial black holes have mass and thus interact with gravity and are effectively invisible due to the fact that they are bounded by a light-trapping surface called an event horizon. That makes them a good fit for dark matter.

However, as Cappelluti and Magaraggia concede, as convinced as they are that this mystery signal indicates the existence of primordial black holes, a lot more evidence of these non-astrophysical black holes will be needed before they can be firmly connected to dark matter.

With U.S.-based LIGO and its gravitational wave detector partners, Virgo in Italy and KAGRA in Japan, set for sensitivity boosts and a future wealth of highly sensitive gravitational wave detectors such as the space-based LISA (Laser Interferometer Space Antenna), on the horizon, this could be merely a case of waiting for technology to catch up to theory. But that is nothing new. Considering that gravitational waves were first predicted by Einstein in 1915 and the first successful detection was only made 100 years later in 2015, hunting these ripples in spacetime has always been a waiting game requiring a lot of patience.

"LIGO picked up what is very strong evidence that these types of black holes exist. But we’ll need to detect another such signal or even several others to get the smoking-gun confirmation that they are real," Cappelluti said. "But what is clear is that they cannot be excluded as being real."

The team's research has been accepted for publication in the Astrophysical Journal and is available as a preprint on the paper repository arXiv.

Our catalog of spacetime ripples "heard" by gravitational wave detectors here on Earth has doubled, scientists say, with newly discovered sources ranging from wobbly black hole mergers to the heaviest black hole collision detected to date.

Back in 1915, Albert Einstein predicted that when the most dense and extreme objects in the universe collide, these events would set the very fabric of space and time (united as a 4-dimensional entity called spacetime) ringing. Then, 100 years later, on Sept. 14, 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first detection of these spacetime ripples — they originated from colliding black holes over 1.3 billion light-years away.

Since then, LIGO and its partner gravitational wave detectors Virgo in Italy and KAGRA (Kamioka Gravitational Wave Detector) in Japan have detected a multitude of gravitational waves from colliding black holes, merging neutron stars, and even the odd "mixed merger" between a black hole and a neutron star. The latest data collection from the LIGO-Virgo-KAGRA (LVK) Collaboration reveals the universe is practically humming with gravitational waves from cosmic collisions.

Each new gravitational-wave detection allows us to unlock another piece of the universe’s puzzle in ways we couldn’t just a decade ago," LVK member Lucy Thomas of the California Institute of Technology (Caltech), said in a statement. "It's incredibly exciting to think about what astrophysical mysteries and surprises we can uncover with future observing runs."

More variety

The data that comprises this catalog, dubbed the Gravitational-Wave Transient Catalog-4.0 (GWTC-4), includes 128 incredibly distant gravitational wave sources. It was collected during the fourth observational run of these gravitational wave detectors, which was conducted between May 2023 and Jan. 2024.

Prior to this, and during the first three observing runs of LIGO, Virgo and KAGRA, scientists had only "heard" 90 potential gravitational wave sources. Excitingly, GWTC-4 could technically have been even larger, as around 170 other gravitational wave detections made by LIGO, Virgo and KAGRA haven't yet made their way into the catalog.

"In the past decade, gravitational wave astronomy has progressed from the first detection to the observation of hundreds of black hole mergers," LIGO spokesperson Stephen Fairhurst, a professor at Cardiff University in the U.K., said in the statement. "These observations enable us to better understand how black holes form from the collapse of massive stars, probe the cosmological evolution of the universe and provide increasingly rigorous confirmations of the theory of general relativity."

One aspect of GWTC-4 that really stands out is the variety of events that created these signals. Within this catalog are gravitational waves from mergers between the heaviest black hole binaries yet, each about 130 times as massive as the sun, lopsided mergers between black holes with seriously mismatched masses, and black holes that are spinning at incredible speeds of around 40% the speed of light. In these cases, scientists think the extreme characteristics of the black holes involved in these mergers are the result of prior collisions, providing evidence of merger chains that explain how some black holes grow to masses billions of times that of the sun.

"This dataset has increased our belief that black holes that collided earlier in the history of the universe could more easily have had larger spins than the ones that collided later," LVK member and MIT scientist Salvatore Vitale said in the statement.

GWTC-4 also includes two new mixed mergers involving black holes and neutron stars.

"The message from this catalog is: We are expanding into new parts of what we call 'parameter space' and a whole new variety of black holes," LVK member Daniel Williams, of the University of Glasgow in the U.K., said in the statement. "We are really pushing the edges, and are seeing things that are more massive, spinning faster, and are more astrophysically interesting and unusual."

The catalog also demonstrates just how sensitive the LVK detectors have become. Some of the neutron star mergers occurred up to 1 billion light-years away, while some of the black hole mergers occurred up to 10 billion light-years away. These detections have allowed scientists to test the theory that first predicted the existence of both black holes and gravitational waves, Einstein's magnum opus theory of gravity, general relativity.

"Black holes are one of the most iconic and mind-bending predictions of general relativity. They shake up space and time more intensely than almost any other process we can imagine observing," LVK member Aaron Zimmerman, of the University of Texas at Austin, said in the statement. "When testing our physical theories, it's good to look at the most extreme situations we can, since this is where our theories are most likely to break down, and where we have the best chance of discovery.

"So far, the theory is passing all our tests. But we’re also learning that we have to make even more accurate predictions to keep up with all the data the universe is giving us."

The LVK results will soon appear in a special edition of the Astrophysical Journal Letters.

A team of scientists says it's possible to use tiny ripples in space and time, or gravitational waves, to measure the rate at which our universe is expanding. This could solve one of the biggest mysteries in physics today, a disparity in calculating this rate known as the "Hubble tension."

Scientists have known since 1998 that not only is the universe expanding, but also that the expansion rate is accelerating. "Dark energy" was introduced as a placeholder name for the mysterious force driving this acceleration, but there's an outstanding issue surrounding the universe's expansion rate in general, even after over two decades and a half of investigation.

A key part in measuring the rate of our universe's expansion is the Hubble constant. The so-called "Hubble tension" arises from the fact that when you measure the Hubble constant starting from the local and modern-day universe — using Type 1a supernovas for your measurements — you get one value. However, when you begin the calculation starting from the distant and ancient cosmos — and use a major framework in physics called the standard model of cosmology to measure the answer— you get another value. Scientists have therefore long been hunting for a third way to measure the Hubble constant as an extra way of checking its true value. And now, a team of researchers from the University of Illinois Urbana-Champaign and the University of Chicago thinks the answer lies with gravitational waves.

"This result is very significant — it's important to obtain an independent measurement of the Hubble constant to resolve the current Hubble tension," team leader Nicolas Yunes, the founding director of Urbana's Illinois Center for Advanced Studies of the Universe (ICASU), said in a statement. "Our method is an innovative way to enhance the accuracy of Hubble constant inferences using gravitational waves."

Why gravitational waves?

The story of gravitational waves begins in 1915 with Albert Einstein's theory of gravity, known as general relativity. General relativity suggests that objects with mass cause the very fabric of spacetime (the four-dimensional unification of space and time) to warp. What we experience as gravity arises from this warping; the larger the mass, the greater the curvature and the stronger the gravitational effect.

However, general relativity also predicts that when objects accelerate in spacetime, this generates ripples that radiate outward at the speed of light. Those are called gravitational waves. Humanity made the first detection of gravitational waves in 2015, thanks to the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the U.S. The detected waves came from the collision and merger of two massive black holes located around 1.3 billion light-years away. Since then, along with its fellow detectors Virgo and the Kamioka Gravitational Wave Detector (KAGRA) in Italy and Japan, respectively, LIGO has detected gravitational waves from many mergers between pairs of black holes, pairs of ultra-dense neutron stars — and even a mixed merger between a black hole and a neutron star.

Gravitational waves have been proposed as a way of gauging the Hubble constant before, but the issue has been that the accuracy hasn't been there. This team thinks their novel approach has that accuracy, and says it will only increase as our gravitational wave detectors become more sensitive.

"It's not every day that you come up with an entirely new tool for cosmology. We show that by using the background gravitational-wave hum from merging black holes in distant galaxies, we can learn about the age and composition of the universe," Daniel Holz of the University of Chicago said. "This is an exciting and completely new direction, and we look forward to applying our methods to future datasets to help constrain the Hubble constant, as well as other key cosmological quantities."

To use gravitational waves to measure the Hubble constant, scientists need to measure the speed at which the events that launch the waves are receding away from us, not just estimate the distance to said events. That requires astronomers tracking down light, or more precisely, electromagnetic radiation, from these events or even from the galaxies hosting the events

Comparing these two forms of astronomy, unified as so-called "multi-messenger astronomy," scientists can get two values for the Hubble constant: one with electromagnetic radiation alone, one with electromagnetic radiation and gravitational waves. If these techniques don't agree, the Hubble tension persists, and scientists know there is something different about the early universe and the modern universe that is currently unaccounted for.

What the team proposes to use in the technique they call the stochastic siren method are background gravitational waves. This can be thought of as the background hum of the universe from a host of more distant collision events underlying that loud crashing orchestra of relatively close massive black hole mergers.

"Because we are observing individual black hole collisions, we can determine the rates of those collisions happening across the universe," Cousins said. "Based on those rates, we expect there to be a lot more events that we can’t observe, which is called the gravitational-wave background."

Cousins and colleagues reason that for lower Hubble constant values, there is a lower volume of space available for collisions to occur, resulting in a higher collision density and thus a stronger gravitational wave background signal. So, if that background can't be detected, that hints at a higher Hubble constant.

Though the LIGO-Virgo-KAGRA conglomerate isn't yet sensitive to detect the gravitational wave background, the team was still able to apply the stochastic siren method to the data gathered by these detectors. They found that this implied higher Hubble constant values and thus a more rapid universal expansion rate.

That was just a proof of concept for the team; the stochastic siren method could really come into its own over the next six years, as sensitivity increases and scientists can tighten the constraints on the Hubble constant. After this period, gravitational wave detectors should be sensitive enough to "hear" much of the gravitational wave background, and this method could have developed enough to provide an independent measure of the Hubble constant, potentially ending the Hubble tension.

"This should pave the way for applying this method in the future as we continue to increase the sensitivity, better constrain the gravitational-wave background, and maybe even detect it," Cousins said. "By including that information, we expect to get better cosmological results and be closer to resolving the Hubble tension."

The team's research appears in the March 11 edition of the journal Physical Review Letters.

Two supermassive black holes on a dizzying death spiral could soon become visible to astronomers after researchers worked out how, while rotating around each other, these dark, massive behemoths could gravitationally lens the stars behind them.

Pretty much every large galaxy hosts a supermassive black hole, ranging in mass from millions of times that of our sun (for example the black hole in our Milky Way galaxy, Sagittarius A*) to billions of solar masses. Ordinarily, galaxies have just one supermassive black hole at their hearts, but when two galaxies merge, their black holes can fall towards each other, eventually coming into each other's orbit and, long after that happens, merging in a burst of gravitational waves.

So far, the only binary supermassive black holes that astronomers have identified are widely separated by hundreds or thousands of light-years. For the future, the European Space Agency has planned a space-based gravitational-wave detector called LISA, the Laser Interferometer Space Antenna, to detect the low frequency gravitational waves emitted by merging supermassive black hole binaries. Chinese scientists have also proposed a similar mission called TianQin. But there has been no other known way of spotting such binaries — until now, perhaps.

"The prospect of identifying in-spiraling supermassive black hole binaries years before future space-based gravitational-wave detectors come online is extremely exciting," said Bence Kocsis of the University of Oxford in a statement. "It opens the door to true multi-messenger studies of black holes, allowing us to test gravity and black-hole physics in entirely new ways."

Kocsis is part of a team of astronomers from Oxford and the Max Planck Institute for Gravitational Physics in Germany who have shown how gravitational lensing by binary black holes can reveal their presence in distant galaxies.

Gravitational lensing is a phenomenon caused by massive objects bending the fabric space-time around them due to the impact of their gravitational pull, altering the path of light moving along this warped fabric such that background objects can appear magnified, or sometimes even split into multiple images.

When there is just a single black hole, a background star has to be perfectly aligned with it in order to be lensed. However, for a binary black hole, the situation changes.

"The chances of starlight being hugely amplified increases enormously for a binary compared to a single black hole," said Kocsis.

A binary black hole acts like a pair of rotating lenses, as the black holes orbit their common center of mass. This produces a diamond-shape zone of quasi-periodic lensing events called the 'caustic curve', and along this curve, the lensing is amplified in intensity.

The result is that background stars aligned with the caustic curve will periodically appear to flash, as their light is amplified on timescales of several years corresponding to the orbital period of the black holes. We probably will not even see the star at other times, so distant are the galaxies hosting binary supermassive black holes.

"As the binary moves, the caustic curve rotates and changes shape, sweeping across a large volume of stars behind it," said Hanxi Wang, a Ph.D. student at Oxford, in the statement. "If a bright star lies within this region, it can produce an extraordinarily bright flash each time the caustic passes over it. This leads to repeating bursts of starlight, which provide a clear and distinctive signature of a supermassive black-hole binary."

However, this situation doesn't remain unchanged forever, because the orbits of the black holes are shrinking.

The black holes lose orbital energy to each other, and this energy is transported away as gravitational waves. As they get closer to each other, the black holes begin to orbit faster and faster. It takes millions of years for two black holes to lose enough orbital energy for them to merge, but this shortening of their orbits could become apparent in changes to the caustic curve. These changes would modify the modulation of the frequency of the lensing events and their peak brightness. The mass of the two black holes could also be encoded into the caustic curve.

The modulation to the frequency and peak brightness of the lensing events would take thousands or millions of years to become noticeable. At best, astronomers can only take a snapshot of any given binary supermassive black hole system. However, observe enough similar systems at different stages in their orbital evolution and the snapshots could be put together to tell a larger story.

Fortunately, the forthcoming detailed surveys of the night sky by the Vera C. Rubin Observatory in Chile and the Nancy Grace Roman Space Telescope that is set for launch in 2027 should be powerful enough to spot many lensing events from binary supermassive black holes in faraway galaxies. Then, when it is operational, hopefully some time in the 2030s, LISA could partner with the survey telescopes to perform a detailed multi-messenger (electromagnetic waves and gravitational waves) census of black holes in the universe that are spiraling to an inevitable merger.

The research was published on Feb. 12 in the journal Physical Review Letters.

A new simulation created using a NASA supercomputer has shown how things get messy for merging neutron stars even before they slam together; their magnetospheres, the most powerful magnetic fields in the known universe, entwine and generate chaos.

Neutron stars are the most extreme stellar objects in the universe, created when massive stars die in powerful supernova explosions. These objects are so dense that a scooped-out teaspoon of their matter would weigh something like 10 million tons, about the same as 85,000 adult blue whales, if brought to Earth.

With that considered, it is hardly surprising that things get extremely violent when two neutron stars slam into each other and merge. In fact, such a collision creates the only environment in the universe so turbulent that gold, silver, plutonium and other metals heavier than iron can be forged. Not even the raging hearts of the most massive stars are capable of this feat of elemental alchemy.

"Just before neutron stars crash, the highly magnetized, plasma-filled regions around them, called magnetospheres, start to interact strongly," team leader Dimitrios Skiathas, a researcher at NASA's Goddard Flight Center, said in a statement. "We studied the last several orbits before the merger, when the entwined magnetic fields undergo rapid and dramatic changes, and modeled potentially observable high-energy signals."

What makes neutron stars so extreme?

When stars with around the same mass as the sun run out of hydrogen, the fuel necessary for nuclear fusion in their cores, their cores collapse and their outer layers swell out and are eventually lost. This leads to the stars ending their lives as smoldering stellar embers called white dwarfs.

However, the situation is different for stars with around 10 times the mass of the sun and more. When their hydrogen-depleted cores collapse, the extra mass generates the pressure and temperatures needed to allow the helium, created in these cores over millions of years of hydrogen fusion, to fuse, forming even heavier elements.

This repeated process of fuel exhaustion, collapse and reignition continues until the massive star's heart is filled with iron. When this final collapse happens, shockwaves ripple out to the star's outer layers, which are blown away in a supernova explosion, taking with them the vast majority of the star's mass.

The result is a stellar remnant with a mass between one and two times the mass of the sun, filled with neutron-rich matter crammed into a width of around 12 miles (20 kilometers). The rapid crushing down of this stellar core doesn't just create a body of incredible density, but also creates magnetic fields that can be 1 quadrillion times stronger than Earth's magnetosphere.

Massive stars are often found in binary pairs with a stellar companion, and in these cases, when both stars die, a neutron star binary is the result. As the two dead stars swirl around each other, they generate ripples in spacetime called gravitational waves, which carry away angular momentum. This results in the neutron star binary tightening. In other words, the stellar remnants move closer, causing them to emit gravitational waves of higher frequencies, losing angular momentum more rapidly and drawing together even faster.

This ends when the neutron stars are close enough to each other for their gravity to take over, leading to an inevitable collision and merger. This causes a blast of high-energy radiation called a gamma-ray burst (GRB), a final screech of gravitational waves, and sends out a spray of neutron-rich matter, which allows a process to occur that generates very heavy but unstable elements. These eventually decay to create gold, silver, and other metals heavier than iron. The decay also creates a glow that astronomers call a kilonova.

The fact that these events are responsible for the creation of some of our most precious and important elements, as well as bright cosmic phenomena like GRBs and kilonovas, means there has been a heavy bias toward studying the aftereffects of neutron star mergers.

Skiathas and colleagues took a different approach, looking in more depth at what happens prior to the neutron stars meeting.

Messy magnetism

To consider the 7.7 milliseconds prior to neutron stars merging, the team turned to NASA's Pleiades supercomputer at NASA’s Ames Research Center, creating over 100 simulations of a system of two neutron stars, each with around 1.4 times the mass of the sun.

"In our simulations, the magnetosphere behaves like a magnetic circuit that continually rewires itself as the stars orbit. Field lines connect, break, and reconnect while currents surge through plasma moving at nearly the speed of light, and the rapidly varying fields can accelerate particles," team member Constantinos Kalapotharakos of NASA Goddard said in the statement. "Following that nonlinear evolution at high resolution is exactly why we need a supercomputer!"

The team's main aim was to investigate how the magnetic fields of these stellar remnants impacted light, or electromagnetic radiation in technical terms, during the final orbits of the neutron stars around each other.

"Our work shows that the light emitted by these systems varies greatly in brightness and is not distributed evenly, so a far-away observer’s perspective on the merger matters a great deal," team member Zorawar Wadiasingh of the University of Maryland, College Park, and NASA Goddard, added in the statement. "The signals also get much stronger as the stars get closer and closer in a way that depends on the relative magnetic orientations of the neutron stars."

The simulations revealed that respective magnetic fields of the neutron stars swept out behind them as they orbited each other, connecting the stellar remnants, then breaking, then reconnecting once again.

The researchers were also able to use Pleiades to simulate how electromagnetic forces impacted the surfaces of the neutron stars. The aim of this was to determine how magnetic stress accumulates in such systems, but future modeling will be needed to determine how magnetic interplay plays a role in the final moments of a neutron star merger.

"Such behavior could be imprinted on gravitational wave signals that would be detectable in next-generation facilities," team member and NASA Goddard researcher Demosthenes Kazanas said in the statement. "One value of studies like this is to help us figure out what future observatories might be able to see and should be looking for in both gravitational waves and light."

The researchers were able to use the simulated magnetic fields to identify the points where the highest-energy emissions were created and how these emissions would propagate through the environment of the neutron star merger.

The researchers found that regions around neutron star mergers produce gamma-rays with high energy, but this radiation was unable to escape. That was because gamma-ray photons, individual particles of light, were rapidly transformed into pairs of electrons and positrons. However, lower-energy gamma-rays were able to escape the neutron star merger along with even lower-energy radiation like X-rays.

This means future gamma-ray space telescopes, particularly those with wide fields of view, could be used to detect signals from neutron stars on the brink of merging. One other way these systems could be studied before a merger in the future is via the detection of gravitational waves.

The NASA/European Space Agency project Laser Interferometer Space Antenna (LISA) could be particularly useful in this regard. Set to launch in the mid-2030s, LISA will be the first space-based gravitational wave detector, benefiting from a much greater sensitivity than the current generation of Earth-based detectors, including the Laser Interferometer Gravitational-Wave Observatory (LIGO).The team's results were published on Nov. 20, 2025 in The Astrophysical Journal.

Scientists may have solved a cosmic mystery that has been troubling them since the James Webb Space Telescope (JWST) began observations back in 2022.

When astronomers started looking back into the early days of the universe with the cutting-edge observatory, they discovered supermassive black holes that appear to have formed prior to the universe being 1 billion years old, something our current models of the cosmos can't explain But a new study has found that a black hole "feeding frenzy" may explain how these cosmic monsters were born so early in the universe's history.

"We found that the chaotic conditions that existed in the early universe triggered early, smaller black holes to grow into the super-massive black holes we see later, following a feeding frenzy which devoured material all around them," research leader Daxal Mehta of Maynooth University said in a statement. "We revealed, using state-of-the-art computer simulations, that the first generation of black holes – those born just a few hundred million years after the Big Bang grew incredibly fast, into tens of thousands of times the size of our sun."

Performing complex computer simulations, this team of researchers found that the turbulent and dense-gas-rich conditions in the first galaxies may have allowed black holes to enter into brief phases of mega-gluttony, exceeding a barrier known as the "Eddington limit." This limit determines how much material can fall to a body like a star or black hole before the radiation generated by that accretion pushes further matter away, emptying the central object's larder of gas and dust, thus cutting off its food supply.

Periods of super-consumption that defy this limit are known as "super-Eddington accretion" and serve as the missing link between black holes that form when massive stars die in supernova explosions and monstrous supermassive black holes.

Supermassive black holes are like six-foot toddlers

Supermassive black holes with masses millions or even billions of times that of the sun sit at the heart of all large galaxies in the modern 13.8 billion-year-old universe, which isn't troubling to explain at all, as they have had plenty of time to grow.

The issue is the discovery of supermassive black holes as early as 500 million years after the Big Bang, a population that the JWST has routinely been uncovering for the last three and a half years. That is because the merger and feeding processes that are thought to allow black holes to achieve supermassive status are thought to take at least 1 billion years.

"It's like seeing a family walking down the street, and they have two six-foot teenagers, but they also have with them a six-foot-tall toddler," research team member and Maynooth University scientist John Regan previously told Space.com. "That's a bit of a problem. How did the toddler get so tall? And it's the same for supermassive black holes in the universe. How did they get so massive so quickly?"

The team's simulations suggest that a super-Eddington feeding frenzy could have allowed the first generation of black holes to gorge on the dense gas of the early cosmos to reach masses of tens of thousands of times that of the sun. While that doesn't get us to supermassive black holes, it provides a significant head start on the merger process that would see black holes of increasing size collide and fuse together to birth an even more massive black hole.

"These tiny black holes were previously thought to be too small to grow into the behemoth black holes observed at the center of early galaxies," Mehta said. "What we have shown here is that these early black holes, while small, are capable of growing spectacularly fast, given the right conditions."

The team's research could help scientists determine whether early supermassive black holes started out as "light seeds," with ten to a few hundred times the mass of our sun, or as "heavy seeds," with as much as 100,000 times the mass of the sun. Previously, it had been theorized that only heavy seeds would be massive enough to facilitate the rapid growth of supermassive black holes.

"Now we're not so sure," Regan said. "Heavy seeds are somewhat more exotic and may need rare conditions to form. Our simulations show that your 'garden variety' stellar mass black holes can grow at extreme rates in the early universe."

The team's research doesn't just suggest a new avenue for supermassive black hole growth, but it also shows how important high-resolution simulations are in our investigation of the early cosmos.

"The early universe is much more chaotic and turbulent than we expected, with a much larger population of massive black holes than we anticipated, too," Regan said.

As for collecting evidence of this theory, that may be a job not for the JWST or any other traditional astronomical device, but for instruments designed to detect the tiny ripples in space known as gravitational waves that mergers such as this radiate. Of particular importance could be the first space-based gravitational wave detector, the Laser Interferometer Space Antenna (LISA), a joint European Space Agency/ NASA mission set to launch in 2035.

"Future gravitational wave observations from that mission may be able to detect the mergers of these tiny, early, rapidly growing baby black holes," Regan concluded.

The team's research was published on Wednesday (Jan. 21) in the journal Nature Astronomy.

Scientists may have "heard" the first tantalizing hints of long-theorized primordial black holes born during the Big Bang. The potential detection of these tiny black holes that could be the size of a coin or even as small as a fraction of the size of an atom came from the detection of ripples in spacetime called gravitational waves by two Earth-based detectors, the Laser Interferometer Gravitational-wave Observatory (LIGO), and Virgo.

The LIGO-Virgo collaboration has been routinely detecting gravitational waves launched through the fabric of space by mergers between black holes and collisions between extreme stellar remnants called neutron stars, since 2012. On Nov. 12, however, the LIGO-Virgo-KAGRA collaboration issued an automated alert for a black hole merger that was anything but routine.

When the signal from the event designated S251112cm was observed, it revealed that one of the objects involved had a mass way too small to be either a stellar-mass black hole or a neutron star, both of which are stellar remnants born from the collapsing core of a dying massive star and have masses greater than that of the sun. "If this turns out to be real, then it's enormous," Durham University theoretical physicist Djuna Croon, who was not involved in the gravitational wave observation, told Science. "This is not an event we can explain by conventional astrophysical processes." However, that "if" is a very, very big one.

Gravitational wave astronomer and LIGO team member, Christopher Berry, shared the LIGO-Virgo alert of Bluesky, writing: "Interesting #GravitationalWave candidate #S251112cm potentially from a *subsolar* mass source."

Oo! Interesting #GravitationalWave candidate #S251112cm potentially from a *subsolar* mass sourceIf real, the source is probably has chirp mass ~0.1–0.87 solar massesFalse alarm rate 1 in 6.2 yrGraceDB gracedb.ligo.org/superevents/...GCN gcn.nasa.gov/circulars/42...Rating 📏🍬[🧪🔭⚛️]

— @cplberry.bsky.social (@cplberry.bsky.social.bsky.social) 2025-11-28T20:06:05.752Z

Later, the University of Glasgow researcher added that there was still a significant chance of this being a false alarm, resulting from noise in the detectors. Current estimates suggest that the rate for false alarms in terms of this type of detection is around one every four years. For signals from "ordinary" black hole and neutron star mergers, which are detected frequently, this is a small margin of error, but for a signal as rare as S251112cm, it casts a large shadow of doubt.

However, primordial black holes have been speculated about for a long time but have thus far proved elusive, and that makes even the slightest chance of a potential detection something that is very exciting indeed.

What are primordial black holes?

Generally, when we use the term "black hole," what we are referring to is a stellar-mass black hole. That is a black hole with a mass between 5 and 100 times the mass of the sun that is born when the core of a massive star with at least 10 solar masses collapses, triggering a supernova that blows away that star's outer layers.

The other common usage of the term "black hole" refers to the supermassive black holes that lurk at the hearts of all large galaxies. With masses larger than millions to billions of suns, supermassive black holes are much too massive to have formed from a single star, so scientists theorize that these cosmic titans grow from repeated mergers between successively larger and larger black holes.

Primordial black holes are believed to have formed long before even the first stars, directly from overly dense pockets in the steaming-hot "soup" of plasma that filled the universe in the first few seconds after the Big Bang.

Primordial black holes have been proposed to have masses ranging from 1/100,000th that of a paperclip to 100,000 times that of the sun, a mass range that encompasses "sub-stellar masses." They are often referred to as "non-astrophysical black holes," which is based on the fact that they don't rely on stars to be created.

If primordial black holes exist, then they could be major players in how the universe has evolved over time, or they could explain one of the most pressing mysteries in modern cosmology: the nature of dark matter.

Dark matter is puzzling to scientists because, despite accounting for around 85% of the matter in the universe, it is effectively invisible because it doesn't interact with electromagnetic radiation. That lack of interaction means that we can only infer the existence of dark matter via its interaction with gravity, which affects space-time and subsequently impacts ordinary matter and light. The lack of direct interaction with light has prompted scientists to search for potential candidates for dark matter outside the Standard Model of particle physics.

Primordial black holes are an appealing dark matter candidate because their existence is within the remit of our current models of the universe, meaning they don't require physics beyond the standard model. Thus far, however, if they exist, primordial black holes have evaded any attempts to detect them, and that may be because they simply aren't around in the modern cosmos anymore.

According to Stephen Hawking, black holes leak heat into the universe in the form of "Hawking Radiation." This causes them to gradually evaporate before a final explosion, but this process slows as the mass of a black hole increases. That would give supermassive black holes lifetimes that exceed the predicted lifespan of the universe. However, it would mean that very light primordial black holes could have evaporated within seconds of their formation, while larger examples could still be evaporating in the cosmos today.

A needle in a cosmic haystack

If this signal is more than a false alarm, then researchers currently can't account for it with the collision of any known astrophysical bodies. The alert from LIGO-Virgo has enabled astronomers to begin searching for an explosion that accompanied the gravitational wave signal. However, the gravitational wave detectors were only able to narrow down the source of this signal to a region of sky equivalent to around 6,000 times the width of the moon, which makes looking for an accompanying electromagnetic signal akin to searching for a needle in a cosmic haystack.

That means for the moment, researchers only have this gravitational wave signal to assess in order to investigate the nature of this merger. That provides more information than it may initially seem, however. Gravitational wave scientists have the opportunity to study the "hum" of gravitational waves preceding the merger to determine the identity of the two objects that were spiraling together.

Unfortunately, we may never know if this really is a signal from a primordial black hole. That is, unless more similar signals are detected, something scientists say is a slim possibility.

"It seems unlikely that we’ll actually know with certainty whether this alert was real or not," Croon concluded.

Scientists have "heard" the symphony of two newborn black holes — each created when its respective parent black holes crashed together and merged. One of those collision events, in fact, was the first of its kind.

The detection of the baby black holes and information about the four parent black holes that forged them came courtesy of ripples in spacetime, or gravitational waves, caused by the violent cosmic events that gave birth to them. Those waves were registered by the LIGO (Laser Interferometer Gravitational-Wave Observatory), Virgo, and KAGRA (Kamioka Gravitational Wave Detector) gravitational wave detectors.

The LIGO-Virgo-KAGRA collaboration detected the first merger, designated GW241011, on Oct. 11, 2024. It was the result of a black hole with around 17 times the mass of the sun crashing into its partner black hole with a mass around seven times that of our star. The event was calculated to have happened around 700 million light-years from Earth. Decoding the resultant gravitational wave signal revealed a couple of things: The masses of the black holes involved as well as the fact that the larger of the pair is one of the most rapidly spinning black holes ever observed.

Less than one month after this groundbreaking detection, on Nov. 11, 2024, the gravitational wave instruments "heard" another newborn black hole screaming after the violent collision of its progenitors. This signal, GW241110, originated from a collision between black holes with 16 and eight times the mass of the sun and occurred about 2.4 billion light-years away. This signal revealed that one of the black holes involved was spinning in the opposite direction of its orbit around the other black hole, the first time such a characteristic has been seen for merging binary black holes.

"Each new detection provides important insights about the universe, reminding us that each observed merger is both an astrophysical discovery but also an invaluable laboratory for probing the fundamental laws of physics," Carl-Johan Haster, an assistant professor of astrophysics at the University of Nevada, Las Vegas (UNLV), said in a statement. "Binaries like these had been predicted given earlier observations, but this is the first direct evidence for their existence."

Both events indicate the existence of so-called second-generation black holes.

"GW241011 and GW241110 are among the most novel events among the several hundred that the LIGO-Virgo-KAGRA network has observed," Stephen Fairhurst, LIGO Collaboration spokesperson and a Cardiff University professor, said in the statement. "With both events having one black hole that is both significantly more massive than the other and rapidly spinning, they provide tantalizing evidence that these black holes were formed from previous black hole mergers."

Black holes: the second generation

The idea that the detected black holes are second-generation comes from the difference in size between the larger black holes and their smaller companions in the two mergers. The more diminutive black holes appear to have been almost half the mass of their companions. The orbit-opposing orientation of the larger black hole's spin in the merger that produced the signal GW241110 is also evidence of a prior merger having produced that dominant black hole.

The process of black hole growth by collision after collision is known as "hierarchical merger." This is believed to occur in densely populated regions like star clusters, where black holes are more likely to meet and coalesce over and over again, resulting in subsequently larger black holes.

GW241011 offers scientists the opportunity to probe the limits of Albert Einstein's 1915 theory of gravity, general relativity, from which the concept black holes and gravitational waves both emerged.

For instance, the rapid rotation of the black hole involved in this particular merger deforms the object, and that leaves a unique impression in the gravitational waves it emits. This means that this event can be compared to general relativity and the predictions physicist Roy Kerr made using Einstein's theory concerning rotating black holes. The black holes of GW241011 conformed to Kerr's solution to general relativity, the study team explains, helping verify it as well as Einstein's magnum opus theory itself in extreme circumstances. This includes confirming for the third time within a gravitational wave signal (GW241011) the "hum" of a higher harmonic, akin to the overtones of musical instruments.

The LIGO-Virgo-KAGRA collaboration also thinks these gravitational wave signals could be key to unlocking something predicted but never before seen — something outside the limits of general relativity.

Plus, the two black hole mergers behind these signals have the potential to reveal more about an unrelated scientific field: particle physics.

Scientists can use rapidly rotating black holes to test the hypothesized existence of ultralight bosons, or particles that exist beyond the Standard Model of particle physics. Should they exist, ultralight bosons should draw the rotational energy from spinning black holes. How much energy these particles extract and how much they slow black holes down is dependent on their mass.

The revelation that the progenitor black hole of the merger behind GW241011 is still rotating at a rapid rate after millions (or even billions) of years since the merger that created it seems to rule out a range of ultralight boson masses.

"The detection and inspection of these two events demonstrate how important it is to operate our detectors in synergy and to strive to improve their sensitivities," Francesco Pannarale, co-chair of the Observational Science Division of the LIGO-Virgo-KAGRA Collaborations and professor at Sapienza University of Rome, said. "The LIGO and Virgo instruments taught us yet some more about how black hole binaries can form in our universe, as well as about the fundamental physics that regulates them at the very essence.

"By upgrading our instruments, we will be able to dive deeper into these and other aspects with the increased precision of our measurements."

The team's research was published on Tuesday (Oct. 28) in the Astrophysical Journal Letters.

Ripples in spacetime from both the merger of supermassive black holes and from the Big Bang can potentially be distinguished by the "beats" one of them plays, a new study suggests.

During the moment of the Big Bang, "quantum fluctuations" in the universe expanded in tandem with space ballooning outward, producing what are known as primordial gravitational waves. In theory, those waves should still ripple through the universe today. They would be joined by gravitational waves coming from various other sources, such as supermassive black holes that have collided and merged in different galaxies. Together, these waves as well as the Big Bang ones should produce a background of faint ripples throughout the cosmos. So, how do we find the ripples?

To search for this gravitational-wave background, astronomers are ingeniously using arrays of pulsars. These are dead stars that spin fast enough for beams of radiation emitted from their magnetic poles to flash repeatedly in our direction, appearing as radio pulses. And importantly, pulsars are highly efficient time-keepers — among the most precise in the universe. Any deviation in their pulses is a sign that something is amiss.

If space is filled with ambient gravitational waves from inflation and supermassive black-hole binaries, then every so often one of these waves is going to ripple its way through the space between us and nearby (on a galactic scale, of course) pulsars. If a number of pulsars in roughly the same region of the sky suddenly show the same deviation in the timing of their pulses, it's a big clue that a gravitational wave has passed between us and them.

In 2023 NANOGrav, the North American Nanohertz Observatory for Gravitational Waves, working alongside similar experiments in Australia, Europe and India, released results that strongly suggest they had detected evidence for the gravitational-wave background using pulsar timing arrays. Although the results are not yet for certain, if and when they are shown to be then the next step will be trying to disentangle all the sources of these ambient gravitational waves.

Hideki Asada and Shun Yamamoto, who are physicists from Hirosaki University in Japan, have an idea about that.

"In our paper, we explored the situation where a nearby pair of supermassive black holes produces a particularly strong signal," said Asada in a statement. "If two such systems have very similar frequencies, their waves can interfere and create a beat pattern, like in acoustics. That feature could, in principle, allow us to distinguish them from the stochastic background of inflation."

The gravitational waves from two supermassive black hole binaries with similar gravitational-wave frequencies (in the nanohertz regime, resulting in wavelengths many light-years long), resulting from the black holes having similar masses and similar separations, could enter a state of superposition as they pass over each other. This would result in constructive and destructive interference as the peaks and troughs of the gravitational waves align, imprinting a modulation on the pulsar timing deviations that could be detected by more sensitive instruments than are currently available.

By distinguishing the contribution from supermassive black hole binaries, astronomers will be able to get a handle on estimating how many such such systems are in the universe and determining their masses. Even more tantalizingly, accounting for gravitational waves from supermassive black holes will separate them from primordial gravitational waves. These were produced during inflation, which took place in the first tiny fraction of a second, and distinguishing primordial them will assist cosmologists in learning more about the dawn of creation.

Asada and Yamamoto's research is published in the Journal of Cosmology and Astroparticle Physics.

Sept. 14, 2015, was one of the most important days in science history. It marked the first-ever detection of gravitational waves, tiny ripples in space-time (the four-dimensional union of space and time), a milestone notched by the Laser Interferometer Gravitational-Wave Observatory (LIGO).

Since that day, LIGO — composed of two highly sensitive laser interferometers located in Hanford, Washington and Livingston, Louisiana — has been joined by two smaller gravitational wave observatories: Virgo, which came online in Italy on Aug. 1, 2017, and the Kamioka Gravitational Wave Detector (KAGRA) located in Japan, in late 2019.

Over the course of four operating runs, separated by shutdowns to allow for improvements and upgrades, the LIGO-Virgo-KAGRA instruments have become so sensitive that they can now measure distortions in space-time caused by gravitational waves that are 1/10,000 the width of a proton, or 700 trillion times smaller than the width of a human hair. Together, the LIGO-Virgo-KAGRA collaboration has now detected over 300 gravitational wave signals, opening a completely new window to the universe that allows scientists to hear some of the most extreme and violent cosmic events.

Here, Space.com takes you through some of the most important gravitational wave breakthroughs that have occurred since 2015.

While these milestones come in no particular order, there is only one place we can start...

1. Proving Einstein right! The first gravitational wave detection

On Sept. 14, 2015, ripples in space-time washed over Earth that were generated by the merger of two black holes, each with a mass of around 30 times that of the sun. This signal, which would come to be known as GW150914 (GW for "gravitational wave" and the following numbers for the date of measurement), had been traveling to our planet for 1.4 billion years.

GW150914's arrival and detection confirmed a theory that was first proposed a century earlier by arguably history's most famous physicist, Albert Einstein, in his 1915 theory of gravity, general relativity.

General relativity predicts that objects with mass cause the very fabric of space-time to warp, with gravity arising from this warp. The larger the mass of an object, the greater the warp in space-time it generates, and thus the stronger its gravitational influence.

But general relativity also suggested that, when objects accelerate, they should generate ripples in space-time — gravitational waves. These would be significant enough to measure only for objects of truly massive status, such as black holes swirling around each other in a binary system and eventually merging.

Announced to the public on Feb. 11, 2016, GW150914 represented further validation of general relativity and confirmed that black hole mergers actually occur, creating more massive "daughter" black holes. The find also gave scientists a separate way to investigate the universe alongside "traditional" astronomy, which relies largely on the detection and study of light.

The achievement would earn Rainer Weiss, who passed away just last month, Kip Thorne and Barry Barish the 2017 Nobel Prize in Physics.

2. Heaviest black hole merger

On Nov. 23, 2023, LIGO-Virgo-KAGRA (LVK) detected the gravitational wave signal GW231123, which involved a clash between black holes with masses 100 and 140 times that of the sun. This collision created a daughter black hole with a mass around 225 times that of the sun, with the missing mass converted toa gravitational wave "screech" (which you can learn more about below).

This was the most massive black hole merger detected in gravitational waves to date, with the prior record holder being 2021's GW190521, which was resulted in a daughter black hole with 140 solar masses.

"This is the most massive black hole binary we've observed through gravitational waves, and it presents a real challenge to our understanding of black hole formation," LVK collaboration member and Cardiff University researcher Mark Hannam said of GW231123. "Black holes this massive are forbidden through standard stellar evolution models.

"One possibility is that the two black holes in this binary formed through earlier mergers of smaller black holes."

3. This neutron star merger was golden!

It's not all black hole mergers for LKV. The gravitational wave detectors have also "heard" ripples in space-time from clashes between neutron stars. These are extreme stellar remnants composed of the densest matter in the known universe that, like stellar-mass black holes, are born when massive stars go supernova and die.

On Aug. 17, 2017, LIGO and Virgo detected a signal, GW170817, representing gravitational waves from a collision between neutron stars located around 130 million light-years from Earth. This was the first detection of gravitational waves from anything other than black holes.

This was an important scientific breakthrough, because it is thought that mergers between neutron stars generate the only environment that is extreme and violent enough to allow the fusion processes that can generate elements heavier than iron, like gold, silver and plutonium.

"It immediately appeared to us the source was likely to be neutron stars, the other coveted source we were hoping to see — and promising the world we would see," David Shoemaker, spokesperson for the LIGO Scientific Collaboration and senior research scientist at the Massachusetts Institute of Technology's (MIT) Kavli Institute for Astrophysics and Space Research, said in a statement at the time. "From informing detailed models of the inner workings of neutron stars and the emissions they produce to more fundamental physics such as general relativity, this event is just so rich. It is a gift that will keep on giving."

GW170817 was humanity's first step toward understanding how the gold in your jewelry box was forged. But this list isn't done with this event just yet; its importance to science goes beyond the first detection of a neutron star merger.

4. Best of both worlds: Multimessenger astronomy is born!

As you might imagine, when stellar remnants as extreme as neutron stars collide, there is quite a burst of energy, and not just in gravitational waves, which can be considered gravitational radiation.

Neutron star mergers are also accompanied by flashes of light that astronomers have dubbed "kilonovas." Thus, the first detection of a neutron star merger in gravitational waves offered scientists the unique opportunity to follow this up with "traditional astronomy," which utilizes different wavelengths of the electromagnetic spectrum.

This led to GW170817 becoming one of the most widely studied astronomical events in history, with nearly one-third of the world’s electromagnetic astronomers chasing the gravitational wave detection via traditional astronomy.

Such work paid off, with NASA's Fermi Gamma-ray spacecraft and Europe's INTEGRAL (International Gamma-Ray Astrophysics Laboratory) both independently detecting a gamma-ray burst designated GRB 170817A erupting from this same merger.

This allowed astronomers to determine that the neutron star merger occurred in the galaxy NGC 4993, located about 140 million light-years away.

This was the first successful application of "multimessenger astronomy," which observes cosmic events using more than one form of messenger — in this case, gravitational waves and electromagnetic radiation. The third spoke in this wheel is messengers in the form of high-energy particles, such as neutrinos or cosmic rays generated by cosmic events.

The fact that each of these "messengers" is created by a different astrophysical process means they have the potential to reveal different information about the same source. That makes multimessenger astronomy a powerful new tool in science.

To date, the event that generated GW170817 and launched GRB 170817A remains the only successful observation of an event in both gravitational waves and electromagnetic radiation.

"It is tremendously exciting to experience a rare event that transforms our understanding of the workings of the universe," France Córdova, then the director of the U.S. National Science Foundation (NSF), which funds LIGO, said in a statement at the time. "This discovery realizes a long-standing goal many of us have had — that is, to simultaneously observe rare cosmic events using both traditional and gravitational-wave observatories."

5. For whom the black hole tolls

The emission of gravitational waves from a binary black hole merger comes in three phases. As these orbiting black holes emit gravitational waves, their orbits tighten due to the loss of angular momentum from the system. This leads to the two black holes eventually colliding and merging, sending out a high-pitched gravitational wave "screech" followed by a diminishing "ringdown" of vibrations lasting for a fraction of a second.

"The [daughter] black hole is similar to a bell that rings, producing a spectrum of multiple fading tones that encode information about the bell," Collin Capano from the Albert Einstein Institute said in a statement back in 2023, after he and his colleagues revealed that they had found strong observational evidence of at least two gravitational-wave frequencies existing in a binary black hole ringdown signal.

This ringdown signal, the aforementioned GW190521, can give details of the mass and spin of a resultant daughter black hole, to great precision.

"Achieving this multimode observation – in other words, the detection of two distinct vibration frequencies of a deformed black hole – has been a welcome surprise. It was widely assumed this would not be possible before the next generation of gravitational-wave detectors," Capino said.

The GW190521 ringdown was also significant because it acted as a test of the idea that black holes can be described by just three characteristics: their mass, spin and electric charge. This theory is immortalized by physicist John Wheeler's infamous phrase: "Black holes have no hair."

"GW190521 passed the test and we found no signs of any black hole physics beyond Einstein’s general theory of relativity," Capino's colleague Julian Westerweck said back in 2023. "It is quite remarkable that a theory that is over 100 years old now continues to work so well."

6. Mix it up! Detecting a black hole-neutron star 'mixed merger'

Everybody loves chocolate, and most of us can't get enough peanut butter, but it is when these two treats are mixed that they really come into their own. It turns out that black hole and neutron star mergers are the cosmic equivalent of chocolate peanut butter cups. No wonder scientists spent so long hunting for them.

On Jan. 5, 2020, LIGO/Virgo detected GW200105_162426, a signal from a neutron star with a mass 1.9 times that of the sun colliding with an 8.9-solar-mass black hole. It occurred five years after the detection of the first black hole-black hole merger, and three years after the first neutron star-neutron star merger.

This was the first evidence of a third kind of stellar remnant merger: a neutron star-black hole collision, or a "mixed merger." With peanut butter cups, one is rarely enough, and that turns out to be true for mixed mergers, too.

The second neutron star-black hole collision event was spotted in the form of the signal GW200115_042309, detected just a few days later on Jan. 15, 2020. This neutron star had an estimated mass 1.5 times that of the sun, with its companion being a 5.7-solar-mass black hole.

"With this new discovery of neutron star-black hole mergers outside our galaxy, we have found the missing type of binary," Astrid Lamberts, a scientist with the French national research agency CNRS at Observatoire de la Côte d’Azur, said in 2021. "We can finally begin to understand how many of these systems exist, how often they merge, and why we have not yet seen examples in the Milky Way."

To date, the LIGO-Virgo-KAGRA collaboration has detected and confirmed just two mixed mergers between a neutron star and a black hole, though there is another possible candidate that hasn't been fully vetted yet.

7. The lightest black hole merger is a mixed mystery

On Aug. 14, 2019, LIGO and Virgo detected the gravitational wave signal GW190814 from a merger that occurred 790 million light-years away.

While one of the objects involved was a black hole of 22 to 24 solar masses, the identity of the second object isn't as clear-cut as in the case of the mixed mergers above. That's because its mass is right in the sweet spot between black holes and neutron stars.

With a mass 2.6 times that of the sun, the other component of this merger was either one of the lightest black holes ever seen or one of the heaviest neutron stars. As such, the fact that it was detected earlier than the two 2020 signals means that GW190814 could actually be the first recorded mixed merger.

The merger remains shrouded in mystery. Astronomers can find no electromagnetic counterpart, meaning this could be two merging black holes or a black hole that has completely devoured a neutron star. Solving this puzzle could help us better understand the cycle of life and death experienced by the most massive stars.

8. This one goes up to 11: The loudest gravitational wave ever!

Proving that the LIGO-Virgo-KAGRA collaboration is still at the cutting edge of gravitational wave science, this entry on our list comes from just this month!

On Sept. 10, 2025, LKV team members announced the detection of GW250114, the result of two merging black holes with masses around 32 times the mass of the sun.

What makes GW250114 remarkable is the fact that it is one of the clearest gravitational wave signals ever. So clear, in fact, that it not only further confirmed the theory of general relativity but also verified the theories of other black hole luminaries.

"GW250114 is the loudest gravitational wave event we have detected to date; it was like a whisper becoming a shout." Geraint Pratten, member of the LIGO-Virgo-KAGRA collaboration and a researcher at the University of Birmingham in England, said in a statement. "This gave us an unprecedented opportunity to put Einstein's theories through some of the most rigorous tests possible — validating one of Stephen Hawking's pioneering predictions that when black holes merge, the combined area of their event horizons can only grow, never shrink."

GW250114 gets on the list because it demonstrates just how far LIGO-Virgo-KAGRA has come over the last 10 years.

Read More: Gravitational wave detector confirms theories of Einstein and Hawking: 'This is the clearest view yet of the nature of black holes'

9. Hearing a cosmic symphony

This one isn't LVK-related, but it is a gravitational wave discovery made during the last 10 years, so it still makes the list.

On June 28, 2023, it was revealed that the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) had detected low-frequency gravitational waves, a historic breakthrough that represents 15 years of searching. NANOGrav uses spinning neutron star pulsars as a timing array to detect the tiny fluctuations in space-time caused by gravitational waves.

The gravitational waves detected by LIGO and its collaborators represent a dramatic single "crash" of cymbals from violent events like collisions and mergers; the low-frequency gravitational wave signal NANOGrav heard is more akin to the gentle background harmony of violins.

The strength of the signal represents a gravitational wave orchestra of hundreds of thousands, maybe even millions, of supermassive black holes swirling around each other and eventually merging in the early universe.

"This finding opens up a new low-frequency window on the gravitational universe which will let us study how galaxies and their central black holes merge and grow with time," National Radio Astronomy Observatory astronomer and NANOGrav researcher Scott Ransom told Space.com in 2023.

10. Proving Einstein ... wrong!?!

This may come as a bit of surprise, but while every gravitational wave discovery made since 2015 has verified Einstein's theory of general relativity, ironically, each has also proved the great physicist wrong, too.

That's because Einstein believed that gravitational waves are so faint and so insubstantial, in terms of the displacement of space-time they cause as they wash through the cosmos at near light-speed, that we would never be able to detect them.

Even some of the scientists who were integral to the development of LIGO and the first detection of gravitational waves weren't initially certain such a feat was possible, agreeing with Einstein.

"Rai Weiss proposed the concept of LIGO in 1972, and I thought, 'This doesn't have much chance at all of working,'" Kip Thorne, an expert on the theory of black holes, said in a statement earlier this month. "We had to invent a whole new technology."

The Laser Interferometer Gravitational-Wave Observatory (LIGO) is celebrating 10 years of cutting-edge gravitational wave science by confirming predictions made by physics luminaries Albert Einstein, Stephen Hawking and Roy Kerr — and potentially revealing a path toward a theory of quantum gravity.

LIGO achieved this latest milestone by detecting gravitational waves, or tiny ripples in spacetime. The existence of gravitational waves was first predicted by Einstein in his 1915 theory of gravity, general relativity. The newly detected ripples resulted from the collision of two black holes, each estimated to have a mass around 32 times that of the sun.

In just four days, on September 14, LIGO will celebrate exactly 10 years since it made the very first detection of gravitational waves. This signal, designated GW150914, had traveled about 1.3 billion years to reach Earth. Its detection represented an entirely new method of astronomy — a way to "hear" spacetime ringing after some of the most powerful events in the cosmos rather than "see" them by relying on electromagnetic radiation. Since then, LIGO and its gravitational wave detecting partners, Virgo and the Kamioka Gravitational Wave Detector (KAGRA), have detected a multitude of gravitational wave signals from other black hole collisions, mergers between neutron stars, and even from two rare "mixed mergers" involving a neutron star and a black hole.

"This is the clearest view yet of the nature of black holes," Maximiliano Isi, a member of the LIGO-Virgo-KAGRA collaboration from the Flatiron Institute's Center for Computational Astrophysics, said in a statement. "We've found some of the strongest evidence yet that astrophysical black holes are the black holes predicted from Albert Einstein’s theory of general relativity."

Black hole theories validated by new gravitational wave signal

This newly detected signal, GW250114, stands among previous detections as one of the clearest gravitational wave signals ever.

"GW250114 is the loudest gravitational wave event we have detected to date; it was like a whisper becoming a shout," Geraint Pratten, member of the LIGO-Virgo-KAGRA collaboration and a researcher at the University of Birmingham, said in a separate statement. "This gave us an unprecedented opportunity to put Einstein's theories through some of the most rigorous tests possible — validating one of Stephen Hawking's pioneering predictions that when black holes merge, the combined area of their event horizons can only grow, never shrink."

Hawking's prediction involves the light-trapping outer boundary of a black hole called the event horizon. This marks the point at which the gravitational influence of the black hole becomes so great that not even light moves fast enough to escape its grip. As gravity is related to mass, the size of an event horizon, also known as the Schwarzschild radius — after Karl Schwarzschild, the first physicist to solve the equations of general relativity and inadvertently predict the existence of black holes — also depends on the mass of a black hole.

The greater the mass, the wider the event horizon.

In 1971, Hawking, along with physicist Jacob Bekenstein, predicted that when black holes merge, the total area of the resultant daughter black hole's event horizon would be larger than the area of the progenitor black holes' event horizons combined; the duo said that event horizon would have an area proportional to its level of disorder or "entropy."

GW250114 revealed that the progenitor black holes had a total surface area of around 93,000 square miles (240,000 square kilometers), which is around the size of the entire U.K. The daughter black hole created by the merger, however, has a surface area of 154,000 square miles (400,000 square kilometers), which is about the size of Sweden.

Another prediction verified by this research comes from New Zealand mathematician Roy Kerr, who developed Kerr geometry from general relativity, which describes empty spacetime around a rotating black hole, or a Kerr black hole.

Following mergers between black holes, these systems enter what scientists call a ringdown phase. This sees the daughter black hole vibrating and emitting gravitational waves at very specific frequencies, akin to the changing "voice" of the black hole. Kerr predicted the "voice" of the black hole could be described by two values alone: the mass of the black hole and its spin.

This really sets black holes apart from other celestial objects, like stars, that must be described using a vast range of characteristics. What is extraordinary about this is the fact that a black hole 1 billion times as massive as the sun can be completely characterized by just two numbers: mass and spin.

"Given the clarity of the signal produced by GW250114, for the first time, we could pick out two 'tones' from the black hole voices and confirm that they behave according to Kerr's prediction, obtaining unprecedented solid evidence for the Kerr nature of black holes found in nature," Gregorio Carullo, member of the LIGO-Virgo-KAGRA collaboration and a researcher at the University of Birmingham, said in the statement.

What is fitting about this new detection is the fact it is so similar to the signal that LIGO detected to make history on September 14, 2015, GW150914.

"The detection of a black hole binary with parameters similar to those of GW150914, but three times louder, only a decade after the breakthrough discovery, is owed to the tremendous technological improvements of our instruments, paving the path for precision astronomy with gravitational waves," said LIGO-Virgo-KAGRA collaboration team member Patricia Schmidt, Associate Professor at the University of Birmingham.

Regular and purposeful improvements have been a constant factor in the life of LIGO, which consists of two detectors in Washington and Louisiana that can now measure distortions in spacetime that are 1/10,000 the width of a proton, or 700 trillion times smaller than the width of a human hair.

"Rai Weiss proposed the concept of LIGO in 1972, and I thought 'this doesn't have much chance at all of working,'" Kip Thorne, an expert on the theory of black holes who won the 2017 Nobel Prize in Physics with Weiss and Barry Barish for the development of LIGO, said in a separate statement. "It took me three years of thinking about it on and off and discussing ideas with Rai and Vladimir Braginsky [a Russian physicist], to be convinced this had a significant possibility of success. The technical difficulty of reducing the unwanted noise that interferes with the desired signal was enormous. We had to invent a whole new technology."

Who could have predicted this technology could have been so successful in opening a new window to our study of the universe? Certainly not Einstein, who, when he theorized the existence of gravitational waves, predicted that no instrument of Earth would ever be sensitive enough to detect these spacetime ripples.

"Just 10 short years ago, LIGO opened our eyes for the first time to gravitational waves and changed the way humanity sees the cosmos," Aamir Ali, a program director in the National Science Foundation (NSF) Division of Physics, which has supported LIGO since its inception, said in the statement. "There's a whole universe to explore through this completely new lens, and these latest discoveries show LIGO is just getting started."

Future improvements for LIGO could include the addition of a planned fourth detector, this time located in India, which will improve the precision with which LIGO-Virgo-KAGRA can localize gravitational wave sources.

As for this latest milestone, it is certain that Einstein, Hawking and Weiss, the latter of whom passed away just last month, would all have been thrilled to see LIGO further validate their work.

"If Hawking were alive, he would have reveled in seeing the area of the merged black holes increase," Thorne added.

The team's research was published on Wednesday (Sept. 10) in the journal Physical Review Letters.

A radical new theory regarding the origin of the universe suggests that gravitational waves, tiny ripples in spacetime first predicted by Albert Einstein back in 1915, could have given rise to cosmic matter, eventually spawning galaxies, stars and planets.

The theory aims to do away with a range of speculative and adjustable parameters within the standard Big Bang theory. The fact that these parameters can be so freely modified is challenging, as it means that scientists can't tell if a model of the beginning of the universe truly predicts observations of the modern cosmos, or if it has simply been adapted to fit this picture.

"For decades, cosmologists have been working on a model, the 'inflationary paradigm,' that suggests the universe expanded at an incredible rate, explaining everything we observe today," team leader Raúl Jiménez of the University of Barcelona told Space.com. "The new model suggests that natural quantum oscillations of spacetime itself, gravitational waves, were sufficient to trigger the tiny density differences that ultimately gave rise to galaxies, stars, and planets."

"The inflationary paradigm can explain why our universe is so homogeneous and isotropic [possessing the same amount of matter at the same density in all directions] and that the fluctuations of a quantum origin of the primordial density arise from the amplification of vacuum fluctuations of a scalar field. This is crucial because these primordial quantum fluctuations are precisely the seeds that explain the entire structure of the universe we see today," team member Daniele Bertacca of the University of Padua told Space.com. "But there's a problem: this theory includes too many 'free' or 'tunable' parameters, which can be adjusted at will.

"Too much flexibility in science can be problematic because it makes it difficult to determine whether a model is truly predicting something or simply adapting, a posteriori [after the fact], to observed data."

Inflation not 'inflaton'

The team's model begins with initial cosmic inflation described by an expanding cosmic state called "De Sitter space," which they explain can be considered as a condensation of "gravitons," the hypothetical particles that convey the force of gravity in a similar way that photons are the "messenger particles" (or gauge bosons) of the electromagnetic force.

This de Sitter spacetime would have decayed completely when its near-equilibrium state, when quantum effects became so strong that they caused the universe to become a chaotic quantum system.

This all represents their model, depending on a single energy scale that goes on to account for all predictions of cosmic evolution.

This does away with the need for a range of hypothetical fields and particles, such as the "inflaton" field, a hypothetical field with a high potential energy that generated a repulsive force in the early universe, which caused rapid and exponential inflation in some Big Bang models. Instead, gravitational waves, as natural quantum oscillations of space-time itself, are enough in this model to create the density fluctuations that lead to matter developing structures like galaxies, stars, and planets.

"This was almost 'magical,' since the only free parameter of the de Sitter scale is its energy scale, and due to its complexity and nonlinearity, this turns out to be linked to the observed level of fluctuations," Bertacca said. "It is precisely the elegance and simplicity of the proposed model, and the absence of free parameters, that are key.

"Furthermore, we expect it to be able to elegantly and naturally explain the energy scale and the time course of inflation. Both determine all observable predictions and are necessary to solve the cosmological horizon and flatness problems."

Of course, this is science, not magic, and when it comes to any scientific theory, verification with observational evidence is key. The team thinks that their model could provide fingerprints that can be detected in astronomical data.

"Like all theoretical models, ours must be confirmed by measurements and observations that researchers can analyze, evaluate, and compare with data from ground-based and space-based experiments today and in the near future," Bertacca said. "These gravitational ripples interact and build complexity over time, leading to testable predictions that researchers can now compare with real data."

Data that could confirm or refute this new model includes measurements of a cosmic fossil called the cosmic microwave background (CMB), a field of radiation left over from an event just after the Big Bang. Observations of the large-scale structure of the universe and measurements of primordial gravitational waves could also make or break this new model.

"Our work provides a minimalist yet powerful, elegant, and potentially falsifiable framework. This is science at its best: clear predictions that future observations can confirm or disprove," Jiménez concluded. "Finally, these new results demonstrate that we may not need speculative ingredients to explain the cosmos, but only a deep understanding of gravity and quantum physics. If the model holds true, it could mark a new chapter in the way we think about the birth of the universe."

The team's research was published in July in the journal Physical Review Research.

Deep in the first moments of the Big Bang, the entire cosmos shook and rumbled. Those quakes still reverberate to the present day. It will take the most sensitive instruments ever imagined to reveal those ripples, but if they are discovered, they will change our understanding of the entire universe.

In 1916, Albert Einstein discovered that his theory of general relativity predicted the existence of gravitational waves — ripples in the fabric of space-time caused by anything with mass that accelerates. But gravity is by far the weakest of the known forces, and gravitational waves are weaker still. So, although the idea of gravitational waves was interesting, Einstein believed they could never be detected.

Nearly a century later, a team of physicists set out to prove that gravitational waves could, indeed, be detected. After 25 years of effort, they developed the Laser Interferometer Gravitational Wave Observatory (LIGO). The detector consists of mile-long lasers tuned precisely to monitor vibrations down to the scale of an atomic nucleus.

In 2015, the team finally hit the jackpot, finding the unmistakable signature of gravitational waves released by merging black holes washing over the instrument.

Despite the minute strength of those gravitational waves when they were detected, they were enormously strong when they were produced. Merging black holes release tremendous amounts of energy. In less than a second, they output as much energy as the entire mass of the sun would if it were converted into pure energy.

There is no flash. No explosion. No detonation. The energy released is entirely invisible, purely in the form of gravitational waves. Within a light-year of that merger, anything caught by the waves would be torn to shreds as the competing gravitational forces overwhelmed them.

But as powerful as those events are, they aren't nearly the strongest gravitational waves the universe has made.

Cosmologists think that in the first few moments of the Big Bang, less than a fraction of a second into the existence of the universe, our cosmos underwent a remarkable transformation. It grew by multiple orders of magnitude — the equivalent to swelling your body to the size of the modern-day observable universe. This event, known as inflation, was over and done with in the blink of an eye, and it set the stage for the entire future history of the universe as we know it.

We do not know what powered inflation, why it started when it did, or why it stopped when it did. But we strongly suspect that it happened, because we have indirect evidence for it. During inflation, everything got bigger, including the sub-microscopic quantum foam that constantly roils and bubbles at the tiniest scales.

Inflation turned that quantum foam into merely small variations in density across the universe. That left a lasting impression, as matter eventually collected into the high-density pockets left over from inflation. The cosmic microwave background, released 380,000 years later, contains a faint memory of that initial imprint, as seen in variations in temperature across the sky.

The statistical properties of those patterns match what we expect from inflation. But still, we don't have a direct view into the event itself.

Thankfully, inflation left more than a fingerprint. It shook the cosmos. It triggered the formation of gravitational waves of such ferocity that nothing in the entire history of the universe could ever compete with.

Those gravitational waves still exist today and gently slosh over Earth. But they are very weak, having been stretched out through billions of years of cosmic expansion. They are also very hard to detect, as they have incredibly long wavelengths.

LIGO can see the vibrations caused by black hole mergers because they are brief and sharp, which makes them stand out clearly against the background noise of all of the natural vibrations within the experiment, like seismic waves and even the conversations of people in the lunchroom.

But gravitational waves from inflation, known as primordial gravitational waves, are too long and slow. They hide beneath the noise. No Earthly detector is ever likely to find them.

That's why the next generation of gravitational wave observatories will be in space. The Laser Interferometer Space Antenna (LISA), which is set to launch in the mid-2030s, will consist of a trio of satellites flying 600,000 to 3 million miles (1 million to 5 million kilometers) apart. They will bounce lasers back and forth, looking for any minute change in their distances as gravitational waves wander through the solar system.

LISA has a number of science goals, including finding the waves created by supernovas and supermassive black holes and hunting for primordial gravitational waves. Nobody knows if it will be successful. We don't know how strong primordial gravitational waves were when they were generated, so we don't know how weak they are in the present day.

Over a decade ago, astronomers proposed a successor to LISA, called the Big Bang Observer (BBO). Instead of just three satellites, the BBO would feature dozens of spacecraft coordinating across the width of the solar system with high-powered, ultraprecise lasers.

The BBO would have the sensitivity to detect pretty much any primordial gravitational wave predicted by our theories of inflation. But as of right now, the BBO is just a proposal with no firm plans for continuing.

So, right now, all hopes are pinned on LISA. With luck, it will provide the first direct view into the earliest moments of cosmic history and reveal detailed information about how inflation happened. It will be a view unlike any other — completely invisible, exceptionally quiet and utterly transformative.

Scientists have detected the most massive merger of black holes ever. This titanic collision, "heard" in ripples in spacetime called gravitational waves, involves black holes so massive that it could challenge current models of the universe.

The merger was detected by the gravitational wave detector network LIGO-Virgo-KAGRA (LVK) on Nov. 23, 2023, during the fourth observing run of these three sensitive laser interferometers located in the US, Italy, and Japan.

The merger event that set spacetime ringing with this gravitational wave signal, designated GW231123, involved progenitor black holes with masses of 100 and 140 times that of the sun. These two were so massive that when they merged, they created a "daughter" black hole 225 times the mass of our sun, with the missing mass converted to energy, propelling gravitational waves that rippled out from the violent event.

Prior to GW231123, the most massive black hole created in a merger and detected in gravitational waves had a mass of 140 times that of the sun. This was detected in 2021 as the signal GW190521.

"This is the most massive black hole binary we've observed through gravitational waves, and it presents a real challenge to our understanding of black hole formation," LVK collaboration and Cardiff University researcher Mark Hannam said in a statement. "Black holes this massive are forbidden through standard stellar evolution models.

"One possibility is that the two black holes in this binary formed through earlier mergers of smaller black holes."

The monstrous masses of these black holes are not the only things that make GW231123 so interesting. The signal seems to indicate that prior to the merger, at least one of the progenitor black holes was spinning rapidly. Perhaps as rapidly as the laws of physics allow, in fact.

"The black holes appear to be spinning very rapidly — near the limit allowed by Einstein's theory of general relativity," LVK member Charlie Hoy of the University of Portsmouth said. "That makes the signal difficult to model and interpret. It's an excellent case study for pushing forward the development of our theoretical tools."

Another record broken for LIGO and partners

The Laser Interferometer Gravitational-wave Observatory (LIGO) is no stranger to making history and breaking records. In 2015, its twin detectors based in Livingston, Louisiana, and Hanford, Washington, made the first ever detection of gravitational waves.

This detection came exactly a century after Einstein had first predicted the existence of gravitational waves in his 1915 theory of gravity, general relativity.

The signal, which would become known as GW150914, was the result of the merger of black holes that created a daughter black hole with a mass around 62 times that of the sun.

Since 2015, LIGO has been joined by the gravitational wave detectors Virgo and the Kamioka Gravitational Wave Detector (KAGRA). This resultant collaboration has now detected over 300 black hole mergers.

Over 200 of these detections have occurred in the fourth operating run of these instruments. As impressive as that is, the high-mass and rapid spin of the black holes that clashed to create GW231123 have pushed the limits of gravitational-wave detection technology and perhaps the bounds of current theoretical models, too.

"This event pushes our instrumentation and data-analysis capabilities to the edge of what's currently possible," LVK member and California Institute of Technology (Caltech) researcher Sophie Bini said. "It's a powerful example of how much we can learn from gravitational-wave astronomy — and how much more there is to uncover."

Fully unlocking the secrets of this signal and others that LVK detected up until the end of its fourth operating run in January 2024 will require the refinement of analysis and interpretation methods.

"It will take years for the community to fully unravel this intricate signal pattern and all its implications," LVK team member Gregorio Carullo of the University of Birmingham said. "Despite the most likely explanation remaining a black hole merger, more complex scenarios could be the key to deciphering its unexpected features.

"Exciting times ahead!"

GW231123 was presented at the 24th International Conference on General Relativity and Gravitation (GR24) and the 16th Edoardo Amaldi Conference on Gravitational Waves in Glasgow, Scotland, on Monday (July 14).

Cosmic echoes from some of the universe's most violent collisions are far more nuanced than scientists had realized, according to new research.

Like the lingering chime of a struck bell, tiny ripples in the fabric of spacetime are created when massive objects like black holes spiral toward each other and merge into a single, larger black hole. These ripples are known as "gravitational waves," and astronomers rely on theoretical models to decode the waves' faint signals, both in the final moments leading up to the merger and in the aftermath.

In theory, as the newly formed black hole behemoth settles into a stable, spinning form, it should radiate a distinctive pattern of gravitational waves — known to astronomers as quasinormal modes (QNMs) — which are shaped by the black hole's intrinsic properties, such as its mass and spin.

Astronomers have long expected these waveforms to follow a predictable pattern, with each mode gradually fading over time — much like the diminishing chime of a struck bell. But in 1997, theorists identified a puzzling exception: one particular mode appeared out of sync, an inconsistency that couldn't be explained by existing models. This anomaly raised the possibility that QNMs do not evolve independently, but instead interact in complex, nonlinear ways, suggesting the internal structure of black holes, and indeed the gravitational waves they emit, might be far more complex than previously thought.

Now, new theoretical work by Hayato Motohashi, an astrophysicist at the University of Tokyo Metropolitan University in Japan, builds on the 30-year mystery. By analyzing the behavior of multiple such QNMs, Motohashi found that the original "dissonance" wasn't a cosmic glitch but rather the result of two modes interacting with each other. This kind of interaction, according to Motohashi, happens regularly across many modes, suggesting it could be a fundamental feature of black hole physics.

Such coupled cosmic ringing is more than just a mathematical curiosity, scientists say. Because gravitational wave signals, including QNMs, are shaped by the geometry of the black hole's spacetime — the "fabric" of the universe surrounding it — analyzing the interactions between modes could lead to more precise "maps" of the black holes themselves.

For example, just as a bell ringing slightly out of tune might indicate a crack or imperfection, deviations in gravitational wave frequencies can point to asymmetries in the black hole's shape. These imperfections, which scientists expect decay over time but leave a temporary imprint on the QNMs, could even reveal signs of new physics, according to the new study.

"Our findings pave the way for rigorous examinations of black holes and the exploration of new physics in gravity," Motohashi wrote in the new study.

This research is described in a paper published April 9in the journal Physical Review Letters.

Scientists have discovered a new way to probe the interiors of neutron stars by using gravitational waves to turn them into "cosmic tuning forks." The reverberations of such ripples in spacetime could reveal the interiors of these extreme stellar remains.

Born when massive stars die, neutron stars have up to two times the mass of the sun crammed into a diameter of about 12 miles (20 kilometers). This means they are composed of the densest matter in the known universe. But because of their extreme characteristics, the interiors of these stellar remnants are shrouded in mystery.

Luciano Rezzolla and their research group at Goethe University Frankfurt theorize that the key to revealing the interiors of neutron stars could lie with collisions between these extreme stellar remnants. More precisely, the team thinks the key is to focus on how the daughter remnant of this violent collision sets spacetime ringing with gravitational waves.

"Just like tuning forks of different material will have different pure tones, remnants described by different equations of state will ring down at different frequencies," Rezzolla said in a statement. "The detection of this signal thus has the potential to reveal what neutron stars are made of."

Ringing spacetime

Gravitational waves were first suggested by Albert Einstein in this 1915 theory of gravity, known as general relativity.

General relativity suggests that gravity arises as a result of mass curving the very fabric of spacetime (the four-dimensional unification of space and time). In addition to this, when massive objects accelerate, they send out ripples in spacetime.

When two neutron stars exist in a binary system, as they circle around one another, they radiate gravitational waves.

This gravitational radiation carries angular momentum away from the binary system, causing the neutron stars to draw together. This increases the frequency of the emitted gravitational waves, meaning the system loses angular momentum quicker and quicker — and shrinks faster and faster.

That continues until the mutual gravity of the neutron stars takes over, and the stellar remnants collide, causing a cataclysmic explosion called a kilonova. This also sends out a scream of gravitational waves.

This process creates a massive, rapidly rotating post-merger remnant that also emits gravitational waves while spinning, albeit in a strong but narrow frequency range.

Rezzolla and colleagues propose these gravitational waves have encoded within them information about the interior of the post-merger remnant. More precisely, the "equation of state" of the nuclear matter within describes how matter behaves at extreme densities and pressures.

Using computer simulations of general relativity, the team found that the amplitude of the post-merger gravitational-wave signal diminishes over time. As this happens, the signal becomes increasingly "pure." This means it begins to converge on a single frequency, akin to a giant tuning fork beginning to resonate after being struck.

The team has dubbed this phase of the signal's evolution a "long ringdown." The researcher theorize that there is a strong connection between the characteristics of the long ringdown and the properties of the densest regions in neutron-star cores.

Analysis of the long ringdown signal therefore reduces the uncertainties in the equation of state of matter at the incredibly high densities found within neutron stars.

"Thanks to advances in statistical modeling and high-precision simulations on Germany’s most powerful supercomputers, we have discovered a new phase of the long ringdown in neutron star mergers," team leader Christian Ecker, a Goethe University researcher, said in the statement. "It has the potential to provide new and stringent constraints on the state of matter in neutron stars."

Gravitational wave detectors like LIGO (Laser Interferometer Gravitational-Wave Observatory) and Virgo have been listening to these ripples in spacetime since 2015. However, a long ringdown signal such as the one discussed in this research is yet to be "heard." The hope is that the next generation of gravitational wave detectors, including the space-based observatory LISA (Laser Interferometer Space Antenna) will be capable of making just such a detection.

"This finding paves the way for a better understanding of dense neutron star matter, especially as new events are observed in the future," Ecker said.

The team's research was published on Feb. 3 in the journal Nature Communications.

You can tell a lot about a human being's ancestry from their general characteristics. A child can have their father's eyes, their mother's smile, or maybe even their grandfather's male pattern baldness (thanks, grandpa).

However, black holes have few defining characteristics — as theoretical physicist John Wheeler put it, "black holes have no hair" (much like your humble author). Of course, though, testing a child's parentage based on physical features is far too subjective — that's typically where DNA tests come in. Such tests can offer a far more scientific way of checking a person's lineage, and new research suggests an analogous ancestry test for black holes.

Rather than relying on a cheek swab or a little blood, however, these cosmic DNA tests utilize tiny ripples in the fabric of spacetime called gravitational waves, first proposed by Albert Einstein 110 years ago.

A team of scientists, led by researchers from the University of Cardiff, has discovered that the ancestry of supermassive black holes that form from a merger chain of progressively larger progenitor black holes could be hidden in their rotations or "spins."

Additionally, the team's method suggests the spin patterns of these black holes could reveal the region of space in which they were born. Even human DNA tests can't tell you what hospital a baby was delivered in!

Gravitational waves, as detected by facilities like the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo Observatory, could be used to "read" this information like the writing on a birth certificate.

"Our study gives us a powerful, data-driven way to identify the origins of a black hole's formation history, showing that the way it spins is a strong indicator of it belonging to a group of high-mass black holes, which form in densely populated star clusters where small black holes repeatedly collide and merge with one another," team member and University of Cambridge researcher Isobel Romero-Shaw said in a statement.

Black hole family trees

Black hole ancestry became a curious question for scientists when they discovered that some black holes are just too massive to have been spawned the usual way: via a dying star.

Stellar-mass black holes with masses between 10 and 100 times that of the sun are born when stars much more massive than the sun run out of fuel needed for nuclear fusion in their cores. Subsequently, these stars collapse under the influence of their own gravity.

Supermassive black holes, however, have masses equivalent to millions or even billions of sun. No single star can collapse to form such a massive black hole, leading to the theory that they form from mergers of smaller black holes.

The first detection of gravitational waves from merging black holes was made by LIGO and Virgo in 2015, 100 years after Einstein predicted them in his theory of gravity, known as general relativity. This and the wealth of mergers "heard" by these facilities since have helped confirm this "growth by merger" theory.

General relativity predicts that objects with mass cause the fabric of space and time, or spacetime, to "warp." Gravity arises from this warping.

Einstein also predicted that when objects accelerate in spacetime, this causes ripples that radiate outward at the speed of light. However, these so-called gravitational waves are only detectable when the objects involved are truly massive — and black holes fit the bill. The merger of black holes is intrinsically linked to the emission of gravitational waves.

Once black holes are close enough together to form a binary as they whirl around one another, this constant acceleration (acceleration is a change in speed and direction, so circular motion represents perpetual acceleration) sets the fabric of spacetime ringing with gravitational waves.

As these binary systems emit gravitational waves, those ripples in spacetime carry away angular momentum. This causes the binary to tighten. In other words, the black holes move closer together.

This causes the binary black holes to emit gravitational waves faster and faster or at increasing frequencies, meaning they get closer and closer. This continues until the mutual gravity of these black holes takes over and they are forced together, colliding and merging.

This merger creates a daughter black hole that is more massive than its parents' but not quite the total of their masses due to a mass loss in a high-frequency "scream" of gravitational waves.

"As we observe more black hole mergers with gravitational wave detectors like LIGO and Virgo, it becomes ever clearer that black holes exhibit diverse masses and spins, suggesting they may have formed in different ways," team leader Fabio Antonini from Cardiff University’s School of Physics and Astronomy said in the statement. "However, identifying which of these formation scenarios is most common has been challenging."

To unravel this mystery, the team looked at data concerning 69 gravitational wave events detected by LIGO and Virgo.

What they found was that the spin of a black hole changes when that black hole reaches a certain mass. This represents a clear mass threshold at which the spin of black holes constantly changes. The pattern uncovered by the team corresponds with models that suggest black holes grow through repeat collisions in densely packed star clusters.

Using the findings, scientists can now refine computer modeling techniques used to simulate the formation and growth of black holes.

When future gravitational wave signals are detected by facilities like LIGO, Virgo, the proposed underground gravitational wave observatory known as the Einstein Telescope, and the forthcoming space-based gravitational wave detector LISA (Laser Interferometer Space Antenna), such refined models can be used to better interpret these signals.

"Collaborating with other researchers and using advanced statistical methods will help to confirm and expand our findings, especially as we move toward next-generation detectors," team member and University of Chicago researcher Thomas Callister said in the statement. "The Einstein Telescope, for example, could detect even more massive black holes and provide unprecedented insights into their origins."

The team's research was published on Tuesday (Jan. 7) in the journal Physical Review Letters.

Nothing is supposed to escape a black hole's event horizon — yet new research suggests it may secretly leak information. That leakage would appear in subtle signatures in gravitational waves, and now we know how to look for them, the study authors say.

In 1976, Stephen Hawking rocked the astrophysics world with his discovery that black holes aren't entirely black. Instead, they emit tiny amounts of radiation and, given enough time, can give off so much that they disappear entirely. But this introduced a massive problem. Information flows into black holes as they consume matter, and that information can't escape. But Hawking radiation doesn't carry any information with it. So what happens to it when the black hole disappears?

This "black hole information paradox" has bedeviled researchers for decades, and they have developed numerous potential solutions. One is known as nonviolent nonlocality. In this scenario, the insides of black holes are connected to their outsides through "quantum nonlocality" — in which correlated particles share the same quantum state — an effect Einstein called "spooky action at a distance." This nonlocality is "nonviolent" because nothing energetic like an explosion or merger that is causing the ensuing gravitational waves — the ripples in space-time outside the black hole. Rather, they are being caused by the quantum connections between the inside and outside of the black hole.

If this hypothesis is true, the space-time around black holes carries tiny perturbations that aren't entirely random. Instead, the variations would be correlated with the information inside the black hole. Then, when the black hole disappeared, the information would be preserved outside it, thus resolving the paradox.

In a recent preprint paper that has not been peer-reviewed yet, researchers at Caltech investigated this intriguing hypothesis to explore how we might be able to test it.

Related: 'Impossible' black holes discovered by the James Webb telescope may finally have an explanation

The researchers found that these nonlocal quantum correlations don't just make an imprint in the space-time around a black hole; they also leave a signature in the gravitational waves released when black holes merge. These signatures exist as tiny fluctuations on top of the main gravitational wave signal, but they have a unique spectrum that clearly separates them from the usual waves.

The researchers went on to outline a program for separating out this special signal. They found that current gravitational wave detectors, like the Laser Interferometer Gravitational-Wave Observatory and the Virgo interferometer, don't have the sensitivity to comprehensively determine if nonviolent nonlocality is an accurate solution to the black hole information paradox. But next-generation instruments that are currently being designed and constructed might be able to do it.

The next step for the research is to build even more-accurate models of how nonviolent nonlocality affects the space-time around realistic black holes. This will provide a precise prediction of what the changes in the gravitational wave signals should look like — and it just might lead to a resolution of the infamous paradox.

The most detailed map of the universe ever created using gravitational waves could reveal hidden black holes, merging supermassive black holes and even the large-scale structure of the cosmos.

The study, conducted by a team led by astronomers from Swinburne University of Technology, also presented the largest-ever galactic-scale detector of gravitational waves, which are basically ripples in spacetime.

This research has provided further evidence of a background "hum" of gravitational waves that permeates the universe. As such, it could grant new insights into the universe's earliest black holes, how they grew, and the impact they had on the evolution of the comic structure.

"Studying the [gravitational wave] background lets us tune into the echoes of cosmic events across billions of years," team member and Swinburne University researcher Matt Miles said in a statement. "It reveals how galaxies, and the universe itself, have evolved over time."

The gravitational wave background that Miles refers to was created by merging supermassive black holes in the early and distant universe. It was first revealed by a gravitational wave detector that taps into a multitude of rapidly spinning neutron stars, or "pulsars," and a precise time-keeping instrument called a pulsar timing array as part of the NANOGrav project.

This new study also relied on a pulsar timing array, though with the aid of the MeerKAT radio telescope in South Africa. By improving detection methods, the nanosecond precision of the MeerKAT Pulsar Timing Array granted the Swinburne-led team a stronger signal than before.

"What we're seeing hints at a much more dynamic and active universe than we anticipated," Miles said. "We know supermassive black holes are out there merging, but now we’re starting to ask: where are they, and how many are out there?"

Gravitational wave map delivers suprises

Gravitational waves were originally predicted by Albert Einstein's in his theory of gravity, called "general relativity." The great physicist's magnum opus theory states that objects with mass cause the very fabric of space and time (united as a 4-dimensional entity called "spacetime") to "warp." It is from such warping that gravity arises.

However, Einstein also predicted that when massive objects accelerate, they cause ripples that spread through spacetime at the speed of light. These are known as gravitational waves.

An example of gravitational wave-emitting systems are binary black holes, which set spacetime ringing as two black holes within each of these systems swirl around one another. As gravitational waves surge away from the binary black holes, they carry with them angular momentum. This energy loss causes the black holes to draw together and emit gravitational waves faster and faster — that is, until the gravity of these cosmic titans takes over, and they collide and merge.

This powerful event sets spacetime ringing with a high-pitched "scream" of gravitational waves.

Black hole mergers are important to understanding the evolution of the cosmos. This is how the supermassive black holes with masses equivalent to millions, or even billions, of suns that sit at the hearts of large galaxies are born.

Pulsars can be used to detect gravitational waves because they can spin up to 700 times per second; they also blast out beams of radiation that sweep the cosmos like the illumination of a cosmic lighthouse.

The key, though, is that a pulsar's spin timing is highly regular, meaning that when a large group of pulsars is considered en masse, they can be used as a precise cosmic clock — a clock sensitive enough to detect tiny fluctuations in spacetime caused by the passage of gravitational waves.

This is how the MeerKAT Pulsar Timing Array allowed the team to create a highly detailed gravitational wave map — and that spacetime-ripple-forged cosmic atlas actually revealed a surprise feature.

The researchers discovered an unexpected anomaly in their map: a "hotspot" that appears to exhibit a "directional bias" for gravitational waves.

Previously, researchers had assumed that the universe's gravitational wave background would have no preferred direction and thus be evenly spread across the sky.

"The presence of a hotspot could suggest a distinct gravitational wave source, such as a pair of black holes billions of times the mass of our sun," said team member and Monash University researcher Rowina Nathan. "Looking at the layout and patterns of gravitational waves shows us how our universe exists today and contains signals from as far back as the Big Bang.

"There's more work to do to determine the significance of the hotspot we found, but this an exciting step forward for our field."

The team's findings have opened the door to new, unpredicted discoveries concerning the structure of the universe.

These discoveries could well be delivered by the galactic-sized gravitational wave detector formed by the MeerKAT Pulsar Timing Array, which will now continue to refine its gravitational wave map.

"By looking for variations in the gravitational wave signal across the sky, we’re hunting for the fingerprints of the astrophysical processes shaping our universe," Kathrin Grunthal , team member and a scientist at the Max Planck Institute for Radio Astronomy, said in the statement.

It turns out that dark matter can sing, but only if it's given a microphone made of a black hole.

The true nature of dark matter has eluded scientists for decades. To explain why dark matter particles haven't come up in any direct detection experiments, physicists have wondered if it may be a kind of particle known as an axion. Axions are predicted in highly theoretical models of the strong nuclear force and also appear in various versions of string theory.

If dark matter is an axion, then each individual particle might be incredibly light — anywhere from one-trillionth to one-thousandth the mass of an electron. This would make the dark matter behave very strangely on a large scale. It would be so light that its quantum wave nature would be prominent. In other words, we wouldn't live in a buzzing swarm of particle-like dark matter but rather in a sloshing sea of dark matter.

While intriguing, this hypothesis makes dark matter even more difficult to detect, because dark matter rarely, if ever, interacts with normal matter. That means we wouldn't see it in our direct detection experiments. One of the only ways we would be able to detect this kind of dark matter would be through its gravitational interactions.

Related: We still don't know what dark matter is, but here's what it's not

Now, a team of researchers says black holes may serve as the perfect test bed for finding this kind of dark matter. They discussed their results in a paper published to the preprint database arXiv in October. (It has not been peer-reviewed or published in a scientific journal yet.)

Black holes and axions would interact with each other in a very surprising way. Through a process known as superradiance, dark matter could steal energy from a rotating black hole. Over time, the dark matter would continue to orbit the black hole and, eventually, collect a dense cloud of dark matter around it.

Once the dark matter sapped enough rotational energy from the black hole, it would slow down, triggering the release of the dark matter particles in a cascade of completely invisible emission. However, while the emitted dark matter would not emit any radiation, it would affect space-time.

The researchers found that the emitted dark matter would ring space-time like a bell around the black hole. It would not be as strong of a signal as the ones that come from black hole mergers, but it just might be detectable with experiments in the solar system.

The emitted gravitational waves would be in the range of frequencies that could be detected by ground-based experiments like the Laser Interferometer Gravitational-Wave Observatory and the Virgo interferometer (for high-mass axions) and with space-based detectors like the upcoming Laser Interferometer Space Antenna (for extremely low-mass axions).

Either way, there is currently no evidence that these axions exist, and we have never detected those kinds of signals. But now that the suggestion is out there, we can comb through existing data and fine-tune future experiments to go hunting for them.

Black hole mergers are beautiful — and some of the most violent events in the cosmos. Here's how the process unfolds.

The story begins with two black holes orbiting far from each other in long, lazy circles. They could have been born as a binary pair of stars, or they may have just randomly encountered each other in the depths of interstellar space. Either way, to merge, they must get close, which means losing a lot of orbital energy.

The first step in stealing energy from the system is through the black holes' interaction with their environment. They are not alone — there are always thin wisps of gas and dust floating around, and sometimes there are even larger objects, like planets or stars. All of these objects interact through gravity with the black hole. Sometimes, they fall in, never to be seen again. Other times, they just barely miss, getting a little boost to their velocities and sapping some of the orbital energy from the black hole. 

Once the black holes get close enough, another process takes over. The black holes stir space-time as they orbit each other, and this stirring releases gravitational waves that emanate from the pair like ripples in a pond. The gravitational waves are incredibly weak, however, and they start to seriously sap energy only when the black holes are very, very close together. 

Related: How dancing black holes get close enough to merge 

This has led astrophysicists to a conundrum called the "final parsec problem." Simulations have shown that gravitational interactions with the environment can bring black holes to within about a parsec (about 3.26 light-years) of each other in a decent amount of time. But within that distance, there simply isn't enough stuff to keep pulling away energy. On the other hand, at the same distance, the gravitational waves are far too weak and would take many multiples of the age of the universe to get the job done.

The final parsec problem is currently an unsolved riddle in astrophysics. But whatever mechanism takes place, eventually black holes get close enough that the gravitational waves can really pull a lot of energy from the system. At this point, the black holes have only a few seconds before they merge.

At these close distances, the black holes start to deform each other. They don't really have surfaces; the event horizons are invisible boundaries that mark the region of no escape. But the shape of the event horizon depends not just on the black hole itself but also the geometry of space-time around it. So, as the black holes begin their deadly dance, the event horizons elongate and stretch toward each other.

We understand what happens next only through complex computer simulations that monitor and track the evolution of the event horizons. In the milliseconds before impact, each black hole sends out a thin tendril — a tiny tunnel of its event horizon — toward its companion. These tendrils meet and merge, forming a bridge between the two black holes, as if they were connected by an umbilical cord.

Very quickly, the bridge widens and the event horizons glue together, like two colliding soap bubbles. Within an instant, the black holes merge into one.

What happens inside is anybody's guess. The center of a black hole is known as a singularity, a point of infinite density. This is where our current understanding of physics breaks down. Simulations show that the singularities quickly find each other, briefly orbit and then merge — but what actually happens is unclear.

Strangely, the newly merged black hole has a mass that's less than the combined masses of the original pair. For example, in 2016, the LIGO Scientific Collaboration detected the first gravitational wave event from merging black holes, discovering that a 36-solar-mass black hole had merged with a 30-solar-mass black hole to create a new one weighing only 63 solar masses.

What happened to the extra three solar masses? That mass got converted into energy in the form of gravitational waves. Someone had to pay for all the energy loss, and it came from the conversion of the black hole's mass itself. In every black hole merger, roughly 5% gets converted into gravitational waves.

For perspective, that's like converting three entire suns into pure energy. When black holes collide, they release more energy than every star in the universe — and all of it happens in complete, utter silence and darkness.

College students' measurements during a "ring of fire" solar eclipse last year have confirmed a long-held prediction about mysterious gravity waves in Earth's atmosphere.The students confirmed the prediction by identifying a signature of the waves in data obtained from the NASA-sponsored Nationwide Eclipse Ballooning Project (NEBP) during the annular solar eclipse on Oct. 14, 2023.

New Mexico was the ideal location to connect the dots between the solar eclipse and the creation of atmospheric gravity waves — ripples that occur in Earth's atmosphere. (These should not be confused with gravitational waves, which are ripples in space-time first proposed by Albert Einstein.)

Related: NASA balloon observes glowing blue clouds in Earth's upper atmosphere (video)

"New Mexico looked especially promising," Jie Gong, a researcher in the Climate and Radiation Laboratory at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and co-investigator of the research on atmospheric gravity waves, said in a statement. "The majority of atmospheric gravity sources are convection, weather systems, and mountains. We wanted to eliminate all those possible sources."

Starting the day before the eclipse, the teams worked in shifts to release a balloon every 15 minutes. The instruments aboard each balloon measured the temperature, humidity, wind direction and speed, and location throughout the different levels of Earth's atmosphere. After studying the information closely for months, the teams identified a pattern indicating that atmospheric gravity waves had occurred.

"We put all the data together according to time, and when we plotted that time series, I could already see the stripes in the signal," Gong said. "I bombarded everybody's email. We were quite excited."

The students' discovery could help improve weather forecasting.

"Climate models are complicated, and they make some assumptions about what atmospheric factors to take into account," Angela Des Jardins, director of the Montana Space Grant Consortium, which led the NEBP, said in the statement. "Understanding how the atmosphere reacts in the special case of eclipses helps us better understand the atmosphere, which in turn helps us make more accurate weather predictions and, ultimately, better understand climate change." 

New research suggests that this current generation of gravitational wave detectors could "hear" the most energetic core-collapse supernovas at distances thousands of times greater than currently possible, as far as 65 million light-years away, beyond the Milky Way and as distant as the Virgo cluster. If possible, this could help scientists determine if the massive dying star that launches the detected supernova leaves a black hole or a neutron star in its wake.

Since the first detection of tiny ripples in spacetime called "gravitational waves" from collisions and mergers of black holes and neutron stars, the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO), Italy's Virgo and Japan's Kamioka Gravitational Wave Detector (KAGRA) have opened up an entirely new window on the cosmos and have created a powerful new form of astronomy that allows scientists to "hear" some of the cosmos' most violent events.

It is predicted that this current generation of gravitational wave detectors, which collectively form the LIGO-Virgo-Kagra (LVK) collaboration, should be able to detect gravitational waves from the supernova explosions that mark the death of massive stars and the birth of either a black hole or a neutron star, albeit only within the limits of the Milky Way. Yet, thus far, they have failed to hear the high-pitched chirping of supernovas within our galaxy.

Why do scientists expect supernovas to communicate with the cosmos via gravitational waves all, though?

Einstein's theory of gravity, 1915's general relativity, suggests that accelerating objects create gravitational waves. That means that when black holes and neutron stars spiral around each other, they emit low-frequency gravitational waves, then produce a high-pitched "chirp" of these spacetime ripples when they collide and merge, usually creating a more massive black hole "daughter."

Supernovas caused by the accelerating collapse of a star's core should also produce a gravitational wave chirp, but unlike the call of mergers between dense stellar remnants like black holes and neutron stars, this cosmic chirping hasn't been "heard" yet.

"Considering the current capabilities of the LVK observatories and our results, we estimate that, under optimal conditions, we could detect up to about one event per year," team member Maurice van Putten, an astrophysicist from Sejong University, said in a statement. "More conservatively and perhaps more realistically, assuming less favorable conditions and taking into account the limited activity cycles of the detectors, we estimate a detection rate of a few events per decade. 

"This is still significantly higher than the two events per century expected in the Milky Way."

Related: 'Vampire stars' explode after eating too much — AI could help reveal why

Black hole or neutron star?

Core collapse supernovas happen when a massive star, at least eight times the mass of the sun, runs out of the fuel for nuclear fusion in its core. This ends the outward pressure of radiation that has supported the star against its own gravity for millions or even billions of years.

As the core crushes down rapidly, shockwaves ripple outward and hit the star's outer layers. This triggers a supernova explosion that causes the majority of the star's mass to be blown away. As the stellar core crushes down further, it either becomes a neutron star or a black hole, depending on the mass it has left after this matter is explosively expelled, the shed material surrounds it as a halo of material called a torus.

The team focused on a particular type of core-collapse supernova, Type-1c supernova. This occurs when a massive star explodes after it has lost its outer envelope of hydrogen and helium. The animation below shows the Type-1c core-collapse supernova process.

A subclass of these supernovas is notable because of how rapidly the shell of exploded material they launch spreads away, with these blasts associated with high-energy flashes of radiation called long-duration gamma-ray bursts (GRBs).

The end result of this subclass of core-collapse supernovas is likely to be a rapidly spinning black hole surrounded by a "reservoir of energy" in the form of angular momentum. The amount of angular momentum possessed by these black holes is vastly greater than that of a neutron star created in the same process.

The authors considered a rapidly rotating black hole surrounded by a dense torus of matter that acts as a middleman in the conversion of angular momentum or "spin" into gravitational radiation, or, in other words, gravitational waves.

As the torus cools, it expands away from the stellar remnant at its heart, and the black hole also loses angular momentum and starts to spin slower. This would be colloquially called "deacceleration," but scientists consider decceleration to be negative acceleration. That means this spin-down should create a negative gravitational wave chirp.

Gravitational wave astronomy hears something standard astronomy can't see

Spin loss of a black hole has been detected as a chirp in gravitational waves before. 

The gravitational wave signal GW 170817 came from the merger of two neutron stars located about 140 million light-years away in the elliptical galaxy NGC 4993. It was followed by a gravitational wave "chirp" designated GW 170817B, which represented the "spin down" of the daughter black hole created by the merger. A similar chirp would be expected in the aftermath of a core-collapse supernova that leaves a rapidly spinning black hole in its wake. 

GW 170817B was detected in 2017 during observing run 2 (O2) of the LVK detectors. During the current run (O4), these detectors are almost twice as sensitive as they were. Additionally, the black hole created in such an event would be more massive than that involved with GW 170817B, creating an easier-to-detect signal. The team calculated that the spin-down of a black hole after a core-collapse supernova with a rapidly expanding torus of material would have a higher energy output, bringing it right into the "sweet spot" for detection by the  LVK detectors.

These factors led the researchers to conclude that the horizon for detecting such a spin-down chirp should be expanded by around a thousand times. This wider net vastly improves the chances of spotting such a signal. 

Such a signal could tell us something about core-collapse supernovas that "traditional astronomy" that relies on electromagnetic radiation can't. It could tell us the nature of the stellar remnant created in the supernova, namely, whether it is a black hole or neutron star.

To illustrate how useful this could be, consider the supernova remnant SN 1987A. This is one of the brightest stellar explosions visible in the sky, it is located right "next door" to the Milky Way in the Large Magellanic Cloud, a nearby dwarf galaxy about 168,000 light-years away, and it has been extensively studied by astronomers since its discovery 37 years ago. Yet, it was only this year, using the James Webb Space Telescope (JWST), that astronomers were able to determine it is a neutron star that looks at the heart of SN 1987A using electromagnetic observations. 

A descending gravitational wave chirp would unambiguously identify a supernova-buried stellar remnant as a black hole, as this is the only cosmic beast capable of chirping like this; neutron stars don't have enough energy to lose to create such a signal. Alternatively, while a non-detection wouldn't prove a stellar remnant is a neutron star, it would provide some evidence to support further investigation.

"Our study indicates that the current generation of LVKs may reveal that some supernovae may, in fact, be brighter in their gravitational wave emission than previously believed," van Putten concluded. "This puts gravitational wave astronomy at the forefront even in fields such as core-collapse supernovas, traditionally the hunting ground of 'electromagnetic' astronomers."

The team's research was published in The Astrophysical Journal Letters.

In July, some of the world's leading astronomers and planetary scientists who specialize in the search for technological intelligence elsewhere in the cosmos gathered to discuss their work at the University of Oxford's physics department.

They were attending this year's annual Breakthrough Discuss conference — a meeting of the minds to unravel how artificial intelligence, astrobiology and space missions may one day come to redefine our understanding of life and humanity's relationship to the cosmos. 

"This was the first time we had the Breakthrough Discuss conference outside the U.S. It speaks to how technosignature science is becoming more universally accepted across universities. It's now a part of mainstream astrophysics," Vishal Gajjar, search for extraterrestrial intelligence (SETI) investigator and project scientist for Breakthrough Listen's international collaboration, told Space.com.  

The conference is associated with other "Breakthrough" initiatives, including the Breakthrough Listen Project, which is a 100 million dollar program that will use some of the world's most advanced telescopes to search about one million nearby star systems for signs of technologically advanced civilizations. Another initiative, Breakthrough Watch, is aiming to characterize a number of Earth-size rocky planets within 20 light-years of Earth to accomplish the same goal of finding life beyond our world. The Discuss conference provides a platform for researchers working in different fields of astrobiology to compare and discuss their work, and to speculate on what the future of this exciting field might hold. 

Related: Could AI find alien life faster than humans, and would it tell us?

The hot topic of this year's conference centered around how artificial intelligence tools can help comb through massive sets of data generated by telescopes and other observatories to identify any indications that we are, in fact, not alone in the universe. 

Historically, scientists with the search for extraterrestrial intelligence (SETI) institute — an organization that focuses on the search for extraterrestrial life —  have had to decide where to look for signals as well as what type of signals to look for in the first place. How would a sufficiently technologically advanced civilization alert their presence to anyone out there listening? How would we locate any such messages? The answers to these questions — answers that would mark the starting point of any quest to find advanced alien life — have been, at best, educated guesses thus far.

But maybe that doesn't need to be the case going forward. 

New developments in artificial intelligence and improved levels of observational capabilities mean enormous volumes of data can be captured and sifted through at record rates. And these advancements, perhaps, may resolve some of the current limitations in how SETI scientists conduct their searches. Rather than taking observational stabs in the dark, it might now just be a matter of waiting for something unexpected. 

Where to look? 

Which part of the sky should a typical technological signature (or technosignature) search focus on? 

This has been a question SETI investigators have sought to answer because, as time and resources are limited when it comes to having access to large and expensive telescopes, investigators want to give themselves the best chance at spotting something potentially interesting. 

Largely thanks to funding, as well as new observatories that capture data from large areas of the sky simultaneously, SETI investigators are overcoming some of these limitations when it comes to the "where" in this equation.

"We are surveying almost a million nearby stars identified by the Gaia mission," Gajjar told Space.com. 

Launched in 2013, Gaia is a space-based telescope that's creating a catalog of over one billion stars in the Milky Way. From this catalog, SETI investigators identified one million nearby stars of various sizes and luminosities to keep track of using some of the world's most powerful ground-based radio and optical telescopes, including the Green Bank Telescope, Parkes Observatory and the MeerKAT Array. 

Scientists aren't only restricting themselves to stars, though. It's possible that a technosignature could come from empty space — from an object that isn't a star or planet but rather something like a spacecraft or probe that might be broadcasting a signal. "We are also broadly looking at the entire galactic plane of the Milky Way and the galactic center as well, where there is the highest concentration of stars," said Gajjar. 

But new technologies can also create new problems. 

When we are searching with such a large number of targets, and therefore collecting such a large volume of data, it's inevitable that we'll generate a massive number of false positive detections from our own technology. For instance, in terms of electromagnetic waves, human technology is constantly producing signals (phone towers, airplanes and drones, to name a few) and these local interferences are also picked up by scientists' technosignature detection mechanisms. So, separating our own signals from those of a potential extraterrestrial source becomes a real challenge. 

Gajjar explains that researchers have successfully been able to train artificial intelligence models on raw data that has been collected so far (which is full of local false positives), so the model can effectively eliminate these signals, reducing the amount of "noise" that researchers must shift through to find interesting signals. "With AI we have been able to remove 99.8% of our own signals," said Gajjar. 

What to look for?  

Deciding what a "typical" technosignature should look like is difficult because, as humans, we may assume another technologically advanced civilization would create a signal with characteristics we use to allude to our own existence. For example, a probe like Voyager 1, or broadcasting radio waves into space. 

However, we ought to get away from our anthropocentric way of thinking. "So far we have been limited in our imagination of what these technological signatures might look like," Gajjar said. 

There are still legitimate scientific reasons, though, why scientists think a signal should have certain features. "Our main argument has to do with energy," says Gajjar. If a civilization wanted to create a "beacon" to disclose its presence, for example, it makes sense that they would want to create a signal that stands out — but doesn't cost extreme amounts of energy to create. 

Consider a signal that taps into the electromagnetic spectrum, the continuum of various wavelengths and energies of radiation. The shorter the wavelength, the more energy is required to generate a signal within its range. It would thus cost a civilization vast amounts of resources to generate a unique signal in the gamma band of the spectrum, but way less to create one in the radio band. On the other hand, though it sounds extreme, what if an extremely advanced civilization could actually move a whole star in such a way that they form unique gravitational waves? Generating ripples in the fabric of spacetime by moving a massive object to alert the rest of the universe to your presence would be astounding of course, but it would take mind boggling amounts of energy to do — according to the physics we know about, at least.

If the signal is too low in energy, though, it can get lost among the background of other electromagnetic sources in the galaxy — so, theoretically, radio signals may not be the optimal way to go despite the relative ease with which they can be generated. "But even within that boundary, there is still a vast amount of possible ways a signal could look," says Gajjar. 

For instance, it also might not be the case that we detect "beacon" signals where a civilization wants to be seen. It could be a "leakage" signal, where we intercept communication intended for a civilization's own internal communicative purposes (our own radio signals are leaking into space in this way, in fact). So, if we want to be agnostic about what a signal could look like, Gajjar says we need to simply search for anomalies. All of them. 

And this is where artificial intelligence comes in.

Researchers have been developing artificial intelligence models capable of identifying anomalous signals in vast astronomical datasets generated by surveys like the ones mentioned earlier. These models work in similar ways to how large language models (LLMs), like ChatGPT, work. LLMs basically predict the most likely word to follow another in a sentence based on large quantities of data, and these anomaly-detectors predict what electromagnetic signals are most likely to follow based on previous observational data.

If you take hundreds of hours of observational data that you have already gathered, and then you train an AI to make probabilistic predictions about what electromagnetic signals should happen next, and then if the new data violates what the AI predicts, that could be considered an anomaly. 

"Machine learning algorithms in particular have proven effective at detecting anomalies but humans still have a crucial role to play," Michelle Lochner, an astrophysicist who also spoke at Breakthrough Discuss, and who develops anomaly detection algorithms told Space.com. It's then the job of astrophysicists to develop an explanation for what could be the cause of this anomalous signal.  

The future

While galactic surveys are already generating large quantities of data for SETI investigators to sift through, even more are on the way. Next year, the Vera C. Rubin Observatories Survey of Space and Time will begin operations and is expected to generate 20 terabytes of data every single night (60 petabytes over 10 years), and ultimately provide “32 trillion observations of 20 billion galaxies.” 

In addition, the Square Kilometer Array, which will exceed the image resolution of Hubble by 50 times, is slated to open in 2028. 

There will be computational challengers for astronomers seeking to identify new potential  targets in large data sets created by current and future observatories, however, "the possibilities for scientific discoveries are enormous and, by combining large datasets, sophisticated AI algorithms and human insights, this may be the time for one of the most important discoveries in human history," said Lochner. 

Gajjar says he is also excited about the future of technosignature science. 

"When Breakthrough Listen started, it changed the whole game and opened up this new area of research and triggered so many opportunities across the world - it's being funded at all levels of academia which was certainly not the case when I finished my P.H.D!"

Technosignature science appears to have entered the mainstream. Researchers wanting to enter the field are finally being taken seriously, and new avenues for funding as well as emerging technologies are turning what was once a field for retired astrophysicists into a respected scientific pursuit. 

"Searching for technological signs of aliens', can sound a bit absurd to say, but why build multibillion dollar telescopes if we can't even check to see if there is another advanced civilization out there? They might even want to say hi."

In 2023, physicists were awed to find nearly imperceptible ripples in the fabric of space and time — united as an entity known as spacetime. They were ripples discovered in association with collections of rapidly spinning neutron stars called "pulsar timing arrays."

This low-frequency background hum of gravitational waves in our universe was originally attributed to a change, or a "phase transition," that occurred shortly after the Big Bang. New research, however, casts doubt on that assumption.

"Theorists and experimentalists have speculated nanohertz gravitational waves originated from a known transition that happened very soon after the Big Bang — a change that generated the masses of all the known fundamental particles," Andrew Fowlie, an assistant professor at Xi'an Jiaotong-Liverpool University, said in a statement. "However, our work uncovers serious problems with that otherwise appealing explanation of their origin."

Phase transitions are sudden changes in a substance's properties, and they typically occur when a particular substance reaches a critical temperature. The phase transition perhaps most familiar to us is the transition of water into ice as temperatures fall below freezing. There are also what are known as "supercool" transitions. With water, a supercool transition occurs when the substance gets "stuck" in its liquid phase, slowing its transformation into ice.

Related: Spacetime ripples detected in 2023 continue to puzzle astronomers. Could they be from the dawn of the universe?

Many scientists believe a "first-order phase transition" occurred at the very beginning of time, triggering the launch of gravitational waves, or ripples in space-time. Those waves, experts think, could therefore be used to determine conditions present during the first epoch of rapid inflation in our universe, or maybe even the conditions present before the Big Bang.

Just a phase?

The concept of gravitational waves dates back to Albert Einstein's 1915 theory of gravity called "general relativity." The great physicist's magnum opus theory predicts that objects with mass have a warping effect on the very fabric of spacetime . Our physical experience of gravity, the theory states, arises from this warping. 

General relativity goes further than this as well, also suggesting that when objects accelerate, they generate ripples in spacetime — aka, gravitational waves. Though this phenomenon is negligible when it comes to the acceleration of objects on a scale we see on Earth, the effect becomes significant when the acceleration involves massive cosmic objects like supermassive black holes and neutron stars. 

For instance, when these objects exist in binary systems — meaning two of them constantly accelerate around one another — they continuously emit gravitational waves until they finally collide and emit a high-pitched "screech" of these ripples. 

Additionally, gravitational waves, like electromagnetic radiation, come in a range of frequencies. High-frequency gravitational waves, like high-frequency light, have shorter wavelengths and are more energetic; low-frequency gravitational waves have longer wavelengths and are less energetic. Low-frequency longwave gravitational waves also have long "periods," which refers to the time between one peak of the wave passing a set point to the next peak passing that point. 

The gravitational waves detected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) pulsar timing array in June 2023 are lower in frequency than the gravitational waves seen coming from supermassive black hole and neutron star mergers routinely detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO), VIRGO,and KAGRA.

This means there must be a different source for these low-frequency nanohertz gravitational waves. The prime suspect? A phase transition just after the Big Bang — a supercool one, to be exact.

"We found that to have created waves with such tiny frequencies, the transition would have to be supercool," Fowlie explained. 

However, there is a problem. Such cosmic supercool transition phases would be a bit unexpected during the period of rapid cosmic inflation (in other words, the universe's expansion) triggered by the Big Bang.

"These slow transitions would struggle to finish, as the transition rate is slower than the cosmic expansion rate of the universe," Fowlie said. "What if the transition sped up at the end? We calculated that even if this helped the transition to end, it would shift the frequency of the waves away from nanohertz."

The researcher also added that, although nanohertz gravitational waves are cool, they are probably not "supercool" in origin.

"If these gravitational waves do come from first-order phase transitions, we now know that there must be some new, much richer physics going on — physics we don't know about yet," Fowlie said.

Fowlie and colleagues believe their research demonstrates that more care is needed to understand supercool phase transitions, especially those that could have occurred at the beginning of the universe.

"Because these are necessarily slow transitions, the usual simplifications of whether transitions complete or not won't work," he said. "There are a lot of subtleties in the connections between the energy scale of the transitions and the frequency of the waves, so we need more careful and sophisticated techniques when considering gravitational waves and supercool transitions.

"Understanding this field will help us understand the most fundamental questions about the origin of the universe."

A better comprehension of supercool phase transitions could also help understand more Earthly and less cosmic phase transitions.

"It also has links to applications that are closer to home, such as understanding how water flows through a rock, the best ways to percolate coffee, and how wildfires spread," Fowlie concluded. 

The team's research is discussed in a paper published in the journal Physical Review Letters.

Binary pairings of small black holes could be used by astronomers in a cosmic game of "hide-and-seek" to hunt much larger, yet more elusive, supermassive black hole binaries. The technique could, therefore, help solve the mystery of how supermassive black holes grew so fast in the early universe.

Detecting black holes is no easy task despite their reputation as fearsome cosmic titans. All black holes are surrounded by a one-way light-trapping boundary called an "event horizon" that ensures they emit no light. Even the supermassive black holes at the hearts of galaxies with masses millions or billions of times that of the sun are only "visible" if they are feasting on a vast amount of surrounding matter or if they are ripping apart an unfortunate star. 

However, light, or "electromagnetic radiation" as it is more accurately known, is only one type of radiation. Another is "gravitational radiation," which comes in the form of tiny ripples that set spacetime humming called "gravitational waves," which humanity is just beginning to detect. That means rather than looking for supermassive black hole pairs in this game of hide-and-seek, astronomers can listen for them instead.

"Our idea basically works like listening to a radio channel. We propose to use the signal from pairs of small black holes similar to how radio waves carry the signal," team leader Jakob Stegmann, a postdoctoral research fellow at the Max Planck Institute for Astrophysics, said in a statement. "The supermassive black holes are the music that is encoded in the frequency modulation (FM) of the detected signal."

Related: Cracking! Some binary black holes may roll around each other in egg-shaped orbits

Small black hole sing soprano

Gravitational waves are a concept that was first suggested by Albert Einstein in general relativity, his 1915 magnum opus theory of gravity.

General relativity suggests that gravity arises when an object with mass "warp" the very fabric of space and time, which Einstein had previously united as a single four-dimensional entity (three spatial dimensions, one dimension of time) called "spacetime."

The bigger the mass, the greater the extremity of the curvature of space an object creates. That explains why planets have a bigger gravitational influence than moons, why stars have a bigger influence than planets, and why black holes have the biggest influence of any single object.

Einstein also predicted that when objects accelerate in spacetime, they set its fabric "ringing" with ripples or gravitational waves. These are completely insignificant for objects with low masses, but when black holes orbit around each other (remembering that circular motion is acceleration), they have enough mass to generate significant gravitational waves.

As these black holes spiral around each other, they emit continuous low-frequency gravitational waves. These gravitational waves carry away angular momentum (or spin), forcing the black holes together, a process called "inspiralling." This increases the frequency of the gravitational waves, thus causing angular momentum to be carried away faster and faster.

That is until the black holes finally collide and merge, an event that sends out a higher frequency "scream" of gravitational waves. 

Even so, Einstein predicted that these spacetime ripples would be too faint to ever detect, especially as they would lose energy as they propagated through the cosmos and black hole mergers occur millions or even billions of light-years away. 

Fortunately, we now know Einstein was wrong.

Since the detection of the first gravitational wave signal by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015, which originated from a binary black hole merger 1.3 billion light-years away, many such black hole collisions have been detected.

But these detections have one thing in common. When they involved black holes, they were always pairs in the stellar-mass black hole range, with masses between three and a few hundred times that of the sun. Supermassive black hole mergers have been elusive for terrestrial gravitational wave detectors like LIGO and its compatriots VIRGO in Italy and the Kamioka Gravitational Wave Detector (KAGRA) in Japan.

Just as our ears have evolved to hear certain frequencies of sound and not others, these instruments can only detect a certain frequency range of gravitational waves. The gravitational waves emitted by swirling pairs of supermassive black holes are too low-frequency for terrestrial gravitational wave detectors to "hear."

In other words, with their gravitational waves, stellar-mass binaries sing soprano, while supermassive pairings sing baritone. 

This team proposes detecting the subtle change in gravitational waves from stellar-mass black hole binaries that are caused by interfering gravitational waves from supermassive binaries. 

These small modulations could, therefore, help reveal supermassive black hole mergers that are currently only detectable as a collective "background hum" using vast collections of rapidly spinning neutron stars called a "pulsar timing array."

"The novel aspect of this idea is to utilize high frequencies that are easy to detect to probe lower frequencies that we are not sensitive to yet," Stegmann said.

The proposal could also help direct the design of future gravitational wave detectors, such as the upcoming NASA and European Space Agency (ESA) space-based detector LISA (Laser Interferometer Space Antenna).

"As the path for the LISA is now set, after adoption by ESA last January, the community needs to evaluate the best strategy for the following generation of gravitational wave detectors," team member and University of Zurich black hole theorist Lucio Mayer said. "In particular, which frequency range they should target – studies like this bring a strong motivation to prioritize a deci-Hz [low-frequency] detector design." 

The team's research was published on Monday (August 5) in the journal Nature.

In 2002, Thomas Hertog, then a theoretical physics graduate student, stepped inside Stephen Hawking's office at the University of Cambridge and saw his supervisor's eyes filled with emotion. 

Hawking's news was also a confession. The famed physicist told his student that his book, "A Brief History of Time," was wrong because it predicted a barren universe unsuitable for life, and he wanted Hertog to help him find a new theory.

So, in the last 16 years of Hawking's life, the duo, along with collaborator James Hartle, developed a new explanation for how our universe came to be. 

Live Science sat down with Hertog, now a professor at KU Leuven in Belgium, to discuss his new book "On the Origin of Time" (Penguin Random House, 2024), his decades-long collaboration with Hawking, and the mind-bending Darwinian view of the universe's origins that their work ultimately produced. 

Ben Turner: When you met Stephen Hawking, he was beginning to think that the picture of the universe's origins he had previously presented in "A Brief History of Time" was flawed, and he wanted to look for a new theory. For readers who might not know, what is the standard conception of how our universe began?

Thomas Hertog: Certainly what's standard is that there's been some sort of Big Bang — a violent, extremely odd beginning. What's been challenging is to describe what exactly happened at the Big Bang. 

What's the novelty of Hawking's contribution in "A Brief History of Time?" What was the key insight he invoked? He came up with a mathematical model of the actual beginning in his famous "no boundary proposal," in which the Big Bang is a true origin. 

Sadly, Hawking's model didn't produce a habitable universe. It was, instead, an empty universe — without stars, without galaxies and without life. So, as you say, by the late '90s, Hawking realized there was a problem with his model. 

BT: A popular answer for how our habitable universe could have formed is that the Big Bang led to eternal cosmic inflation with different pockets of expanding space-time — a multiverse — and that our universe just happens to be one of the pockets where the laws of physics balanced out in just the right way to produce life. Why didn't this idea suit Hawking? 

TH: These multiverse models are not falsifiable, even in principle. That's not because we can't look at the early universe and check it out; it's because multiverse models do not make unambiguous predictions of what we should see in this universe. 

Related: Stephen Hawking wanted scientists to 'make black holes' on Earth. Physics says it's possible. 

BT: So how did you and Hawking meet and begin to collaborate? You met him when you were a master's student. What was that like? He was already a legend by this time. 

TH: Yes, he was already pretty famous. I met him because, well, I grew up in Belgium, and there was no cosmology going on in Belgium in the late '90s. Stephen and his colleagues, Martin Rees and those folks, had established a kind of mecca for cosmology at Cambridge. So I had a professor who told me, "Look, if you're into cosmology, go to Cambridge."

At Cambridge, it was very well known that whoever came top of the master's class would get an invitation to go talk to Stephen, and that's what happened [to me]. So he took me on as his PhD student. 

But, of course, the real collaboration started later, when we found ourselves on the same scientific wavelength and interested in the deeper problems to do with the Big Bang. It just happened: You find yourselves on the same wavelength, interested in the same problems, perhaps sharing some sort of intuition. As theoretical physicists, you're always performing thought experiments on each other, and after a while, you develop a common understanding.

BT: Past theories of the Big Bang have framed the universe as if they're looking at it from an "objective," godlike perspective. The theory you and Hawking began working on shifted that perspective to one more like our own — an observer somewhere in the universe. That made you take quantum mechanics, as well as string theory, as your starting point. What did beginning this way teach you? 

TH: When you take a God's-eye of the universe, you are going to be looking for a prior explanation of why the entire cosmos should be doing what it's doing — some Platonic mathematical truth that looms over the entire universe. 

But when you take what you call a more human perspective, a perspective of an observer within the universe, it's very different. You'll be taking a more historical perspective. You're not asking, "Why should the universe be this way?" but "How did it all come about?" 

If you use quantum mechanics to reconstruct that history all the way back to the Big Bang, that historical perspective begins to play out at the level of the laws of physics themselves. And that's, of course, a surprise. We thought the laws of physics were fixed and immutable, but if you go back in time, they begin to simplify. In a sense, they begin to evaporate, even the structure. 

That structure, encoded in the laws of physics, begins to disappear until ultimately — and this is the crux of our hypothesis — even the distinction between time and space blurs. The laws of our universe's evolution, the standard laws of physics, close themselves; they cease to be. Physics itself disappears. 

It's a Darwinian turning. In biology, we go back along the tree of life to life's origin, and the laws of biology also disappear. That's because those laws are emergent properties of biological evolution. We claim that the laws of physics are also emergent properties of a much earlier evolution. 

BT: That's going to strike people as very strange. In biology, selective pressure plays the role of spurring biological laws to evolve. What's causing physical laws to evolve?

TH: The act of observation in quantum mechanics. You're going to ask me, "But wait a minute — who's observing?" Because clearly, in the early universe, there is no human observer. But we all know that the act of observation in quantum mechanics comes from the environment itself — it's the interactions between the particles and the forces. 

Even a single photon can perform an act of observation in quantum mechanics. It can convert a range of possible histories into a tangible, concrete reality. 

BT: According to your theory, when we wind time back to the Big Bang, physical laws fold in on themselves and time itself loses its identity — that gives it an origin point. Einstein particularly disliked this notion. Why did he object to it?

TH: When Einstein and his contemporaries were running the evolution of the universe backwards in time, they were doing this using Einstein's own theory in a classical, deterministic manner. They ran into what they call the singularity [where the equations describing the universe broke down]. The origin of time, the Big Bang, seemed to not be part of science. 

Related: Tweak to Schrödinger's cat equation could unite Einstein's relativity and quantum mechanics, study hints

When Stephen and I ran the evolution of the universe backwards, we did it in a quantum mechanical way. This agrees with Einstein until you reach the earlier stages where our picture is very, very different. The laws of physics never really break down [in Hertog, Hartle and Hawking's picture]; they just gradually disappear. I think Einstein would be okay with that.

BT: Key to your idea of time having an origin is that it's an emergent property from the interactions of many quantum particles at the edge of the observable universe. The universe is like a disk expanding outward, and at the edge of that disk are qubits, particles containing all the universe's information. The play of these particles beams time into our universe from that furthest edge — like a cosmic hologram. Can you explain the holographic principle a bit more? 

TH: So the way we read the past of the universe is from a holographic perspective. The holographic screen is an abstract representation of our reality, and as we zoom out further and further from that screen, it corresponds to going back in time. The picture gets more coarse-grained, you lose information, you lose pixels, and the Big Bang is the limit where you run out of information. The beginning of the world is really an epistemic horizon where science (from the holographic perspective) simply doesn't reach further back. 

And, of course, that fits in very well with the story that I told you earlier — that the laws of physics, along with time and space, disappear as we reach the Big Bang, the origin of physics. The holographic implementation of our vision made it click together. 

That's how theoretical physics works. In retrospect, you start off with a lot of intuition, and you mold this into a mathematical framework that is consistent and that allows you to ultimately predict new phenomena. This is where current research is going: How can we test this model? How can we find fossils of this very early evolution? 

BT: That's actually my next question.

TH: [Laughs] I feared. 

BT: So where can we look? Before the cosmic microwave background (CMB), the universe was completely opaque. How do we peer beyond that microwave fuzz?

TH: The cosmic microwave background gives you a picture of the universe 380,000 years after the Big Bang, when it became transparent. But this early phase of evolution that I'm talking about happens much sooner, so you have to peer through [the CMB]. And you can't do this with light, electromagnetic waves.

But gravitational waves go through everything, so you can hope to look further backward. In principle, there's no limit — you can look all the way back to the Big Bang and unlock this deeper layer of evolution. 

BT: Say we are able to. What might we see?

TH: We've hypothesized. How? Well, the way I envision this early stage is a little bit like a branching, diversifying tree of physical laws. Each of these branchings is really the birth of a new kind of force — one force splits into two with new particles and more structure. Some of these branches are pretty violent, coming with bursts of gravitational waves which are not localized to one place and appearing as background radiation, much like the cosmic microwave background. 

It's the entire universe transitioning into a new state when it cools and expands, and it's accompanied by a strong burst of inflation. 

BT: Your theory describes physical laws evolving quickly when the universe was dense and hot, and there were plenty of interactions or "observations" between particles. But if these laws still have the capacity to evolve, does that have any implications for how the universe ends?

TH: The short answer is, of course, that I don't know. But if you challenge me a little bit, I can say something very speculative: If the laws of physics were not determined, fixed and immutable in the past, it's natural to expect they won't be eternal. So, even though that evolution is suppressed now (because the universe is cold), it's not infinitely suppressed. It's not gone.

BT: We've spoken a lot about intuition in physics. The one you shared with Hawking fueled this collaboration and enabled you to finish your theory, even as Hawking slowly lost his ability to use his artificial voice. How did you do that?

TH: It's a little bit like being in a marriage, right? Or really any long-term relationship — you can guess one another's thoughts. Towards the end, that happened to us, as well. We developed an intimacy when it came to cosmology and its fundamental problems. In the later stages, we developed a nonverbal layer of communication in which I could fire yes-or-no questions at Stephen and read his facial expressions. 

This developed in a fairly spontaneous manner, but it was only possible because, in the late '90s and early '00s, we had some very good years in which Stephen could speak fairly fluently through his speech synthesizer. He really dragged me into his thinking about these paradoxes associated with the multiverse.

BT: Do you think his ability to move outside problems and intuit them is what made him such a great physicist?

TH: Stephen's intuition was grounded in 15 years of doing a lot of calculations. It didn't come to him from heaven. It was rooted in the early stages of his career. 

Of course, there's something genius that happened in the early '80s, when he lost his ability to write equations. He had the capacity and the stubbornness to retrain himself to perform theoretical physics in a very unique way. It was more intuition-based, more distant from the equations than others, and with the ability to visualize shapes and geometries in his head. His true glory lies in that, with this new language, he was able to arrive at certain discoveries which were very difficult to reproduce with equations.

Editor's note: This interview has been condensed and edited for clarity.

After its “birth” in the Big Bang, the universe consisted mainly of hydrogen and a few helium atoms. These are the lightest elements in the periodic table. More-or-less all elements heavier than helium were produced in the 13.8 billion years between the Big Bang and the present day.

Stars have produced many of these heavier elements through the process of nuclear fusion. However, this only makes elements as heavy as iron. The creation of any heavier elements would consume energy instead of releasing it.

In order to explain the presence of these heavier elements today, it’s necessary to find phenomena that can produce them. One type of event that fits the bill is a gamma-ray burst (GRB) – the most powerful class of explosion in the universe. These can erupt with a quintillion (10 followed by 18 zeros) times the luminosity of our Sun, and are thought to be caused by several types of event.

GRBs can be subdivided into two categories: long bursts and short bursts. Long GRBs are associated with the deaths of massive and fast-rotating stars. According to this theory, the fast rotation beams material ejected during the collapse of a massive star into narrow jets that move at extremely fast speeds. 

Related: Scientists unravel mysteries of gamma-ray bursts — the universe's most powerful explosions

The short bursts last only a few seconds. They are thought to be caused by the collision of two neutron stars – compact and dense “dead” stars. In August 2017, an important event helped support this theory. Ligo and Virgo, two gravitational wave detectors in the US, discovered a signal that seemed to be coming from two neutron stars moving in for a collision.

A few seconds later, a short gamma-ray burst, known as GRB 100817A, was detected coming from the same direction in the sky. For a few weeks, virtually every telescope on the planet was pointing at this event in an unprecedented effort to study its aftermath.

The observations revealed a kilonova at the location of GRB 170817A. A kilonova is a fainter cousin of a supernova explosion. More interestingly, there was evidence that many heavy elements were produced during the explosion. The authors of a study in Nature that analysed the explosion showed that this kilonova seemed to produce two different categories of debris, or ejecta. One was composed primarily of light elements, while another consisted of heavy elements.

We’ve already mentioned that nuclear fusion can only feasibly produce elements as heavy as iron in the periodic table. But there’s another process which could explain how the kilonova was able to produce even heavier ones.

Rapid neutron-capture process, or r-process, is where the nuclei (or cores) of heavier elements such as iron capture many neutron particles in a short time. They then rapidly grow in mass, yielding much heavier elements. For r-process to work, however, you need the right conditions: high density, high temperature, and a large number of available free neutrons. Gamma ray bursts happen to provide these necessary conditions.

However, mergers of two neutron stars, like the one that caused the kilonova GRB 170817A, are very rare events. In fact, they may be so rare as to make them an unlikely source for the abundant heavy elements we have in the universe. But what of long GRBs?

A recent study investigated one long gamma ray burst in particular, GRB 221009. This has been dubbed the BOAT – the brightest of all time. This GRB was picked up as a pulse of intense radiation sweeping through the solar system on October 9 2022.

The BOAT sparked a similar astronomical observation campaign as the kilonova. This GRB was 10 times more energetic then the previous record holder, and so close to us that its influence on the Earth’s atmosphere was measurable on the ground and comparable to a major solar storm.

Among the telescopes studying the aftermath of the BOAT was the James Webb Space Telescope (JWST). It observed the GRB about six months after it exploded, so as not to be blinded by the afterglow of the initial burst. The data JWST collected showed that, despite the event’s extraordinary brightness, it was caused by a merely average supernova explosion.

In fact, previous observations of other long GRBs indicated that there is no correlation between the brightness of the GRB and the size of the supernova explosion associated with it. The BOAT seems no exception.

The JWST team also inferred the number of heavy elements produced during the BOAT explosion. They found no indication of elements produced by the r-process. This is surprising as, theoretically, the brightness of a long GRB is thought to be associated with the conditions in its core, most likely a black hole. For very bright events –- especially one as extreme as the BOAT –- the conditions should be right for the r-process to occur.

These findings suggest that gamma ray bursts may not be the hoped-for crucial source of the universe’s heavy elements. Instead, there must be a source or sources still out there.

Physicists consider black holes one of the most mysterious objects that exist. Ironically, they’re also considered one of the simplest. For years, physicists like me have been looking to prove that black holes are more complex than they seem. And a newly approved European space mission called LISA will help us with this hunt.

Research from the 1970s suggests that you can comprehensively describe a black hole using only three physical attributes – their mass, charge and spin. All the other properties of these massive dying stars, like their detailed composition, density and temperature profiles, disappear as they transform into a black hole. That is how simple they are.

The idea that black holes have only three attributes is called the “no-hair” theorem, implying that they don’t have any “hairy” details that make them complicated.

Related: What is a black hole event horizon (and what happens there)?

Hairy black holes? 

For decades, researchers in the astrophysics community have exploited loopholes or work-arounds within the no-hair theorem’s assumptions to come up with potential hairy black hole scenarios. A hairy black hole has a physical property that scientists can measure – in principle – that’s beyond its mass, charge or spin. This property has to be a permanent part of its structure.

About a decade ago, Stefanos Aretakis, a physicist currently at the University of Toronto, showed mathematically that a black hole containing the maximum charge it could hold – called an extremal charged black hole – would develop “hair” at its horizon. A black hole’s horizon is the boundary where anything that crosses it, even light, can’t escape.

Aretakis’ analysis was more of a thought experiment using a highly simplified physical scenario, so it’s not something scientists expect to observe astrophysically. But supercharged black holes might not be the only kind that could have hair.

Since astrophysical objects such as stars and planets are known to spin, scientists expect that black holes would spin as well, based on how they form. Astronomical evidence has shown that black holes do have spin, though researchers don’t know what the typical spin value is for an astrophysical black hole.

Using computer simulations, my team has recently discovered similar types of hair in black holes that are spinning at the maximum rate. This hair has to do with the rate of change, or the gradient, of space-time’s curvature at the horizon. We also discovered that a black hole wouldn’t actually have to be maximally spinning to have hair, which is significant because these maximally spinning black holes probably don’t form in nature.

Detecting and measuring hair 

My team wanted to develop a way to potentially measure this hair – a new fixed property that might characterize a black hole beyond its mass, spin and charge. We started looking into how such a new property might leave a signature on a gravitational wave emitted from a fast-spinning black hole.

A gravitational wave is a tiny disturbance in space-time typically caused by violent astrophysical events in the universe. The collisions of compact astrophysical objects such as black holes and neutron stars emit strong gravitational waves. An international network of gravitational observatories, including the Laser Interferometer Gravitational-wave Observatory in the United States, routinely detects these waves.

Our recent studies suggest that one can measure these hairy attributes from gravitational wave data for fast-spinning black holes. Looking at the gravitational wave data offers an opportunity for a signature of sorts that could indicate whether the black hole has this type of hair.

Our ongoing studies and recent progress made by Som Bishoyi, a student on the team, are based on a blend of theoretical and computational models of fast-spinning black holes. Our findings have not been tested in the field yet or observed in real black holes out in space. But we hope that will soon change.

LISA gets a go-ahead 

In January 2024, the European Space Agency formally adopted the space-based Laser Interferometer Space Antenna, or LISA, mission. LISA will look for gravitational waves, and the data from the mission could help my team with our hairy black hole questions.

Formal adoption means that the project has the go-ahead to move to the construction phase, with a planned 2035 launch. LISA consists of three spacecrafts configured in a perfect equilateral triangle that will trail behind the Earth around the Sun. The spacecrafts will each be 1.6 million miles (2.5 million kilometers) apart, and they will exchange laser beams to measure the distance between each other down to about a billionth of an inch.

LISA will detect gravitational waves from supermassive black holes that are millions or even billions of times more massive than our Sun. It will build a map of the space-time around rotating black holes, which will help physicists understand how gravity works in the close vicinity of black holes to an unprecedented level of accuracy. Physicists hope that LISA will also be able to measure any hairy attributes that black holes might have.

With LIGO making new observations every day and LISA to offer a glimpse into the space-time around black holes, now is one of the most exciting times to be a black hole physicist. 

In 2015, the iconic Laser Interferometer Gravitational-Wave Observatory (LIGO) made the first-ever tangible detection of gravitational waves. The waves were the result of two black holes colliding far away in the universe; since then, a wealth of such signals from merging black holes, neutron stars and even a couple of mixed mergers between the two have been spotted.

Yet, despite the success of LIGO — located in two U.S. sites and supported by the Virgo detector, located in Italy, and Japan's Kamioka Gravitational Wave Detector (KAGRA) — astronomers have only been able to confirm one of these gravitational-wave-producing events using "traditional" light-based astronomy. That event was the merger of two neutron stars, which produced the gravitational wave signal GW170817.

Now, a team of scientists from the University of Minnesota has developed software upgrades that can help alert astronomers to merger events just 30 seconds after gravitational waves are picked up on Earth. This early-alert system should allow more merger events to be followed up on with light-based astronomy.

Related: Gravitational waves reveal 1st-of-its-kind merger between neutron star and mystery object

"With this software, we can detect the gravitational wave from neutron star collisions that are normally too faint to see unless we know exactly where to look," Andrew Toivonen, team member and a Ph.D. student at the University of Minnesota Twin Cities School of Physics and Astronomy, said in a statement. "Detecting the gravitational waves first will help locate the collision and help astronomers and astrophysicists to complete further research."

What are gravitational waves?

Gravitational waves are tiny ripples in the fabric of space and time, the two of which are united as a single, four-dimensional entity called "spacetime." Such ripples were first predicted by Albert Einstein in his 1915 theory of gravity, general relativity.

General relativity predicts that gravity arises from objects with mass that warp the very fabric of spacetime. The greater the mass, the more extreme the curve, thus explaining why stars have a greater gravitational influence than planets do.

Einstein also theorized that, when objects accelerate, they cause spacetime to ripple. These ripples are only perceptible when truly massive objects accelerate — objects like neutron stars and black holes that swirl around each other in binary systems and emit gravitational waves as they do so. This continuous emission of gravitational waves, Einstein said, would carry away angular momentum and cause the ultradense objects to draw together and eventually merge, a collision that sends out a high-pitched "scream" of gravitational waves. 

Einstein, however, thought that even gravitational waves from objects significant enough to generate them would be too faint to ever be detected here on Earth. 

Fortunately, he was wrong. 

Still, spotting gravitational waves is still no mean feat. After all, neutron star and black hole binaries are located millions (sometimes even billions) of light-years away, and gravitational waves lose energy as they travel through the cosmos.

For LIGO to detect gravitational waves from these events, this massive laser interferometer consists of two L-shaped arms, each 2.5 miles (4 kilometers) long. When in phase, laser light shines down each of these arms. This means that when the beams meet, the peaks and troughs of their waves line up, and the laser light is amplified, which is something called "constructive interference." 

However, if a gravitational wave passes over one of these lasers and space is squeezed and stretched, then the laser passing over this section of space would be knocked out of phase, meaning troughs meet peaks, and vice versa, resulting in "destructive interference" and thus no amplification.

The changes LIGO picks up to "hear" gravitational waves are 0.0001 times the width of a proton, particles that sit at the hearts of atomic nuclei. To put this in "standard" astronomy terms, that is equivalent to measuring the distance to the nearest star, Proxima Centauri, about 4.2 light-years away, with an quantitative accuracy equal to the width of a human hair.

LIGO, Virgo and KAGRA are currently in their fourth operating run, which began on May 24, 2023, and is set to last until Feb. 2025. Between each of the previous operating runs, scientists in the LIGO/Virgo/KAGRA collaboration have upgraded the software used to detect the shape of gravitational wave signals, track how the signal evolves, and then estimate the masses of the neutron stars or the black holes that smashed together to create the signal. This software also sends an alert out to other scientists.

Thanks to simulations created using data collected from observation periods one through three, as well as artificially generated gravitational wave signals, the team now knows upgrades can made to the observation software that allows alerts to go out within 30 seconds of a gravitational wave detection during observation. Such upgrades will impact observation period four.

That should help astronomers track the locations of these events in the sky with light-based astronomy, something no gravitational wave detector can currently do, and determine how collisions between the most exotic and mysterious objects in the cosmos evolve over time. 

This is unlikely to be the end of the upgrades to gravitational wave detection alerts. At the end of this current operating run, LIGO/Virgo/KAGRA collaboration scientists will use data collected in nearly two years of "listening" to a universal symphony of colliding black holes and neutron stars to improve alert speed even further. 

The team's research was published in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS).

This article was originally published at The Conversation. The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.

Amy Lien is an Assistant Professor of Physics at the University of Tampa whose research interests lie in understanding how the universe begins and evolves through the most energetic astrophysical explosions: gamma-ray bursts (GRBs), supernovas, merging neutron stars and black holes. 

When faraway stars explode, they send out flashes of energy called gamma-ray bursts that are bright enough that telescopes back on Earth can detect them. Studying these pulses, which can also come from mergers of some exotic astronomical objects such as black holes and neutron stars, can help astronomers like me understand the history of the universe.

Space telescopes detect on average one gamma-ray burst per day, adding to thousands of bursts detected throughout the years, and a community of volunteers are making research into these bursts possible.

On Nov. 20, 2004, NASA launched the Neil Gehrels Swift Observatory, also known as Swift. Swift is a multiwavelength space telescope that scientists are using to find out more about these mysterious gamma-ray flashes from the universe. 

Related: Brightest gamma-ray burst ever seen a 1-in-10,000-years event that's 'absolutely monstrous,' scientists say

Gamma-ray bursts usually last for only a very short time, from a few seconds to a few minutes, and the majority of their emission is in the form of gamma rays, which are part of the light spectrum that our eyes cannot see. Gamma rays contain a lot of energy and can damage human tissues and DNA.

Fortunately, Earth’s atmosphere blocks most gamma rays from space, but that also means the only way to observe gamma-ray bursts is through a space telescope like Swift. Throughout its 19 years of observations, Swift has observed over 1,600 gamma-ray bursts. The information it collects from these bursts helps astronomers back on the ground measure the distances to these objects.

Looking back in time 

The data from Swift and other observatories has taught astronomers that gamma-ray bursts are one of the most powerful explosions in the universe. They’re so bright that space telescopes like Swift can detect them from across the entire universe.

In fact, gamma-ray bursts are among one of the farthest astrophysical objects observed by telescopes.

Because light travels at a finite speed, astronomers are effectively looking back in time as they look farther into the universe.

The farthest gamma-ray burst ever observed occurred so far away that its light took 13 billion years to reach Earth. So when telescopes took pictures of that gamma-ray burst, they observed the event as it looked 13 billion years ago.

Gamma-ray bursts allow astronomers to learn about the history of the universe, including how the birth rate and the mass of the stars change over time.

Types of gamma-ray bursts 

Astronomers now know that there are basically two kinds of gamma-ray bursts – long and short. They are classified by how long their pulses last. The long gamma-ray bursts have pulses longer than two seconds, and at least some of these events are related to supernovae – exploding stars.

When a massive star, or a star that is at least eight times more massive than our Sun, runs out of fuel, it will explode as a supernova and collapse into either a neutron star or a black hole.

Both neutron stars and black holes are extremely compact. If you shrank the entire Sun into a diameter of about 12 miles, or the size of Manhattan, it would be as dense as a neutron star.

Some particularly massive stars can also launch jets of light when they explode. These jets are concentrated beams of light powered by structured magnetic fields and charged particles. When these jets are pointed toward Earth, telescopes like Swift will detect a gamma-ray burst.

On the other hand, short gamma-ray bursts have pulses shorter than two seconds. Astronomers suspect that most of these short bursts happen when either two neutron stars or a neutron star and a black hole merge.

When a neutron star gets too close to another neutron star or a black hole, the two objects will orbit around each other, creeping closer and closer as they lose some of their energy through gravitational waves.

These objects eventually merge and emit short jets. When the short jets are pointed toward Earth, space telescopes can detect them as short gamma-ray bursts.

Classifying gamma-ray bursts 

Classifying bursts as short or long isn’t always that simple. In the past few years, astronomers have discovered some peculiar short gamma-ray bursts associated with supernovae instead of the expected mergers. And they’ve found some long gamma-ray bursts related to mergers instead of supernovae.

These confusing cases show that astronomers do not fully understand how gamma-ray bursts are created. They suggest that astronomers need a better understanding of gamma-ray pulse shapes to better connect the pulses to their origins.

But it’s hard to classify pulse shape, which is different than pulse duration, systematically. Pulse shapes can be extremely diverse and complex. So far, even machine learning algorithms haven’t been able to correctly recognize all the detailed pulse structures that astronomers are interested in.

Community science 

My colleagues and I have enlisted the help of volunteers through NASA to identify pulse structures. Volunteers learn to identify the pulse structures, then they look at images on their own computers and classify them.

Our preliminary results suggest that these volunteers – also referred to as citizen scientists – can quickly learn and recognize gamma-ray pulses' complex structures. Analyzing this data will help astronomers better understand how these mysterious bursts are created.

Our team hopes to learn about whether more gamma-ray bursts in the sample challenge the previous short and long classification. We'll use the data to more accurately probe the history of the universe through gamma-ray burst observations.

This citizen science project, called Burst Chaser, has grown since our preliminary results, and we’re actively recruiting new volunteers to join our quest to study the mysterious origins behind these bursts.

Read the original article at The Conversation.

Astronomers announced on April 5 that they may have detected a collision between a neutron star and a lightweight mystery object — an object larger than the largest known neutron star, but smaller than the smallest known black hole. The finding sheds light on objects that exist in this murky realm, which was long thought to be empty but, in recent times, has revealed otherwise.

More specifically, a signal detected in a pocket of the universe roughly 650 million light-years from Earth indicates a rare merger between a neutron star and what astronomers suspect is a surprisingly lightweight black hole. The pair would have danced around one another and merged about 650 million years ago, generating ripples in the fabric of space and time known as gravitational waves. These waves were sensed and flagged on May 29, 2023 by a network of antennas in Japan, Italy and the U.S. associated with the LIGO-Virgo-KAGRA (LVK) collaboration. 

"These are rare events," Evan Goetz, a LIGO researcher at the University of British Columbia (UBC) in Canada, told Space.com. "It's very exciting for the community to study as the first one of its type."

Related: Heaviest pair of black holes ever seen weighs 28 billion times more than the sun

The black hole candidate, which is about 2.5 to 4.5 times heavier than our sun, is heavier than the established limit of 2.5 suns for a neutron star — but lighter than the lightest known black hole, which weighs about five solar masses. This places the newfound object within the "mass gap," a mysterious region that separates the heaviest neutron stars from the lightest black holes.

This discovery "hints at this 'mass gap' being less empty than astronomers previously thought," Michael Zevin, an astrophysicist at the Adler Planetarium, said in a statement. 

Black holes, small and big, are born from the violent deaths of immensely massive stars. A few models of how stars evolve, however, predict black holes with masses within the "mass gap" range cannot directly form from such stellar deaths.  

"It does appear that it could be possible now with these observations," Goetz said. Perhaps, he says, astronomers need to tweak the models — or maybe "we really do have a more complicated evolution of a heavy neutron star that evolved into a black hole."

"It's hard to know just from this one example," he said.

In early 2020, astronomers announced the first conclusive detection of gravitational waves created by a collision which involved a stellar remnant right in the mass gap range. However, the discovery team couldn't classify the object with conviction at the time, concluding it could be either the biggest known neutron star or the smallest known black hole.

As for the latest finding, astronomers say they cannot pinpoint just where in the sky the mammoth objects merged because only one LVK detector was recording data when the signal was detected. Nevertheless, the finding has raised hopes that there may be many more such mass-gap objects out there waiting to be discovered.

"There is a lot more potentially we could find and a lot more to look forward to," Heather Fong, a LIGO researcher at UBC, told Space.com.

After a short maintenance break, LVK detectors resumed measuring ripples in space-time on April 10. The LIGO team anticipates observing over 200 gravitational wave signals by February 2025, including hints of a few objects within the elusive mass-gap range.

The discovery was presented at the American Physical Society meeting on Friday (April 5) and is awaiting peer review.

Editor's update 4/11: This is the first merger confirmed between a mass-gap object and a neutron star; mergers between black holes and neutron stars have been detected in the past. This article has been updated to reflect that.

Scientists may soon be able to detect the most mysterious entity in the universe using a fleet of next-generation satellites, a new theoretical study suggests.

Dark matter — a poorly understood substance that does not emit, absorb or reflect light but exerts a clear gravitational influence on other matter — dominates the universe. Despite being more than five times more abundant in space than ordinary matter, dark matter's composition and properties remain entirely unknown.

To address this problem, Hyungjin Kim a theoretical physicist at the German Electron Synchrotron (DESY) accelerator center, proposed searching for dark matter particles using gravitational wave detectors — instruments designed to measure subtle ripples in the fabric of space-time that were first predicted by Albert Einstein.

Related: What is dark matter?

Dark matter as waves

There are many hypotheses about the nature of dark matter particles, which accumulate in huge quantities to form so-called halos in galaxies. In the new paper, published in December 2023 in the Journal of Cosmology and Astroparticle Physics, Kim assumed that these particles may be extremely light, as many popular theories of dark matter predict.

Related: Physicists want to use gravitational waves to 'see' the beginning of time

"Ultralight particles appear commonly in many beyond-the-Standard-Model theories," Kim told LiveScience via email. Some of these particles are "perfect candidates'' for dark matter, raising some interesting implications for how the elusive entity might behave, he added.

"Unlike other 'particle' dark matter candidates, ultralight dark matter particles behave more similarly to classical [electromagnetic] waves," Kim said.

The wave properties of dark matter could lead to unexpected behaviors. In particular, recent theoretical studies suggest that the density of dark matter within a galactic halo should undergo random changes, jostling entire galaxies and potentially leaving subtle clues about dark matter's makeup.

"Imagine waves in the ocean; we see all the time there are fluctuations at the surface of the ocean, and it evolves in unpredictable ways," Kim said. "The same would happen in the ultralight dark matter halo,” with the resulting fluctuations potentially extending millions of times the distance between the Earth and the sun, Kim added.

If dark matter is ultralight, and if it indeed behaves like a wave, then scientists could potentially detect its movements with  gravitational wave detectors.

Gravitational wave detectors come to the rescue

According to Einstein's theory of general relativity, gravitational waves are ripples in the fabric of space-time.

When such a wave passes through a gravitational wave detector, it alters the geometry of the space inside, temporarily changing the distance between two mirrors or other similar objects placed inside the detector. This minute change enables scientists to detect the gravitational wave's presence.

In his study, Kim suggests this distance could be altered not only by a gravitational wave but also by a moving dark matter fluctuation, which could attract the mirrors with its gravitational field much like Earth attracts celestial bodies traveling around it.

"These fluctuations randomly move within the solar system, and continuously bombard gravitational wave detectors," Kim said.

To see if modern gravitational wave detectors could theoretically detect the influence of ultralight dark matter, Kim calculated how dark matter particles of varying sizes might perturb space-time. Kim had to explore a wide range of masses — from about 16 to 28 orders of magnitude smaller than the mass of an electron.

His theoretical analysis showed that for all these masses, existing detectors such as the  Laser Interferometer Gravitational-Wave Observatory (LIGO), which helped prove the existence of gravitational waves in 2015, would not be able to detect dark matter fluctuations because their sensitivity is too low.

However, there are several projects for future gravitational wave detectors that will be located in space, and the distance between their satellites will not be several miles, like the distance between the mirrors of LIGO, but around a million times bigger. If this distance changes even by a small fraction, the magnitude of the change should be so large that dark matter's influence should be measurable.

"What I found is that the dark matter fluctuations bombardment could leave a distinctive signal in gravitational wave detectors, and potentially future space-borne detectors might be able to test the hypothesis of ultralight dark matter," Kim said. "My proposal utilizes future space-borne gravitational wave detectors, such as Laser Interferometer Space Antenna (LISA)."

With LISA currently scheduled to launch in the mid-2030s, this theory may be more than a decade away from being testable. However, Kim added, in the meantime there may be other ways of detecting dark matter's influence on space-time.

"I am currently investigating the prospect of rapidly rotating neutron stars as another way to probe such fluctuations," he said.

Humanity's first space-based gravitational wave detector has received the go-ahead. 

The Laser Interferometer Space Antenna (LISA) mission, which consists of three spacecraft that together form a single gravitational wave detector, is a collaboration between NASA and the European Space Agency (ESA). It's set to launch in the mid-2030s.

The adoption of LISA was announced by ESA on Jan. 25, and recognizes that the mission concept and associated technology are sufficiently advanced. Getting the green light means scientists can start building the spacecraft as well as its required instruments; work will begin in Jan. 2025 after a European industrial contractor has been selected for construction.

The three spacecraft of LISA will trail Earth as our planet orbits the sun, forming an equilateral triangle in space. Each side of this triangle which will be a staggering 1.6 million miles (2.6 kilometers) long. The LISA craft will fire laser beams down these sides, which will experience minute-long alterations as gravitational waves pass over them, squashing and squeezing the very fabric of space.  

"LISA is an endeavor that has never been tried before," LISA lead project scientist Nora Lützgendorf said in a statement. "Using laser beams over distances of several kilometers, ground-based instrumentation can detect gravitational waves coming from events involving star-sized objects — such as supernova explosions or the merging of hyper-dense stars and stellar-mass black holes. To expand the frontier of gravitational studies, we must go to space."

Related: The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery

Gravitational waves were first predicted by Albert Einstein, arising from his 1915 theory of gravity: General relativity.

This revolutionary theory suggested that gravity arises as the result of objects with mass curving the very fabric of space and time, which are actually united as a single entity called spacetime. The greater the mass of an object, the larger the curvature that object causes — and thus the greater its gravitational influence. General relativity therefore explains, for instance, why stars have a stronger gravity than planets, but black holes have a stronger gravity than stars.

In addition to this, general relativity says that when a body in space accelerates, this movement creates ripples in spacetime that radiate outwards. These gravitational waves are insignificant unless the accelerating objects are massive compact objects (like black holes or neutron stars) orbiting around one other and eventually colliding. Collapsing massive stars can, which also trigger supernova explosions, can send out some substantial waves.

Yet, even in the most extreme cases, Einstein believed that gravitational waves would still be too faint to detect from Earth. He was wrong about that.

Fortunately, in Sept. 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the U.S. and the Virgo interferometer in Italy indeed detected ripples in spacetime. These waves were formed from the collision and merger of two black holes 29 and 36 times the mass of the sun, located over 1 billion light-years away.

And. since then, gravitational wave astronomers have detected signals from a multitude of events, including more black hole mergers, neutron star collisions and even mixed mergers between a black hole and a neutron star. 

LISA is ready to take this achievement even further as a space-based interferometer with the sensitivity to "hear" gravitational waves from merging black holes, neutron stars and supernovas at much greater distances than is possible for Earth-based detectors. This also means it'll be able to search for waves sourced from events further back in time.

"Thanks to the huge distance traveled by the laser signals on LISA and the superb stability of its instrumentation, we will probe gravitational waves of lower frequencies than is possible on Earth, uncovering events of a different scale, all the way back to the dawn of time," Lützgendorf said.

As well as detecting gravitational waves from more distant sources, LISA should also provide astronomers with the sensitivity to investigate closer and less extreme events such as the merger of compact white stars, which are born when smaller stars like the sun die.

"For centuries, we have been studying our cosmos through capturing light. Coupling this with the detection of gravitational waves is bringing a totally new dimension to our perception of the Universe," LISA project scientist Oliver Jennrich said in the statement. "If we imagine that, so far, with our astrophysics missions, we have been watching the cosmos like a silent movie, capturing the ripples of spacetime with LISA will be a real game-changer, like when sound was added to motion pictures."

Scientists are still hunting for the source of the faint, persistent hum of gravitational waves discovered reverberating through the Milky Way last year. Those waves could point to more than one tantalizing source, new research suggests.

The discovery team, the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, collaboration, strongly suspects the ripples in space-time were created from merging supermassive black holes, each a billion times more massive than our sun. These are known as binary pairs. If that's indeed the case, ongoing work would help estimate the locations of the celestial cosmic beasts, as well as their masses.

However, "finding one binary will not rule out the cosmological origin," study co-author Juan Urrutia of the National Institute of Chemical Physics and Biophysics in Estonia told Space.com. To that end, he and his colleagues studied the NANOGrav data and found that, in addition to the orbiting black hole hypothesis, three proposed cosmological sources seem to explain the data. More on all of those in just a bit; the big picture is that this suggests the gravitational wave signal could be a muddled mix of different sources. 

"This is a big potential problem because many signals are quite similar."

Related: The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery

Cosmological sources for spacetime ripples 

The aforementioned exotic, high-energy cosmological processes that took place in the early universe include "cosmic strings," "phase transitions" and "domain walls." 

Importantly, the latter two are thought to have unfolded shortly after the Big Bang — yet before the event's leftover radiation diffused across the universe. Thus, if the new findings pan out, and one of the sources are those domain walls, scientists say the detected signal would actually be the closest we've gotten to accessing the beginning of the universe.

Further, the cosmological processes outlined by the new study could also help the ongoing hunt for dark matter and dark energy, which together make up 95% of the universe but remain invisible to human eyes. 

"As [domain walls] move and evolve, they carry a lot of energy and emit gravitational waves," said Urrutia. At some point, however, they decay and you end up with "clumps" of dark matter, he added.

The possibility that the detected signal could be from domain walls is especially intriguing, as these complex structures were originally proposed over 50 years ago as one way to explain why the universe contains far more matter than antimatter, the latter of which refers to sort of "opposite" matter. Unlike normal, or baryonic, matter that's composed of positive protons and negative electrons, antimatter is composed of negative protons and positive electrons.  

What's especially weird when it comes to antimatter is that because antimatter and baryonic matter are supposedly fully symmetrical, the Big Bang should've had a 50/50 chance of producing either. That means our universe, theoretically, should consist of equal amounts of both. But it doesn't. Baryonic matter totally dominates the cosmos. 

On the other hand, studying phase transitions allows scientists to peer into many of the various phases that early universe went through to produce the baryonic electrons, protons and neutrons we know of today. Similar to how water boils when heated, cosmic phase transitions were triggered by the variation of temperatures in the universe, and "bubbles" interacted with each other to produce sound waves as well as gravitational waves, perhaps like the one recently detected.

Because the signals from the diverse sources seem to be similar, teasing them out of the detected gravitational waves is no easy task — made harder by the limits of our telescopes. The Laser Interferometer Gravitational-Wave Observatory (LIGO), a pair of research facilities in the United States and our current best gravitational wave detector, is designed to spot high-frequency waves. 

To spot more of the low-frequency waves like the ones recently observed, scientists are gearing up for the Laser Interferometer Space Antenna (LISA), a European three-satellite network launching in 2037. According to a NASA description, LISA would measure changes in position "that are less than the diameter of a helium nucleus over a distance of a million miles."

Another space experiment proposed in 2020, the Atomic Experiment for Dark Matter and Gravity Exploration, or AEDGE, may help in the search for gravitational waves in frequencies between those that can be "heard" by LISA and LIGO.

For these future detectors to deliver on their promise, it is crucial for scientists to have concrete predictions on what to look for and how to interpret the data, said Urrutia.

"There is a huge effort from the community to get all these calculations as precise as possible for when these experiments are ready to launch."

This research is described in a paper accepted for publication in the journal Physical Review D. 

The gravitational pull from black holes is  so strong that nothing can escape its grasp. So could we ever harness the gargantuan power of black holes as a source of energy?

In a new study, scientists propose two ways to use black holes as energy sources someday. They predicted processes for extracting energy from black holes by using their rotational and gravitational properties.

"We know that we can extract energies from black holes, and we also know that we can inject energy into them, which almost sounds like a battery," lead author Zhan Feng Mai, a postdoctoral researcher at the Kavli Institute for Astronomy and Astrophysics at Peking University, told Live Science.

In the first hypothetical scenario, scientists would "charge" the black hole by injecting it with massive, electrically charged particles. These charges would continue being sucked in until the black hole itself had an electric field that began repelling any additional charges that they attempted to inject, the scientists explained in the study, published Nov. 29 in the journal Physical Review D.

Related: Supermassive black hole at the heart of the Milky Way is approaching the cosmic speed limit, dragging space-time along with it

When this electromagnetic repulsion was greater than the gravitational pull of the black hole, scientists would consider it "fully charged." In keeping with Einstein's theory of general relativity, which says that mass can be treated as equivalent to energy, the black hole’s available energy would come from a combination of the electrical charges injected into it as well as the mass of those electrical charges.

"The black hole battery is transforming the energy of the particle's mass into charge energy," Mai said.

The researchers calculated the efficiency of the recharging process to be 25%, meaning that black hole batteries could transform about a quarter of the mass inputted into available energy in the form of an electric field. This would make the efficiency of the battery around 250 times higher than that of an atomic bomb, the team calculated.

To extract the energy, the researchers would utilize a process known as superradiance, which is based on the theory that space-time is literally dragged around the rotation of a spinning black hole because of its intense gravitational field.

Gravitational or electromagnetic waves that entered this region of rotation would get dragged along too, but assuming they had not yet passed the black hole's event horizon — the boundary beyond which nothing, not even light, can escape — some waves might be deflected with more energy than they initially carried, the researchers wrote. This process would convert the black hole's rotational energy, determined by its mass, into the waves that are deflected.

The other method of harnessing a black holes' energy would involve extracting that energy in the form of so-called Schwinger pairs, or paired particles that form spontaneously in the presence of an electric field.

If we started with a fully charged black hole, the electric field near the event horizon might be so strong that it would spontaneously create an electron and positron, which is like an electron but with an opposite charge, Mai explained. If the black hole were positively charged, the positron would be shot out from the black hole due to repulsion. That runaway particle could then, theoretically, be collected as energy.

Mai said he does not know if we will ever see a battery like this, but the theoretical exercise was inspired by scientists' previous attempts to theoretically extract energy from black holes.

"We see the black hole as a place where quantum mechanics and gravity have to somehow get together," Daniele Faccio, a physicist at the University of Glasgow who was not involved in the study, told Live Science. "By looking at them from the perspective of energy mining, we can understand a little more about what's going on."

Scientists have discovered gravitational waves stemming from a black hole merger event that suggest the resultant black hole settled into a stable, spherical shape. These waves also reveal the combo black hole may be much larger than previously thought. 

When initially detected on May 21, 2019, the gravitational wave event known as GW190521 was believed to have come from a merger between two black holes, one with a mass equivalent to just over 85 suns and the other with a mass equivalent to about 66 suns. Scientists believed the merger therefore created an approximately 142 solar mass daughter black hole.

Yet, newly studied spacetime vibrations from the merger-created black hole, rippling outward as the void resolved into a proper spherical shape, seem to suggest it's more massive than initially predicted. Rather than possess 142 solar masses, calculations say it should have a mass equal to around 250 times that of the sun. 

These results could ultimately help scientists better test general relativity, Albert Einstein's 1915 theory of gravity, which first introduced the concept of gravitational waves and black holes. “We are really exploring a new frontier here," Steven Giddings, a theoretical physicist at the University of California, said in a statement.

Related: How dancing black holes get close enough to merge 

Gravitational waves and general relativity 

General relativity predicts that objects with mass warp the very fabric of space and time  —  united as a single, four-dimensional entity called "spacetime”" —  and that "gravity" as we perceive it arises from the curvature itself.

Just as a bowling ball placed on a stretched rubber sheet causes a more extreme "dent" than a tennis ball would, a black hole causes more curvature in spacetime than a star does, and a star causes more curvature than a planet does. In fact, a black hole, in general relativity, is a point of matter so dense it causes curvature of spacetime so extreme that, at a boundary called the event horizon, not even light is fast enough to escape the inward dent.

This isn't the only revolutionary prediction of general relativity, however. Einstein also predicted that when objects accelerate, they should set the very fabric of spacetime ringing with ripples called gravitational waves. And again, the more massive the objects involved, the more extreme the phenomenon is. This means when dense bodies like black holes spiral around one another, constantly accelerating due to their circular motion, spacetime rings around them like a struck bell, humming with gravitational waves.

These ripples in spacetime carry away angular momentum from the spiraling black holes, and that, in turn, causes the black holes' mutual orbits to tighten, drawing them together and increasing the frequency of the gravitational waves emitted. Spiraling closer and closer, the black holes finally merge, creating a daughter black hole and sending a high-frequency "chirp" of gravitational waves echoing out through the cosmos. 

But there was one thing Einstein got wrong about gravitational waves. The great physicist believed that these ripples in spacetime would be so faint that they would never be detected here on Earth after traveling across the universe for millions, or even billions, of light years.

Yet, in Sept. 2015, the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) based in Washington and Louisiana showed Einstein was incorrect. They detected GW150914, gravitational waves associated with merging black holes located around 1.3 billion light-years away. The gravitational wave signal was detected as a change in the length of one of LIGO's 2.5 miles (4 kilometers) long laser arms, equivalent to a thousandth the width of a proton.

Remarkably, since then, LIGO and its fellow gravitational wave detectors, Virgo in Italy and KAGRA in Japan, have detected many more such  events, reaching the point at which they are detecting one gravitational wave event each week. Although, even amongst this cornucopia of gravitational wave detections, GW190521 stands out. 

A special gravitational wave event 

The merging frequency of the two black holes behind the GW190521 signal, which are located as far away as 8.8 billion light-years from Earth, was so low it was only during the final two orbits of the black holes that the frequency became high enough to reach the sensitivity limits of LIGO and Virgo.

The team behind this new investigation  —  which is not part of the LIGO/Virgo Collaboration  —  wanted to know what information about the violent collision and merger of these black holes may be locked away in this signal.

They found that the instant the black holes collided, the resultant black hole was created with a lopsided shape. Black holes are only stable when they have a spherical shape, meaning that within milliseconds of the merger, the daughter black hole would have to assume the shape of a sphere. 

Just as the shape of a bell determines the frequency at which it rings, the team said that as this new black hole changed shape and stabilized, the frequencies of the gravitational waves it rang out were shifted. These so-called "ring down" gravitational waves contained information about the mass of the daughter black hole and also the rate at which it is spinning. 

This means that ring-down gravitational waves from such a merger offer scientists an alternative way to measure the properties of merging black holes, in contrast to the traditional method of using the gravitational waves created during the spiraling process. 

The team found two separate ring-down frequencies in the gravitational wave signal GW190521, which, when considered together, give the created black hole a mass of 250 solar masses. That means it's considerably more massive than estimated by using the spiraling gravitational waves. The detection of these ringdown gravitational waves was shocking even to the team behind these findings.

"I never thought I would ever see such a measurement in my lifetime,” Badri Krishnan, co-author of the research and a physicist at Radboud University, said.

The team’s research is detailed in a paper published on Nov. 28 in the journal The Physical Review Letters. 

Scientists have determined the possible effects of a neutron star collision happening near Earth, finding that these so-called kilonovas could be real killers that would doom humanity. But don't worry, the collision would have to be really close to wreak havoc on our world. Nonetheless, here's what would probably go down.

"We found that if a neutron star merger were to occur within around 36 light-years of Earth, the resulting radiation could cause an extinction-level event," Haille Perkins, team leader and a scientist at the University of Illinois Urbana-Champaign, told Space.com.

Neutron star clashes that create bursts of light, called kilonovas, are considered  the most violent and powerful events in the known universe. This is perhaps unsurprising, given that neutron stars are the collapsed remnants of dead stars and are made of matter so dense a teaspoon of one brought to Earth would weigh about 10 million tons. That's equivalent to 350 Statues of Liberty balanced on a spoon.

Not only do these dead star mergers create blasts of gamma rays and showers of charged particles moving at near-light speeds , known as cosmic rays,  but they also generate the only environments we know of turbulent enough to forge elements heavier than lead, like gold and platinum. These elements can’t even be created at the incredible ultra-high temperatures and pressures found in the hearts of massive stars. 

Further, neutron star mergers set the very fabric of space "ringing" with ripples called gravitational waves, which can be detected here on Earth — even after traveling across billions of light years.

"Neutron stars can exist in binary systems, and when they merge, they produce a rare but spectacular event," Perkins said.

Related: What happens when neutron stars collide? Astronomers may finally know 

The team’s research was based on observations of the neutron star merger behind gravitational wave signal GW 170817, picked up by Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2017, and  gamma-ray burst GRB 170817A. 

Occurring about 130 million light-years away, this is the only neutron star merger thus far seen in electromagnetic radiation and heard in gravitational waves, making it a natural choice for investigating these powerful events.

A killer-nova? 

Neutron star merger gamma rays are arguably the most obviously threatening aspect of these events. That's because this type of radiation carries enough energy to strip electrons from atoms, a process called ionization. And these ionizing blasts of radiation could easily destroy the Earth’s ozone layer, resulting in our planet receiving lethal doses of ultraviolet radiation from the sun.

Perkins and her colleagues determined gamma rays coming from neutron star mergers — in twin narrow jets from either side of the merger — would pretty much roast any living thing that falls directly in their path for a distance of about 297 light-years. Fortunately, however, that effect has an extremely narrow range. In other words,  it really would take a "direct hit" from a jet to give rise to such dramatic effects. But, there's another issue.

These jets are cocooned with gamma radiation in general, which would also affect the ozone layer of Earth if our planet was in their wider path — within about  13 light-years of them. . This "off-axis" gamma-ray cocoon's ozone damage would also take 4 years to recover from. All in all, the gamma-ray cocoon strike would leave the Earth's surface exposed to harmful ultraviolet light for nearly half a decade.

Though gamma-ray effects of neutron star mergers are relatively short-lived, there is also another form of ionizing radiation these emissions give rise to, which is less energetic but longer-lasting. 

When the jets of gamma rays hit gas and dust around stars, called the interstellar medium, this creates powerful X-ray emissions called the X-ray afterglow. Such X-ray emission lives longer than  gamma-ray emissions and could also ionize the ozone layer, the team says. This, therefore, is arguably more lethal. Earth would need to be quite close to this afterglow before we have to be concerned about our fate, however — within a distance of 16.3 light years to be exact. 

And we haven't gotten to the worst part yet. 

The most threatening effect of the neutron star smash-up that the team discovered comes from those highly energetic charged particles, or cosmic rays, that spread away from the event's epicenter in the form of an expanding bubble. Were these cosmic rays to strike Earth, they would strip the ozone layer and leave the planet vulnerable to being blasted by ultraviolet rays for a period of thousands of years. 

This would qualify as an extinction-level event, and Earth could be affected even if our planet were around 36 light-years away. 

"The specific distance of safety and component that is most dangerous is uncertain as many of the effects depend on properties like viewing angle to the event, the energy of the blast, the mass of material ejected, and more," Perkins continued. "With the combination of parameters we select, it seems that the cosmic rays will be the most threatening."

Again, don’t panic just yet! 

Before lamenting that the end is nigh, it is worth weighing the apocalyptic picture painted by the impact of neutron star mergers against some other factors surrounding these events. 

"Neutron star mergers are extremely rare but quite powerful, and this, combined with the relatively small range of lethality, means an extinction caused by a binary neutron star merger should not be a concern of the people on Earth," Perkins assured. 

To get a picture of this rarity, throughout the 100 billion stars in the Milky Way, scientists have thus far only found one potential kilonova progenitor system, CPD-29 2176, which is located about 11,400 light-years from Earth.

"There are several other more common events like solar flares, asteroid impacts, and supernova explosions that have a better chance of being harmful," Perkins continued.

She added that some of these other events have been associated with mass extinction events on Earth already, with the most striking example of this being the impact of a massive asteroid that wiped out the non-avian dinosaurs and three-quarters of life on Earth around 66 million years ago in the Cretaceous-Tertiary extinction event.

Where this research does have important connotations is in the search for life elsewhere in the universe, as it certainly gives us an idea of the systems that aren’t likely to enjoy the conditions needed to support life. (Life as we know it, at least.)

"Their conclusion that kilonovas could have a similar lethality to supernovas, but are much less common, coincides with what I believe would be likely to be the case," Niels Bohr Institute Cosmic Dawn Center scientist Darach Watson, who also studies kilonovas and was not involved in this research, told Space.com. "So overall, this is likely to be more of a threat for planets in old galaxies where the star-formation has ended, not so much in the Milky Way."

As for the team behind this research, Perkins explained that the next step is to observe more of these neutron star collision events. 

"Currently, we only have one confirmed detection of a kilonova from a binary neutron star merger, so any more observations will constrain the unknowns," she concluded.

The team’s research is published on the open-access paper repository arXiv. 

A new crystal can bend light like a black hole would, causing the light to bow away from its usual straight path.

This phenomenon, called pseudogravity, could be used in 6G communication technology, according to the authors of the new study, published Sept. 28 in the journal Physical Review A. This next-generation communication would transmit information wirelessly at ultrahigh speeds. Because the crystal mimics what happens when light passes by black holes and other ultradense space objects, the new technique could also be used to study so-called quantum gravity, a theory that would unite quantum mechanics and Albert Einstein's theory of relativity.

According to relativity, light and other electromagnetic waves can be influenced by gravitational forces. This is called gravitational lensing, and astronomers use it all the time to study massive space objects such as quasars. Recreating such an effect in a laboratory environment is difficult, given the need for a huge amount of mass, but scientists have long suspected they could mimic the phenomenon using crystalline materials.

Related: Mysterious dark energy is spread evenly across the cosmos

To do so, Kyoko Kitamura, a professor in the graduate school of engineering at Tohoku University in Japan, and her colleagues started with photonic crystals, which are crystals of two or more arrangements that are arrayed in a regular, grid-like pattern and are capable of slowing light as it passes through them. The team gradually distorted these crystals, disrupting the crystalline lattice, and then shined beams of light through the crystals and watched them deflect.

"Much like gravity bends the trajectory of objects, we came up with a means to bend light within certain materials," Kitamura said in a statement.

Manipulating light in this way is one potential pathway for next-generation communications technology, which will require sending information in the terahertz range, or above 100 gigahertz. (5G technology maxes out at 71 gigahertz.) Researchers believe that creative manipulation of light is one way to reach these frequencies. The new material could also have applications in research.

"Academically, the findings show that photonic crystals could harness gravitational effects, opening new pathways within the field of graviton physics," study co-author Masayuki Fujita, an associate professor at Osaka University in Japan, said in the statement.

A graviton is the hypothetical quantum particle that mediates the force of gravity. No such particle has been observed yet, nor have scientists entirely worked out what this particle would even look like in theory.

Earth’s premier gravitational wave detector just received a major upgrade that will significantly improve its ability to spot ripples in the fabric of space and time — undulations created by collisions between black holes or neutron stars, and sometimes, between both.

What this means is, during the next run of the Laser Interferometer Gravitational-Wave Observatory (LIGO), the instrument will be able to detect more merger events between these massive stellar remnants that form when enormous stars collapse at the end of their lives. LIGO will also be able to spot such impacts across greater distances thanks to the new upgrade, tracking gravitational waves that have rippled through spacetime for billions of years. 

"We can now reach a deeper universe and are expected to detect about 60 percent more mergers than before," LIGO lab researcher, Wenxuan Jia, told Space.com. "LIGO will certainly detect farther-away binary coalescence events. With lower noise and higher signal-to-noise ratio, we can further constrain the parameters of the compact objects that merged together billions of years ago. 

"The upgrade also increases our chances of detecting sub-stellar mass black holes in the universe. The latest experimental upgrade will benefit our detection of astrophysical signals in nearly every way."

Related: 2 merging supermassive black holes spotted at ‘cosmic noon’ in early universe 

LIGO first became world-famous in Sept. 2015, when it detected gravitational waves from merging black holes for the first time. These ripples had traveled for around 1.4 billion years, squashing and squeezing spacetime as they made their way through the universe. 

But since then, LIGO, and its partner gravitational wave detector Virgo, have detected signals from far more merging black hole pairs, colliding neutron stars, and mixed mergers between the two.

LIGO researchers are particularly excited for the new upgrade, however, because this pushes the instrument beyond what is called the "quantum limit" — a first for a gravitational wave detector.

What is the 'quantum limit?' 

LIGO is designed to measure an incredibly small change that can occur between lasers within two arms on the detector. The change occurs when gravitational waves ripple over those arms, Jia explained. 

Basically, the instrument produces one laser and splits it into two beams that separately travel through the pair of 4-kilometer arms. The two beams then come into phase while traveling through the two arms, meaning they perfectly line up despite being in different places. Then, when the beams get reflected back via mirrors built into the device, they reunite and the peaks and troughs of the wavelengths meet. But here's where that length change comes in.

"The gravitational wave is a perturbation to spacetime," Jia explained. "When it propagates through the LIGO detector, it will change the length difference of the two 4-kilometer arms of LIGO as if it’s stretching one arm while shortening the other arm."

To be clear, the actual arms of LIGO aren't changing length. The lasers within them are — and this length change also forces the beams to increase in amplitude during the part when they reunite. So, in other words, the length difference resulting from the passage of gravitational waves leads to an amplitude change. And that causes the lasers' power to change as well. 

However, because gravitational waves "squash and stretch" space, if they wash over these lasers, the length of the arms gets altered only by a very small amount so it's pretty hard to measure with standard mechanisms. But the effect is still observable thanks to the fact that light wavelength changes are reflected by the phase of the light. Remember the bit about both lasers lining up, being in the same phase? Well, a small change in arm length would mean that, when the light arrives back at the detector, it is no longer in phase  —  the peaks and troughs of the wavelength cancel each other out in  a process called destructive interference.

There is another problem, however. The variations in length of these arms can sometimes get infinitesimally small — trillions of times smaller than a human hair, falling into sizes we see in  the subatomic and quantum realms. 

The weaker the gravitational wave, the less it changes the arm length and the smaller the effect on the laser. And when this effect gets really tiny, it bumps up against a principle in quantum physics called Heisenberg’s Uncertainty Principle, which says there is a limit to how precisely we can measure a correlated pair of physical quantities called "observables." 

Practically, it means some gravitational waves have remained outside LIGO's abilities. But now, there seems to be a workaround.

"There exists a minimal uncertainty, or noise, when you want to measure the phase of a laser beam over time," Jia said. "A laser light has two observables, namely amplitude and frequency. We are doing a phase or frequency measurement with LIGO, and so we don’t care so much about the amplitude."

Jia added that Heisenberg’s Uncertainty Principle actually allows for a trade-off The team reduce the uncertainty of one observable they want  —  frequency  —  at the expense of increasing the uncertainty of the other one they do not  —  amplitude.

"We can reduce the frequency uncertainty by 'squeezing' the light. This clever idea allows us to surpass the minimal uncertainty, or the quantum limit, of the LIGO detector," Jia added. 

"Since I was a young grad student, I’d heard about this kind of idea, but for 20 years, I didn’t think about it much. It seemed very ‘sci-fi' because, in school, you learn there’s a Heisenberg Uncertainty limit and you can only measure things so well before you reach that limit," Rana Adhikari, a professor of physics at the California Institute of Technology (Caltech) an told Space.com. "If you try to measure things more precisely, you disturb things. With this new upgrade, we’re able to basically measure as strongly as we want to. We can put full laser power into the system."

Adhikari added that his dream is to upgrade LIGO to the point at which it can detect gravitational waves at incredibly low sensitivities and, through this, access bigger black holes that may have existed in the very early universe. And the Caltech researcher also believes LIGO can be pushed even further in terms of sensitivity.

"Everything really depends on the quality of your materials. You can make really, really pure mirror materials, and then there’s almost no limit to how precisely you can measure things and push on the same technique, just doing it better and better over the years," Adhikari explained. "At that point, where the sensitivity has been upped by a factor of 10, the way that the LIGO works, you would be able to see black hole mergers from very, very early in the universe when the first galaxies were forming."

The achievement isn’t just a big deal for LIGO and the detection of gravitational waves; it also tells physicists that if you are willing to do a deal, it’s possible to surpass the quantum limit without violating the Uncertainty Principle.

"It's a very exciting event for me and the whole LIGO 'squeezing' team. A tremendous effort has been made to achieve this milestone over many years," Jia added. "It felt surreal to see the whole system working at first after we put together all subsystems carried out by many other teams. The success of frequency-dependent squeezing is not possible without such an awesome team and collaboration."

The team’s research was published in September in the journal Physical Review X.

A new cosmic atlas contains precise details of nearly 400,000 galaxies in the Milky Way’s general neighborhood. And, beyond being of immense use for astronomers seeking hard data, the atlas also features beautiful images that are free for the public to access online and get to know our corner of the universe. 

Called the Siena Galaxy Atlas (SGA), this digital atlas was created using data from three astronomical surveys collected between 2014 and 2017 at Cerro Tololo Inter-American Observatory (CTIO) and Kitt Peak National Observatory (KPNO). Together, those surveys are known as  the DESI Legacy Surveys.

Cosmic atlases of this type help astronomers spot patterns that help categorize new discoveries, such as stars that suddenly flare and then disappear  —  so-called “transients." Plus, such atlases allow astronomers to identify which objects are contenders for detailed follow-up investigations. These databases must also be constantly updated to keep up with those discoveries, especially in the modern age when telescope technologies continue to rapidly improve. 

That's where the SGA's merit comes in. This atlas represents peak accuracy, promising to be a gold mine of galactic information for scientists aiming to investigate everything from the births and evolutions of galaxies to the distribution of dark matter and propagation of gravitational waves through space.  

"Nearby large galaxies are important because we can study them in more detail than any other galaxies in the universe; they are our cosmic neighbors," John Moustakas, SGA project leader and a physics professor at Siena College, said in a statement. "Not only are they strikingly beautiful, but they also hold the key to understanding how galaxies form and evolve, including our very own Milky Way galaxy."

Related: James Webb Space Telescope detects quartz crystals in an exoplanet’s atmosphere 

Standing on the shoulders of giants

Mapping the night sky is a practice that dates back centuries, with other notable cosmic atlases being the 1774 Catalogue des Nébuleuses et des Amas d’Étoiles (Catalogue of Nebulae and Star Clusters) created by French astronomer Charles Messier, the New General Catalogue of Nebulae and Clusters of Stars (NGC), devised by John Louis Emil Dreyer in 1888 and, more recently, 1991's Third Reference Catalogue of Bright Galaxies, in which galaxies still bear the prefixes Messier (M) and NGC in their names, indicating they originally belonged to previous catalogs..

The SGA stands on the shoulders of this astronomical legacy — but, whereas historic atlases have relied on antiquated equipment and photographic plates, the surveys that informed the SGA depended on state-of-the-art digital images captured by technology. For instance, this atlas was created with information from the Dark Energy Camera (DECam) on the Víctor M. Blanco 4-meter Telescope in Chile, the Mosaic3 camera on the Nicholas U. Mayall 4-meter Telescope and the 90Prime camera on the Bok 2.3-meter Telescope. Additional data was provided to the atlas by NASA’s Wide-field Infrared Survey Explorer (WISE) satellite.

All together, these surveys cover an area amounting to 20,000 square degrees, which is equivalent to almost half of the night sky over Earth. This means the SGA comprises an absolutely huge amount of cosmic information in one place, including the locations, shapes and sizes of hundreds of thousands of large galaxies relatively close to the Milky Way. 

What really sets SGA apart is the accuracy of all that data, thanks to being built on images captured with highly sensitive instruments. SGA is also the first cosmic atlas to feature the light profiles of galaxies  —  a curve that describes how the brightness of the galaxy changes from its brightest point, usually at the center, to its dimmest, commonly at its edge.

"Previous galaxy compilations have been plagued by incorrect positions, sizes, and shapes of galaxies and also contained entries that were not galaxies but stars or artifacts," Arjun Dey, a project scientist and astronomer at the NOIRLab, said in the statement.."The SGA cleans all this up for a large part of the sky. It also provides the best brightness measurements for galaxies, something we have not reliably had before for a sample of this size.

"The SGA is going to be the pre-eminent digital galaxy atlas for large galaxies."

Some specific projects astronomers could undertake with SGA data include investigations into how stars form in differently shaped galaxies and how the distribution of dark matter  —  a mysterious form of matter that dominates our  universe but remains effectively invisible  —  determines the positions of galaxies and how they cluster. The SGA could also help astronomers find the sources of gravitational wave signals detected on Earth because these faint ripples in the very fabric of space and time wash over our planet after traveling for millions of light years.

"The public release of these spectacular data contained in the atlas will have a real impact not only on astronomical research but also on the public’s ability to view and identify relatively nearby galaxies," NOIRLab Project director Chris Davis said. "Dedicated amateur astronomers will particularly love this as a go-to resource for learning more about some of the celestial targets they observe."

This article was originally published at The Conversation. The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.

Valerio Faraoni is a Professor of Physics & Astronomy at Bishop's University. Andrea Giusti is a postdoctoral fellow at the Swiss Federal Institute of Technology Zurich.

Einstein's theory of gravity — general relativity — has been very successful for more than a century. However, it has theoretical shortcomings. This is not surprising: the theory predicts its own failure at spacetime singularities inside black holes — and the Big Bang itself.

Unlike physical theories describing the other three fundamental forces in physics — the electromagnetic and the strong and weak nuclear interactions — the general theory of relativity has only been tested in weak gravity.

Deviations of gravity from general relativity are by no means excluded nor tested everywhere in the universe. And, according to theoretical physicists, deviation must happen.

Related: Was Einstein wrong? The case against space-time theory

Deviations and quantum mechanics

According to Einstein, our universe originated in a Big Bang. Other singularities hide inside black holes: Space and time cease to have meaning there, while quantities such as energy density and pressure become infinite. These signal that Einstein’s theory is failing there and must be replaced with a more fundamental one.

Naively, spacetime singularities should be resolved by quantum mechanics, which apply at very small scales.

Quantum physics relies on two simple ideas: point particles make no sense; and the Heisenberg uncertainty principle, which states that one can never know the value of certain pairs of quantities with absolute precision — for example, the position and velocity of a particle. This is because particles should not be thought of as points but as waves; at small scales they behave as waves of matter.

This is enough to understand that a theory that embraces both general relativity and quantum physics should be free of such pathologies. However, all attempts to blend general relativity and quantum physics necessarily introduce deviations from Einstein’s theory.

Therefore, Einstein’s gravity cannot be the ultimate theory of gravity. Indeed, it was not long after the introduction of general relativity by Einstein in 1915 that Arthur Eddington, best known for verifying this theory in the 1919 solar eclipse, started searching for alternatives just to see how things could be different.

Einstein’s theory has survived all tests to date, accurately predicting various results from the precession of Mercury’s orbit to the existence of gravitational waves. So, where are these deviations from general relativity hiding?

Cosmology matters

A century of research has given us the standard model of cosmology known as the Λ-Cold Dark Matter (ΛCDM) model. Here, Λ stands for either Einstein’s famous cosmological constant or a mysterious dark energy with similar properties.

Dark energy was introduced ad hoc by astronomers to explain the acceleration of the cosmic expansion. Despite fitting cosmological data extremely well until recently, the ΛCDM model is spectacularly incomplete and unsatisfactory from the theoretical point of view.

In the past five years, it has also faced severe observational tensions. The Hubble constant, which determines the age and the distance scale in the universe, can be measured in the early universe using the cosmic microwave background and in the late universe using supernovae as standard candles.

These two measurements give incompatible results. Even more important, the nature of the main ingredients of the ΛCDM model — dark energy, dark matter and the field driving early universe inflation (a very brief period of extremely fast expansion originating the seeds for galaxies and galaxy clusters) — remains a mystery.

From the observational point of view, the most compelling motivation for modified gravity is the acceleration of the universe discovered in 1998 with Type Ia supernovae, whose luminosity is dimmed by this acceleration. The ΛCDM model based on general relativity postulates an extremely exotic dark energy with negative pressure permeating the universe.

Problem is, this dark energy has no physical justification. Its nature is completely unknown, although a plethora of models has been proposed. The proposed alternative to dark energy is a cosmological constant Λ which, according to quantum-mechanical back-of-the-envelope (but questionable) calculations, should be huge.

However, Λ must instead be incredibly fine-tuned to a tiny value to fit the cosmological observations. If dark energy exists, our ignorance of its nature is deeply troubling.

Alternatives to Einstein’s theory

Could it be that troubles arise, instead, from wrongly trying to fit the cosmological observations into general relativity, like fitting a person into a pair of trousers that are too small? That we are observing the first deviations from general relativity while the mysterious dark energy simply does not exist?

This idea, first proposed by researchers at the University of Naples, has gained tremendous popularity while the contending dark energy camp remains vigorous.

How can we tell? Deviations from Einstein gravity are constrained by solar system experiments, the recent observations of gravitational waves and the near-horizon images of black holes.

There is now a large literature on theories of gravity alternative to general relativity, going back to Eddington’s 1923 early investigations. A very popular class of alternatives is the so-called scalar-tensor gravity. It is conceptually very simple since it only introduces one additional ingredient (a scalar field corresponding to the simplest, spinless, particle) to Einstein’s geometric description of gravity.

The consequences of this program, however, are far from trivial. A striking phenomenon is the “chameleon effect,” consisting of the fact that these theories can disguise themselves as general relativity in high-density environments (such as in stars or in the solar system) while deviating strongly from it in the low-density environment of cosmology.

As a result, the extra (gravitational) field is effectively absent in the first type of systems, disguising itself as a chameleon does, and is felt only at the largest (cosmological) scales.

The current situation

Nowadays the spectrum of alternatives to Einstein gravity has widened dramatically. Even adding a single massive scalar excitation (namely, a spin-zero particle) to Einstein gravity —and keeping the resulting equations “simple” to avoid some known fatal instabilities — has resulted in the much wider class of Horndeski theories, and subsequent generalizations.

Theorists have spent the last decade extracting physical consequences from these theories. The recent detections of gravitational waves have provided a way to constrain the physical class of modifications of Einstein gravity allowed.

However, much work still needs to be done, with the hope that future advances in multi-messenger astronomy lead to discovering modifications of general relativity where gravity is extremely strong.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

A simulated map of the Milky Way as it would appear in gravitational waves has given a powerful impression of what future space-based detectors will observe.

Over 90 gravitational-wave events have been detected so far by the triumvirate of ground-based detectors — the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the U.S., Virgo in Italy and KAGRA in Japan. All these detected events are mergers of stellar-mass black holes and/or neutron stars in distant galaxies; no gravitational-wave events have been found coming from our Milky Way galaxy.

However, our galaxy is filled with so-called ultracompact binaries, which used to be binary stars but which have since evolved to become stellar remnants.

Related: The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery

"Binary systems … fill the Milky Way, and we expect many of them to contain compact objects like white dwarfs, neutron stars and black holes in tight orbits," said Cecilia Chirenti, of the University of Maryland and NASA's Goddard Space Flight Center, in a statement. "But we need a space observatory to 'hear' them because their gravitational waves hum at frequencies too low for ground-based detectors."

Earth-bound observatories such as LIGO are able to detect gravitational waves of frequencies between 5 and 20,000 Hertz. Ultracompact binaries in our galaxy, as they spiral around each other and eventually merge, have frequencies in the range of milliHertz.

Several space-based gravitational-wave detectors are in the works. The European Space Agency's Laser Interferometer Space Antenna (LISA) is at the forefront, with launch projected to be in the 2030s, while Chinese scientists also have two mission concepts, named TianQin and Taiji, respectively.

Chirenti is part of a team, led by Kaitlyn Szekerczes of the Gravitational Astrophysics Laboratory at NASA Goddard, who have now simulated the intensity and frequency of gravitational waves emitted by ultracompact binaries in the Milky Way. The resulting image shows how observatories such as LISA will be able to study the Milky Way in gravitational waves just like astronomers study it in X-rays, gamma rays and so on. The simulated image shows ultracompact binaries concentrated in the plane of the Milky Way's spiral disc and spilling out into the galactic halo.

"Our image is directly analogous to an all-sky view of the sky in a particular type of light, such as visible, infrared or X-rays," said team-member James Ira Thorpe, who is also based at NASA Goddard. "The promise of gravitational waves is that we can observe the universe in a totally different way, and this image really brings that home."

To date, astronomers know of only a handful of ultracompact binaries with orbital periods less than an hour, which would place the compact objects close enough to each other to emit detectable gravitational waves. Finding them is difficult, because neutron stars and black holes do not emit much light. This is where LISA will come in: ultracompact binaries should radiate brightly in gravitational waves, allowing LISA to discover tens of thousands of them.

The shorter the orbital period of an ultracompact binary, the higher the frequency and the lower the amplitude of the gravitational waves they emit. If they are really close together, there may even be some mass transfer between the two objects that astronomers could follow up on with optical, X-ray and gamma-ray telescopes. Scientists refer to this fusing of electromagnetic and gravitational-wave observations as "multi-messenger astronomy."

Details of the simulated image were published in a paper in The Astronomical Journal this past June.

In astrophysics, there's a saying that "black holes have no hair." 

This means that, in the theory of general relativity, black holes are exceptionally simplistic objects. All you need to describe a black hole is its mass, its electric charge and its spin rate. With those three numbers alone, you have everything you could ever know about black holes. In other words, they're bald — they have no extra information.

This aspect of black holes is extremely frustrating to astrophysicists, who desperately want to understand how these cosmic behemoths work. But because black holes have no "hair," there's no way to learn more about them and what makes them tick. Alas, black holes remain some of the most puzzling and mysterious objects in the universe.

But this concept of "no-hair" black holes relies on our current understanding of general relativity, as originally formulated by Albert Einstein. This picture of relativity focuses on the curvature of space-time. Any entity with mass or energy will bend space-time around it, and that bending instructs those entities how to move.

Related: Stephen Hawking's famous black hole paradox may finally have a solution

This is not the only way to construct a theory of relativity, however. There's an entirely different approach that instead focuses on the "twistiness," rather than on the curvature, of space-time. In this picture, any entity with mass or energy twists up space-time around it, and that twisting instructs other objects how to move.

The two approaches, one based on curvature and the other based on twistiness, are mathematically equivalent. But because Einstein developed the curvature-based language first, it's much more widely used. The twistiness approach, known as "teleparallel" gravity for its mathematical use of parallel lines, offers a lot of room for intriguing theoretical insights that aren't obvious in the curvature approach.

As an example, a team of theoretical physicists recently explored how teleparallel gravity could approach the problem of black hole hairiness. They detailed their work in a paper published to the preprint database arXiv in July. (The research has yet to be peer-reviewed.)

The team examined potential extensions of general relativity using what's called a scalar field — a quantum object that inhabits all of space and time. A famous example of a scalar field is the Higgs boson, which is responsible for giving many particles their masses. There may be additional scalar fields that inhabit the universe and subtly alter how gravity works, and physicists have long used these scalar fields in attempts to explain the nature of cosmic mysteries such as dark matter and dark energy.

In regular curvature-based general relativity, there are only so many ways to add scalar fields. But in teleparallel gravity, there are many more options. This research team discovered a way to add scalar fields to general relativity using the teleparallel framework. Then, they used that approach to investigate if these scalar fields, which would otherwise be invisible, might show up near black holes.

The end result: The scalar fields added to general relativity, when explored through the teleparallel lens, gave black holes some hair.

The "hair" in this case is the presence of a strong scalar field near the event horizon of a black hole. Crucially, this scalar field carries information about the black hole inside it, which would allow scientists to understand more about black holes without having to plunge inside them.

Now that the researchers have identified how to give black holes some hair, they next need to work on the observational consequences of these results. For example, future gravitational wave observations might reveal subtle signatures of these scalar fields in the collisions of black holes.

Scientists developing more sensitive next-generation gravitational wave detectors struggle with technical challenges that might be easily overcome by putting such detectors on the moon. Here is why a gravitational wave observatory on Earth's natural companion might be a good idea. 

Gravitational waves are ripples in spacetime triggered by collisions between the most massive objects known to exist in the universe — black holes and neutron stars, the odd leftovers birthed from the deaths of the most giant stars. 

First detected in 2015, these cosmic signals can be picked up by sophisticated detectors such as the U.S. Laser Interferometer Gravitational-Wave Observatory (LIGO) or the European VIRGO detector in Italy. But the current detectors only detect the most powerful and relatively nearby collisions that occurred within 7 billion light-years from Earth, according to Einstein Online. 

Scientists are already developing larger, more sensitive machines that would be able to detect even the most distant cosmic clashes, as well as those caused by collisions between smaller black holes and lighter neutron stars. There's one issue: We have yet to figure out how to eliminate the ever-present background noise produced by our planet. One researcher thinks that the problem could be solved by putting a gravitational wave detector on the moon. 

Related: We could hunt gravitational waves on the moon if this wild idea takes off

Speaking at the Astronomy from the Moon conference held earlier this year in London, professor Jan Harms, a gravitational wave-detection expert at Italy's Gran Sasso Science Institute, said that since these next-generation detectors want to provide much better sensitivity than LIGO and VIRGO, they need to be not only larger, but also better protected from seismic noise, the constant hum of our planet and the life it harbors. 

"Gravitational-wave detectors are sensitive to tiny forces that no other experiment can see, which makes them very susceptible to their environment (even in space and on the moon)," Harms told Space.com in an email. "The moon offers ideal conditions, and we are very lucky that it is so close to Earth. For example, we know that the moon is seismically extremely quiet, above all because of the lack of ocean and atmosphere, but also because moonquakes are less frequent and weaker compared to earthquakes."

Moreover, to maximize the sensitivity of these devices, the next-generation gravitational-wave detectors should be cooled to extremely low temperatures close to absolute zero, the temperature equivalent to minus 459.67 degrees Fahrenheit (minus 273.15 degrees Celsius) at which point atoms stop moving. But the cooling technologies required to reach such temperatures create vibrations, which then make it more difficult to detect the subtle space-time ripples. 

Inside the moon's permanently shadowed polar craters, on the other hand, temperatures are naturally extremely cold and stable enough to host such sensitive equipment. 

"Exploiting these physical conditions on the moon will make it possible to open an important frequency band from 0.1Hz to 1Hz to gravitational-wave observations, which cannot be observed on Earth," Harms explained. "There is a lot of breakthrough science we expect to be able to do with observations in this band."

In addition to sensing merging black holes and neutron stars, a gravitational-wave detector on the moon could be able to spot collisions of smaller, less massive objects, such as white dwarfs, the cooling remnants of smaller stars such as our sun, Harms wrote. Scientists think that such collisions are responsible for certain types of supernovas, the most powerful explosions known to occur in the universe, but they don't yet have any evidence of that.

By being able to observe mergers of smaller black holes at the most distant reaches of the universe, scientists would also be able to fill gaps in their understanding of the formation of supermassive black holes, the gaping monsters devouring matter at the center of galaxies.

"At the moment, we do not have a unique and fully satisfactory answer to how supermassive black holes were formed," Harms added. 

Harms and his team are already developing a concept of a lunar gravitational-wave detector, which, Harms believes, could launch to the moon within the next decade. Called the Lunar Gravitational Wave Antenna, the detector would use the entire moon as a giant receptor of gravitational waves. A sensitive sensor placed inside one of the cold, shaded lunar craters would then measure the minuscule vibrations of the moon caused by passing gravitational waves. 

"There are no major technological hurdles to overcome to realize this detector," Harms wrote. 

Scientists from other astronomy disciplines are looking at the moon and its polar craters as well, seeing them as the best destination that could allow them to push the boundaries of what is possible with existing ground and space-based telescopes. If all these ideas come to fruition, it might soon get a little crowded in those frosty, eternally quiet moon craters. 

Black holes have a preference for forming around two "universal" masses equivalent to about nine and 16 times the mass of our sun, according to a new study of the frequency of the gravitational-wave ‘chirps’ released when two black holes collide and merge. These findings could ultimately pave the way for an independent measure of the expansion of the Universe.

Since 2015, 90 gravitational-wave events have been identified by detectors at sites specifically built to find these information-rich ripples in spacetime. This includes laboratories such as the Laser Interferometer Gravitational-wave Observatory (LIGO) in the US, its sister site, Virgo, in Italy and the Kamioka Gravitational-Wave Detector (KAGRA) in Japan. Each merger produces what's known as a chirp, which is a blast of gravitational waves that rapidly increase in frequency as two black holes spiral closer and closer around one another before colliding and merging. The frequency and amplitude of this chirp is connected to the mass of the black holes that have merged; their combined mass is sometimes referred to as the "chirp mass."

"When two black holes merge, they produce gravitational waves that can be 'heard' on Earth," Eva Laplace, an astrophysicist at the Heidelberg Institute for Theoretical Studies in Germany and an author on the study, told Space.com. "By listening to these chirps and analyzing them, it is possible to measure the combined mass of distant merging black holes."

Related: The universe is humming with gravitational waves. Here's why scientists are so excited about the discovery

Stellar-mass black holes form during the death of a massive star. While in some cases a massive star will explode as a supernova and leave behind a compact neutron star, in other cases there is no explosion. Instead, the star's core collapses under gravity so severely it forms a black hole that eventually causes the rest of the star to cave in around it.

The masses of these resulting black holes determine the frequency of the gravitational-wave chirp that is emitted when they merge, and are also related to the mass of the stars that formed them. One would therefore expect a wide range of stellar mass-black holes to exist in the universe, reflecting the various masses of their progenitor stars, and indeed this is mostly the case. However, astronomers have been baffled to find more black holes associated with gravitational-wave events that have masses around 8–9 solar masses and 14–16 solar masses, but for some reason, hardly any with masses in between.

Now, new research conducted by Laplace alongside fellow astrophysicists Fabian Schneider, and Philip Podsiadlowski, also of the Heidelberg Institute for Theoretical Studies in Germany, addresses this apparent preference for merging black holes to converge on certain masses over others.

"What our study shows is that there is always a gap in black-hole masses between 9 and 16 solar masses,” Schneider told Space.com.

What happens inside massive stars

The existence of the mass gap is dictated by what is happening inside a massive star as it nears the end of its life. 

Young stars "burn" hydrogen in their cores via their intrinsic nuclear fusion processes; in massive stars, the dominant version of this process is known as the carbon–nitrogen–oxygen (CNO) cycle. This refers to a long chain of reactions involving hydrogen, plus those elements, that eventually produce helium and release a lot of energy to power the star. However, once the star’s core runs out of hydrogen, its energy production falters. Without enough energy to hold the star up, the core begins to contract under gravity. This increases the core's temperature by millions of degrees Celsius, until it's hot and dense enough to begin burning helium and temporarily halt the contraction.

At this stage, the star is like an onion, with various layers. Its core is burning helium. Around the core is a non-burning layer of helium, and ordinarily, around that is a shell that’s still burning some leftover hydrogen to produce even more helium that sinks to the stellar core. This extra helium further increases the core’s mass and temperature, speeding up the nuclear reactions which control the star’s evolution. Ultimately, this leads to a supernova and usually either a neutron star or a lone black hole, depending upon the compactness of the star’s core (in the case of stars with 130-250 solar masses and fairly primitive chemical compositions, they can sometimes explode and utterly destroy themselves in a so-called pair-instability supernova, leaving nothing behind).

By contrast, black-hole mergers are the product of massive binary star systems. During the time that they still exist as stars, the close companions are able to steal matter from each other, stripping away each other’s hydrogen-burning shell. Without this shell, a star’s core doesn’t gain that extra helium, which changes the star’s evolutionary trajectory. Conditions inside the core of a star that has lost its hydrogen shell are such that thermal neutrinos – tiny, ghost-like particles that spontaneously form – escape the star, carrying some of the core’s thermal energy with them. This lowers the temperature of the core and slows down nuclear reactions. The result is a decrease in energy production which allows the core to gravitationally contract some more. This leads to a very dense core that, when the star exhausts all its nuclear fuel and dies, can collapse to form a black hole.

In a binary system, this can lead to two black holes that eventually merge with a chirp of gravitational waves.

“Because of a complex interplay between neutrino losses, nuclear burning and core contraction, we find that stars of specific core masses are more prone to collapsing to black holes rather than exploding as a supernova and leaving behind a neutron star,” said Schneider.

This interplay leads to common black hole masses, according to the calculations of Schneider, Laplace and Podsiadlowski. In their models, the black hole masses tend to converge on two values, which are 9 and 16 times the mass of our Sun. These values are very close to the peaks that have been observed in the gravitational-wave data, which are at about 8 and 14 solar masses, so they don't exactly match, but remain within observational uncertainties.

Measuring the expansion of the universe

A prevalence of certain masses of black hole not only tells us about the physics of massive stars, but it also gives astronomers another way to measure the expansion rate of the universe, known as the Hubble constant. This has come under the spotlight in recent years because different methods give contradictory values for the Hubble constant.

The frequency of a gravitational-wave chirp depends mostly on the combined masses of the black holes involved, but a portion of it is also connected to their redshift, which tells us their distance, because the farther away they are, the more the expansion of the universe has shifted them to longer wavelengths. 

Until now, it had been impossible to disentangle the black hole masses from the redshift in the chirp. However, knowing that a large proportion of black holes have these universal masses gives scientists an advantage. 

“We can then take a statistical approach to decouple the masses from the redshift,” said Schneider. This technique would require a larger sample of gravitational-wave events than we currently have, but in principle, would provide a means of measuring the Hubble constant from the redshift that is independent of methods involving standard candles such as Type Ia supernovae.

A larger sample of gravitational-wave events may be coming soon. A new observing run involving LIGO, Virgo and KAGRA that will last 20 months has recently begun, and the aim is to discover another 300 events. We will know soon enough whether the new results enhance the peaks in the distribution around the universal masses and the gap between them.

The findings were published in The Astrophysical Journal Letters. 

Earlier this year, after 15 years of searching, scientists finally heard the background hum of low-frequency gravitational waves that fill our universe. Now, the hard work of searching for the source of these ripples in spacetime can begin. 

Currently, the primary suspects in this case are pairings of supermassive black holes with masses millions, or even billions, of times that of the sun. However, that doesn't mean that there isn't room for a few unusual suspects, which could potentially point us toward new physics. 

The breakthrough was made by researchers from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration who analyzed 68 rapidly spinning neutron stars, also known as pulsars, that sweep radiation over Earth at regular intervals. This sort of activity allows the pulsars to be turned into a very precise cosmic clock called a "pulsar timing array." 

As gravitational waves ripple throughout the cosmos, they cause the very fabric of space and time, or spacetime, to squash and squeeze. Data surrounding this effect can be combined with those pulsar timing arrays to eventually create a detectable signal or "spectra" for scientists to study.

Related: The gravitational wave background of the universe has been heard for the 1st time

This isn't the first time gravitational waves have been detected, however. Previous identifications have been made with the Laser Interferometer Gravitational-Wave Observatory (LIGO) starting in 2015, but the thing is, those involved higher-frequency, shorter-wavelength gravitational waves from different kinds of sources such as stellar mass black holes. 

"The biggest thing that sets these gravitational waves detected by NANOGrav apart is the wavelength. These gravitational waves are much longer," Scott Ransom, a National Radio Astronomy Observatory (NRAO) astronomer and former chair of NANOGrav, told Space.com.

Ransom also compared the difference to the familiar phenomenon of electromagnetic frequencies: "If you think about it in terms of the electromagnetic spectrum, NANOGrav is like radio astronomy, and LIGO is like X-ray astronomy."

This difference in wavelength frequency between the two types of gravitational waves is enormous. 

To put that into perspective, gravitational waves detected by LIGO express wavelengths that are thousands of miles (or km) in length and hold frequencies of milliseconds to seconds. The new gravitational waves detected by NANOGrav, by contrast, have wavelengths on a scale of trillions of miles (or km). This is similar to the distance between the sun and its neighboring star, Proxima Centauri, a staggering 20 light-years in length. Plus, NANOGrav gravitational wavelengths have frequencies on scales of years instead of mere seconds.

Practically, what this means is scientists need to build over 15 years of NANOGrav data to confirm a low-frequency gravitational wave detection. But, when it happens, it's worth the wait. 

That's because these results have the capacity to point us toward new information about our universe.

"Pulsar timing array experiments are definitely long-term experiments that you have to be very patient with because our signal slowly grows with time," Ransom said. "The detection of low-frequency gravitational waves means they're from very different sources to the LIGO and Virgo sources, which are stellar mass black holes and neutron star mergers."

The main suspect: Supermassive black hole binaries

Ransom is part of a collaboration of researchers that believe low-frequency gravitational waves, including those detected by NANOGrav, may originate from a pretty incredible source. They could come from, the team argues, hundreds of thousands of supermassive black hole pairings that, over the 13.8-billion-year course of cosmic history, came close enough together that they've merged. 

Ransom explained that gravitational waves would come from supermassive black hole pairings as the objects orbit one another for hundreds of thousands of years. But that wave emission would someday stop, he said, when the waves have stolen enough angular momentum from the black holes to cause the latter to merge. 

"That's when they were pumping out these big, massive gravitational waves and losing their energy so that eventually, they're going to collide," Ransom said. "But we didn't know how many of these supermassive black hole binaries there were, and we didn't know how massive they were, and all of that determines when you're going to finally detect gravitational waves."

The team's detection matched what scientists have long-expected to discover about supermassive black hole binaries. 

"For many decades, theorists have hypothesized that supermassive black hole binaries should produce a signal with characteristics just like what NANOGrav and other pulsar timing arrays are seeing," Luke Zoltan Kelly, a Northwestern University theoretical astrophysicist and NANOGrav researcher, told Space.com. "For most of the community, supermassive black hole binaries are a natural best guess for what’s producing the gravitational wave background."

That doesn't mean it's "case closed" just yet, though. Researchers need more than circumstantial evidence to point the finger at something as major as the presence of supermassive black hole binaries in our universe. 

"The challenge with binaries is that no example of a supermassive black hole binary has been fully confirmed before, so we don’t actually know for certain that these incredible objects exist," Zoltan Kelley said. "That being said, it would be an even bigger puzzle if they don’t exist because we know that galaxies contain supermassive black holes, and we know that galaxies merge as a normal part of their evolution."

Compounding this cosmic mystery is the fact that there are other teams on the case with more exotic suspects for low-frequency gravitational wave emitters in mind. 

The (un)usual suspects

Zoltan Kelley pointed out to Space.com that besides binaries, there are a number of new models in cosmology and in particle physics that, under the right circumstances, could also produce a similar gravitational wave background to that detected by NANOGrav.

"For example, axion or 'fuzzy' dark matter, cosmic strings, inflationary phase transitions, and many others," the Northwestern astrophysicist said. "What’s really exciting about these possibilities is that each of these models is an attempt to explain some of the biggest current mysteries of our universe."

These include the nature of dark matter — a form of matter that makes up around 85% of the "stuff" in the universe but remains effectively invisible because it doesn't interact with light — and the puzzle of what has caused the expansion of the universe to start accelerating again after the Big Bang. 

"In my mind, it’s a win-win. If the background is cosmological, it will completely transform our understanding of the universe and of fundamental physics - potentially one of the biggest discoveries in the history of science," Zoltan Kelley continued.  "If instead, the background is from boring old supermassive black hole binaries then we get to explore how the most massive objects in the universe are able to form pairs and spiral together while emitting more energy than most entire galaxies do over their lifetimes."

Fortunately, there is a way in which scientists may be able to confirm the source of these gravitational waves, particularly if the waves have a preferred direction in the sky, meaning they come in stronger and brighter from one direction.

"The next step is determining for certain where these gravitational waves are coming from," Zoltan Kelley said. "We think we can do that by mapping the differences in their amplitudes across different parts of the sky. If binaries are producing the background, then we expect to be able to see differences in brightness from different parts of the sky, while if the source is cosmological — like cosmic strings or phase transitions — then the gravitational wave background should be almost perfectly uniform across the sky."

The Northwestern astrophysicists added that the team is also excited about taking this investigation a step further by possibly detecting the individual black holes that came together to make this gravitational wave background in the first place.

"This would be an incredible opportunity to find these sources using traditional electromagnetic telescopes," Zoltan Kelley added. "We’re working very hard to figure out what types of electromagnetic signatures would hint at the presence of a binary black hole within an active galactic nucleus (AGN) or quasar so that we can start searching for them."

Even before the NANOGrav researchers can finally nail down the sources of low-frequency gravitational waves, the mere detection of two varieties of these spacetime rumbles is astounding. It is especially striking when Albert Einstein, the man who first predicted gravitational waves in his 1915 theory of gravity general relativity, also predicted these ripples in spacetime would be too faint to ever be detected by humanity.

"I think it's amazing for one thing that the state of astronomical technology and research has advanced so much that within a decade, we opened up two completely independent windows of the gravitational wave universe," Ransom concluded. 

Details of the 15-year-long hunt for low-frequency gravitational waves by NANOGrav are revealed in a paper published on Aug. 1 in The Astrophysical Journal Letters.