There may be an easier way to describe how black holes "leak" energy than the theory Stephen Hawking proposed — and the newly suggested process is similar to how we describe a boiling pot of water. This simple (well, relatively simple) description could be used to model black holes in many situations such as during their formation, mergers with other black holes, eventual evaporation and even explosive death.

In the 1970s, legendary theoretical physicist Stephen Hawking wrote a letter to the journal Nature entitled "Black hole explosions?" explaining how these objects may leak thermal radiation, evaporate and eventually implode at the end of their lives. This radiation eventually became known as Hawking radiation.

But in new research, scientists have suggested an alternative to Hawking radiation. It involves describing the increase in disorder, or entropy, of black holes. Boiling water, as an example, is also often described based on its increase in entropy. For black holes, this measure of entropy is connected to characteristics like spin and energy, which means it could be used to understand how these cosmic titans respond to different events.

"Hawking's laws of black hole mechanics provided a satisfying connection between extreme and ordinary physics and have been the paradigm for 50 years, but they have a serious limitation," team leader Abhay Ashtekar of the Eberly College of Science at Penn State University said in a statement. "They were formulated for black holes at equilibrium — or unchanging over time — but black holes are constantly changing; they form, merge, and eventually evaporate. We wanted to find a way to overcome this limitation and extend the laws to black holes that are out of equilibrium."

Black holes, Einstein and Hawking

To investigate the origins of black holes, one has to go back to history's most famous physicist (sorry Hawking, you're number two), Albert Einstein.

In 1915, Einstein revealed his theory of gravity, general relativity. One consequence of the equations underpinning that theory was the possibility of a singularity, a point at which the equations of general relativity go to infinity. This represents the heart of a black hole.

Another consequence of the general relativity equations is a region of space around this singularity at which gravity is so extreme that the escape velocity of the area increases to a value greater than the speed of light. That region is the light-trapping outer boundary of the black hole known as the event horizon, which prevents us from ever seeing the singularity at the heart of the black hole or receiving information from it. In fact, until Hawking's work in 1974, this is why it was proposed that nothing at all can escape a black hole.

"The laws of black hole mechanics came directly from Einstein's equations," team member Daniel E. Paraizo, a graduate student in physics at Penn State, said. "Because you cannot see into a black hole, it seemed that there could be an infinite number of ways to make a black hole, making their entropy infinite as well. They were also thought to only absorb energy and never radiate, so their temperature was zero."

However, the advent of Hawking radiation somewhat changed this paradigm. By suggesting that black holes actually radiate thermal energy, Hawking redefined them in such a way that suddenly the laws of thermodynamics could be applied to black holes.

"This changed the thinking about the thermodynamic properties of black holes from a sort of mathematical concept described by equations, to being more of a physical reality," Paraizo said. "This opened the door to finding analogies in black holes of entropy and temperature used in thermodynamics."

In Hawking's recipe for black holes, the area of the event horizon is proportional to its temperature and entropy, and is inversely proportional to its mass and its spin.

"There is a problem, though," team member Jonathan Shu, also from Penn State, said in the statement. "These analogies only really work for a black hole that is at equilibrium. In dynamic situations, event horizons can form and grow in what we call flat regions of space-time, where nothing is happening."

Shu added that a consequence of this is the properties of black holes cannot be determined just by the local physics of the black hole. Instead, determining the properties of black holes relies on the prediction of events that may or may not happen in the future.

"Therefore, the area of event horizons cannot be a measure of the physical entropy of dynamical black holes," Shu argues. "If we want to understand black holes that are growing, evaporating and merging, we need a viable alternative."

For the team, this meant replacing the event horizon of a black hole with something they call a "dynamical horizon," already used when scientists simulate black holes. Now, the first law of thermodynamics — which states the energy of a closed system cannot be created or destroyed but rather can only change forms — can be applied to black holes even when they are involved in dynamic acts. It also means black holes are subject to the second law of thermodynamics, which says the total entropy of an isolated system will always increase over time, during their birth, merger and death.

"This allows us to extend the first and second laws of thermodynamics to black holes that are not at equilibrium, thereby overcoming the limitations of the paradigm that has been used for over half a century," Ashtekar said. "We can apply these generalized laws to better understand evaporating black holes in quantum theory and black hole mergers."

The team's research was published in June in the journal Physical Review Letters.

When the beacons were lit in "The Lord of the Rings: The Return of the King," the city of Gondor called to Rohan for aid, spelling doom for Sauron and his legions. However, when the beacons of supermassive black hole systems named for these locations in J.R.R. Tolkien's "Lord of the Rings" novels were lit up, it was exceptionally good news for scientists.

The supermassive black hole binaries Gondor, officially designated SDSS J0729+4008, and Rohan, SDSS J1536+0411, were discovered by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) using a new technique that uses the background hum of ripples in space called "gravitational waves" in conjunction with observations of quasars, which are powered by feeding supermassive black holes.

The logic behind this is that supermassive black hole binaries, which spiral together to lead to collisions and mergers, emit gravitational waves of increasing frequency as their orbits shrink, creating a background hum of gravitational waves. The resultant mergers seem to be five times more likely to be found in quasars.

That makes quasars beacons that can indicate the unification of supermassive black holes. If one of these beacons radiates gravitational waves like the lit beacons of Gondor, it indicates binary black holes are present. Thus, this detection technique offers scientists a method to create a cosmic map of these merging titans.

"Our finding provides the scientific community with the first concrete benchmarks for developing and testing detection protocols for individual, continuous gravitational wave sources," NANOGrav team member Chiara Mingarelli said in a statement.

Mingarelli and colleagues hunted for supermassive black hole binaries using their new approach in 114 Active Galactic Nuclei (AGNs), the bright central regions of galaxies where supermassive black holes are ravenously feasting on surrounding gas and dust.

Mingarelli explained the reason for the unusual name choice for these black hole systems: "The names come from both people and pop culture. Rohan was first, for Rohan Shivakumar, the Yale student who first analyzed it, and Gondor was next, because, well — the beacons were lit!"

NANOGrav, which first detected a gravitational wave background in 2023, will spend the coming months hunting and identifying supermassive black hole binaries. The team thinks that even a relatively small catalog of black hole mergers could help create a gravitational wave background map. This research could also help scientists better understand galaxy mergers, the physics of black holes and the nature of gravitational waves themselves.

"Our work has laid out a roadmap for a systemic supermassive black hole binary detection framework," Mingarelli said. "We carried out a systematic, targeted search, developed a rigorous protocol — and two targets rose to the top as examples motivating follow-up."

The team's results were published on Feb. 5 in The Astrophysical Journal Letters.

Only a handful of supermassive black holes have been confirmed by scientists, but the universe could be filled with billions of these gravitational giants. 

Theoretically, if you compress a sufficient amount of matter into a small enough space, it will create such a powerful gravitational field that nothing — not even light — can escape from it. That’s the basic idea behind black holes, and it’s so bizarre that for many years people thought they couldn’t possibly exist in reality, according to University of Texas in Austin.

Yet today we know the universe is filled with them — perhaps as many as one for every ten visible stars, according to Live Science. A few of those black holes are truly enormous, with masses millions of times greater than the sun. Here we take a closer look at the strange world of supermassive black holes.

How big are supermassive black holes?

It’s impossible to observe a black hole directly, because — as their name suggests — they don’t emit any light or other radiation. But they can be detected via their gravitational effect on visible stars in their neighbourhood, which orbit around the black hole much faster than they would around a normal object of similar size. 

By measuring the speed of stars close to the black hole, astronomers can estimate its mass. That’s how they know, for example, that the black hole at the center of our own galaxy has a mass around four million times that of the sun, according to NASA.

As big as that sounds, it’s really quite tiny compared to the largest supermassive black holes that have been measured — some of which approach 100 billion solar masses,

Supermassive black hole examples

Black holes at the center of galaxies

Rather than devouring anything that ventures too close to them, the black holes at the centers of most galaxies only give away their existence through subtle effects on nearby stars. In an active galaxy, however, the supermassive black hole behaves a lot differently. 

When surrounded by a swirling "accretion disk" of rapidly rotating gas and dust, matter is constantly spiralling down into the black hole. In the process, it releases enormous amounts of energy, sometimes even outshining the rest of the galaxy. 

In 2019 the Event Horizon Telescope succeeded in photographing one such galaxy — Messier 87 — producing a direct image of the accretion disc. The ominous shadow of the galaxy’s 6.5 billion solar mass black hole is clearly visible, quite literally as a "black hole" at the center of the disk.

How supermassive black holes are formed

Movies often portray black holes as giant cosmic vacuum cleaners, relentlessly sucking in other material until there’s nothing left. If that was how real black holes worked, there’d be no mystery as to where the supermassive kind came from: once an "ordinary" black hole had formed from stellar collapse, it would simply grow and grow until it reached enormous size. But real black holes don’t suck matter in like this; they merely attract it with the same law of gravity as a normal object of the same mass.

Their exceptional nature comes from the fact that they’re super-condensed and the force of gravity increases as distance decreases. So it’s possible for an orbiting object to stray into a region where gravity becomes incredibly strong. At larger distances, however, a black hole’s gravity is perfectly normal. But if a black hole is incapable of sucking in distant matter, how does it ever grow to supermassive size? At present no one knows the answer to this, although there are several promising theories.

Although they may not be the rapacious predators portrayed in sci-fi, we know that some black holes do absorb new material and that’s what’s going on in the accretion disks of active galaxies, for example. Occasionally pairs of black holes crash into each other and merge to produce a single, larger black hole and we know that from the evidence of gravitational waves, which have been observed on a regular basis since they were first discovered in 2015.

But accretion and mergers, while undoubtedly part of the solution, aren’t enough in themselves to explain the observational evidence for supermassive black holes. That’s because we know the first active galaxies — which must have been powered by central black holes – were formed very early in the life of the universe. For example, a supermassive black hole of a billion solar masses is believed to have existed in one galaxy more than 12 billion years ago — around 90% of the way back to the Big Bang.

It’s possible that the stellar life cycle, which is so crucial to the standard model of black hole formation, had nothing to do with the creation of the oldest supermassive black holes. Instead, they may have formed almost immediately from the gravitational collapse of an enormous cloud of gas — one that already contained as much matter as millions of stars. According to this theory, a "direct collapse black hole" of this kind would have taken around 150 million years to form — the blink of an eye in cosmic terms.

Another hypothesis invokes the idea of primordial black holes, which are theorised to have been created in the Big Bang itself. These are sometimes proposed as a possible explanation for dark matter, and are generally assumed to have been quite small in size. However, they might have served as the basic seeds from which present-day supermassive black holes grew .

Additional resources

For more information about black holes check out " Death by Black Hole - and Other Cosmic Quandaries" by Neil deGrasse Tyson and "Gravity's Fatal Attraction: Black Holes in the Universe" by Mitchell Begelman and Martin Rees. 

Bibliography

Patchen Barss, "The mysterious origins of Universe's biggest black holes", BBC, August 2021. 

University of Texas in Austin, "History of Black Holes", accessed January 2022. 

Stephen Batttersby, "Monster munch: How did black holes get vast so fast?", New Scientist, March 2013. 

Alison Klesman, "What are primordial black holes?", Astronomy, July 2019.

ESA, "​​What happens when two supermassive black holes merge?", May 2019. 

NASA, "Exploring Active Galactic Nuclei", February 2016. 

Shobha Kaicker, "How do astronomers calculate the mass of a black hole?", Astronomy, April 2020. 

NASA, "Black Holes", March 2022. 

NASA, "Active Galaxies", September 2021. 

NASA, "Stars", September 2021. 

Take three black holes and throw them into the disk surrounding a supermassive black hole and things get really weird, really fast.

That's the conclusion of new research digging into a particularly strange gravitational wave event that scientists observed in May 2019 and are still trying to understand. Gravitational waves are the ripples in space-time caused by, among other dramatic events, the mergers of black holes. But this particular observation didn't match other collisions scientists have caught: it resulted in a black hole in the mid-size range that scientists can barely see, much less explain, and some force was stretching the typically circular dance as the behemoths approached each other.

"The gravitational wave event GW190521 is the most surprising discovery to date. The black holes' masses and spins were already surprising, but even more surprising was that they appeared not to have a circular orbit leading up to the merger," Imre Bartos, a physicist at the University of Florida and co-author on the new research, said in a statement. (Astronomers name gravitational wave signals with the date they were observed, so GW190521 marks a gravitational wave detected on May 21, 2019.)

Related: 8 ways we know that black holes really do exist

In the earliest analysis of the strange signal, scientists had already suspected that the merger occurred in a pocket of space rich with black holes. Astronomers know of two types of black holes. One, dubbed stellar black holes, form from dying stars and contain perhaps a dozen times the mass of our sun. Supermassive black holes, in contrast, hide at the center of some galaxies (including our Milky Way) and can contain millions of times the mass of their puny counterparts.

The result of the May 2019 merger appeared to be an intermediate black hole, a size category that ranges from perhaps 100 to 1,000 times the mass of our sun. Scientists had never yet managed to study and could not explain how such an object might form. In response to the detection, astronomers suggested that one of the colliding black holes was itself the result of a collision, pushing the final product into the mysterious intermediate range at 142 times the mass of the sun.

To get two sequential collisions, the astronomers first analyzing GW190521 proposed that the event occurred near what scientists call an active galactic nucleus — a particularly dynamic supermassive black hole anchoring a galaxy, where smaller black holes might proliferate.

The new research supports that suggestion, approaching the situation from a different angle.

These scientists wanted to understand how it was that the two black holes weren't actually circling each other as they collided — instead, their orbits were eccentric or elliptical, more ovals than circles. That, too, was strange: Astronomers thought that the massive gravitational forces involved when two black holes are colliding should have forced these two onto circular paths.

So physicists behind the new research set about modeling black hole collisions. And while their calculations suggested that three black holes at random were unlikely to result in an eccentric collision, something changed when they considered the environment of an active galactic nucleus.

This type of feature also hosts a disk of matter surrounding the supermassive black hole, like a much more massive model of the solar system. Where the solar system has planets, an active galactic nucleus has stellar black holes scattered throughout the disk in what the new research identifies as essentially a two-dimensional system.

Under those conditions, the probability of an eccentric merger in the models shot up — as much as 100 times, co-author Johan Samsing, an astrophysicist at the Niels Bohr Institute in Denmark, said in the statement. At that rate, perhaps half of mergers in the disks of active galactic nuclei would be eccentric rather than circular, making the exceptional observation of May 2019 much less surprising.

"In these environments, the typical velocity and density of black holes is so high that smaller black holes bounce around as in a giant game of billiards and wide circular binaries cannot exist," co-author Bence Kocsis, an astrophysicist at the University of Oxford in the United Kingdom said in the statement.

The researchers noted that the probability of eccentric mergers in their model varies with characteristics of the disk surrounding the supermassive black hole. Next up, they said, is spotting ever more black hole collisions to analyze.

The research is described in a paper published Wednesday (March 9) in the journal Nature.

Email Meghan Bartels at mbartels@space.com or follow her on Twitter @meghanbartels. Follow us on Twitter @Spacedotcom and on Facebook.

Isolated supernovae, exploding alone in the empty space between galaxies, may be the result of star pairs ejected from their homes as galaxies merge, new research suggests.

"This story has taken lots of twists and turns, and I was surprised every step of the way," Ryan Foley, principle investigator of the new study, said in a statement from the Hubble Space Telescope news center.

Foley and his colleagues studied massive stellar explosions that normally take place inside the confines of a parent galaxy, but were instead lighting up the sky between galaxies, presenting an ongoing puzzle for astronomers. What happened to the stars that created these explosions? How were they ejected them from their homes, and what caused them to explode? The researchers think they have some answers. [Supernova Photos: Great Images of Star Explosions]

Swing your partner, do-si-do

Dramatic stellar explosions known as supernovae often occur when massive stars run out of fuel to burn and end their lives with a bang. Foley, an assistant professor of astronomy at the University of Illinois at Urbana-Champaign, was interested in isolated explosions that were caused by a slightly different mechanism: the remnants of burned-out stars called white dwarfs, merging together and creating the explosion. This suggested that both stars had somehow left their galaxy together, before their explosion, the statement said. Some had traveled as far as half a million light-years from their birth place.

Foley studied 13 lonely supernova to determine how fast the binary stars must have been moving before the explosion. To do this, he looked at observational data taken at the Lick Observatory in California, and the W. M. Keck Observatory and the Subaru Telescope, both in Hawaii. He found that the doomed stars reached speeds similar to stars tossed from the Milky Way by the supermassive black hole at its center, more than 5 million miles per hour (7 million kilometers per hour).

To determine what might eject these stellar twins, Foley studied the galaxies where they started out.

"Whatever put the star system in the state [location] where it's about to explode is related to the center of the galaxy it came from," Foley said.

Observations of the parent galaxies revealed signs of recent mergers, where two galaxies had collided to form a single new one. Archived images captured by NASA's Hubble Space Telescope showed that each parent galaxy had a supermassive black hole at its center. Foley concluded that, as the galaxies united, the supermassive black holes also combined. The gravity of this new binary black hole interacted with the binary stars and cast them from their home galaxy.

"You have two dancing partners, they do-si-do, and one pair gets flung away," Foley said.

"The white dwarf and its partner are ejected out like from a slingshot, and after traveling at a high speed for about 50 million years, explode in the middle of nowhere."

A shorter fuse

The time scale of this destructive process (about 50 million years) is much shorter than the time scientists think a white dwarf merger would take under normal circumstances, the statement said. The ejection could also explain what sped up the explosive process between the binary stars.

"Many of these stars will be flung far away, and those ejected stars in surviving binary systems will orbit even closer after the encounter," the statement from the Hubble news center said. The closer the two stars get, the faster they spin around one another, the stronger the gravitational pull becomes between them. The force of the gravity pulls material from one star onto the surface of the other, and previous work has shown that this accumulation of matter eventually causes the growing white dwarf to explode as a supernova.

"The interaction with the black holes shortens that fuse," Foley said.

While the black hole at the center of the Milky Way only ejects about one star every 100 years, a binary supermassive black hole may toss out around 100 stars every year.

Though the source of the isolated supernovae has been identified, some mysteries remain. The explosions tend to produce more than five times the amount of calcium produced by other supernovae and significantly less iron. Most supernovae create elements that are heavier than calcium, such as iron and nickel, in their hearts. The lack of heavy elements suggests that these isolated explosions have lower energy than other supernovae.

Foley hopes that by tracking down more of these isolated supernovae, scientists could also find more binary supermassive black hole systems, which are rare and interesting phenomena that could provide clues about several mysteries of physics, such as general and special relativity.

"These supernovae could be the bread crumbs to find our way to these supermassive binary black holes, and we could potentially find them in much higher numbers," Foley said.

The findings were published online on Aug. 12, in the journal Monthly Notices of the Royal Astronomical Society.

Follow Nola Taylor Redd on Twitter @NolaTRedd. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.