The ripples in space-time known as gravitational waves have already helped answer major questions regarding the nature of matter and black holes. And upcoming gravitational-wave observatories both on Earth and in space could soon help solve some of the greatest mysteries in science.
"We're going to be able to learn a lot about the universe," said Cole Miller, an astrophysicist at the University of Maryland, College Park.
The existence of gravitational waves was first predicted by Albert Einstein in 1916. According to Einstein's theory of general relativity, gravity results from how mass warps space and time. When any object with mass moves, it generates gravitational waves that travel at the speed of light, stretching and squeezing space-time along the way.
Gravitational waves are extremely weak, making them extraordinarily difficult to sense, and even Einstein was uncertain whether they really existed and if they would get detected. After decades of work, researchers succeeded in discovering the first direct evidence of gravitational waves in 2015 using the Laser Interferometer Gravitational-Wave Observatory (LIGO).
LIGO uses a pair of detectors — one in Hanford, Washington, and the other in Livingston, Louisiana — to sense the distortions that gravitational waves cause as they move through matter. Each detector is shaped like a gigantic L, with legs about 2.5 miles (4 kilometers) long. The legs of each detector are normally the same length, so laser beams take the same amount of time to travel down each. However, if gravitational waves pass through Earth — making the detector legs expand and contract by about one-ten-thousandth the diameter of a proton — atomic clocks can detect the split-second differences in time it takes for laser beams to zip down one leg of the detector versus the other.
Because LIGO's detectors are about 1,865 miles (3,000 km) apart, it can take up to 10 milliseconds for a gravitational wave to cross from one detector to the other. Scientists can use this difference in arrival times to deduce where the gravitational waves come from. As more gravitational-wave detectors come online — such as the Virgo facility near Pisa, Italy — researchers will get better at pinpointing the sources of gravitational waves.
The gravitational waves that LIGO can best detect are the most powerful ones, which are released when extraordinarily massive objects collide with one another. The first gravitational waves LIGO detected were from colliding black holes, and with Virgo, it detected merging neutron stars. These findings have shed light on everything from the nature of gravity to the origin of most of the elements heavier than iron.
The immediate future
Not only are LIGO and Virgo continuing to sense bursts of gravitational waves, but they are getting upgrades to increase their sensitivity to detect even more events. In addition, other gravitational-wave observatories are coming online soon. Japan has built KAGRA, which is planned to join the LIGO and Virgo network in 2019, and LIGO-India will hopefully be operational by the mid-2020s, Miller said.
"More detectors means a better determination of the direction of gravitational-wave sources," Miller told Space.com.
Beyond 2025, scientists are discussing two advanced gravitational-wave observatories — the Einstein Telescope and the Cosmic Explorer. The plan for the Einstein Telescope is to build it underground to reduce the amount of noise experienced from seismic vibrations, whereas the aim for Cosmic Explorer is to use cryogenic systems to help cut down the noise experienced from heat on its electronics. Decreasing noise in turn boosts sensitivity to gravitational-wave events.
The next generation of ground-based gravitational-wave observatories may detect mergers of intermediate-mass black holes, ones that are hundreds to thousands of times more massive than the sun. Previous research suggested that intermediate-mass black holes are the building blocks of the supermassive black holes millions to billions of times the sun's mass found at the hearts of galaxies such as the Milky Way, "but their existence has not yet been conclusively proven," Miller said.
Space, the final frontier
The current and planned ground-based gravitational-wave observatories are all sensitive to wavelengths of about 60 miles (100 km), the kind generated by neutron stars and black holes up to a few dozen times the mass of the sun. However, scientists have long planned space-based gravitational-wave observatories with detectors separated by vast distances that could sense even longer wavelengths, the type released by supermassive black holes.
One space-based gravitational-wave observatory under development is the European Space Agency's Laser Interferometer Space Antenna (LISA) mission, scheduled to launch in 2034. LISA will consist of a constellation of three satellites in orbit around the sun and trailing Earth. Inside each satellite is a cube that will fall freely through space, tracing a path that will only get perturbed by gravitational waves. These satellites will carefully monitor the position of each cube to look for signs of space-time ripples.
Each of LISA's satellites will be millions of miles away from each other. In principle, LISA will be able to detect gravitational waves with wavelengths of about 18 million miles (30 million km) from the mergers of black holes 10,000 to 10 million times the mass of the sun, Miller said. The hope is that LISA and similar proposed projects, such as TianQin from China, can help shed light on the mergers of galaxies.
"We can learn how galaxies and supermassive black holes assemble," Miller said.
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Expect the unexpected
However, what scientists are likely most looking forward to with gravitational-wave observatories is the unexpected. "We may see unanticipated types of sources of gravitational waves, or see the sources we generally knew about with twists that might surprise us," Miller said.
For example, the hearts of black holes are infinitely dense, infinitely small points known as singularities. And researchers have long suspected that infinitely dense, infinitely thin rings known as cosmic strings might also exist. "If cosmic strings are real, we may be able to detect gravitational waves from them over a broad range of energies," Miller said.
In addition, scientists have long suggested the possible existence of primordial black holes, ones born less than a second after the Big Bang, when dense clumps of the newborn universe collapsed under their own gravity. Such black holes would have masses just that of asteroids or planets.
"If we saw gravitational waves from something, say, half the mass of the sun, there would be no other way to produce them but primordial black holes," Miller said.
Furthermore, some physicists have suggested that black holes may not actually exist. If they do not, then gravitational-wave observatories may discover their actual nature, Miller said.
Generally, researchers think most black holes form when massive stars finish burning their fuel and rapidly collapse under their own weight, getting crushed into singularities that are hidden by invisible boundaries known as event horizons from which nothing can return. (The Event Horizon Telescope recently got the first-ever look at one of these points of no return recently, imaging the boundary of the supermassive black hole at the core of the M87 galaxy.)
However, the possibility of singularities clashes with laws of physics suggesting that destruction of information is impossible, including information encoded within anything falling into black holes.
As such, physicists have suggested that when massive stars die, they may instead form structures with names such as "black stars" and "gravastars." These alternatives resemble black holes in nearly every way but lack singularities and event horizons, sidestepping the conundrums these aspects of black holes raise.
One way to find out whether black hole alternatives exist is by analyzing gravitational waves unleashed by what scientists currently think are merging black holes. As black holes spiral toward one another, they should each give off gravitational waves, but their event horizons should absorb those directly falling onto them. However, since black hole alternatives lack event horizons, they can reflect gravitational waves, and gravitational-wave observatories could detect these "echoes," Miller said.
If such echoes are discovered, they could yield insights into both general relativity and quantum physics. Doing so could finally help lead to a model of "quantum gravity" marrying both long-disparate theories, Miller added.
Miller and his colleague Nicolás Yunes, of Montana State University in Bozeman, detailed this research online April 24 in the journal Nature.
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