Giant Gravitational Waves: Why Scientists Are So Excited

An artist's rendition of a pair of supermassive black holes emitting gravitational waves.

The collision of two holes in supermassive blocks emits gravitational waves in this artist illustration.Credit: Aurore Simonnet for the NANOGrav collaboration

On June 29, four separate teams of scientists made an announcement14 that promises to shake up astrophysics: they had seen strong hints of very long gravitational waves warping the Galaxy.

Gravitational waves are ripples in the fabric of spacetime that arise when large masses accelerate. They were first detected in 2015, but the latest evidence suggests monstrous ripples with wavelengths of 0.3 parsecs (1 light-year) or more; the waves detected so far have wavelengths of tens to hundreds of kilometres.

Here Nature reports what these monstrous gravitational waves might mean for our understanding of the cosmos and how the field might evolve.

How do the newly announced gravitational waves differ from those astronomers had already discovered?

Gravitational waves were first detected by the twin detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Louisiana and Washington state. They sensed ripples produced by two black holes spiraling into each other and merging. Since then LIGO and its Virgo counterpart in Europe have reported dozens of similar events.

For the latest findings, the authors relied on special beacon stars called millisecond pulsars. The teams monitored changes over more than a decade in the distances between Earth and millisecond pulsars in the Milky Way, comparing signals from arrays of dozens of beacon stars. These pulsar timing arrays (PTAs) are sensitive to long waves of 0.3 parsecs or more.

And while LIGO and Virgo spot evidence of the late stages of individual merger events, regularly spaced waves emanating from a definite direction in the sky, the four PTA collaborations have thus far found only a stochastic background, a constant jostling in random directions. This is comparable to the random splash of water on the surface of a pond caused by rain.

What is the origin of the waves?

The most likely explanation for the stochastic background seen by PTAs is that it’s produced by many pairs of supermassive black holes orbiting each other in the hearts of distant galaxies, says Sarah Burke-Spolaor, an astrophysicist at West Virginia University in Morgantown. .

Most galaxies are thought to host one of these monstrous black holes, with a mass millions or billions of times that of the Sun. And astronomers know that many galaxies have merged throughout the history of the universe. Hence, some galaxies must have ended up with two supermassive black holes, known as black hole binaries.

The researchers also calculated that in the crowded center of such a galactic merger, each black hole would transfer some of its momentum to surrounding stars, either hurling them out at high speeds or simply dragging them around. As a result, the two black holes would eventually slow down and end up orbiting each other at distances of about 1 parsec, explains Chiara Mingarelli, a gravitational-wave astrophysicist at Yale University in New Haven, Connecticut.

However, only paired black holes that come as close to each other as 1 parsec would contribute to the PTA signal. They must be separated by a milliparsec to emit detectable gravitational waves, Mingarelli says. Theories as to how this would happen are speculative, however, and whether binaries can do this has been an open question, known as the final parsec problem. If you don’t get past the final parsec problem, you don’t get gravitational waves, Mingarelli says.

Scientists will now try to verify that the PTA signal actually comes from binary supermassive black holes. If this could be confirmed, it would be evidence that supermassive black holes can get very close to each other in nature.

This result would be of fundamental importance, says Monica Colpi, an astrophysicist at the University of Milano-Bicocca in Italy, who shows that thousands of binary black hole systems in the Universe have somehow solved the final parsec problem. It would be the discovery that such a population exists.

What would such binary black holes mean for LISA, the European-designed space detector?

Pairs of supermassive black holes that got close enough to emit gravitational waves would eventually collide and merge. This is because gravitational waves themselves would carry energy and momentum away from black holes, turning their orbits into spirals. In hundreds or tens of thousands of years, each of the pairs would eventually collide.

Colpi says this could be good news for the Laser Interferometer Space Antenna (LISA), a trio of probes the European Space Agency plans to launch in the mid-1930s.

As black holes spiral inward, the frequencies of their gravitational waves will increase and, in some cases, enter the LISA sensitivity spectrum. LISA will be sensitive to wavelengths between 3 million km and 3 billion km shorter than the wavelengths that can be detected by PTAs, although still much longer than those seen by ground-based detectors. So LISA was able to witness many of these mergers during her mission.

Black hole mergers could also help explain how some of the black holes got so big: They are themselves the result of previous mergers.

Could something other than binary black holes produce the stochastic background?

There is a plethora of exotic physics theories that predict a similar omnidirectional background of waves emanating from all directions in space. These sources may make up part or even most of the signal. Possibilities include some types of dark matter and even cosmic strings, hypothetical infinitely subtle flaws in the curvature of space-time. Cosmic strings could develop knots, which could eventually break apart, producing gravitational waves.

One of the more intriguing alternative explanations is a cosmic gravitational wave background originating in the early Universe, says Burke-Spolaor. Telescopes that see across the electromagnetic spectrum from radio waves to rays are limited in how far they can peer, and therefore how far into the past they can see. This is because, long before galaxies and stars existed, an opaque ionized gas filled the cosmos. This blocks astronomers’ view of what happened in the Universe during its first 400,000 years or so.

But gravitational waves can travel through any medium. Consequently, any such waves created since the first instant after the Big Bang could still be present and be detectable as part of a stochastic background, providing a window into the extreme physics of the Big Bang. This is just amazing to me, says Burke-Spolaor. Who knows what’s back there.

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