Cast your mind back four years, and gravitational waves were the talk of the town. On September 14, 2015, the first detection of these ripples in space-time was made by the LIGO-Virgo collaboration, revealed months later to deserved global fanfare. Now with the fourth anniversary of that discovery approaching, the field has matured dramatically with dozens of subsequent detections made—and the prospect of even more thrilling discoveries on the horizon.
The field “has exploded,” says Nergis Mavalvala from the Massachusetts Institute of Technology. “I’ve really been amazed at what we’ve been able to achieve. It’s staggering both on the astrophysics side, and the immense improvements to the instruments that have come about.”
Including that first discovery, a total of 23 confirmed gravitational-wave detections have been made to date across three observing runs. Within those, 20 have been black hole mergers, two have been neutron star mergers, and one is the suspected first known instance of a merger between a black hole and a neutron star. Each has been exciting in its own right, but the sheer volume of detections—moving from one a month to nearly one a week, thanks to upgrades to LIGO in 2018 and 2019 that improved its sensitivity—is hugely impressive. By some estimates gravitational-wave observatories could catch a merger every hour by 2023. “It’s hard to overstate how explosive the growth of gravitational-wave astronomy has been,” says Ben Farr of the University of Oregon.
Thanks to that explosive growth, remarkable progress is being made across multiple subdisciplines of astrophysics. Daniel Holz from the University of Chicago says that the study of black hole mergers, which now seems almost “boring” due to the high volume of observed events, is being nonetheless transformed. “We’re moving very rapidly into the area of population and statistics,” he says. “Instead of analyzing one, we’re analyzing a whole bunch of them. Now we get to see what is the distribution method, how many are big, how many are small. Even if they continue to be ‘boring,’ the distribution of the boring events is fascinating.”
The first neutron star merger observed by LIGO and Virgo, meanwhile, has helped researchers probe some fundamental aspects of the universe itself. Christopher Berry from Northwestern University notes that gamma rays from the event were detected by other telescopes 1.6 seconds after the gravitational waves, which allowed for an unprecedented test of the speed of gravity versus the speed of light. “We’d expect a little difference in their arrival time because they weren’t necessarily created at the same time,” he says. “But the fact that it was 1.6 seconds allowed us to test that the speed of light and the speed of gravity really are the same thing, as predicted in general relativity.”
Another way scientists have hoped to probe relativity is to see gravitational waves that have been “gravitationally lensed” by a massive object. Just as light can be bent and magnified when it passes through the gravitational fields of galaxies and other massive objects, gravitational waves should be warped in the same way, too. Last month astronomers were momentarily abuzz at the possible detection of such an event when two similar looking gravitational-wave signals washed over Earth just 21 minutes apart—a hint the waves might have been from the same source and had been lensed. Unfortunately, further examination showed that the back-to-back signals had come from two different directions in the sky, but astronomers remain hopeful of spotting such an event in the future, although it might not be easy. “The probability of a chanced alignment is really small,” says Asantha Cooray, a researcher at the University of California, Irvine who is unaffiliated with the LIGO/Virgo collaboration. “You would have to [do hundreds of observations] to see one of these things.”
Lensing events are not the only future discoveries anticipated by astronomers. One of the most enticing is the possible detection of gravitational waves caused by a detonating supernova. Such an event, however, would probably need to happen in our galaxy in order for LIGO and Virgo to detect it. “That happens roughly once a century,” Holz says. “And it hasn’t happened as far as we can tell in the last four years. So that’s something that we still have to wait for. It’ll happen someday.”
More ambitiously, scientists also think it might be possible to someday see primordial gravitational waves, left over from the first fractions of a second after the big bang. Such waves would allow researchers to look back further than ever before toward the birth of the universe. “The earliest light that reaches us as observers was [emitted] when the universe was 400,000 years old,” Mavalvala says. “Whereas gravitational waves have been streaming to us since the earliest moments after the big bang.” Unfortunately, the signatures of such waves should be so incredibly feeble that only so-called third-generation gravitational-wave detectors, such as the planned Cosmic Explorer in the U.S. or the Einstein Telescope in Europe, would be capable of detecting them.
And, of course, the gravitational-wave deluge is revealing unexpected new mysteries as well. One example is the unknown origin of the black holes involved in these mergers, known as stellar-mass black holes as they are several times the mass of our Sun. “Naively you’d like to think that well, this category of black holes are the remnants of ordinary stars going supernova,” Mavalvala says. “But we know that ordinary stars can’t grow to be tens of solar masses big. So that formation scenario is not easily supported.” Another possibility is that they are the result of mergers of smaller black holes, but that requires an unfathomably large population of smaller black holes in the universe to account for the number of mergers seen. The true answer remains elusive for now.
Four years on, the growth of gravitational-wave astronomy shows no signs of slowing. “I think it’s been a revolution,” Berry says. “We’ve really opened our eyes to what’s out there, that was invisible, that we can only reveal through gravitational waves.” And with the detections piling up higher and higher, there is plenty more to come.