The roar of gravitational waves from merging supermassive black holes has been heard in the universe for the first time

After 15 years of collecting data in a galaxy-sized experiment, scientists have “heard” the perpetual chorus of gravitational waves rippling through the universe for the first time — and it’s louder than expected.

The groundbreaking discovery was made by scientists with the North American Nanohertz Gravitational-Wave Observatory (NANOGrav) who closely monitor stars called pulsars that act as celestial metronomes. The newly discovered gravitational waves—ripples in the fabric of space-time—are the most powerful ever measured: they carry nearly a million times as much energy as single-detected gravitational waves from black hole and neutron star mergers. Through experiments like LIGO and Virgo.

Nanograph scientists report in a series of new papers that appear today in Nanograph Astrophysical Journal Letters.

says NANOGrav scientist Chiara Mingarelli, who worked on the new findings while she was an assistant research scientist at the Center for Computational Astrophysics (CCA) in New York City. “This is the first ever evidence of a gravitational wave background. We have opened a new observation window on the universe.”

The existence and formation of background gravitational waves–long theoretical but unheard of–offers a treasure trove of new insights into long-standing questions, from the fate of pairs of supermassive black holes to the frequency of galaxy mergers.

For now, NANOGrav can only measure the overall gravitational-wave background rather than the radiation from individual “singers.” But even this brought surprises.

“The gravitational wave background is about twice as high as I expected,” says Mingarelli, now an assistant professor at Yale University. “It’s really on the higher end of what our models can create out of just supermassive black holes.”

The deafening size may result from experimental limitations or heavier and more abundant supermassive black holes. But there’s also the possibility that something else is generating strong gravitational waves, Mingarelli says, such as mechanisms predicted by string theory or alternative explanations for the birth of the universe. “What’s next is all,” she says. “This is only the beginning.”

A galaxy-wide experience

Getting to this point has been a challenge for the NANOGrav team for years. The gravitational waves they caught are unlike anything previously measured. Unlike the high-frequency waves detected by ground-based instruments such as LIGO and Virgo, the gravitational wave background is composed of ultra-high-frequency waves. The rise and fall of a wave can take years or even decades. Since gravitational waves travel at the speed of light, an individual wavelength can be tens of light years.

No experiment on Earth could detect such massive waves, so the NANOGrav team looked to the stars instead. They closely watched pulsars, the ultra-dense remnants of massive stars that have gone supernova. Pulsars act like stellar beacons, shooting beams of radio waves from their magnetic poles. Because pulsars spin so quickly (sometimes hundreds of times per second), those beams sweep across the sky, appearing from our location on Earth as rhythmic pulses of radio waves.

The pulses hit the ground like a perfect timing metronome. The timing is so precise that when Jocelyn Bell measured the first radio waves in 1967, astronomers thought they might be signals from an alien civilization.

When a gravitational wave passes between us and the pulsar, it throws off the radio wave timing. This is because, as Albert Einstein predicted, gravitational waves stretch and compress space as they ripple through the universe, changing the distance that radio waves must travel.

For 15 years, NANOGrav scientists from the United States and Canada have been timing radio wave pulses from dozens of millisecond pulsars in our galaxy using the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia and the Very Large Array in New Mexico. The new findings are the result of a detailed analysis of a group of 67 pulsars.

“Pulsars are actually very faint radio sources, so we need thousands of hours a year on the world’s largest telescopes to do this experiment,” says Maura McLaughlin of West Virginia University, co-director of the Physics Frontiers Nanograph Center. “These findings are made possible by the National Science Foundation’s (NSF) ongoing commitment to these highly sensitive radio observatories.”

Background detection

In 2020, with just over 12 years of data, NANOGrav scientists began to see hints of a signal, an extra “hum” common to the timing behavior of all pulsars in the array. Now, after three years of additional observations, they have accumulated tangible evidence for the existence of the gravitational-wave background.

“Now that we have evidence of gravitational waves, the next step is to use our observations to study the sources that produce this buzz,” says Sarah Vigeland of the University of Wisconsin-Milwaukee, chair of the NANOGrav Detection Working Group.

The most likely sources of gravitational wave background are pairs of supermassive black holes captured in a death spiral. These black holes are really massive, containing billions of the sun’s mass. Almost all galaxies, including our own Milky Way, have at least one giant galaxy at their core. When two galaxies merge, supermassive black holes can meet and begin to orbit each other. Over time, their orbits narrow as gas and stars pass between the black holes and steal energy.

Eventually, supermassive black holes get so close that the energy theft stops. Some theoretical studies have argued for decades that black holes then crash indefinitely when they are separated by a parsec (about three light-years) apart. This proximity-but-not-cigar theory has come to be known as the last parsec problem. In this scenario, only rare combinations of three or more supermassive black holes lead to mergers.

However, pairs of supermassive black holes can have a trick up their sleeves. They can emit energy like powerful gravitational waves as they orbit each other until they eventually collide in a catastrophic conclusion. “Once two black holes get close enough to be seen by pulsar timing arrays, nothing can stop them from merging in just a few million years,” says Luke Kelley of the University of California, Berkeley, lead of the NANOGrav astrophysics group.

The existence of the gravitational-wave background found by NANOGrav seems to support this prediction, which could put the recent parsec problem to rest.

Because pairs of supermassive black holes are formed by galaxy mergers, their abundance of gravitational waves will help cosmologists estimate how often galaxies collide throughout the history of the universe. Mingarelli, postdoctoral researcher Deborah C. Judd of CCA and the University of Connecticut and colleagues studied the intensity of the gravitational wave background. They estimate that hundreds of thousands or perhaps a million or more supermassive black hole binaries populate the universe.

alternative sources

Not all gravitational waves detected by NANOGrav are necessarily from pairs of supermassive black holes. Other theoretical proposals also predict waves in the very low frequency range. String theory predicts, for example, that one-dimensional faults called cosmic strings may have formed in the early universe. These strings can dissipate energy by emitting gravitational waves. Another suggestion is that the universe began not with the Big Bang but rather with the Big Bounce, in which the negative universe collapsed in on itself before expanding outward. In such an origin story, the gravitational waves from the accident would still ripple through space-time.

There’s also the possibility that pulsars aren’t the perfect gravitational-wave detectors that scientists think they are, and that instead they may have some unknown asymmetry skewing the NANOGrav results. “We can’t walk up to a pulsar and turn it on and off again to see if something’s wrong,” Mingarelli says.

The NANOGrav team hopes to explore all possible contributors to the newly discovered gravitational wave background as they continue to monitor pulsars. The group plans to break down the background based on the frequency of the waves and their origin in the sky.

international effort

Fortunately, the NANOGrav team is not alone in its quest. Several papers published today by collaborations using telescopes in Europe, India, China and Australia point to the same gravitational-wave background signals in their data. Through the International Pulsar Timing Matrix consortium, individual groups pool their data to better characterize the signal and identify its sources.

“Our combined data will be even more powerful,” says Stephen Taylor of Vanderbilt University, who co-led the new research and is currently chairing the NANOGrav collaboration. “We are excited to discover the secrets they will reveal about our universe.”