This artist's impression shows a pair of supermassive black holes circling each other and sending out gravitational waves, which affect the bright, shining pulsars.

This artist's impression shows a pair of supermassive black holes circling each other and sending out gravitational waves, which affect the bright, shining pulsars. / Aurore Simonet for NANOGrav

Scientists say they are starting to find signs of an elusive type of rumbling through space that could be created by the biggest, baddest black holes in the universe.

The discovery means that astrophysicists may have opened a whole new window onto supermassive black holes. These mysterious, extremely dense objects, millions to billions of times more massive than the sun, sit at the center of galaxies like our own.

When two galaxies merge, the enormous black holes at their centers are thought to come together and circle each other in a spinning dance that sends giant waves spiraling out.

These waves are like the ripples that move through a pond if you toss in a rock — only these waves move through the very fabric of the universe, and researchers have been eager to study them.

"We've been on a mission for the last fifteen years to find a low-pitched hum of gravitational waves resounding throughout the universe," says Stephen Taylor, a Vanderbilt University astrophysicist who serves as the chair of a team of researchers known as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). "We're very happy to announce that our hard work has paid off."

Other research groups using telescopes in Europe, Australia, India, and China also say they're starting to see hints of these waves.

A new class of space ripples

Until now, scientists have only been able to detect gravitational waves created by much smaller black holes. The first were seen in 2015, when a research consortium registered the waves created by the merger of two black holes that were each about 30 times as massive as the sun.

That landmark discovery showed that gravitational waves truly existed, fulfilling a prediction made by Albert Einstein in 1916 and giving researchers a new way to study exotic phenomena like black holes and neutron stars.

The initial detection of those gravitational waves relied on a pair of specially-built devices, in Louisiana and Washington, that sent lasers down two 2.5-mile "arms," or tubes. When a gravitational wave rolled through and stretched space, these detectors could catch the incredibly slight change in the distance traveled by the lasers.

That approach worked to find gravitational waves that stretched roughly 2,000 miles long, says Jeff Hazboun, an astrophysicist at Oregon State University. But this wouldn't work to find the kind of long-wavelength gravitational waves created by supermassive black holes — the kind whose wavelength is 4 light years long, or "20 million million miles," says Hazboun. To catch wavelengths that long, a detector would have to have "arms" that stretched as long as half of the galaxy.

So researchers decided to turn the galaxy itself into a kind of detector, by taking advantage of its existing weirdness.

"We get to hack the galaxy," says Hazboun, a member of the NANOGrav team, which has nearly 100 members from the U. S, Canada, and a dozen other countries. "That is one of the most exciting things about this project for me."

NANOGrav's technique relies on monitoring pulsars, which are the super-dense, spinning cores of dead stars. Each pulsar is small, about the size of a city, but it spins hundreds of times a second, sending out beams of radio emissions that regularly sweep the sky.

"Each time their beam crosses our line of sight, we see a pulse signal," says NANOGrav collaboration member Thankful Cromartie of Cornell University. "These pulses arrive at stunningly regular intervals."

The intervals are so regular that scientists can predict exactly when a pulse should arrive at Earth. They can then look for tiny deviations from that expected arrival time.

"And if that pulse is a little bit late or a little bit early, then we may be able to attribute that to a gravitational wave passing through," says Hazboun, who explains that a gravitational wave will stretch or compress space-time, changing the distance that a pulse has to travel to get to Earth.

In their latest analysis, which is being published in a series of papers in The Astrophysical Journal Letters, the researchers looked at data from about 70 pulsars.

What they found is a pattern of deviations from the expected pulsar beam arrival timings that suggests gravitational waves are jiggling space-time as though it's a vast serving of Jell-O.

"It's really hard to attribute that the waves are coming from one direction or another," says Hazboun.

Rather than seeing one wave come rolling in, like someone standing on a beach, he says, it's more like the experience of swimming out in a choppy ocean.

The researchers don't yet know what's creating these waves. What they see is consistent with predictions about supermassive black holes, but it could be something even more unusual.

"The theorists have really had a lot of fun coming up with models that can produce very similar types of gravitational wave signals," says Luke Zoltan Kelley, a theoretical astrophysicist at Northwestern University and NANOGrav.

He says the possibilities range from cosmic strings to dark matter to primordial black holes that formed soon after the Big Bang.

'We have been lucky, so why not them?'

The new work convinced Gabriela González of Louisiana State University, a member of the Laser Interferometer Gravitational-Wave Observatory (LIGO) scientific collaboration, which now routinely detects gravitational wave signals from much less massive black holes.

"They have done several very sophisticated analyses," she says. "They all confirm the same observation. There are gravitational waves here."

The nature of this kind of evidence for gravitational waves means that certainty grows as more data from pulsars gets collected, she says, adding that a few years ago, the data published by this group seemed to be trending in this direction.

"They had seen very strong evidence for some kind of rumbling in the galaxy. They couldn't confirm that it was due to gravitational waves, but there was something there," she says. "So we have been expecting this for several years now."

And the NANOGrav researchers are already poring over a dataset that includes a couple more years' worth of observations.

"We expect the gravitational wave evidence that we've seen in this 15-year dataset to be even stronger in that one," says Maura McLaughlin, an NANOGrav astrophysicist at West Virginia University.

The NANOGrav collaboration, which is funded in large part by the National Science Foundation, also plans to merge their findings with similar efforts by researchers overseas, as part of a group called the International Pulsar Timing Array.

That effort should be complete in the next year or two, says McLaughlin, and would add information on even more pulsars to the mix.

In addition to providing stronger evidence of the gravitational wave background signal, she says, it might even let researchers zero in on the location of one particular source, like a pair of nearby supermassive black holes.

For that to happen, says González, "they would have to be lucky. Although, we have been lucky, so why not them?"

Scientists could then try to observe them with telescopes to learn more about them, much like they did in 2017 when detectors registered gravitational waves from the collision of two neutron stars. That allowed astronomers to point their telescopes in that direction and witness the faint glow of this never-before-seen event.

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