This artist’s impression shows a pair of supermassive black holes orbiting each other and emitting gravitational waves, affecting bright pulsars.
Aurore Simonet for NANOGrav
hide captions
switch captions
Aurore Simonet for NANOGrav
This artist’s impression shows a pair of supermassive black holes orbiting each other and emitting gravitational waves, affecting bright pulsars.
Aurore Simonet for NANOGrav
Scientists say they are beginning to find signs of an elusive kind of rumbling in space that could be produced by the largest, ugliest black holes in the universe.
This discovery means that astrophysicists may have opened a whole new window on supermassive black holes. These mysterious, extremely dense objects, millions to billions of times more massive than the sun, lie at the center of galaxies like our own.
When two galaxies merge, the massive black holes at their centers are expected to converge and spin around each other in a spinning dance, creating giant spiraling waves.
These waves are like ripples that move across a pond if you throw a stone in — only these waves move through the very fabric of the universe, and researchers have been eager to study them.
“For the past fifteen years we’ve been on a mission to find the deep hum of gravitational waves reverberating throughout the universe,” said Stephen Tayloran astrophysicist from Vanderbilt University who is the chair of a group of researchers known as the North American Nanohertz Gravitational-Wave Observatory (NANOGrav). “We are delighted to announce that our hard work has paid off.”
Other research groups using telescopes in Europe, Australia, India and China also say they are starting to see signs of these waves.
A new layer of spatial ripples
So far, scientists have only been able to detect gravitational waves produced by much smaller black holes. First seen in 2015, when a research consortium register waves produced by the merger of two black holes, each 30 times more massive than the sun.
That landmark discovery shows that gravitational waves really do exist, fulfilling Albert Einstein’s 1916 prediction and giving researchers a new way to study strange phenomena like black holes. and neutron stars.
The initial detection of those gravitational waves relied on a pair of specially built instruments, in Louisiana and Washington, sending lasers down two 2.5-mile “arms” or tubes. As a gravitational wave rolls through and stretches space, these detectors are able to capture extremely small changes in the distance the lasers travel.
That approach worked to find gravitational waves spanning about 2,000 miles, say Jeff Hazboun, an astrophysicist at Oregon State University. But this won’t work for finding the kind of long-wavelength gravitational waves produced by supermassive black holes — the kind with wavelengths four light-years, or “20 million million miles,” Hazboun said. To capture such long wavelengths, a detector would have to have an “arm” half the length of the galaxy.
So the researchers decided to turn the galaxy itself into a sort of detector, by taking advantage of its existing weirdness.
“We can hack the galaxy,” says Hazboun, a member of the NANOGrav group, has nearly 100 members from the United States, Canada, and dozens of other countries. “That’s one of the most exciting things about this project for me.”
NANOGrav’s technique relies on tracking pulsars, which are super-dense, spinning cores of dead stars. Each pulsar is small, about the size of a city, but it rotates hundreds of times a second, emitting beams of radio emission that regularly sweep across the sky.
“Every time their beam passes through our field of vision, we see a pulse signal,” said collaborator NANOGrav Cromartie is grateful of Cornell University. “These pulses come at amazingly regular intervals.”
The intervals are so regular that scientists can accurately predict when a pulse will reach Earth. They can then look for small deviations from the expected arrival time.
“And if that pulse is a little bit later or a little earlier, then we can refer to it as a passing gravitational wave,” Hazboun explains, explaining that gravitational waves would stretch or compress space-time. , which changes the distance a pulse must travel to reach Earth.
In their latest analysis, published in a series of articles in Astrophysics MagazineThe researchers looked at data from about 70 pulsars.
What they found was a false pattern from the expected pulsar beam’s arrival time, showing the gravitational wave wobbling through space-time as if it were a large amount of Jell-O.
“It’s hard to assume that the waves are coming from one direction or the other,” Hazboun said.
Instead of seeing a wave roll in, like someone standing on a beach, he said, it was more like the experience of swimming in an ocean full of crashing waves.
Researchers still don’t know what causes these waves. What they saw was consistent with predictions of a supermassive black hole, but it could be something more unusual.
“Theoreticians have been really happy to come up with models that can generate very similar types of gravitational wave signals,” says Luke Zoltan Kelleya theoretical astrophysicist at Northwestern University and NANOGrav.
He says possibilities range from cosmic strings to dark matter to primordial black holes that formed shortly after the Big Bang.
‘We were lucky, so why aren’t they?’
Convincing new job Gabriela Gonzalez of Louisiana State University, a member of the Laser Interferometer Gravitational-Wave Observatory (LIGO) scientific collaboration, which now regularly detects gravitational wave signals from much less massive black holes .
“They did some very complex analysis,” she said. “All confirm the same observation. There are gravitational waves here.”
The nature of this type of evidence for gravitational waves means that certainty increases as more data is collected from pulsars, adding that a few years ago, data published by this group seems to be trending in this direction.
“They saw very clear evidence of some sort of rumbling in the galaxy. They couldn’t confirm that it was due to gravitational waves, but there was something there,” she said. “So we’ve been looking forward to this for several years now.”
And the NANOGrav researchers studied a dataset that included observations that would be valid for several more years.
“We expect the evidence for gravitational waves that we’ve seen in this 15-year dataset to be even stronger than in that dataset,” said Maura McLaughlina NANOGrav astrophysicist at West Virginia University.
The NANOGrav collaboration, which is largely funded by the National Science Foundation, also plans to merge their findings with similar efforts by researchers abroad, as part of a joint plan. work. group called the International Pulsar Time Array.
McLaughlin says that effort will be complete in the next year or two, and will add information about even more pulsars to the mix.
In addition to providing stronger evidence for a gravitational wave background signal, she said, it could also allow researchers to focus on the location of a specific source, such as a pair of supermassive black holes. large nearby.
For that to happen, González said, “they’re going to have to be lucky. Although, we were lucky, so why wouldn’t they?”
Scientists can then try to observe them with telescopes to learn more about them, just like they do. do in 2017 when detectors found 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 unprecedented event.
