The universe is humming with gravitational radiation — a very low-frequency rumble that rhythmically stretches and compresses spacetime and the matter embedded in it.
That is the conclusion of several groups of researchers from around the world who simultaneously published a slew of journal articles in June describing more than 15 years of observations of millisecond pulsars within our corner of the Milky Way galaxy. At least one group — the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) collaboration — has found compelling evidence that the precise rhythms of these pulsars are affected by the stretching and squeezing of spacetime by these long-wavelength gravitational waves.
"This is key evidence for gravitational waves at very low frequencies," says Vanderbilt University's Stephen Taylor, who co-led the search and is the current chair of the collaboration. "After years of work, NANOGrav is opening an entirely new window on the gravitational-wave universe."
Gravitational waves were first detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015. The short-wavelength fluctuations in spacetime were caused by the merger of smaller black holes, or occasionally neutron stars, all of them weighing in at less than a few hundred solar masses.
The question now is: Are the long-wavelength gravitational waves — with periods from years to decades — also produced by black holes?
In one paper from the NANOGrav consortium, published Aug. 1 in The Astrophysical Journal Letters (ApJ Letters), University of California, Berkeley, physicist Luke Zoltan Kelley and the NANOGrav team argued that the hum is likely produced by hundreds of thousands of pairs of supermassive black holes — each weighing billions of times the mass of our sun — that over the history of the universe have gotten close enough to one another to merge. The team produced simulations of supermassive black hole binary populations containing billions of sources and compared the predicted gravitational wave signatures with NANOGrav's most recent observations.The black holes' orbital dance prior to merging vibrates spacetime analogous to the way waltzing dancers rhythmically vibrate a dance floor. Such mergers over the 13.8-billion-year age of the universe produced gravitational waves that today overlap, like the ripples from a handful of pebbles tossed into a pond, to produce the background hum. Because the wavelengths of these gravitational waves are measured in light years, detecting them required a galaxy-sized array of antennas — a collection of millisecond pulsars.
"I guess the elephant in the room is we're still not 100% sure that it's produced by supermassive black hole binaries. That is definitely our best guess, and it's fully consistent with the data, but we're not positive," said Kelley, UC Berkeley assistant adjunct professor of astronomy. "If it is binaries, then that's the first time that we've actually confirmed that supermassive black hole binaries exist, which has been a huge puzzle for more than 50 years now."
"The signal we're seeing is from a cosmological population over space and over time, in 3D. A collection of many, many of these binaries collectively give us this background," said astrophysicist Chung-Pei Ma, the Judy Chandler Webb Professor in the Physical Sciences in the departments of astronomy and physics at UC Berkeley and a member of the NANOGrav collaboration.
Ma noted that while astronomers have identified a number of possible supermassive black hole binaries using radio, optical and X-ray observations, they can use gravitational waves as a new siren to guide them where in the sky to search for electromagnetic waves and conduct detailed studies of black hole binaries.Ma directs a project to study 100 of the closest supermassive black holes to Earth and is eager to find evidence of activity around one of them that suggests a binary pair so that NANOGrav can tune the pulsar timing array to probe that patch of the sky for gravitational waves. Supermassive black hole binaries likely emit gravitational waves for a couple of million years before they merge.
Other possible causes of the background gravitational waves include dark matter axions, black holes left over from the beginning of the universe — so-called primordial black holes — and cosmic strings. Another NANOGrav paper appearing in ApJ Letters today lays out constraints on these theories."Other groups have suggested that this comes from cosmic inflation or cosmic strings or other kinds of new physical processes which themselves are very exciting, but we think binaries are much more likely. To really be able to definitively say that this is coming from binaries, however, what we have to do is measure how much the gravitational wave signal varies across the sky.
Binaries should produce far larger variations than alternative sources," Kelley said. "Now is really when the serious work and the excitement get started as we continue to build sensitivity. As we continue to make better measurements, our constraints on the supermassive black hole binary populations are just rapidly going to get better and better."
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