Astrophysicists have found the best evidence yet for a low-frequency “hum” of gravitational waves rippling through the cosmos, based on 15 years’ worth of ultra-precise measurements checking the timing of radio pulses from distant stars.
The evidence, newly published in the Astrophysical Journal Letters, comes from several teams of researchers working in the U.S. and Canada as well as Europe, India, Australia and China.
The teams monitored radio emissions from a total of 115 ultra-dense, spinning stars known as pulsars. Nearly 70 of those pulsars were observed by the North American Nanohertz Observatory for Gravitational Waves, known as NANOGrav.
“This is key evidence for gravitational waves at very low frequencies,” Vanderbilt University’s Stephen Taylor, who co-led the search and is the current chair of the NANOGrav Collaboration, said today in a news release. “After years of work, NANOGrav is opening an entirely new window on the gravitational-wave universe.”
The NANOGrav Collaboration has more than 170 members, including researchers from the University of Washington at Bothell, Oregon State University and the University of British Columbia. Jeff Hazboun, a physics professor at Oregon State who previously served as a postdoctoral researcher at UW-Bothell, said working on NANOGrav has been “really wonderful.”
“I’d definitely say that big problems nowadays really require a lot of people to work on them, and collaborations are a great way to get things done,” he told GeekWire. “You know, we were using Zoom two years before the pandemic.”
Hazboun said his primary role is to “make sure that what we’re seeing is what we think we’re seeing, and understanding how sensitive our array of pulsars is as a detector for gravitational waves.”
A detector of galactic proportions
Pulsars send out radio waves in a rotating beam as the stars spin —somewhat like the rotating spotlight in a lighthouse. The fastest-spinning pulsars rotate hundreds of times a second, which means they can serve as ultra-precise cosmic clocks.
In the mid-1990s, scientists figured out that the timing of the radio flashes from pulsars could theoretically be used to detect gravitational waves created by powerful phenomena such as the interactions between two supermassive black holes.
Such waves should create subtle ripples of distortion in the fabric of spacetime, in accordance with Albert Einstein’s theory of general relativity. To observers on Earth, it would look as if the timing of the radio pulses was thrown off ever so slightly.
Earth-based experiments — such as the Laser Interferometer Gravitational-wave Observatory, or LIGO — have detected high-frequency gravitational waves associated with small to medium-sized black holes. But supermassive black holes are a different matter. They’re millions of times more massive than the black holes that LIGO has been targeting. It’d be impossible to use an Earth-based detector to look for the low-frequency gravitational waves that those monster black holes are thought to emit as they interact.
“A really important thing to note is that our detector doesn’t lie solely on Earth, or even in the solar system,” Taylor told reporters during a teleconference about the results. “We’re using a decades-old discipline of timing pulsars throughout the Milky Way, and we’re effectively building a galaxy-scale gravitational-wave antenna from them.”
It’s not an easy task. “Pulsars are actually very faint radio sources, so we require thousands of hours a year on the world’s largest telescopes to carry out this experiment,” said West Virginia University’s Maura McLaughlin, co-director of the NANOGrav Physics Frontiers Center. NANOGrav’s radio observations were made using the Green Bank Telescope in West Virginia, the Very Large Array in New Mexico, and the Arecibo Observatory in Puerto Rico (which collapsed in late 2020).
Don’t use the D-word
Over the years, NANOGrav and other collaborations have been gradually closing in on a confirmed detection of low-frequency gravitational waves. But McLaughlin said she and her colleagues weren’t quite ready yet to use the D-word.
“We’re not reporting a ‘detection,'” she told reporters. “We’re being very careful with our language, and we are calling this ‘evidence’ for gravitational waves.”
The reason for her caution has to do with the statistical analysis of the findings so far. A key measure for the significance of results has to do with standard deviation, also known as “sigma.” Physicists would like to see a rating of 5-sigma for an official discovery, like the discovery of the Higgs boson in 2012. But none of the pulsar timing groups has yet reported 5-sigma results.
China’s research team is the closest, with a confidence level of 4.6-sigma — which translates into a 2-in-a-million chance that the report is just a false alarm. “However, it’s only based on three years of data on a very short dataset, so it’s quite difficult to really assess the validity or pull any astrophysics out of it,” McLaughlin said.
NANOGrav’s researchers expect to clear the 5-sigma hurdle once the results from all of the pulsar timing groups are combined, probably in the next year or two. “We’re looking forward to the boost in gravitational-wave sensitivity that this kind of data combination is going to be able to afford us,” said Cornell University’s Thankful Cromartie, chair of NANOGrav’s pulsar timing working group.
What could be causing the hum?
Based on the analysis so far, the NANOGrav team says the best way to describe the gravitational-wave patterns would be as a background hum — analogous to overlapping voices in a crowd, or the din you hear when musicians in an orchestra are tuning their instruments.
“Now that we have evidence for gravitational waves, the next step is to use our observations to study the sources producing this hum,” said Sarah Vigeland, an astrophysicist at the University of Wisconsin at Milwaukee. “One possibility is that the signal is coming from pairs of supermassive black holes, with masses millions or billions of times the mass of our sun. As these gigantic black holes orbit each other, they produce low-frequency gravitational waves.”
But that’s not the only possibility. Northwestern University’s Luke Zoltan Kelley, chair of NANOGrav’s astrophysics working group, said the results are “also consistent with new physics — gravitational waves produced by cosmological or inflationary processes in the very early universe.” Theorists have already come up with alternate explanations.
NANOGrav and the other pulsar timing groups will need to collect more data to determine which possibility makes the most sense.
“The answer is probably much more complicated, that it’s really a mixture of different processes, and it’s unlikely that supermassive black hole binaries aren’t in the mix somewhere,” Taylor said. “We’re trying to put a constraint on all of these different parameters and all of these different processes at the same time.”
What’s next?
Hazboun said pulsar timing techniques and Earth-based interferometers like LIGO are providing a variety of ways to look at different ends of the gravitational-wave spectrum.
“It’s akin to seeing something from the rest of the universe in radio, or in ultraviolet, or in the infrared,” he explained. “You need different detectors to see different parts of the electromagnetic spectrum. And it’s the same thing with gravitational waves.”
In the decades to come, still more types of detectors are expected to fill in gaps in the gravitational-wave spectrum.
“There are a couple of Chinese projects [TianQin and Taiji] and a European-NASA project [LISA] to build space-based gravitational-wave detectors,” Hazboun said, “and those are actually sensitive in the region of the frequency band between pulsar timing at the really low end and LIGO at the high end.”
Studies in Astrophysical Journal Letters about the NANOGrav 15-year data set:
- Evidence for a Gravitational-Wave Background
- Observations and Timing of 68 Millisecond Pulsars
- Detector Characterization and Noise Budget
- Search for Signals from New Physics
Other studies focusing on low-frequency gravitational waves:
- Searching for the Nano-Hertz Stochastic Gravitational Wave Background with the Chinese Pulsar Timing Array Data Release I
- Search for an Isotropic Gravitational-wave Background with the Parkes Pulsar Timing Array
- The Gravitational-wave Background Null Hypothesis: Characterizing Noise in Millisecond Pulsar Arrival Times with the Parkes Pulsar Timing Array
In coordination with other pulsar timing groups, members of the NANOGrav team will discuss their results during a live-streamed presentation at 10 a.m. PT Thursday.
