After nearly two decades of listening, astronomers are finally starting to “hear” the rumbles of gravitational waves they believe emanate from the behemoths of our universe: supermassive black holes.
The result comes from a National Science Foundation–sponsored initiative known as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Since 2004 NANOGrav has monitored metronomelike flashes of light from a Milky Way–spanning network of dead stars known as pulsars. Forged from the hearts of exploding massive stars, these city-size orbs weigh as much as an entire sun and can spin thousands of times per second. This makes them remarkably accurate timekeepers—and ideal sentinels for the especially large ripples in spacetime predicted to emerge from merging supermassive black holes.
Such gravitational waves are distinct from the kinds that were previously reported from the Laser Interferometer Gravitational-Wave Observatory (LIGO) and other Earth-based detectors. For one thing, the waves spotted via pulsars wouldn’t all be traceable to individual merger events: they would form the so-called gravitational-wave background, the ambient rustling of spacetime built up from cumulative mergers throughout the cosmos. Another important distinction is that in their crest-to-trough span, each of these waves should be approximately the size of our solar system—which counterintuitively makes them much harder to detect. Washing over pulsar-strewn space, these gargantuan swells in spacetime could betray their presence via minuscule offsets to the dead stars’ spins, allowing observers to glimpse them through painstaking measurements. In a collection of five papers released today, that is essentially what NANOGrav claims to have done.
“It’s incredibly exciting because we think we’re starting to open up this new window on the gravitational-wave universe,” says Sarah Vigeland, an astrophysicist at the University of Wisconsin–Milwaukee and a member of NANOGrav.
(The collaboration’s work to date hasn’t quite met the statistical gold standard of how physicists evaluate the robustness of a finding. So for now, scientists working on the project are modestly claiming “evidence for” the gravitational-wave background, not a full-fledged detection. But they’re confident that milestone will come with additional observations.)
NANOGrav is just one of several different pulsar timing array projects underway around the globe. All these endeavors follow the same basic blueprint: they use radio telescopes to monitor dozens of superpredictable pulsars for years on end to catch tiny variations in their rhythmic spinning.
“We can create these models that basically let us know the time of arrival to precisions that rival atomic clocks,” says Thankful Cromartie, an astrophysicist at Cornell University and a member of NANOGrav. “So we know when there’s something happening, something at play that’s causing the pulsars to tick a little bit off-time”—something like gravitational waves stretching and shrinking the space between Earth and each pulsar.
That makes for a remarkably elegant natural experiment. “You don’t need to build this billion-dollar detector; you just need to put together a radio telescope and look out into the universe,” says Caitlin Witt, an astrophysicist at Northwestern University and a NANOGrav member.
Although pulsar timing arrays don’t require extremely specialized detectors, they do require patience. Building on previous NANOGrav papers from 2020 that reported a more borderline signal that was consistent with expectations for the gravitational-wave background, the latest results include 15 years’ worth of data from the North American collaboration. NANOGrav is now monitoring 68 different pulsars that form a natural gravitational-wave detector roughly the size of our galaxy. (The “new” data in the project’s analysis run through August 2020, when the iconic radio telescope at Puerto Rico’s Arecibo Observatory began its slide toward collapse and ceased observations. The Canadian Hydrogen Intensity Mapping Experiment has since joined NANOGrav to bolster its capabilities.)
But despite the volume of data and today’s hopeful announcement, scientists are only just beginning to detect the gravitational-wave background, and still have more questions than answers.
For example, while consensus holds that supermassive black hole pairs are the specific astrophysical sources responsible for most of the gravitational-wave background, conclusive evidence for this remains elusive.
“You can think of each individual supermassive black hole binary as one instrument, and the gravitational-wave background is the symphony of all of them added together,” says Maura McLaughlin, an astrophysicist at West Virginia University and a member of NANOGrav. But other “instruments” might exist, too, and they could conceivably contribute just as much, if not more, to the cosmic cacophony of giant gravitational waves.
By analyzing the symphony’s “sound,” scientists hope to determine how many such instruments are playing and even begin to understand what those supermassive black hole binaries look like. And because scientists believe these binaries emerge as a consequence of collisions between supermassive-black-hole-hosting galaxies, NANOGrav’s work should shed light on the hierarchical assembly of large galaxies, including the Milky Way.
But other, stranger phenomena, such as cosmic strings or massively inflated quantum fluctuations from right after the big bang, could also be contributing to the gravitational-wave background. Scientists don’t yet have enough data to tell the difference or to know how much signal comes from what type of source.
A particularly puzzling aspect of the gravitational-wave background signal NANOGrav is reporting is that it’s surprisingly strong—about twice as powerful as predicted. If the more esoteric explanations don’t pan out, and the signal is purely from supermassive black hole binaries, its unexpected strength could mean these behemoths themselves are larger or more plentiful than scientists had surmised.
Such a finding could inspire new efforts to find proof of merging supermassive black holes in more traditional telescope data, too, says Jenny Greene, an astrophysicist at Princeton University, who was not involved in the new research. “It’s a bit embarrassing: we expect that [supermassive] black holes should be merging, but we really haven’t been able to find observational evidence,” she says. “If there are this many binaries, we really ought to be able to find them, so I think it’s going to spur new efforts in that regard.”
In order to sort out the signal’s sources, scientists will need to spend even more time watching even more pulsars. “It’s kind of like if you dig up a dinosaur skeleton, and then you start to dust it off. At first you’re like, ‘Oh, this looks cool.’ And then the more dust you remove, the more you can start to see the skeleton,” says Chiara Mingarelli, an astrophysicist at Yale University and a NANOGrav member. “Right now we definitely know that we found a dinosaur skeleton, but maybe we don’t know what kind of dinosaur it is yet.”
Despite that uncertainty, the scientists are sure the signal is real and comes from gravitational waves because of a unique fingerprint that has only emerged in the newest batch of NANOGrav data. In 1983 researchers calculated that a gravitational-wave background signal would vary slightly—but predictably—when seen through different pairs of pulsars, depending on each pulsar’s location in the sky, as compared with where the other pulsar appeared. That correlation is what NANOGrav scientists say they’re now seeing in their data. “That’s the really exciting new piece here, and it starts to give you confidence that they really are detecting the merging black holes,” Greene says.
As NANOGrav and other pulsar timing arrays continue their work, scientists are hoping not only to understand what category of objects are creating the gravitational-wave background but also to begin seeing the signals from distinct pairs of supermassive black hole emerging from the background noise.
“The real test is going to be in the detection of individual events,” says Shobita Satyapal, an astrophysicist at George Mason University, who was not involved in the new research and calls it exciting.
NANOGrav scientists are also excited to continue working with collaborators at similar pulsar timing array experiments in Australia, Europe and India to combine all these groups’ observations into one even stronger detector in a project dubbed the International Pulsar Timing Array. “I suspect that the findings will be even more robust when they’re combined—at least, that’s the hope,” says Priyamvada Natarajan, an astrophysicist at Yale and a member of NANOGrav.
Other, newer detectors are also joining the hunt. They include China’s powerful Five-hundred-meter Aperture Spherical radio Telescope (FAST), which began observations in 2016. “What’s really important for detecting [individual supermassive black hole binary systems] is to have a very high-powered telescope that can take very precise timing of our best pulsars,” Mingarelli says. “Right now the FAST telescope in China is really leading the way for that.”
Future observatories may also contribute as pulsar-timing work continues. The Square Kilometer Array in Australia and South Africa is due to begin operations by 2027. And North American scientists are hoping for their own new observatory: a project called Deep Synoptic Array–2000 that astronomers have proposed building in Nevada. Whatever the source, the most important task will be to gather more and better data about more pulsars, which will help pin down the gravitational waves that are invisibly rippling through the universe.
“There’s a lot of work still to do over the next decades,” McLaughlin says. “Really, this is by no means the end of the story—this is just the beginning.”