For the first time, physicists have found what they are calling “compelling evidence” for low-frequency gravitational waves, ripples in space-time usually caused by massive cosmic objects orbiting each other. The waves likely emanated from pairs of some of the most gigantic black holes in the universe and jostled other deep space objects enough to create a subtle signal that scientists could pick up.
The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) published its new findings in a series of papers today in the Astrophysical Journal Letters. The team will present their results to the public on Thursday afternoon at the National Science Foundation and on YouTube. The NANOGrav team coordinated with international colleagues, with separate collaborations in Europe, India, Australia, and China releasing similar findings at the same time. The consistency among the groups lends weight to their conclusions, which are that these long-theorized waves actually exist.
“We’ve been on a mission for the last 15 years to find a low-pitched hum of gravitational waves resounding throughout the universe and washing through our galaxy to warp space-time in a measurable way. We’re very happy to announce that our hard work has paid off,” said Stephen Taylor, the NANOGrav chair, at a press briefing on June 27.
The NANOGrav measurement is consistent with predictions from Albert Einstein’s theory of general relativity, Taylor said. According to that theory, black holes spiraling into each other should cause wrinkles in the fabric of space-time, and those distortions should propagate outward at the speed of light. But a century ago, detecting such a thing from Earth seemed virtually impossible. And indeed, those nearly imperceptible waves were not found until 2015, when the US-based Laser Interferometer Gravitational-Wave Observatory collaboration, or LIGO, exhilarated the physics world by detecting one.
The LIGO group, along with the Virgo collaboration in Europe, have since found dozens more, mostly from merging pairs of star-sized black holes, as well as a couple of mergers between black holes and neutron stars. But the gravitational waves the NANOGrav scientists look for are very different: They’re measured at much lower frequencies, and they probably originate from supermassive black holes, gargantuan objects lying at the center of most galaxies, including our own, and weighing as much as hundreds of millions or even billions of suns. In the publications released by NANOGrav and the other teams, the scientists describe their analysis while also showing how gravitational waves are permeating the cosmos. They also speculate about other possible origins if they don’t come from large black holes after all—exotic candidates like cosmic strings or cosmic inflation.
NANOGrav and its international counterparts, like the European Pulsar Timing Array, measured the gravitational waves’ signal by making use of pulsars scattered about the galaxy. Sometimes called “space lighthouses,” pulsars are the cores of dead, massive stars that have collapsed under their own weight and gone supernova. Some of them rotate hundreds of times per second while beaming radiation from their magnetic axes. Researchers use those pulses as incredibly precise cosmic clocks, pinpointing the pulsars’ locations.
The NANOGrav team was essentially able to turn the Milky Way into a giant gravitational wave detector by measuring the signals from these pulsars to determine when a wave nudged them. The collision of enormous black holes—or some other extremely energetic process—generates gravitational waves that ever-so-slightly squeeze and stretch space-time, tweaking the intervals between pulsar blips. NANOGrav researchers measured those minuscule changes among 68 pulsars, then correlated them, finding a pattern that is likely the sign of low-frequency gravitational waves. The other collaborating teams did the same with separate sets of pulsars.
It took more than a decade of data collection and analysis for the teams to reduce their measurement uncertainties and to be sure that they’d spotted a real sign of gravitational waves rather than some other cosmic phenomenon or mere noise. The NANOGrav team, which includes nearly 200 people, conducted a statistical analysis and found less than one-in-a-thousand odds that the signal they observed could happen by chance. The other collaborations found similar levels of statistical significance.
While these are very likely to be signs of real gravitational waves from colossal black holes, the teams are reluctant to use the word “detection” to describe their findings. Nine years ago, the US-based BICEP2 collaboration, using a telescope at the South Pole, claimed to have detected primordial gravitational waves coming from the big bang, only to find that their signal actually came from pesky dust grains in the Milky Way—and that has made researchers circumspect about their conclusions. “The gravitational wave community is very cautious about these kinds of things,” says Scott Ransom, an astronomer with the National Radio Astronomy Observatory and former chair of NANOGrav.
For their measurements, the NANOGrav team made use of several radio telescopes: the Green Bank Observatory in West Virginia, the Very Large Array in New Mexico, and the huge Arecibo Observatory in Puerto Rico, an iconic instrument that collapsed in 2020. The other teams used radio telescopes in five European countries, India, China, and Australia. More telescopes have recently joined the effort, including CHIME in Canada and MeerTime in South Africa.
The collaboration between scientists in the US and China is notable, says Ransom. While a controversial 2011 law called the Wolf Amendment forbids NASA from working directly with Chinese entities because of security concerns, such restrictions don’t apply to National Science Foundation–funded efforts like NANOGrav. “The politics have made some of our collaborations tricky,” Ransom says. “We have to figure out a way to work together, because the science is definitely better when we do that. It’s terrible being hamstrung by politics.”
The teams coordinate with each other through a sort of super-collaboration called the International Pulsar Timing Array. While the group’s geographic span makes it challenging for the scientists to communicate across time zones, they’re able to combine their data sets, improving their precision and their confidence in their measurements. “One cannot construct a galaxy-sized gravitational wave telescope in your backyard,” wrote Michael Keith, an astrophysicist on the European Pulsar Timing Array executive committee, in an email to WIRED. “It takes a combined effort of hundreds of astronomers, theorists, engineers, and administrators to study the universe at this scale.”
