Astronomers Suspect Colliding Supermassive Black Holes Left the Universe Awash in Gravitational Waves

A decade ago, physicists using twin detectors in Washington State and Louisiana detected gravitational waves—subtle ripples in space-time—for the first time. The landmark discovery, confirming Albert Einstein’s century-old prediction of such waves, was recognized with the 2017 Nobel Prize in Physics.

Gravitational waves are created whenever massive objects undergo any kind of acceleration. In the case of those first detections, the signal was produced by pairs of black holes within our galaxy that had been orbiting one another and eventually merged. Although gravity holds us down on Earth, the force is actually very weak, and so gravitational waves only distort space by a very small amount. In the case of the black hole mergers recorded in 2015 by the gravitational-wave detectors, known together as LIGO (Laser Interferometer Gravitational-wave Observatory), the passing waves caused the arms of the L-shaped detectors, each 2.5 miles long, to stretch and contract by an amount smaller than the width of an atom. Physicists describe these as high-frequency, short-wavelength gravitational waves: They jiggled the LIGO detectors only very briefly, corresponding to the few seconds that it took for the black holes to merge in a final death spiral.

Now, astronomers have recorded the faint background hum from a different kind of gravitational wave. These are lower-frequency, longer-wavelength gravitational waves that appear to be coming from every direction in the sky. While the LIGO detections involved collisions between black holes only a bit more massive than the sun, the gravitational waves responsible for this background hum are thought to result from orbiting pairs of supermassive black holes millions of times more massive than our sun, scattered about the cosmos. The data also suggests the hum is slightly more intense in the Southern Hemisphere sky—one of several puzzles raised by the latest findings.

While theorists long suspected this gravitational-wave hum should exist, the evidence for it has only accumulated gradually as radio telescopes known as “pulsar timing arrays” recorded enough data to tease out the faint signal from various sources of radio noise. The latest such findings were published this past December in a series of papers in the Monthly Notices of the Royal Astronomical Society. The scientists reported using data from the MeerKAT radio telescope array in South Africa to track the slight changes in the timing of radio waves coming from more than 80 pulsars—rapidly spinning, ultra-dense neutron stars. As low-frequency gravitational waves drift by, the pulsars are nudged very slightly, altering the timing of their radio wave emissions by a tiny amount. Astronomers can then use the shifts in those signal times to infer the properties of the gravitational waves.

The passing gravitational waves “stretch or contract the universe by around 20 meters [about 65 feet] or so,” says Matthew Miles, an astrophysicist at Swinburne University of Technology in Australia and lead author on two of the papers.

The pulsars, located hundreds or even thousands of light-years away, act as a kind of detector—one that’s far larger than anything humans could ever build.

“Nature has provided us with an observatory on a galactic scale,” says Joey Key, an astrophysicist at the University of Washington Bothell, who was not involved in the current study.

Earlier work published in 2023 by the NANOGrav collaboration, which uses data from the Green Bank telescope in West Virginia and two other active North American radio telescope facilities, had recorded the first hints of this gravitational wave hum after analyzing some 15 years of accumulated pulsar data—but the more sensitive MeerKAT array has achieved comparable results in less than five years, Miles says.

“Because of the sensitivity [of MeerKAT] and the number of pulsars that we’re able to look at with this array, we were able to get a similar result in just four and a half years, which is really exciting,” he says.

In contrast to the signals from 2015 that lasted just a few seconds, the more recently detected low-frequency gravitational waves have wavelengths of about a light-year, meaning it takes about a year for two consecutive wave-crests to pass by—thus requiring at least several years of observations to be certain the signal is actually there. Another difference is that the pairs of supermassive black holes responsible for the recently detected waves are thought to be in relatively stable orbits; theorists believe they may orbit each other for thousands of years before finally merging, emitting low-frequency gravitational waves as they orbit.

Floor Broekgaarden, an astrophysicist at the University of California San Diego, compares the search for these waves to the challenge of hearing specific sounds at a crowded Christmas market. Suppose you’re standing at the edge of the market, trying to hear an orchestra and choir performing on the other side of the plaza. In this analogy, the high-frequency waves discovered in 2015 are like the soprano singers, whose voices would likely be the first thing you’d hear. “And now, all of a sudden, you can hear the bass instruments,” she says. The background hum of gravitational waves “adds a new sound to our discovery space.”

The data from MeerKAT “is really exciting,” Broekgaarden says. “It means the evidence is building up that this humming exists, and these supermassive black holes exist.”

Theorists have long suspected that most galaxies, including our own Milky Way, harbor supermassive black holes in their centers. The black hole in the heart of the Milky Way, known as Sagittarius A* (pronounced “Sagittarius A-Star”), was discovered in the mid-1970s via its strong radio wave emissions. In 2022, astronomers used the Event Horizon Telescope to photograph a disk of matter surrounding the object, confirming that it really is a black hole. When galaxies collide, their black holes are believed to end up orbiting each other, releasing gravitational waves in the process.

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But the new study has also brought puzzles. For starters, the background hum appears to be slightly more intense than expected. This could mean more supermassive black holes are out there than predicted, or that the black holes are, on average, more massive than theorists had calculated, and are therefore producing a stronger signal than expected.

A second puzzle is that the intensity of the background hum appears to be increasing over time. “That shouldn’t really happen, because these things should change on scales of tens of millions of years,” says Miles. One possibility, he suggests, is that something is producing gravitational waves relatively close to our solar system, and so the motion of our solar system relative to this source is causing the apparent change. But for now “we don’t have a very good explanation,” he says. “It’s a very exciting mystery.”

A third puzzle is that the background hum of gravitational waves appears to be slightly asymmetric, being somewhat more intense in the Southern Hemisphere sky than in the Northern Hemisphere sky—what astronomers describe as a gravitational-wave “hot spot.” One possibility is that a particular pair of colliding galaxies in the southern sky, containing an orbiting pair of supermassive black holes, just happens to be closer than the others, and its stronger signal is skewing the data. “At the moment, we can’t really say with any confidence if this is just a statistical anomaly, or whether this [hot spot] is really there,” Miles says.

Astronomers hope that future projects may finally solve these mysteries, especially as gigantic facilities like the Square Kilometer Array, an array of radio telescopes in South Africa and Australia, comes online in 2027. Rather than an all-sky hum, Miles predicts that such projects may reveal individual pairs of colliding supermassive black holes. “In the next decade, I think we should be able to see direct signals from supermassive black hole binaries, and not just this sort of incoherent jumble that we’re seeing now,” he says.

At the very least, better data will likely tell astronomers more about the size and abundance of supermassive black hole binaries, says Broekgaarden. “We want to know how many of these monstrous black holes are out there,” she says. Gravitational wave astronomy “might tell us things about the universe that we had no way of knowing before.”

Dan Falk | | READ MORE

Dan Falk is a science journalist based in Toronto. His books include The Science of Shakespeare and In Search of Time.