Astronomers don’t usually jump out of bed when they receive alerts in the middle of the night that, somewhere far away, two black holes have smacked into each other and sent shock waves coursing through the universe. These days, the detection of colliding black holes verges on routine, and astronomers know what to do: Go back to sleep.
They will rouse in the case of a merger between neutron stars—leftover cores of giant stars that ran out of fuel and imploded—so that they can summon telescopes to scan the sky in search of a burst of light. But with a pair of black holes, there’s not much to see. Usually.
Astronomers announced today that they have spotted what might be a spark from the collision of two faraway black holes. If confirmed, the discovery would mark the first time that astronomers have captured light produced by the joining of the darkest objects in the universe.
But black holes don’t emit light; they trap it. Entire stars can become shredded and swallowed during an unlucky encounter with a black hole. How could any encounter between two black holes produce the kind of light our eyes can see?
It happened last May—from our perspective, at least, because it takes many millions of years for the aftermath of such events to reach Earth. The gravitational waves made themselves known, as usual, at the observatories in the United States and Italy specifically designed to detect them. At about the same time, an observatory in California, programmed to spot luminous objects in the sky, was making its nightly rounds. It caught a bright flare in the darkness. Astronomers traced the flash to the center of a distant galaxy, where a supermassive black hole lies, surrounded by a glowing disk of swirling gas and dust.
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The disk is home to smaller black holes, and sometimes they get close enough to collide. The researchers behind the new paper, published today in Physical Review Letters, believe that when two of these black holes smashed together, the force of the impact warped the space around them and sent gravitational waves rippling outward, like rings in a pond. The black holes merged into a single, larger black hole and shot through the disk of gas, jostling nearby matter and heating it up. The sudden move made the surrounding gas glow even brighter—bright enough for telescopes on Earth to spot a flare. “The reason we get any light from this is because the black-hole binary merger is embedded in a much more exciting neighborhood,” says Mansi Kasliwal, an astronomy professor at Caltech and one of the authors of the new paper.
The researchers believe that the flash of light and gravitational waves erupted here, from the same event.
The flare “is illuminating the invisible,” Rosanne Di Stefano, an astrophysicist at Harvard University who was not involved in the research, told me. “It’s providing light in a region where something fascinating has happened, and a fascinating object still exists—the merger.”
The piece of sky the researchers studied is awash in galaxies, and astronomers aren’t surprised when they catch supernovas, the luminous explosions of dying stars, popping off in the vicinity. The astronomers say they have considered and ruled out supernovas and several explanations for the glow. They also point to the behavior of the black hole at the center of the galaxy in question; when it’s feeding on nearby material, the black hole can put on some spectacular light shows, but observations show that the object remained quite stable for 15 years before that sudden surge last spring.
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Still, Di Stefano said she’s not yet convinced that the merger is the reason for the flare. To her, the signature of the flare described in this research resembles a burst she encountered in her own work with black holes. That flare, she said, was produced when a supermassive black hole passed in front of another, nearby supermassive black hole. Though the two massive objects never actually collided, their movements warped the bright disk of cosmic material around one of the black holes, so that it appeared, to Earth, like a sudden, bright flash.
Astronomers found the flash described in the new paper in archival telescope data. By the time they realized that the light might have come from a black-hole mash-up, months later, the glow was already fading, and growing too faint for more detailed study. But they will soon have an opportunity to do it again: The team predicts, based on computer simulations, that the black hole that was forged from this powerful collision and kicked out will zoom back into the disk of hot material sometime in the next couple of years. The event would produce another brilliant flash of light, and this time, astronomers will be standing by to catch it.
For the moment, researchers are in the dark about any new collisions; the gravitational-wave detectors were paused in late March because of the coronavirus pandemic and haven’t yet resumed operations. If telescopes spot other intriguing flares in the night sky in the coming months, there will be no way to check whether the light might be streaming from a pair of the most mysterious objects in the universe spiraling together in a space-warping clash.
“We won’t know what we missed, except that the universe is so big that there’s rarely anything that’s so special that it would only happen once,” says Maya Fishbach, a graduate research fellow at the University of Chicago who studies gravitational-wave astronomy, and was not involved in the new findings. “I’m sure whatever we missed, there’ll be another system that’s very similar to it, that when we turn it back on again, we’ll catch it then.”
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