Astronomers have finally spotted a pair of supermassive black holes headed for an almighty smash-up, though whether they ever will collide remains to be seen.
The giant black holes are around 2.5 billion years away and each of them is more than 800 million times more massive than our Sun. As they draw closer to each other, they will start to emit their own gravitational waves through space-time. Those ripples will join up with the as-yet-not-entirely detected background noise of gravitational waves from other supermassive black holes.
“Supermassive black hole binaries produce the loudest gravitational waves in the universe,” says co-discoverer Chiara Mingarelli, an associate research scientist at the Flatiron Institute’s Center for Computational Astrophysics in New York City, in a statement. “Gravitational waves from supermassive black hole pairs are a million times louder than those detected by LIGO.”
Which would all be great, if we could actually witness this cosmic event. However, it’s going to take around 2.5 billion years for the black holes to actually start producing the waves.
The black hole binary is 2.5 billion years away, so we’re seeing our Universe 2.5 billion years ago when we look at them now. In the actual present day, the gravitational waves are already happening, but even at light speed, they won’t reach us for billions of years. Mindblowing, isn’t it?
That doesn’t mean it’s a dud discovery though. Observing this pair’s past should help scientists to figure out how many nearby supermassive black holes are emitting gravitational waves that we might have a hope of detecting right now. The discovery provides an anchor point for estimating how many supermassive black hole pairs are within detection distance of Earth, whereas previous estimates relied on computer models of how often galaxies merge.
The search for gravitational waves is hugely important because it will help astronomers to answer some major outstanding questions about the Universe – not least of which is whether supermassive black holes ever do actually merge or if they stay locked into orbit around each other.
“It’s a major embarrassment for astronomy that we don’t know if supermassive black holes merge,” says study co-author Jenny Greene, a professor of astrophysical sciences at Princeton. “For everyone in black hole physics, observationally this is a long-standing puzzle that we need to solve.”
Once supermassive black holes get close enough to each other, they stop swapping gas and stars and stealing each other’s energy and everything slows right down. The final parsec problem theory suggests that all black hole binaries will stall out at around a parsec apart (3.2 light years) and time will stretch out into as-good-as-infinity.
We know that rare groups of three or more supermassive blackholes can merge, but because of this ultra-slow-mo collision course, we don’t know if two can. Once they’re that close to each other, they become really hard to distinguish, so astronomers searching the sky can’t find these stalled pairs.
If this theory is wrong and supermassive black holes are merging, astronomers expect that the gravitational wave background is more like a clamour.
“It’s a bit like a chaotic chorus of crickets chirping in the night,” says Goulding. “You can’t discern one cricket from another, but the volume of the noise helps you estimate how many crickets are out there.”
From this discovery, the team predicts that there are optimistically about 112 nearby supermassive black holes emitting gravitational waves. The first detection of the gravitational wave background from supermassive black holes should therefore come within the next five years or so. If it doesn’t, that would be evidence that the final parsec problem may be insurmountable.