When a pair of black holes merge, the resulting larger black hole can be sent hurtling away at extraordinary speeds – and now astronomers have seen it happen.
at the Max Planck Institute for Gravitational Physics in Germany and his colleagues found this fast-moving black hole by taking a second look at data from the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and its corresponding observatory in Italy, called Virgo. These observatories measure gravitational waves, ripples in space-time caused by the motions of massive objects.
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The signal that Varma and his colleagues studied is designated GW200129. It came from two black holes orbiting one another that spiralled inwards and smashed together, resulting in a single, larger black hole. They found that, before the merger, the black holes were spinning, and their spin axes weren’t aligned with one another or the axis running through the point in space around which they orbited.
This gives us a hint as to where the original black holes may have formed. “Isolated systems tend to give you aligned spins, according to models,” says at the University of Mississippi, who wasn’t involved in this research. “When we see these misaligned spins, that’s a hint that this binary may have formed in a more crowded environment like a globular cluster” – a dense clump of old stars.
That misalignment is also a deciding factor behind what happens to the final black hole. When such black holes merge, the momentum held by the spin has to go somewhere, and it ends up being split between the gravitational waves emitted in the collision and the final black hole.
The final merger can be compared to the firing of a cannon, says at the University of Milano-Bicocca in Italy. “When the cannonball flies, the cannon recoils in the opposite direction,” he says. “When the black holes emit gravitational waves, those carry some linear momentum – the gravitational waves are the cannonball and the black hole that is left behind is the cannon.”
Researchers have calculated that this “kick” effect should be able to give black holes speeds of hundreds of kilometres per second, but this is the first observational evidence. Varma and his colleagues calculated that the final object’s speed was at least about 700 kilometres per second and probably closer to 1500 kilometres per second, which may be fast enough to propel it out of its home galaxy.
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This evidence that black holes can recoil after mergers is important, because removing a black hole from the crowded environment where it was born means that it won’t be around to participate in more mergers. This makes it difficult to explain some of the larger black holes LIGO has spotted, which we would expect to result from a series of mergers in a row.
It also means that the cosmos is full of black holes zooming around at extreme speeds, but that shouldn’t be a cause for concern for us. “Space is so extraordinarily vast that there is basically no chance that on Earth we’ll encounter anything like this,” says Varma. “This one is happening billions of light years away, so even if it was pointed directly at Earth, we wouldn’t have to start worrying about it any time soon. But it’s pointed away from Earth.”
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