Same black hole can collide with its kin multiple times, lopsided merger suggests

For black holes, a collision doesn’t have to be a oncein-a-lifetime experience. On 12 April 2019, scientists detected a new black hole merger using a trio of gravitational-wave detectors. Astrophysicists have spotted such events before, but something about the signals was different: the two black holes that collided were incredibly unevenly matched, with the larger about three times the size of the smaller.

“This event is an oddball the universe has thrown at us – it was something we didn’t see coming,” said Salvatore Vitale, a physicist at the Massachusetts Institute of Technology. “But nothing happens just once in the universe. And something like this, though rare, we will see again, and we’ll be able to say more about the universe.”

Vitale and his colleagues suspect that the strange collision occurred after the larger black hole itself was the product of a black hole merger. The initial event sent a large black hole bouncing around a neighbourhood packed with more black holes, enabling the uneven collision. That’s a very different story than scientists’ two main scenarios for black hole mergers, which both encourage fairly even matches. Vitale and his colleagues used two different models to evaluate whether the traditional merger scenarios could create an event like the unbalanced merger.

Scientists analysing the uneven collision suspect that hierarchical mergers couldn’t happen just anywhere, but instead must occur in a relatively dense neighbourhood, where black holes can easily interact. “This merger must have come from an unusual place,” Vitale said. “As LIGO [the Laser Interferometer Gravitational-Wave Observatory] and Virgo continue to make new detections, we can use these to learn new things about the universe.”