When three black holes get intimate, their relationship is bound to be complex. According to the first full simulation of this cosmic ménage à trois, it is likely to end in sudden violence.
The simulation, performed by researchers at the Rochester Institute of Technology in New York, US, shows that these three-way collisions should send out distinctive pulses of gravitational waves, which might be detected within a decade.
Encounters between three or more black holes have been modelled before using simplified physics. But this is the first time it has been done using full-on general relativity, Einstein's powerful and highly complicated theory of gravity.
That enabled the team to predict what happens when the holes get very close to one another, and even collide. "The only way to see mergers is through a fully relativistic simulation," says Manuela Campanelli, lead author of the study.
To simplify their task, the team chose to model three black holes of equal mass, and with no spin. First they looked at a pair of holes orbiting each other closely, with a third looping around them in the same direction. The third hole swoops in, collides and merges with one of the inner pair, flinging the remaining hole out into a distant, stable orbit.
Head-on collision
Then the simulation was changed, so that the third hole starts out moving in the opposite direction. The outcome is completely different, as two rapid collisions leave all three holes merged into one.
Each collision shakes the fabric of spacetime, sending out a burst of gravitational waves - although it turns out to be a relatively quiet event compared with collisions when only two black holes involved.
A simple pair of orbiting black holes will gradually spiral in towards one another and merge in a relatively prolonged final whirl, radiating gravitational waves all the time. In a three-hole mêlée, however, the end tends to come more quickly and directly.
"These black holes basically approach in a head-on collision, so there's not a lot of time to radiate," Campanelli told New Scientist. Nevertheless, these are still among the most energetic events in the universe, she adds.
Dense clusters
Such complex encounters might be taking place inside large star clusters, where the black-hole remnants of heavy stars are thought to swarm.
They would send out a distinctive gravitational wave signal. Two collisions leading to a merger of all three holes would send two blasts of waves in quick succession, for example.
Multiple mergers of small black holes might be detected within the next few years by the Laser Interferometer Gravitational Wave Observatory.
And if three supermassive black holes decide to collide at the heart of a distant galaxy, that would be picked up by a planned space-based gravitational-wave detector called LISA (Laser Interferometer Space Antenna).
Journal reference: Physical Review D (forthcoming)
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