Recoiling black holes could move at nearly one-tenth the speed of light

The cosmic sinkholes gain speed from being kicked by gravitational waves

An illustration of the convergence of two black holes and yellow gravitational waves rippling through space.

The convergence of two black holes will send gravitational waves rippling through space (illustrated). If moving predominately in a single direction, those waves can shoot the newly unified black hole into space at high speed.

C. Henze/NASA

Collisions between black holes can launch newly melded cosmic sinkholes at speeds up to nearly one-tenth the speed of light, researchers report August 18 in Physical Review Letters. Moving that fast — about 28,500 kilometers per second — it would take about 13 seconds to complete the average trip from Earth to the moon. The findings may help researchers figure out how much energy can get released when black holes converge.

Black holes merge after coming so close together that they become ensnared in each other’s gravitational pull. The couple enters a cosmic waltz, spiraling ever tighter and stirring up ripples in spacetime known as gravitational waves (SN: 6/28/23; SN: 1/21/21).

If the pair’s convergence blows gravitational waves preferentially in one direction, then the emerging black hole will recoil in the opposite direction at high speed, like kickback from a gun (SN: 4/25/22).

Previous research had indicated the blowback from such twisting mergers could hurl black holes at up to about 5,000 kilometers per second. For the new study, astrophysicists Carlos Lousto and James Healy, of the Rochester Institute of Technology in New York, probed black hole interactions that were more head-on, akin to particles zipping towards each other in a particle accelerator.

The duo simulated more than 1,300 approaches. These included close flybys, direct collisions and everything in between. The mashups that almost didn’t happen — grazing collisions that still resulted in merging — produced the fastest recoiling black holes, the researchers found.

Such collisions might be possible with three or more black holes, Lousto says. “Two black holes orbiting in opposite directions around a larger black hole might eventually collide,” he explains, dispatching their descendant with the quickest of quick kicks.

Nikk Ogasa is a staff writer who focuses on the physical sciences for Science News. He has a master's degree in geology from McGill University, and a master's degree in science communication from the University of California, Santa Cruz.

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