When two black holes merge, the resulting larger black hole usually shoots away from its birthplace, but it immediately slows down in some cases, according to computer simulations. In the 4 June *Physical Review Letters*, a team offers an explanation for this puzzling deceleration. They suggest that the moving black hole can radiate gravitational waves preferentially in the forward direction as a result of asymmetry in the curvature of spacetime around it. That slows the black hole like a spacecraft firing retro-rockets. The paper provides an intuitive explanation for the slow-down with a much-simplified simulation that the team believes can be generalized.

Two black holes that are close enough will mutually orbit and eventually spiral inward toward each other, sending off ever-stronger gravitational waves (ripples in spacetime), until they collide and merge into a larger black hole. If the gravitational waves radiate mostly in one direction at the time of the merger, they “kick” the new black hole in the opposite direction. But some simulations have shown an “anti-kick” following the initial kick–the new black hole shoots away but soon slows down. Researchers haven’t had a clear physical explanation for the anti-kick.

Luciano Rezzolla of the Max-Planck Institute for Gravitational Physics in Potsdam, Germany, and his colleagues, realized that they could understand the slow-down if they simplified the problem. They ignored the pre-merger “death spiral” and treated the two black holes as mutually attracting billiard balls of different sizes that simply collide and stick together. Researchers already know that the smaller black hole will accelerate more than the larger one and send more radiation in the direction the small one was moving. This radiation generates the initial kick. But Rezzolla realized that the new black hole’s event horizon–the boundary in spacetime that even light cannot pass–is not perfectly spherical because of the mismatch in the original black hole masses. The region where the smaller black hole arrived will bulge out, a situation that can’t last.

“Black holes, when they are isolated, they long to be perfect, without any deformation,” Rezzolla says. Studies from the 1970s showed that a black hole with a perturbation on its horizon can emit gravitational waves to dissipate energy and remove the bulge. Such black holes are said to “ring” like a bell emitting sound waves. When the merged and kicked black hole emits gravitational waves from its forward-pointing bulge, the effect is equivalent to tapping on the brakes.

To verify this thought experiment quantitatively, Rezzolla and his colleagues ran a computer simulation of this head-on collision and included another simplification. They mathematically described the spacetime around the merged black hole as that of a so-called white hole. A white hole is like an inverted black hole: instead of absorbing all matter and radiation that hits the horizon, a white hole constantly emits radiation in all directions. But for simulations, it has several technical advantages over a black hole, and the bulge and associated radiation are similar. “This simpler scenario allows us to be much more rigorous and precise,” Rezzolla says.

Although their model was simplified, the researchers think it can be applied to black holes in general. Even the most complicated mergers can ultimately be reduced to bulges in the merged black hole’s horizon, they say.

Astrophysicist David Merritt of the Rochester Institute of Technology thinks the study is “certainly important and worth doing.” But when applying the model to more complicated black holes, “I would urge caution,” he says. “It would be nice to present a similar qualitative picture of the anti-kick in more generic black hole mergers, mergers involving spin for instance.”