One of the most spectacular discoveries obtained from numerical simulations of black holes in the framework of Einstein’s general relativity are the so-called superkicks. These arise from the net emission of linear momentum in gravitational waves during the inspiral and coalescence of two black holes. Just like firing a bullet imparts a recoil on the shooter, the single black hole resulting in from the coalescence will recoil from the preferential emission of gravitational waves in a specific direction. Superkicks are the most dramatic version of this effect and are generated when the black holes are rapidly rotating but in opposite directions. The recoil can then reach magnitudes of up to 3700 km/s. This is particularly remarkable, since this velocity is large enough to kick the black hole out of even the most massive galaxies. Superkicks can thus explain astrophysical observations of black holes displaced relative to the centre of their host galaxy or quasars (luminous objects sourced by black holes) exhibiting significant blue or redhift relative to their environment. Superkicks also affect the assembly of black holes throughout cosmological history.

In our work, we have shown that moderate values of the orbital eccentricity can even further enhance the superkick magnitudes by about 25%, leading to kicks well above 4000 km/s. Our results have consequences for the ongoing programs of observing sources of gravitational waves. Larger kicks are able to eject a larger fraction of black holes from their hosts, stellar clusters or galaxies, and can reduce the expected stochastic gravitational-wave background targeted in pulsar timing observations. The superkick effect is illustrated by the figure below, taken from one of our simulations at the time shortly after the two black holes have merged into one.