Gravitational Recoil and Astrophysical Impact

Asymmetric emission of gravitational waves from astrophysical sources leads to a net flux of linear momentum from the source and, by momentum conservation, imparts a gravitational recoil on the emitting source. Numerical relativity simulations have revealed that this effect can lead to astonishingly large kick velocities, so-called superkicks, of several thousand km/s in the inspiral and merger of black-hole binaries. We here discuss the calculation of the recoil in black-hole spacetimes and the astrophysical repercussions of such large kicks, in particular related to the possible displacement or ejection of supermassive black holes from their host galaxies. We also discuss possible mechanisms that would make superkicks less likely to occur in astrophysical binaries and thus explain why most, if not all, galaxies observed in this context appear to harbor a black hole at their center.