the eye of Nova's mind

When two black holes merge, waves of gravitational radiation ripple outward through the galaxy at the speed of light. Because the waves are emitted mainly in one direction, the black hole itself is pushed in the opposite direction, much like the recoil that accompanies the firing of a rifle or the launching of a rocket. The black hole is booted from its normal location in the nucleus of the galaxy. If the kick velocity is high enough, the black hole can escape the galaxy completely.

[...] The new discovery is also important because it indirectly proves that black holes do in fact merge and that the mergers are sometimes accompanied by large kicks. This process had been postulated by theory, but never before confirmed via direct observation. Another implication of the discovery is that there must be galaxies without black holes in their nuclei - as well as black holes which float forever in space between the galaxies. This raises new questions for the scientists: Did galaxies and black holes form and evolve jointly in the early Universe? Or was there a population of galaxies which had been deprived of their central black holes? And if so, how was the evolution of these galaxies different from that of galaxies that retained their black holes?