NASA, ESA, AND A. FEILD (STSCI) / SCIENCE PHOTO LIBRARY NASA, ESA, AND A. FEILD (STSCI) / SCIENCE PHOTO LIBRARY
Illustration of a process by which gravitational waves could eject a black hole from the centre of a galaxy. At left two galaxies, each with a central black hole, merge. The two black holes then settle at the centre of the newly-formed galaxy and orbit each other in a binary system. This produces gravitational waves, seen emanating outwards from the centre of the galaxy. Over time the black holes move closer together. If they are of unequal mass and rotation rate the gravitational waves they produce are emitted more strongly in one direction. Eventually the black holes merge. The energy emitted by the merger propels the black hole in the opposite direction to that of the strongest gravitational waves.
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