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New study shows how black holes can be ejected from a galaxy

When black holes collide, an enormous burst of gravitational radiation results as they violently merge into one massive black hole. The 'kick' that occurs during the collision could knock the black hole clear out of its galaxy. A new study describes the consequences of such an intergalactic collision.
The kick -- a consequence of relativity -- occurs because gravitational waves emitted during the final plunge are anisotropic, producing recoil. The effect is maximized when one black hole is appreciably larger than the other one. While astrophysicists have been aware of this phenomenon since the 1960s, until now no one has had the analytical tools necessary to accurately calculate the size of the effect. The first accurate calculation of the size of the kicks was reported in a companion paper by Favata, Hughes and Holz, which also appears online.