Black hole mergers in the universe

In star clusters, black holes become the most massive objects within a few tens of millions of years; dynamical relaxation then causes them to sink to the cluster core, where they form binaries. These black-hole binaries become more tightly bound by superelastic encounters with other cluster members, and are ultimately ejected from the cluster. The majority of escaping black-hole binaries have orbital periods short enough and eccentricities high enough that the emission of gravitational radiation causes them to coalesce within a few billion years. We predict a black-hole merger rate of about 3 x 10(-7) per year per cubic megaparsec. For the first-generation Laser Interferometer Gravitational-Wave Observatory (LIGO-I), this implies one or two detections during the first two years of operation. For LIGO-II, the rate rises to roughly I detection per day.