Gravitational thermodynamics and black-hole mergers

Abstract

Black holes become the most massive objects early in the evolution of star clusters. Dynamical relaxation then causes them to sink to the cluster core, where they form binaries which become more tightly bound by superelastic encounters with other cluster members. Ultimately, these binaries are 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 waves causes them to coalesce within a few billion years. The rate at which such collisions occur is on the order of 10 7 per year per cubic megaparsec. This implies event rates for gravitational-wave detectors substantially greater than current estimates of the corresponding rates from neutron-star mergers or black-hole mergers stemming from pure binary evolution.