Institute for Advanced Study/Princeton University Joint Astrophysics Colloquium

The advanced detectors of gravitational waves detected several black hole binary mergers one neutron star merger. Such binaries are either formed by evolution of massive binary systems or through dynamical processes in dense stellar systems. I will focus on the dynamical scenarios originating from globular clusters. The galactic nuclei star clusters (NC) could also provide suitable environment for the dynamical binary formation, but they are likely to be much less efficient than globular clusters. In globular clusters, the black holes become the most massive components within 10 million years after the birth of the clusters. The black holes experience dynamical friction that causes central concentration of massive black hoes in the central parts of the clusters in a short time scale. Black hole binaries can be formed either direct capture or by three-body processes in dense core. Under typical conditions of globular clusters, three-body processes are much more efficient than direct capture. The binaries experience subsequent interactions with surrounding stars (mostly black holes) and the orbit shrinks due to such interactions. Eventually the binaries get ejected along with surrounding black holes, leading to nearly complete desertification of black holes in the cluster in a few billion years. The ejected binaries undergo passive evolution outside the cluster through gravitational radiation. Some fraction of the them will merge within Hubble time. We expect the mass ratios of two black holes in binaries would be close to unity. Also the spin directions are expected to be randomly oriented relative to the orbital angular momentum vector. iBinaries composed of a black hole and a neutron star will be very rare among dynamical binaries. I will also describe the gravitational waves arising from the hyperbolic or parabolic encounters between black holes and detectability of such events.