Mind the Binaries

A pair of closely orbiting black holes will eventually collide and merge. Such mergers are expected to be one of the strongest sources of gravitational waves—ripples in the fabric of spacetime first predicted by Albert Einstein in 1916 but never observed directly. Carl Rodriguez from Northwestern University, Illinois, and colleagues now underscore the importance of a factory of binary black holes that is typically neglected in estimates of the frequency of these mergers: dense groups of stars, known as globular clusters, that orbit the core of their host galaxies. Their work indicates that such mergers may provide one of the most common signals to be detected by instruments such as the Advanced Laser Interferometer Gravitational Wave Observatory (Advanced LIGO), which is expected to start collecting its first data in the coming fall.