On the Gravitational Wave Background from Black Hole Binaries after the First LIGO Detections

The detection of gravitational waves from the merger of binary black holes by the LIGO Collaboration has opened a new window to astrophysics. With the sensitivities of ground based detectors in the coming years we can only detect the local black hole binary mergers. The integrated merger rate can instead be probed by the gravitational-wave background, the incoherent superposition of the released energy in gravitational waves during binary-black-hole coalescence. Through that, the properties of the binary black holes can be studied. In this work we show that by measuring the energy density $\Omega_{GW}$ (in units of the cosmic critical density) of the gravitational-wave background, we can search for the rare $\sim 100 M_{\odot}$ massive black holes formed in the Universe. In addition, we can answer how often the least massive BHs of mass $> 3 M_{\odot}$ form. Finally, if there are multiple channels for the formation of binary black holes and if any of them predicts a narrow mass range for the black holes, then the gravitational-wave-background spectrum may have features that with the future Einstein Telescope can be detected.