Using the Binary Black Hole Population to Study Cosmology and the Stochastic Gravitational-Wave Background*

The stochastic gravitational-wave background is the overall signal of gravitational waves produced by a superposition of all binary black hole coalescences in the universe. We use results from the Laser Interferometer Gravitational-Wave Observatory (LIGO) to analyze the population of binary black hole coalescences and the gravitational-wave background expected to be caused by them. In analyzing the population of binary black hole coalescences, we consider several mass and redshift population models, using hierarchical Bayesian inference to infer the population. With these population models, we compute the expected stochastic gravitational-wave background, finding agreement with previous analyses. However, since the catalogs are limited to low-redshift binary black holes, there is significant uncertainty in the empirically-predicted gravitational-wave background. Finally, using a phenomenological model for the mass and redshift population models, we explore how changing the model behavior at high redshifts affects the predicted gravitational-wave background spectrum. Such analyses can be used to anticipate what the stochastic gravitational-wave background caused by stellar-mass binary black holes could look like once LIGO achieves the necessary sensitivity to detect such a background.