Distinguishing cosmology, astrophysics and systematics: the Bayesian search for a primordial gravitational-wave background in next-generation observatories

RESUMO

Next-generation gravitational-wave observatories like Cosmic Explorer will provide an unprecedented opportunity to probe the cosmological stochastic gravitational-wave background, providing insight into the early Universe that is inaccessible with other means. We investigate the prospects for measuring a Gaussian cosmological gravitational-wave background in Cosmic Explorer in the presence of a distinguishable, non-Gaussian foreground of binary black hole mergers, using a Bayesian search that simultaneously fits binary black hole and background parameters. We successfully recover a Gaussian cosmological background with a signal-to-noise ratio of 5 in one day of simulated data in Cosmic Explorer without contamination from unresolved signals or data exclusion. However, when introducing waveform systematic errors to our analysis, our inference on the stochastic background parameters becomes biased and can falsely infer the presence of a background when there is none. The bias is driven by signal mismatches at low frequencies (<20 Hz), especially when the recovery waveform is missing physical effects included in the injected signals. While we propose a metric for diagnosing if measurements will be strongly biased by waveform systematics, our results suggest improvements in waveform modeling or other mitigation techniques will be required to accurately recover a primordial stochastic gravitational-wave background in next-generation detectors.