Precision cosmology with primordial GW backgrounds in presence of astrophysical foregrounds

TL;DR

This paper explores how astrophysical foregrounds impact the detection of primordial gravitational wave backgrounds, showing that with proper modeling, sensitivity loss is limited, enabling precision cosmology.

Contribution

It introduces a simple method to account for astrophysical foregrounds in primordial GW background detection, emphasizing the importance of modeling black hole binary contributions.

Findings

Sensitivity degradation is limited to a factor of a few.

Foreground modeling is crucial for accurate primordial GW detection.

Optimal filtering can effectively subtract foreground components.

Abstract

The era of Gravitational-Wave (GW) astronomy will grant the detection of the astrophysical GW background from unresolved mergers of binary black holes, and the prospect of probing the presence of primordial GW backgrounds. In particular, the low-frequency tail of the GW spectrum for causally-generated primordial signals (like a phase transition) offers an excellent opportunity to measure unambiguously cosmological parameters as the equation of state of the universe, or free-streaming particles at epochs well before recombination. We discuss whether this programme is jeopardised by the uncertainties on the astrophysical GW foregrounds that coexist with a primordial background. We detail the motivated assumptions under which the astrophysical foregrounds can be assumed to be known in shape, and only uncertain in their normalisation. In this case, the sensitivity to a primordial signal can be computed by a simple and numerically agile procedure, where the optimal filter function subtracts the components of the astrophysical foreground that are close in spectral shape to the signal. We show that the degradation of the sensitivity to the signal in presence of astrophysical foregrounds is limited to a factor of a few, and only around the frequencies where the signal is closer to the foregrounds. Our results highlight the importance of modelling the contributions of eccentric or intermediate-mass black hole binaries to the GW background, to consolidate the prospects to perform precision cosmology with primordial GW backgrounds.