Unraveling Cosmological Anisotropies within Stochastic Gravitational Wave Backgrounds

TL;DR

This paper assesses the potential of future gravitational wave detectors to measure anisotropies in the cosmological stochastic gravitational wave background, which could reveal primordial inhomogeneities and their origins.

Contribution

It introduces a Fisher analysis framework to evaluate the detectability of cosmological SGWB anisotropies amidst astrophysical foregrounds and detector noise.

Findings

Upcoming detectors can effectively identify anisotropic cosmological SGWBs.

Fisher analysis estimates parameter uncertainties for cosmological and astrophysical components.

Shot noise in astrophysical foregrounds presents challenges but can be mitigated.

Abstract

Identifying the anisotropies in a cosmologically sourced stochastic gravitational wave background (SGWB) would be of significance in shedding light on the nature of primordial inhomogeneities. For example, if SGWB carries isocurvature fluctuations, it would provide evidence for a multi-field inflationary origin of these inhomogeneities. However, this is challenging in practice due to finite detector sensitivity and also the presence of the astrophysical foregrounds that can compete with the cosmological signal. In this work, we explore the prospects for measuring cosmological SGWB anisotropies in the presence of an astrophysical counterpart and detector noise. To illustrate the main idea, we perform a Fisher analysis using a well-motivated cosmological SGWB template corresponding to a first order phase transition, and an astrophysical SGWB template corresponding to extra-galactic binary mergers, and compute the uncertainty with which various parameters characterizing the isotropic and anisotropic components can be extracted. We also discuss some subtleties and caveats involving shot noise in the astrophysical foreground. Overall, we show that upcoming experiments, e.g., LISA, Taiji, Einstein Telescope, Cosmic Explorer, and BBO, can all be effective in discovering plausible anisotropic cosmological SGWBs.