Testing the Early Universe with Anisotropies of the Gravitational Wave Background

TL;DR

This paper explores how small and intermediate-scale gravitational wave anisotropies can be used to probe the particle content of the early universe during inflation, emphasizing the role of non-Gaussianities and cross-correlations with CMB data.

Contribution

It develops a phenomenological framework for analyzing gravitational wave anisotropies from inflation, including non-Gaussianities and cross-correlations, with realistic forecasts for upcoming detectors.

Findings

Projected constraints on primordial non-Gaussianity parameter $F_{NL}$ for future GW detectors.

Identification of key quantities affecting the detectability of inflationary anisotropies.

Demonstration of the impact of astrophysical backgrounds on inflationary signal detection.

Abstract

In this work we analyse in detail the possibility of using small and intermediate-scale gravitational wave anisotropies to constrain the inflationary particle content. First, we develop a phenomenological approach focusing on anisotropies generated by primordial tensor-tensor-scalar and purely gravitational non-Gaussianities. We highlight the quantities that play a key role in determining the detectability of the signal. To amplify the power of anisotropies as a probe of early universe physics, we consider cross-correlations with CMB temperature anisotropies. We assess the size of the signal from inflationary interactions against so-called induced anisotropies. In order to arrive at realistic estimates, we obtain the projected constraints on the non-linear primordial parameter $F_{\rm NL}$ for several upcoming gravitational wave probes in the presence of the astrophysical gravitational wave background. We further illustrate our findings by considering a concrete inflationary realisation and use it to underscore a few subtleties in the phenomenological analysis.