Isotropic background and anisotropies of gravitational waves induced by cosmological soliton isocurvature perturbations

TL;DR

This paper studies the isotropic background and anisotropies of gravitational waves caused by cosmological soliton isocurvature perturbations, highlighting the role of non-Gaussianity and proposing new observational signatures.

Contribution

It derives the universal shape of the GW energy-density spectrum and the angular power spectrum of anisotropies, emphasizing the impact of non-Gaussianity and the distinct lack of CMB cross-correlation.

Findings

The GW spectrum has a universal shape within the perturbative regime.

Non-Gaussianity significantly influences GW anisotropies.

Isocurvature-induced GWs are nearly uncorrelated with the CMB.

Abstract

Cosmological solitons are widely predicted by scenarios of the early Universe. In this work, we investigate the isotropic background and anisotropies of gravitational waves (GWs) induced by soliton isocurvature perturbations, especially considering the effects of non-Gaussianity in these perturbations. Regardless of non-Gaussianity, the energy-density fraction spectrum of isocurvature-induced GWs approximately has a universal shape within the perturbative regime, thus serving as a distinctive signal of solitons. We derive the angular power spectrum of isocurvature-induced GWs to characterize their anisotropies. Non-Gaussianity plays a key role in generating anisotropies through the couplings between large- and small-scale isocurvature perturbations, making the angular power spectrum to be a powerful probe of non-Gaussianity. Moreover, the isocurvature-induced GWs have nearly no cross-correlations with the cosmic microwave background, providing a new observable to distinguish them from other GW sources, e.g., GWs induced by cosmological curvature perturbations enhanced at small scales. Therefore, detection of both the isotropic background and anisotropies of isocurvature-induced GWs could reveal important implications for the solitons as well as the early Universe.