One $\mu$ to rule them all: CMB spectral distortions can probe domain walls, cosmic strings and low scale phase transitions

TL;DR

This paper introduces a novel method using CMB spectral distortions to detect gravitational effects of domain walls, cosmic strings, and phase transitions, providing a new window into dark sectors and their anisotropies.

Contribution

It develops an analytic framework to estimate CMB spectral distortions caused by gravitationally coupled anisotropies, applicable to various topological defects and phase transition scenarios.

Findings

Method can probe previously unconstrained parameter space.

Spectral distortions are sensitive to anisotropies in dark sectors.

Complementary to gravitational wave searches.

Abstract

We present a new probe of purely gravitationally coupled sectors with large anisotropies. These anisotropies are damped via gravitational interactions with the baryon-photon fluid, which is heated up in the process. The injected heat causes measurable distortions of the cosmic microwave background spectrum. We give analytic estimates for the size of the distortions and outline how to calculate them from first principles. These methods are applied to anisotropies in the form of a domain wall/cosmic string network or caused by a first order phase transition or scalar field dynamics. We find that this method can potentially probe large regions of previously unconstrained parameter space and is very much complementary to up-coming searches of gravitational waves caused by such dark sectors.