Probing early-universe phase transitions with CMB spectral distortions

TL;DR

This paper investigates how early-universe phase transitions, modeled by the $O(N)$ nonlinear sigma model, can produce spectral distortions in the CMB, providing potential observational signatures of such transitions.

Contribution

It calculates the chemical potential distortion caused by scaling seed networks from early-universe phase transitions, applicable to the $O(N)$ model and similar scenarios.

Findings

Chemical potential distortion $ul \, 2 imes 10^{-9}$ for saturated CMB anisotropy constraints.

Spectral distortions are generated by Silk damping of acoustic waves sourced by metric perturbations.

Results are robust across different scaling seed models.

Abstract

Global, symmetry-breaking phase transitions in the early universe can generate scaling seed networks which lead to metric perturbations. The acoustic waves in the photon-baryon plasma sourced by these metric perturbations, when Silk damped, generate spectral distortions of the cosmic microwave background (CMB). In this work, the chemical potential distortion ($\mu$) due to scaling seed networks is computed and the accompanying Compton $y$-type distortion is estimated. The specific model of choice is the $O(N)$ nonlinear $\sigma$-model for $N\gg 1$, but the results remain the same order of magnitude for other scaling seeds. If CMB anisotropy constraints to the $O(N)$ model are saturated, the resulting chemical potential distortion $\mu \lesssim 2\times 10^{-9}$.