Dark Radiation in Spectator Axion-Gauge Models

TL;DR

This paper explores how hidden gauge fields coupled with an axion during inflation can produce dark radiation, affecting gravitational wave signals and offering testable predictions for future cosmological observations.

Contribution

It analyzes the evolution of axion and hidden gauge fields during inflation, highlighting the potential for hidden gauge bosons to contribute to dark radiation and influence gravitational wave signals.

Findings

Hidden gauge bosons can produce significant dark radiation.

Dark radiation constraints can test axion-gauge models.

Subdominant sourced gravitational waves still impact dark radiation.

Abstract

In the framework of axion-gauge fields models, primordial gravitational wave perturbations could be generated during the inflationary epoch from not only the quantum fluctuation of gravitons but also the dynamics of hidden gauge fields coupled with an axion field. We investigate the evolution of the axion and the gauge field of an additional hidden SU(2) gauge group and those energy densities during and after the inflation. We show that the extra radiation component of the hidden gauge bosons produced by the axion decay can be sizable in the cases where the gauge fields sourced additional gravitational waves is subdominant. We point out that future measurements of the dark radiation energy, such as CMB-S4, can impose significant constraints on this cosmological scenario.