Axions in the Early Universe

TL;DR

This paper reviews axion production in the early universe, improves calculation methods, and updates cosmological bounds on the QCD axion mass, highlighting its potential observable signatures.

Contribution

It introduces improved calculations for axion production rates and applies them to refine cosmological bounds on the QCD axion mass.

Findings

Enhanced accuracy in axion production rate calculations

Updated constraints on QCD axion mass from cosmology

Predicted observable signatures in cosmological data

Abstract

The Peccei-Quinn solution to the strong CP problem provides a motivated framework rich in cosmological consequence. Thermal axion production is unavoidable if there is a thermal bath at early times. Scattering and decay processes of bath particles can dump relativistic axions in the primordial plasma, and they can leave observable signatures in cosmological observables probing both the early and the late universe if produced with a significant abundance. We present recent and significant improvements for the calculation of the axion production rate for different scenarios and apply these results to predict the abundance of produced axions. Finally, we provide updated cosmological bounds on the QCD axion mass.