Revisiting signatures of thermal axions in nonstandard cosmologies

TL;DR

This paper examines how nonstandard cosmologies affect the thermal production of hadronic axions, updating interaction rates and deriving new constraints on axion properties in various early universe scenarios.

Contribution

It provides the first constraints on axion thermalization in early matter-dominated cosmologies and refines bounds in low-temperature reheating models.

Findings

Thermalization of axions can be delayed to higher masses in nonstandard cosmologies.

Improved constraints on axion masses around the eV scale in low-temperature reheating scenarios.

First constraints on axion thermalization in early matter-dominated cosmologies.

Abstract

We revisit the formation of a thermal population of hadronic axions in nonstandard cosmologies, in light of the recent developments in obtaining continuous and smooth interaction rates for both the gluon and photon couplings. For certain cosmological histories, such as low-temperature reheating (LTR) and kination-like scenarios, the thermalization of the axion can be severely delayed to higher masses. In the case that thermal equilibrium is achieved, we improve the constraints on LTR for axion masses around the eV scale with respect to previous works and we constrain for the first time early matter-dominated (EMD) cosmologies. We also point out the possibility of having the co-existence of cold and warm dark matter populations of axions in kination-like scenarios in the eV mass range.