Thermal axion constraints in non-standard thermal histories

TL;DR

This paper explores how non-standard thermal histories, like low-temperature reheating and kination, affect cosmological constraints on thermally-produced axions, showing that such histories can significantly loosen existing bounds.

Contribution

It analyzes the impact of non-standard thermal histories on axion constraints, highlighting scenarios where these constraints are relaxed compared to standard cosmology.

Findings

Axion constraints are loosened in low-temperature-reheating scenarios.

Entropy generation prior to radiation domination suppresses axion abundance.

Kination scenarios cause modest changes to axion bounds.

Abstract

There is no direct evidence for radiation domination prior to big-bang nucleosynthesis, and so it is useful to consider how constraints to thermally-produced axions change in non-standard thermal histories. In the low-temperature-reheating scenario, radiation domination begins at temperatures as low as 1 MeV, and is preceded by significant entropy generation. Axion abundances are then suppressed, and cosmological limits to axions are significantly loosened. In a kination scenario, a more modest change to axion constraints occurs. Future possible constraints to axions and low-temperature reheating are discussed.