Axion constraints in non-standard thermal histories

TL;DR

This paper explores how non-standard thermal histories, like low-temperature reheating and kination, can relax cosmological constraints on axion properties, affecting their viability as dark matter candidates.

Contribution

It introduces and analyzes the impact of non-standard thermal histories on axion relic abundance and cosmological constraints, expanding the understanding beyond the standard radiation-dominated model.

Findings

Reheating temperatures below 35 MeV lift the large-scale structure limit on axion mass.

Non-standard histories significantly relax cosmological constraints on axions.

Future observations could further constrain or detect axions in these scenarios.

Abstract

It is usually assumed that dark matter is produced during the radiation dominated era. There is, however, no direct evidence for radiation domination prior to big-bang nucleosynthesis. Two non-standard thermal histories are considered. In one, the low-temperature-reheating scenario, radiation domination begins as late as 1 MeV, and is preceded by significant entropy generation. Thermal axion relic abundances are then suppressed, and cosmological limits to axions are loosened. For reheating temperatures less than 35 MeV, the large-scale structure limit to the axion mass is lifted. The remaining constraint from the total density of matter is significantly relaxed. Constraints are also relaxed for higher reheating temperatures. In a kination scenario, a more modest change to cosmological axion constraints is obtained. Future possible constraints to axions and low-temperature reheating from the helium abundance and next-generation large-scale-structure surveys are discussed.