Axion cold dark matter in non-standard cosmologies

TL;DR

This paper explores how non-standard early universe cosmologies affect the parameter space and properties of axion cold dark matter, highlighting potential observational probes of pre-BBN cosmological history.

Contribution

It analyzes axion dark matter in Low Temperature Reheating and kination cosmologies, revealing how these scenarios alter allowed parameter ranges and potential observational signatures.

Findings

LTR cosmology allows more axion parameter space if PQ symmetry breaks during inflation

Kination cosmology restricts axion parameter space compared to standard cosmology

Axion velocity dispersion can distinguish between different pre-BBN cosmologies

Abstract

We study the parameter space of cold dark matter axions in two cosmological scenarios with non-standard thermal histories before Big Bang nucleosynthesis: the Low Temperature Reheating (LTR) cosmology and the kination cosmology. If the Peccei-Quinn symmetry breaks during inflation, we find more allowed parameter space in the LTR cosmology than in the standard cosmology and less in the kination cosmology. On the contrary, if the Peccei-Quinn symmetry breaks after inflation, the Peccei-Quinn scale is orders of magnitude higher than standard in the LTR cosmology and lower in the kination cosmology. We show that the axion velocity dispersion may be used to distinguish some of these non-standard cosmologies. Thus, axion cold dark matter may be a good probe of the history of the Universe before Big Bang nucleosynthesis.