Axion dark matter with thermal friction

TL;DR

This paper investigates how thermal friction affects axion dark matter evolution, revealing that it can relax constraints on axion parameters, alter density perturbations, and impact axion minicluster formation, expanding viable models.

Contribution

It introduces the role of thermal friction in axion cosmology, showing its effects on relic density, perturbations, and string dynamics, which broadens the parameter space for axion dark matter models.

Findings

Thermal friction causes exponential decay of axion energy before oscillation.

It relaxes overclosure constraints, allowing lighter axions without tuning.

Thermal friction modifies axion density perturbations and minicluster sizes.

Abstract

Cosmological evolution of axion field in the early universe might be significantly affected by a thermal friction induced by the axion coupling to thermalized hidden sector. We examine the effects of such a thermal friction on axion dark matter density and its perturbation when the thermal friction dominates over the Hubble friction until when the axion field begins to oscillate around the potential minimum. We show that in the presence of sizable thermal friction there can be an exponential decay phase of the axion field before the oscillation phase, during which the axion energy density is efficiently dissipated into hidden thermal bath. Consequently, the previously excluded parameter region due to overclosing relic axion density becomes cosmologically viable with thermal friction. In particular, a QCD axion much lighter than $\mu$eV is viable without tuning the initial misalignment angle. We also find that thermal friction can affect the density perturbation of axion dark matter in various ways. For instance, it can alleviate the large-scale isocurvature bound on axion dark matter in the pre-inflationary PQ breaking scenario, which would make the pre-inflationary axion dark matter compatible with high scale inflation over a wide range of model parameters. In the post-inflationary PQ breaking scenario, thermal friction can also significantly change the scaling behavior of axionic strings, and therefore the typical size of the resultant axion miniclusters.