Recent Perspectives on Axion Cosmology

TL;DR

This paper reviews current cosmological constraints on axions, focusing on their production mechanisms, mass ranges, and the viable parameter space for axion dark matter within cosmological and astrophysical bounds.

Contribution

It provides a comprehensive overview of axion cosmology, highlighting the standard thermal production scenario and alternative models with wider mass ranges.

Findings

Axion mass for dark matter is around 100 microelectronvolts with uncertainties.

Cosmological lower bounds allow axion masses below 1-10 meV.

Alternative cosmologies permit a broader, indefinite axion mass range.

Abstract

We review current cosmological constraints on the axion. We describe the basic mechanisms by which axions are created in the early universe, focussing on the standard thermal scenario where the dominant process is through axion radiation by a string network. A dark matter axion in this case would have a mass $\ma \sim 100 \microeV$, with specified large uncertainties. This cosmological lower bound leaves a viable window for the axion below the astrophysical upper limit, $\ma \lapp 1-10 \meV$. We also discuss alternative axion cosmologies which allow a much wider, but indefinite, mass range.