The cold axion populations

TL;DR

This paper systematically discusses the different sources of cold axions in the universe, their contributions to cosmological energy density, and their velocity dispersions, highlighting uncertainties in axion mass estimates.

Contribution

It provides a comprehensive analysis of axion populations from vacuum realignment, string decay, and wall decay, emphasizing their velocity differences and mass estimates.

Findings

Axions contribute significantly to the cosmological energy density.

Two groups of cold axions differ in velocity dispersion by a factor of about 1000.

Estimated axion mass for critical density is around 6 x 10^{-6} eV with large uncertainties.

Abstract

We give a systematic discussion of the contributions to the cosmological energy density in axions from vacuum realignment, string decay and wall decay. We call these the cold axion populations because their kinetic energy per particle is at all times much less than the ambient temperature. In case there is no inflation after the Peccei-Quinn phase transition, the value of the axion mass for which axions contribute the critical energy density for closure is estimated to be of order $6 \cdot 10^{-6}$ eV, with large uncertainties. It is emphasized that there are two groups of cold axions differing in velocity dispersion by a factor of order $10^3$.