Axion Cosmology

TL;DR

This paper surveys the cosmological implications of axions, detailing thermal production, the evolution of the axion field, and the domain-wall issue, followed by the formation and properties of cold axions, axion miniclusters, and isocurvature constraints. It analyzes two cosmological scenarios—PQ symmetry breaking before or after inflation—elucidating how vacuum realignment, string decay, and wall decay contribute to the cold axion density and velocity dispersion, and it discusses resolutions to the domain-wall problem (N=1, inflation, or small PQ breaking). The work provides approximate relic densities and velocity dispersions, highlights the role of axion miniclusters in structure formation and detection, and derives constraints on inflationary scales from isocurvature bounds. Overall, the results have direct implications for axion dark matter searches and the possible clumping of dark matter in the form of miniclusters.

Abstract

Outline: 1. Thermal axions 2. Axion field evolution 3. The domain wall problem 4. Cold axions 5. Axion miniclusters 6. Axion isocurvature perturbations

Figures (1)

• Figure 1: Processes which produce thermal axions in the early universe