Dark matter axions

TL;DR

This paper discusses the role of axions as a dark matter candidate, highlighting their unique properties like Bose-Einstein condensation and potential explanations for observed cosmic phenomena.

Contribution

It reviews recent developments showing axions form a BEC and may explain certain astrophysical observations not accounted for by standard cold dark matter.

Findings

Axions can thermalize and form a Bose-Einstein condensate.

Axion BEC may produce net rotation in dark matter halos.

Axion properties could explain cosmic microwave anisotropy alignments.

Abstract

The hypothesis of an `invisible' axion was made by Misha Shifman and others, approximately thirty years ago. It has turned out to be an unusually fruitful idea, crossing boundaries between particle physics, astrophysics and cosmology. An axion with mass of order $10^{-5}$ eV (with large uncertainties) is one of the leading candidates for the dark matter of the universe. It was found recently that dark matter axions thermalize and form a Bose-Einstein condensate (BEC). Because they form a BEC, axions differ from ordinary cold dark matter (CDM) in the non-linear regime of structure formation and upon entering the horizon. Axion BEC provides a mechanism for the production of net overall rotation in dark matter halos, and for the alignment of cosmic microwave anisotropy multipoles. Because there is evidence for these phenomena, unexplained with ordinary CDM, an argument can be made that the dark matter is axions.