The Rethermalizing Bose-Einstein Condensate of Dark Matter Axions

TL;DR

This paper proposes that axions from the early universe thermalize and form a Bose-Einstein condensate due to gravitational interactions, leading to distinct dark matter properties and explaining galactic caustic rings.

Contribution

It introduces the concept that dark matter axions thermalize into a BEC, providing a new perspective on dark matter behavior and galactic structure.

Findings

Axions thermalize via gravitational self-interactions at ~500 eV.

Axion BEC properties differ from other cold dark matter candidates.

Explains galactic caustic rings as a consequence of axion BEC presence.

Abstract

The axions produced during the QCD phase transition by vacuum realignment, string decay and domain wall decay thermalize as a result of their gravitational self-interactions when the photon temperature is approximately 500 eV. They then form a Bose-Einstein condensate (BEC). Because the axion BEC rethermalizes on time scales shorter than the age of the universe, it has properties that distinguish it from other forms of cold dark matter. The observational evidence for caustic rings of dark matter in galactic halos is explained if the dark matter is axions, at least in part, but not if the dark matter is entirely WIMPs or sterile neutrinos.