Bose-Einstein Condensation of Dark Matter Axions

TL;DR

This paper demonstrates that dark matter axions can thermalize and form a Bose-Einstein condensate, leading to unique effects on structure formation and cosmic anisotropies.

Contribution

It provides a theoretical framework showing axions form a BEC in the universe and explores implications for cosmic structure and anisotropy alignment.

Findings

Axions thermalize and form a Bose-Einstein condensate in the early universe.

Axion BEC affects non-linear structure formation.

Mechanism for rotation and alignment in dark matter halos and cosmic microwave anisotropies.

Abstract

We show that cold dark matter axions thermalize and form a Bose-Einstein condensate. We obtain the axion state in a homogeneous and isotropic universe, and derive the equations governing small axion perturbations. Because they form a BEC, axions differ from ordinary cold dark matter 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.