Bose Einstein condensation of the classical axion field in cosmology?

TL;DR

This paper investigates whether gravitational interactions can cause axions, a dark matter candidate, to form a Bose-Einstein condensate during early universe structure formation, challenging previous thermalisation estimates.

Contribution

The study analyzes the entropy generation of axions via gravitational interactions during linear structure formation, providing a new perspective on their thermalisation process.

Findings

Gravitational interactions at linear order do not confirm rapid thermalisation of axions.

The anisotropic stress indicates a short dissipation scale for axions.

Previous estimates of gravitational thermalisation rates are not supported.

Abstract

The axion is a motivated cold dark matter candidate, which it would be interesting to distinguish from weakly interacting massive particles. Sikivie has suggested that axions could behave differently during non-linear galaxy evolution, if they form a Bose-Einstein condensate, and argues that ``gravitational thermalisation'' drives them to a Bose-Einstein condensate during the radiation dominated era. Using classical equations of motion during linear structure formation, we explore whether the gravitational interactions of axions can generate entropy. At linear order in Newton's constant, we interpret that the principle activities of gravity are to expand the Universe and grow density fluctuations. To quantify the rate of entropy creation we use the anisotropic stress to estimate a short dissipation scale for axions, which does not confirm previous estimates of their gravitational thermalisation rate.