Relaxation times for Bose-Einstein condensation in axion miniclusters

TL;DR

This paper derives kinetic equations for scalar dark matter condensation into Bose stars, showing that gravitational and self-interactions can lead to Bose-Einstein condensation within the universe's lifetime, with self-interactions only slightly affecting the timescale.

Contribution

It introduces a new formalism for kinetic equations that accurately describe gravitational and self-interactions in high-occupancy scalar dark matter regimes.

Findings

Condensation into Bose stars can occur within the universe's age.

Self-interactions only slightly reduce the condensation time.

Derived kinetic equations are valid for high-occupancy regimes of scalar dark matter.

Abstract

We study the Bose condensation of scalar dark matter in the presence of both gravitational and self-interactions. Axions and other scalar dark matter in gravitationally bound miniclusters or dark matter halos are expected to condense into Bose-Einstein condensates called Bose stars. This process has been shown to occur through attractive self-interactions of the axion-like particles or through the field's self gravitation. We show that in the high-occupancy regime of scalar dark matter, the Boltzmann collision integral does not describe either gravitaitonal or self-interactions, and derive kinetic equations valid for these interactions. We use this formalism to compute relaxation times for the Bose-Einstein condensation, and find that condensation into Bose stars could occur within the lifetime of the universe. The self-interactions reduce the condensation time only when they are very strong.