Axions: Bose Einstein Condensate or Classical Field?

TL;DR

This paper clarifies that axion dark matter is inherently a classical field, not a Bose Einstein condensate, and that the condensation process is negligible, resolving confusion in the literature.

Contribution

It demonstrates that axion dark matter from the misalignment mechanism is a classical field, negating the need for Bose Einstein condensation as a separate process.

Findings

Axion dark matter is a classical field, not a BEC.

Condensation or evaporation rates of axions are negligible.

Features attributed to BEC are inherent in the classical field description.

Abstract

The axion is a motivated dark matter candidate, so it would be interesting to find features in Large Scale Structures specific to axion dark matter. Such features were proposed for a Bose Einstein condensate of axions, leading to confusion in the literature (to which I contributed) about whether axions condense due to their gravitational interactions. This note argues that the Bose Einstein condensation of axions is a red herring: the axion dark matter produced by the misalignment mechanism is already a classical field, which has the distinctive features attributed to the axion condensate (BE condensates are described as classical fields). This note also estimates that the rate at which axion particles condense to the field, or the field evaporates to particles, is negligeable.