Do Dark Matter Axions Form a Condensate with Long-Range Correlation?

TL;DR

This paper critically examines the claim that dark matter axions form a long-range correlated Bose-Einstein condensate, concluding that due to attractive interactions, such long-range order is unlikely.

Contribution

It clarifies the difference between long-range order from repulsive interactions and localized clumps from attractive interactions in axion condensates.

Findings

Axions primarily have attractive interactions leading to localized structures.

Long-range correlation in axion condensates is unlikely due to attractive interactions.

Theoretical analysis emphasizes the importance of field theory dynamics over Boltzmann equations.

Abstract

Recently there has been significant interest in the claim that dark matter axions gravitationally thermalize and form a Bose-Einstein condensate with cosmologically long-range correlation. This has potential consequences for galactic scale observations. Here we critically examine this claim. We point out that there is an essential difference between the thermalization and formation of a condensate due to repulsive interactions, which can indeed drive long-range order, and that due to attractive interactions, which can lead to localized Bose clumps (stars or solitons) that only exhibit short range correlation. While the difference between repulsion and attraction is not present in the standard collisional Boltzmann equation, we argue that it is essential to the field theory dynamics, and we explain why the latter analysis is appropriate for a condensate. Since the axion is primarily governed by attractive interactions -- gravitation and scalar-scalar contact interactions -- we conclude that while a Bose-Einstein condensate is formed, the claim of long-range correlation is unjustified.