Axion-like particle dark matter in the linear regime of structure formation

TL;DR

This paper investigates how axion-like particles (ALPs) as dark matter influence cosmic structure formation in the linear regime, highlighting potential observational differences especially for very light ALPs compared to traditional dark matter models.

Contribution

It models ALPs as a fluid with unique velocity equations and analyzes their impact on structure formation, emphasizing differences from point-like dark matter, particularly for very light ALPs.

Findings

Negligible differences for QCD axions in structure formation

Potential observable effects for very light ALPs

ALP fluid dynamics differ from collisionless dark matter

Abstract

If axion-like particles (ALPs) constitute a major part of dark matter (DM), due to the bosonic nature and a relative small mass, they could behave differently from the point like dark matter particles on the formation of the cosmic structures. When studying the structure formation, it is often useful to consider DM as a special fluid with a given density and a given velocity. ALP fluid obeys a same continuity equation comparing to the point-like collisionless DM but has a different first-order velocity equation. In the linear regime of structure formation, the resulted observational differences are negligible for the QCD axions but can be interesting for very light ALPs.