Axion Stars

TL;DR

This paper reviews the properties of axion stars, hypothetical Bose-Einstein condensates of axions that could constitute dark matter, discussing their formation, characteristics, and implications for dark matter detection.

Contribution

It provides a comprehensive summary of the theoretical understanding of axion stars, including their formation, properties, and potential role in dark matter.

Findings

Axion stars can be gravitationally bound or self-interaction bound.

Low-energy axions are effectively described by a classical nonrelativistic field theory.

Axion stars could constitute a significant fraction of dark matter.

Abstract

The particle that makes up the dark matter of the universe could be an axion or axion-like particle. A collection of axions can condense into a bound Bose-Einstein condensate called an axion star. It is possible that a significant fraction of the axion dark matter is in the form of axion stars. This would make some efforts to identify the axion as the dark matter particle more challenging, but it would also open up new possibilities. We summarize the basic properties of isolated axion stars, which can be gravitationally bound or bound by self-interactions. Axions are naturally described by a relativistic field theory with a real scalar field, but low-energy axions can be described more simply by a classical nonrelativistic effective field theory with a complex scalar field.