Formation of Relativistic Axion Stars

TL;DR

This paper investigates the formation of relativistic axion stars through numerical simulations and models their distribution, revealing potential gravitational wave detectability and a mass gap between axion stars and black holes.

Contribution

It presents the first numerical relativity simulations of axion star formation with aspherical initial conditions and develops a toy model to predict their distribution in the universe.

Findings

Axion stars can form from gravitational collapse with properties linked to initial mass and decay constant.

Detectable gravitational waves from axion stars are possible for certain axion mass ranges.

A mass gap exists between axion stars and black holes for specific decay constants.

Abstract

Axions and axion-like particles are compelling candidates for the missing dark matter of the universe. As they undergo gravitational collapse, they can form compact objects such as axion stars or even black holes. In this paper, we study the formation and distribution of such objects. First, we simulate the formation of compact axion stars using numerical relativity with aspherical initial conditions that could represent the final stages of axion dark matter structure formation. We show that the final states of such collapse closely follow the known relationship of initial mass and axion decay constant $f_a$. Second, we demonstrate with a toy model how this information can be used to scan a model density field to predict the number densities and masses of such compact objects. In addition to being detectable by the LIGO/VIRGO gravitational wave interferometer network for axion mass of $10^{-9} < m_a < 10^{-11}$ eV, we show using peak statistics that for $f_a < 0.2M_{pl}$, there exists a "mass gap" between the masses of axion stars and black holes formed from collapse.