Miniclusters in the Axiverse

TL;DR

This paper explores the formation, mass range, and properties of axion-like particle miniclusters, showing they can be very light or up to hundreds of solar masses, depending on formation scenarios and cosmological parameters.

Contribution

It provides a comprehensive analysis of minicluster masses and formation mechanisms, including PQ symmetry breaking and hidden sector phase transitions, expanding understanding of axion dark matter structures.

Findings

Miniclusters can range from very light to hundreds of solar masses.

Formation is possible from PQ symmetry breaking fluctuations.

Alternative formation via hidden sector phase transitions.

Abstract

If dark matter is an axion-like-particle a significant fraction of the present day relic abundance could be concentrated in compact gravitationally bound miniclusters. We study the minicluster masses compatible with the dark matter relic density constraint. If they form from fluctuations produced by PQ symmetry breaking, minicluster masses up to hundreds of solar masses are possible, although over most of the parameter space they are much lighter. The size of these objects is typically within a few orders of magnitude of an astronomical unit. We also show that miniclusters can form if an axion gets mass from a hidden sector with a first order phase transition that takes a relatively long time to complete. Therefore they can appear in models where PQ symmetry is broken before inflation, compatible with large axion decay constants and string theory UV completions.