Pre-inflationary QCD axion stars after moduli domination

TL;DR

Early matter domination causes density fluctuations in pre-inflationary QCD axion dark matter, leading to the formation of dense axion stars that could produce observable astrophysical signals.

Contribution

This work demonstrates that early matter domination can induce axion star formation from pre-inflationary axion fluctuations in a string-theory-motivated framework.

Findings

Density fluctuations induce axion star formation at matter-radiation equality.

Axion stars can contain up to 50% of dark matter.

Transient signals at low axion masses could indicate a non-standard cosmological history.

Abstract

The growth of adiabatic density perturbations during an era of early matter domination induces $\mathcal{O}(1)$ fluctuations in pre-inflationary QCD axion dark matter across a broad, string-theory-motivated parameter space. Remarkably, at $\Lambda$CDM matter-radiation equality the scale of these perturbations coincides with the quantum Jeans scale, so they collapse to solitonic ``axion stars''. These axion stars have densities up to $10^4\,\mathrm{eV}^4$, and, including their surrounding halos, they contain as much as $50\%$ of dark matter. Direct searches for a smooth axion background can be suppressed, but transient enhancements or indirect astrophysical signals at axion masses $m_a\lesssim 10^{-5}\,{\rm eV}$ would point to a non-standard cosmological history.