Axion Cosmology with Early Matter Domination

TL;DR

This paper explores how an early matter-dominated epoch in the universe's history impacts axion cosmology, leading to the formation of axion miniclusters in different parameter ranges and with larger masses, which could inform early universe conditions.

Contribution

It demonstrates that early matter domination alters axion minicluster formation, expanding the parameter space and increasing the maximum minicluster mass, with implications for detection and cosmological history.

Findings

Axion miniclusters can form at lower reheating temperatures below 58 MeV.

The upper bound on minicluster mass increases from 10^{-10} to 10^{-7} solar masses.

Early matter domination affects axion mass range and small-scale inhomogeneity.

Abstract

The default assumption of early universe cosmology is that the postinflationary universe was radiation dominated until it was about 47000 years old. Direct evidence for the radiation dominated epoch extends back until nucleosynthesis, which began during the first second. However there are theoretical reasons to prefer a period of earlier matter domination, prior to nucleosynthesis, e.g. due to late decaying massive particles needed to explain baryogenesis. Axion cosmology is quantitatively affected by an early period of matter domination, with a different axion mass range preferred and greater inhomogeneity produced on small scales. In this work we show that such increased inhomogeneity can lead to the formation of axion miniclusters in axion parameter ranges that are different from those usually assumed. If the reheating temperature is below $58$ MeV, axion miniclusters can form even if the axion field is present during inflation and has been previously homogenized. The upper bound on the typical initial axion minicluster mass is raised from $10^{-10} M_{\odot}$ to $10^{-7} M_{\odot}$, where $M_{\odot}$ is a solar mass. These results may have consequences for indirect detection of axion miniclusters, and could conceivably probe the thermal history of the universe before nucleosynthesis.