Axion Cosmology with Early Matter Domination
  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.
