ALP Dark Matter in a Primordial Black Hole Dominated Universe
  We investigate the phenomenological consequences of axion-like particle (ALP)
dark matter with an early matter domination triggered by primordial black holes
(PBHs). We focus on light BHs with masses smaller than $\sim 10^9~$g which
fully evaporate before Big Bang nucleosynthesis. We numerically solve the
coupled Boltzmann equations, carefully taking the greybody factors and BH
angular momentum into account. We find that the entropy injection from PBH
evaporation dilutes the ALP relic abundance originally produced via the vacuum
misalignment mechanism, opening the parameter space with larger scales $f_a$
or, equivalently, smaller ALP-photon couplings $g_{a\gamma}$, within the reach
of future detectors as ABRACADABRA, KLASH, ADMX, and DM-Radio. Moreover, the
ALP minicluster masses can be several orders of magnitude larger if the early
Universe features an PBH dominated epoch. For the relativistic ALPs produced
directly from Hawking radiation, we find that their contribution to the dark
radiation is within the sensitivity of next generation CMB experiments. For the
sake of completeness, we also revisit the particular case of the QCD axion.
