Primordial Black Holes from QCD Axion Bubbles
  We propose a scenario in which a strong Peccei-Quinn (PQ) symmetry breaking
in the early universe results in large inhomogeneities of the initial QCD axion
field value, leading to the formation of very dense axion bubbles. Some of the
axion bubbles subsequently collapse into primordial black holes (PBHs). The
spatially homogeneous part of the QCD axion explains dark matter of the
universe, while the PBHs arising from the axion bubbles can explain the LIGO
events or the seed of supermassive black holes. Interestingly, the mass of PBH
is determined by the axion decay constant; for $f_a = 10^{17} (10^{16})$ GeV,
the PBH mass is heavier than about $10 (10^4) M_\odot$. In addition, axion
miniclusters are also formed from the axion bubbles more abundantly than PBHs,
and their masses are expected to be heavier than in the usual scenario based on
the spontaneous breaking of the PQ symmetry after inflation.
