Primordial Black Holes from Axion Domain Wall Collapse
arxiv(2024)
摘要
The QCD axion can solve the Strong CP Problem and be the dark matter of our
universe. If the PQ symmetry breaking scale associated with the axion is below
the inflationary reheating temperature, axion strings and domain walls populate
the universe. Most of these strings and walls decay away into axion dark
matter, but a small subset of the walls will be self-enclosed surfaces that are
not attached to any strings. These enclosed walls can collapse in on
themselves, compressing a large amount of energy into a small volume and
potentially forming primordial black holes (PBHs). We study the number density
and dynamics of these self-enclosed walls, taking into account their size
distribution, Hubble expansion, asphericities, and all stages of domain wall
dynamics using a combination of semi-analytic and numerical approaches. We find
that axion models with a high axion decay constant f_a, such as those of
interest in early matter-dominated cosmologies, yield a PBH abundance
potentially observable by future gravitational lensing surveys. We note that
the formalism developed here is also useful for predicting relic PBH abundances
in other models that exhibit unstable domain walls.
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