Primordial black hole formation by vacuum bubbles II
  The discoveries of LIGO/Virgo black holes in recent years have revitalized
the study of primordial black holes. In this work, we investigate a mechanism
where primordial black holes are formed by vacuum bubbles that randomly
nucleate during inflation through quantum tunneling. After inflation, these
bubbles typically run into the ambient radiation fluid with a large Lorentz
factor. In our previous work, we assumed the bubble fields are strongly coupled
to the standard model particles so that the bubble wall is impermeable. Here we
complete this picture by considering bubbles interacting with the fluid only
through gravity. By studying the scenario in several limits, we found that
black holes could form in either subcritical or supercritical regime. Depending
on the model parameters, the resulting mass spectrum of the black holes could
be wide or narrow, and may develop two peaks separated by a large mass range.
With different spectra, these black holes may account for the LIGO/Virgo black
holes, supermassive black holes, and may play an important role in dark matter.
