Primordial black hole formation by vacuum bubbles
  Vacuum bubbles may nucleate during the inflationary epoch and expand,
reaching relativistic speeds. After inflation ends, the bubbles are quickly
slowed down, transferring their momentum to a shock wave that propagates
outwards in the radiation background. The ultimate fate of the bubble depends
on its size. Bubbles smaller than certain critical size collapse to ordinary
black holes, while in the supercritical case the bubble interior inflates,
forming a baby universe, which is connected to the exterior region by a
wormhole. The wormhole then closes up, turning into two black holes at its two
mouths. We use numerical simulations to find the masses of black holes formed
in this scenario, both in subcritical and supercritical regime. The resulting
mass spectrum is extremely broad, ranging over many orders of magnitude. For
some parameter values, these black holes can serve as seeds for supermassive
black holes and may account for LIGO observations.
