  Vacuum bubbles may nucleate and expand during the inflationary epoch in the
early universe. After inflation ends, the bubbles quickly dissipate their
kinetic energy; they come to rest with respect to the Hubble flow and
eventually form black holes. The fate of the bubble itself depends on the
resulting black hole mass. If the mass is smaller than a certain critical
value, the bubble collapses to a singularity. Otherwise, the bubble interior
inflates, forming a baby universe, which is connected to the exterior FRW
region by a wormhole. A similar black hole formation mechanism operates for
spherical domain walls nucleating during inflation. As an illustrative example,
we studied the black hole mass spectrum in the domain wall scenario, assuming
that domain walls interact with matter only gravitationally. Our results
indicate that, depending on the model parameters, black holes produced in this
scenario can have significant astrophysical effects and can even serve as dark
matter or as seeds for supermassive black holes. The mechanism of black hole
formation described in this paper is very generic and has important
implications for the global structure of the universe. Baby universes inside
super-critical black holes inflate eternally and nucleate bubbles of all vacua
allowed by the underlying particle physics. The resulting multiverse has a very
non-trivial spacetime structure, with a multitude of eternally inflating
regions connected by wormholes. If a black hole population with the predicted
mass spectrum is discovered, it could be regarded as evidence for inflation and
for the existence of a multiverse.
