  In theories with a broken discrete symmetry, Hubble sized spherical domain
walls may spontaneously nucleate during inflation. These objects are
subsequently stretched by the inflationary expansion, resulting in a broad
distribution of sizes. The fate of the walls after inflation depends on their
radius. Walls smaller than a critical radius fall within the cosmological
horizon early on and collapse due to their own tension, forming ordinary black
holes. But if a wall is large enough, its repulsive gravitational field becomes
dominant much before the wall can fall within the cosmological horizon. In this
"supercritical" case, a wormhole throat develops, connecting the ambient
exterior FRW universe with an interior baby universe, where the exponential
growth of the wall radius takes place. The wormhole pinches off in a time-scale
comparable to its light-crossing time, and black holes are formed at its two
mouths. As discussed in previous work, the resulting black hole population has
a wide distribution of masses and can have significant astrophysical effects.
The mechanism of black hole formation has been previously studied for a
dust-dominated universe. Here we investigate the case of a radiation-dominated
universe, which is more relevant cosmologically, by using numerical simulations
in order to find the initial mass of a black hole as a function of the wall
size at the end of inflation. For large supercritical domain walls, this mass
nearly saturates the upper bound according to which the black hole cannot be
larger than the cosmological horizon. We also find that the subsequent
accretion of radiation satisfies a scaling relation, resulting in a mass
increase by about a factor of 2.
