Primordial black hole formation during first-order phase transitions - art. no. 124014.

Primordial black holes (PBHs) may form in the early universe when pre-exist ing adiabatic density fluctuations enter into the cosmological horizon and recollapse. It has been suggested that PBH formation may be facilitated whe n fluctuations enter into the horizon during a strongly first-order phase t ransition which proceeds in approximate equilibrium. We employ general-rela tivistic hydrodynamics numerical simulations in order Co follow the collaps e of density fluctuations during first-order phase transitions. We find tha t during late stages of the collapse fluctuations separate into two regimes , an inner part existing exclusively in the high-energy density phase with energy density epsilon(h), surrounded by an outer part which exists exclusi vely in the low-energy density phase with energy density epsilon(h)-L, wher e L is the latent heat of the transition. We confirm that the fluctuation d ensity threshold delta epsilon/epsilon required for the formation of PBHs d uring first-order transitions decreases with increasing L and falls below t hat for PBH formation during ordinary radiation dominated epochs. Our resul ts imply that, in case PBHs form at all in the early universe, their mass s pectrum is most likely dominated by the approximate horizon masses during e pochs when the universe undergoes phase transitions. [S0556-2821(99)01010-3 ].