Dynamics of primordial black hole formation - art. no. 124013.

We present a numerical investigation of the gravitational collapse of horiz on-size density fluctuations to primordial black holes (PBHs) during the ra diation-dominated phase of the early Universe. The collapse dynamics of thr ee different families of initial perturbation shapes, imposed at the time o f horizon crossing, is computed. The perturbation threshold for black hole formation, needed for estimations of the cosmological PBH mass function, is found to be delta(c) approximate to 0.7 rather than the generally employed delta(c) approximate to 1/3 if delta is defined as Delta M/M-h, the relati ve excess mass within the initial horizon volume. In order to study the acc retion onto the newly formed black holes, we use a numerical scheme that al lows us to follow the evolution for long times after formation of the event horizon. In general, small black holes (compared to the horizon mass at: t he onset of the collapse) give rise to a fluid bounce that effectively shut s off accretion onto the black hole, while large ones do not. In both cases , the growth of the black hale mass owing to accretion is insignificant. Fu rthermore, the scaling of black hole mass with the distance from the format ion threshold, known to occur in near-critical gravitational collapse, is d emonstrated to apply to primordial black hole formation. [S0556-2821(99)073 10-5].