Primordial black hole formation with full numerical relativity

We study the formation of black holes from subhorizon and superhorizon perturbations in a matter dominated universe with 3+1D numerical relativity simulations. We find that there are two primary mechanisms of formation depending on the initial perturbation's mass and geometry via direct collapse of the initial overdensity and via post-collapse accretion of the ambient dark matter. In particular, for the latter case, the initial perturbation does not have to satisfy the hoop conjecture for a black hole to form. In both cases, the duration of the formation the process is around a Hubble time, and the initial mass of the black hole is M-BH similar to 10(-2) (H-1MPI2). Post formation, we find that the PBH undergoes rapid mass growth beyond the self-similar limit M-BH proportional to H-1 at least initially. We argue that this implies that most of the final mass of the PBH is accreted from its ambient surroundings post formation.