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 ∼10-2 H -1 M Pl2. Post formation, we find that the PBH undergoes rapid mass growth beyond the self-similar limit M BH α 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.