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
$\textit{direct collapse}$ of the initial overdensity and via
$\textit{post-collapse accretion}$ 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 $M_\mathrm{BH} \sim 10^{-2} H^{-1} M_\mathrm{Pl}^2$. Post
formation, we find that the PBH undergoes rapid mass growth beyond the
self-similar limit $M_\mathrm{BH}\propto H^{-1}$, 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.
