Threshold for PBH formation in the type-II region and its analytical estimation

Abstract

We numerically simulate the formation of Primordial Black Holes (PBHs) in a radiation-dominated Universe under the assumption of spherical symmetry, driven by the collapse of adiabatic fluctuations, for different curvature profiles $\zeta$. Our results show that the threshold for PBH formation, defined as the peak value of the critical compaction function ${\mathcal{C}}_c(r_m)$ (where $r_m$ is the scale at which the peak occurs), does not necessarily asymptotically saturate to its maximum possible value in the type-I region for sufficiently sharp profiles. Instead, the threshold is found in the type-II region with ${\mathcal{C}}_c(r_m)$ being a minimum. We find, for the cases tested, that this is a general trend associated with profiles that exhibit extremely large curvatures in the linear component of the compaction function ${\mathcal{C}}_l(r)\equiv -4r{\zeta }^{\prime }(r)/3$ shape around its peak $r_m$ (spiky shapes). To measure this curvature at $r_m$, we define a dimensionless parameter, $\kappa \equiv -r_m^2{\mathcal{C}}_l^{\prime \prime }(r_m)$, and we find that the thresholds observed in the type-II region occur for sufficiently large $\kappa$ for the profiles we have used, contrary to expectations. By defining the threshold in terms of ${\mathcal{C}}_{l,c}(r_m)$, we extend previous analytical estimations to the type-II region, which is shown to be accurate within a few percent when compared to the numerical simulations for the tested profiles. Our results suggest that current PBH abundance calculations for models where the threshold lies in the type-II region may have been overestimated due to the general assumption that it should saturate at the boundary between the type-I and type-II regions.

9 pages and 6 figures. v4: minor comments added. Published in Phys. Rev. D

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