Masaaki Shimada | Searcharxiv

arXiv2026

COSMOS: A numerical relativity code specialized for PBH formation

Primordial black holes (PBHs) are black holes generated in the early universe without having gone through stellar evolution. In the standard formation process, PBHs are formed from super-horizon primordial fluctuations with non-linearly large initial amplitude. In order to simulate the non-linear gravitational dynamics of PBH formation, one has to rely on numerical relativity solvers to approximate the solution of the Einstein equations. COSMOS is a C++ package for solving the Einstein equations in 3+1 dimensions, providing simple tools for the simulation of PBH formation. In order to resolve the collapsing region, non-Cartesian scale-up coordinates and a fixed mesh-refinement procedure are implemented. In COSMOS, a massless scalar field and a perfect fluid with a linear equation of state are implemented as matter fields. To achieve a practically acceptable computational speed, OpenMP is used for the parallelization. COSMOS has no other dependencies, which makes for an easier installation.

gr-qc↗

arXiv2024

Primordial Black Hole Formation from Type II Fluctuations with Primordial Non-Gaussianity

This study investigates the formation of primordial black holes (PBHs) resulting from the collapse of adiabatic fluctuations with large amplitudes and non-Gaussianity. Ref. \cite{Uehara:2024yyp} showed that fluctuations with large amplitudes lead to the formation of type B PBHs, characterized by the existence of the bifurcating trapping horizons, distinct from the more common type A PBHs without a bifurcating trapping horizon. We focus on the local type non-Gaussianity characterized by the curvature perturbation $ζ$ given by a function of a Gaussian random variable $ζ_{\rm G}$ as $βζ=-\ln(1-βζ_{\rm G})$ with a parameter $β$. Then we examine how the non-Gaussianity influences the dynamics and the type of PBH formed, particularly focusing on type II fluctuations, where the areal radius varies non-monotonically with the coordinate radius. Our findings indicate that, for $β>-2$, the threshold for distinguishing between type A and type B PBHs decreases with increasing $β$ similarly to the threshold for black hole formation. Additionally, for large positive values of $β$, the threshold for type B PBHs approaches that for type II fluctuations. We also find that, for a sufficiently large negative value of $β\lesssim-4.0$, the threshold value is in the type II region of $μ$, i.e., there are fluctuations of type II that do not form black holes. Lastly, we calculate the PBH mass for several values of $β$. Then we observe that the final mass monotonically increases with the initial amplitude within the parameter region of type A PBHs, which differs from previous analytical expectations.

gr-qc↗