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.It has been hypothesized that PBHs may be candidates for black holes and compact objects of various masses in the universe or a major component of dark matter.In particular, PBHs have been attracting much attention in the recent development of gravitational wave observation.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 (Okawa et al., 2014;Yoo et al., 2013) provides simple tools for the simulation of PBH formation (see COSMOS-S for a spherically symmetric version of COSMOS, which is not discussed in this paper).COSMOS is a C++ package for solving the Einstein equations in 3+1 dimensions.It was originally translated from SACRA code (Yamamoto et al., 2008) into C++ and has been developed specifically for the simulation of PBH formation.1 Past publications that use COSMOS for simulation include Yoo et al. (2013), Okawa et al.