Spins of primordial black holes formed in the matter-dominated phase of the Universe

Abstract

Angular momentum plays very important roles in the formation of PBHs in the matter-dominated phase if it lasts sufficiently long. In fact, most collapsing masses are bounced back due to centrifugal force, since angular momentum significantly grows before collapse. As a consequence, most of the formed PBHs are rapidly rotating near the extreme value $a_{*}=1$, where $a_{*}$ is the nondimensional Kerr parameter at their formation. The smaller the density fluctuation $\sigma_{H}$ at horizon entry is, the stronger the tendency towards the extreme rotation. Combining the effect of angular momentum with that of anisotropy, we estimate the black hole production rate. We find that the production rate suffers from suppression dominantly due to angular momentum for a smaller value of $\sigma_{H}$, while due to anisotrpopy for a larger value of $\sigma_{H}$. We argue that matter domination significantly enhances the production of PBHs despite the suppression. If the matter-dominated phase does not last so long, the effect of the finite duration significantly suppresses PBH formation and weakens the tendency towards large spins. (abridged)