Accreting Primordial Black Holes: Dark Matter Constituents

Abstract

We show scenarios in which primordial black hole accretion under the magnetorotational instability (MRI) uniquely relates the density of the early Universe to the abundance of present day dark matter. We demonstrate via long duration general relativistic magnetohydrodynamic (GRMHD) simulations that MRI-dominated accretion at least hundreds of gravitational radii from black holes can occur under conditions expected in the Positronium Era. We thereby identify that the positronium plasma that existed 0.01 s to 14 s into the Big Bang can serve as the primary source of mass that augmented primordial black hole seeds to 10^16-10^17g black holes contributing to dark matter today. This population of black holes, in turn, radiates in a manner consistent with the observed gamma ray background. At a time of uncertainty about the role of new kinds of particles, the better understood primordial black hole MRI accretion process may be the best way to pin down how much dark matter mass lies behind horizons versus new dark sector particles.