Evolution of primordial black holes in an adiabatic FLRW universe with gravitational particle creation

TL;DR

This paper models the evolution of primordial black holes in an adiabatic FLRW universe considering gravitational particle creation, analyzing accretion and evaporation effects on PBH mass over cosmic time.

Contribution

It provides an analytic solution for PBH mass evolution during radiation era with accretion and establishes bounds on accretion efficiency considering gravitational particle creation.

Findings

PBH mass increases rapidly due to radiation accretion early on

Accretion rate decreases as the universe evolves

Hawking evaporation eventually dominates, reducing PBH mass

Abstract

We study the evolution of primordial black holes (PBHs) in an adiabatic FLRW universe with dissipation due to bulk viscosity which is considered to be in the form of gravitational particle creation. Assuming that the process of evaporation is quite suppressed during the radiation era, we obtain an analytic solution for the evolution of PBH mass by accretion during this era, subject to an initial condition. We also obtain an upper bound on the accretion efficiency $\epsilon$ for $a \sim a_r$, where $a_r$ is the point of transition from the early de Sitter era to the radiation era. Furthermore, we obtain numerical solutions for the mass of a hypothetical PBH with initial mass 100 g assumed to be formed at an epoch when the value of the Hubble parameter was, say, 1 km/s/Mpc. We consider three values of the accretion efficiency, $\epsilon=0.23,0.5$, and $0.89$ for our study. The analysis reveals that the mass of the PBH increases rapidly due to the accretion of radiation in the early stages of its evolution. The accretion continues but its rate decreases gradually with the evolution of the Universe. Finally, Hawking radiation comes into play and the rate of evaporation surpasses the accretion rate so that the PBH mass starts to decrease. As the Universe grows, evaporation becomes the dominant phenomenon, and the mass of the PBH decreases at a faster rate. As argued by Debnath and Paul, the evaporated mass of the PBHs might contribute towards the dark energy budget of the late Universe.