Constraints on Primordial Black Holes From Big Bang Nucleosynthesis Revisited

Abstract

As space expands, the energy density in black holes increases relative to that of radiation, providing us with motivation to consider scenarios in which the early universe contained a significant abundance of such objects. In this study, we revisit the constraints on primordial black holes derived from measurements of the light element abundances. Black holes and their Hawking evaporation products can impact the era of Big Bang Nucleosynthesis (BBN) by altering the rate of expansion at the time of neutron-proton freeze-out, as well as by radiating mesons which can convert protons into neutrons and vice versa. Such black holes can thus enhance the primordial neutron-to-proton ratio, and increase the amount of helium that is ultimately produced. Additionally, the products of Hawking evaporation can break up helium nuclei, which both reduces the helium abundance and increases the abundance of primordial deuterium. Building upon previous work, we make use of modern deuterium and helium measurements to derive stringent constraints on black holes which evaporate in $t_{\rm evap} \sim 10^{-1}$ s to $\sim 10^{13}$ s (corresponding to $M \sim 6\times 10^8$ g to $\sim 2 \times 10^{13}$ g, assuming Standard Model particle content). We also consider how physics beyond the Standard Model could impact these constraints. Due to the gravitational nature of Hawking evaporation, the rate at which a black hole evaporates, and the types of particles that are produced through this process, depend on the complete particle spectrum. Within this context, we discuss scenarios which feature a large number of decoupled degrees-of-freedom (\ie~large hidden sectors), as well as models of TeV-scale supersymmetry.