Ultra-fast growth of primordial black holes through radiative absorption

TL;DR

This paper demonstrates that primordial black holes can rapidly grow in the early universe through radiative absorption, potentially explaining the formation of supermassive black holes shortly after the Big Bang.

Contribution

It introduces a full Stefan-Boltzmann law-based model of PBH growth via radiative absorption, highlighting a new dominant growth mechanism in the early universe.

Findings

PBHs can grow from $10^6 M_\u2299$ to $10^{10} M_a$ within about 58 days.

Radiative absorption dominates PBH growth when environment temperature exceeds PBH temperature.

The model provides a natural explanation for the early formation of SMBHs across different mass scales.

Abstract

We show that Schwarzschild primordial black holes (PBHs) formed in the radiation-dominated era can grow extremely rapidly through $\textit{radiative absorption}$ governed by the full Stefan-Boltzmann law. By introducing a principle of isonomy - ensuring identical particle species dependence for Hawking emission and absorption - we find that, whenever the temperature of the PBH environment is larger than the PBH horizon temperature, PBHs generically gain mass. In particular, for PBH masses following the critical collapse mass-scaling law with critical exponent $\gamma_\mathrm{crit}$, with $\gamma_\mathrm{crit} \in (0.33, 0.49)$, the aforementioned radiative absorption mass growth mechanism produces a striking effect: PBHs forming with a mass $10^6M_\odot$ during BBN can reach $\mathcal{O}(10^{10} M_\odot)$ within $\mathcal{O}(10^{6} \mathrm{s})$ ($\sim $ 58 days). Interestingly enough, small deviations from $\gamma_\mathrm{crit}$, depending itself on the number of relativistic species present in the primordial plasma, yield a continuous PBH mass spectrum providing us ultimately with a single, Standard-Model-based explanation for the origin of stellar-mass, intermediate-mass, and supermassive black holes (SMBHs), and naturally accounting for the early appearance of SMBHs. The Schwarzschild treatment presented here can be extended to spherically symmetric cosmological black holes, indicating that radiative absorption is a dominant and previously overlooked PBH growth channel in the early Universe.