Primordial black holes in loop quantum cosmology: The effect on the threshold

TL;DR

This paper estimates how loop quantum cosmology affects primordial black hole formation thresholds, revealing a significant reduction in the critical density perturbation needed, and proposes using PBHs as probes for quantum gravity effects.

Contribution

The study provides the first semi-analytical estimate of PBH formation thresholds within loop quantum cosmology, highlighting quantum effects on early universe black hole formation.

Findings

Quantum effects reduce the PBH formation threshold by up to 50%.

The Barbero-Immirzi parameter influences the critical density perturbation.

Ultra-light PBHs affected by LQG can serve as probes for quantum gravity.

Abstract

Primordial black holes form in the early Universe and constitute one of the most viable candidates for dark matter. The study of their formation process requires the determination of a critical energy density perturbation threshold $\delta_\mathrm{c}$, which in general depends on the underlying gravity theory. Up to now, the majority of analytic and numerical techniques calculate $\delta_\mathrm{c}$ within the framework of general relativity. In this work, using simple physical arguments we estimate semi-analytically the PBH formation threshold within the framework of quantum gravity, working for concreteness within loop quantum cosmology (LQC). In particular, for low mass PBHs formed close to the quantum bounce, we find a reduction in the value of $\delta_\mathrm{c}$ up to $50\%$ compared to the general relativistic regime quantifying for the first time to the best of our knowledge how quantum effects can influence PBH formation within a quantum gravity framework. Finally, by varying the Barbero-Immirzi parameter $\gamma$ of loop quantum gravity (LQG) we show its effect on the value of $\delta_\mathrm{c}$ while using the observational/phenomenological signatures associated to ultra-light PBHs, namely the ones affected by LQG effects, we propose the PBH portal as a novel probe to constrain the potential quantum nature of gravity.