Primordial Black Holes without fine-tuning from a light stochastic spectator field

TL;DR

This paper proposes a new mechanism for primordial black hole formation driven by quantum fluctuations of a light spectator field during inflation, avoiding fine-tuning and offering testable observational predictions.

Contribution

It introduces a stochastic spectator field model that naturally produces PBHs with a broad mass distribution without fine-tuning, relying on anthropic selection.

Findings

Produces an extended PBH mass distribution peaking at solar mass scale.

Predicts observable gravitational wave signatures from black hole mergers.

No parameter fine-tuning required, relying on stochasticity and anthropic selection.

Abstract

We investigate a mechanism of primordial black hole (PBH) formation that avoids any dependence on specific inflationary features or exotic physics. In this scenario, the required large curvature fluctuations leading to PBH formation are generated after inflation by the quantum fluctuations of a light stochastic spectator field during inflation, when this field transiently dominates the energy density. We calculate the dynamics of such a spectator field during and after inflation, the distribution of induced curvature perturbations and their non-Gaussian tails leading to the copious production of PBHs. For a plateau-like potential, this scenario produces an extended PBH mass distribution with a peak at the solar-mass scale when one takes into account the effects of the thermal history. What is remarkable in this scenario is the absence of parameter fine-tuning. Instead, it invokes an anthropic selection over all the realizations of PBH abundances predicted by the field stochasticity. This scenario offers a novel perspective for the formation of PBHs with minimal ingredients and without the need of fine-tuning. It is amenable to observational tests, notably with the gravitational-wave observations of black hole mergers and of a background at nanoHertz frequency, as recently observed by pulsar timing arrays.