Primordial Black Holes from Inflation with a Spectator Field

TL;DR

This paper investigates how a spectator scalar field influences primordial black hole production during inflation, revealing that it can enhance perturbations and alter the typical ultra-slow roll dynamics, thus affecting PBH formation.

Contribution

It demonstrates that a spectator field can prevent ultra-slow roll conditions and still promote PBH formation through curvature and isocurvature perturbation interplay.

Findings

Spectator fields can prevent USR but still enhance PBH-related perturbations.

The model shows a degeneracy allowing compatibility with CMB constraints and PBH production.

Perturbation growth is maintained or enhanced via isocurvature to curvature transfer.

Abstract

How is the production of primordial black holes (PBHs) in single-field models of inflation impacted by the presence of additional scalar fields? We consider the effect of a spectator field - a free scalar field with sub-Hubble mass, no direct coupling to the inflaton, and which makes a subdominant contribution to the total energy density - in the context of single-field models of inflation featuring a transient phase of ultra-slow roll (USR) evolution. Despite the modest title, a spectator field can have a dramatic impact: the slow-roll evolution of the spectator prevents the combined inflaton-and-spectator system from entering into USR, which naively might be expected to preclude the production of PBHs. However, we demonstrate that the growth of perturbations is maintained or enhanced by the spectator, through the rich interplay of curvature and isocurvature perturbations. We show in a model-independent way that the single-field phase of ultra-slow-roll is replaced by two turns in field space encompassing a phase of tachyonic instability for the isocurvature perturbations and a transfer of power from isocurvature to curvature modes. Furthermore, we highlight a degeneracy between the fine-tuning of the feature in the inflaton potential and the parameters of the spectator, leading to an overall resilience of model predictions to parameter variations. This makes it easier for the underlying PBH model to accommodate both high-precision CMB constraints and production of PBHs in the asteroid-mass range.