Mechanisms for Primordial Black Hole Production in String Theory

TL;DR

This paper explores string theory-based inflation models that can produce primordial black holes by temporarily violating slow-roll conditions, potentially explaining dark matter through PBH populations.

Contribution

It identifies two string-inspired mechanisms—axion potential features and warp factor steps—that enable PBH production during inflation, aligning with observational constraints.

Findings

String axion models can generate PBHs matching dark matter abundance.

Warp factor steps in DBI inflation can amplify curvature perturbations for PBH formation.

Models are consistent with Planck CMB observations while producing PBHs.

Abstract

We consider mechanisms for producing a significant population of primordial black holes (PBHs) within string inspired single field models of inflation. The production of PBHs requires a large amplification in the power spectrum of curvature perturbations between scales associated with CMB and PBH formation. In principle, this can be achieved by temporarily breaking the slow-roll conditions during inflation. In this work, we identify two string setups that can realise this process. In string axion models of inflation, subleading non-perturbative effects can superimpose steep cliffs and gentle plateaus onto the leading axion potential. The cliffs can momentarily violate the slow-roll conditions, and the plateaus can lead to phases of ultra slow-roll inflation. We thus achieve a string motivated model which both matches the Planck observations at CMB scales and produces a population of light PBHs, which can account for an order one fraction of dark matter. In DBI models of inflation, a sharp increase in the speed of sound sourced by a steep downward step in the warp factor can drive the amplification. In this scenario, discovery of PBHs could indicate non-trivial dynamics in the bulk, such as flux-antibrane annihilation at the tip of a warped throat.