Uphill inflation

TL;DR

This paper explores a novel mechanism called uphill inflation, where the inflaton climbs its potential, leading to rapid growth of perturbations and the production of abundant ultra-light primordial black holes, with less fine tuning.

Contribution

It introduces uphill inflation as a new scenario for PBH formation, analyzing quantum diffusion effects and showing high PBH abundance with reduced fine tuning.

Findings

Ultra-light black holes are produced with high abundance.

The PBH production does not depend on the inflation energy scale.

The scenario requires less fine tuning compared to other models.

Abstract

Primordial black holes (PBH) may form from large cosmological perturbations, produced during inflation when the inflaton's velocity is sufficiently slowed down. This usually requires very flat regions in the inflationary potential. In this paper we investigate another possibility, namely that the inflaton climbs up its potential. When it turns back, its velocity crosses zero, which triggers a short phase of ``uphill inflation'' during which cosmological perturbations grow at a very fast rate. This naturally occurs in double-well potentials if the width of the well is close to the Planck scale. We include the effect of quantum diffusion in this scenario, which plays a crucial role, by means of the stochastic-$\delta N$ formalism. We find that ultra-light black holes are produced with very high abundances, which do not depend on the energy scale at which uphill inflation occurs, and which suffer from substantially less fine tuning than in alternative PBH-production models. They are such that PBHs later drive a phase of PBH domination.