Sudden braking and turning with a two-field potential bump: primordial black hole formation

TL;DR

This paper explores how a two-field inflation model with a Gaussian potential bump can amplify curvature perturbations, leading to primordial black hole formation and gravitational waves, while reducing fine-tuning issues.

Contribution

It extends single-field PBH formation models to two-field inflation, demonstrating the effects of potential bumps and turns on curvature perturbations and PBH production.

Findings

Potential bump reduces inflaton speed, amplifying curvature perturbations.

Sudden turns during inflation have minimal impact on the power spectrum.

Enhanced small-scale perturbations can produce primordial black holes.

Abstract

We investigate the amplification of curvature perturbations in a two-field inflation model featuring a Gaussian potential bump. When the inflaton encounters a potential bump along the inflationary trajectory, its rolling speed is generally reduced, potentially causing a violation of the slow-roll condition. Consequently, the original decaying modes of comoving curvature perturbations during the slow-roll phase start growing, and lead to enhanced small-scale density perturbations which can produce amounts of primordial black holes (PBHs) and associated scalar-induced gravitational waves. In addition, inflaton also undergoes sudden turnings at the encounter of the Gaussian potential bump, which is insignificant to the overall curvature power spectrum due to the short duration of these turns. Our paper offers a simple example of the extension of a bump-like potential for PBH formation in a single-field inflation to a two-field case, which helps alleviate the fine-tuning of initial conditions to some extent.