Primordial non-Guassianity in inflation with gravitationally enhanced friction

TL;DR

This paper studies how gravitationally enhanced friction during inflation amplifies curvature perturbations, leading to increased primordial non-Gaussianity, which influences primordial black hole formation but has little effect on secondary gravitational waves.

Contribution

It provides the first detailed analysis of primordial non-Gaussianity in inflation models with gravitationally enhanced friction, highlighting its impact on PBH formation.

Findings

Non-Gaussianity is significantly larger than in standard slow-roll inflation.

The non-Gaussianity parameter varies from negative to positive near the power spectrum peak.

Primordial non-Gaussianity enhances PBH formation with negligible effect on SIGWs.

Abstract

The gravitationally enhanced friction can reduce the speed of the inflaton to realize an ultra-slow-roll inflation, which will amplify the curvature perturbations. The amplified perturbations can generate a sizable amount of primordial black holes (PBHs) and induce simultaneously a significant background gravitational waves (SIGWs). In this paper, we investigate the primordial non-Gaussianity of the curvature perturbations in the inflation with gravitationally enhanced friction. We find that when the gravitationally enhanced friction plays a role in the inflationary dynamics, the non-Gaussianity is noticeably larger than that from the standard slow-roll inflation. During the regime in which the power spectrum of the curvature perturbations is around its peak, the non-Gaussianity parameter changes from negative to positive. When the power spectrum is at its maximum, the non-Gaussianity parameter is near zero ($\sim \mathcal{O}(0.01)$). Furthermore, the primordial non-Gaussianity promotes the formation of PBHs, while its effect on SIGWs is negligible.