One Small Step for an Inflaton, One Giant Leap for Inflation: a novel non-Gaussian tail and primordial black holes

TL;DR

This paper reveals that tiny steps in the inflaton potential during single-field inflation can produce highly non-Gaussian tails in the curvature perturbation distribution, significantly impacting primordial black hole formation.

Contribution

It introduces a non-perturbative $ ext{delta }N$ analysis showing how small potential steps cause large non-Gaussian tails, a novel insight into inflationary perturbations.

Findings

Tiny upward steps generate highly non-Gaussian tails.

Non-Gaussian tail effects depend sensitively on potential features.

Primordial black hole abundance can be dramatically affected.

Abstract

We report a novel prediction from single-field inflation that even a tiny step in the inflaton potential can change our perception of primordial non-Gaussianities of the curvature perturbation. Our analysis focuses on the tail of probability distribution generated by an upward step transition between two stages of slow-roll evolution. The nontrivial background dynamics with off-attractor behavior is identified. By using a non-perturbative $\delta N$ analysis, we explicitly show that a highly non-Gaussian tail can be generated by a tiny upward step, even when the conventional nonlinearity parameters $f_{NL}$, $g_{NL}$, etc. remain small. With this example, we demonstrate for the first time the sensitive dependence of non-perturbative effects on the tail of probability distribution. Our scenario has an inconceivable application to primordial black holes by either significantly boosting their abundance or completely forbidding their appearance.