Ultra Slow-Roll Inflation and the non-Gaussianity Consistency Relation

TL;DR

Ultra slow-roll inflation challenges the non-Gaussianity consistency relation but faces physical issues, making its observational relevance questionable.

Contribution

This paper classifies ultra slow-roll inflation as a one-parameter model and analyzes its properties and observational predictions.

Findings

Power spectrum remains scale-invariant.

Bi-spectrum is of local type with fnl=5(3-ns)/4.

Model is unstable and difficult to normalize.

Abstract

Ultra slow-roll inflation has recently been used to challenge the non-Gaussianity consistency relation. We show that this inflationary scenario belongs to a one parameter class of models and we study its properties and observational predictions. We demonstrate that the power spectrum remains scale-invariant and that the bi-spectrum is of the local type with fnl=5(3-ns)/4 which, indeed, represents a modification of the consistency relation. However, we also show that the system is unstable and suffers from many physical problems among which is the difficulty to correctly WMAP normalize the model. We conclude that ultra slow-roll inflation remains a very peculiar case, the physical relevance of which is probably not sufficient to call into question the validity of the consistency relation.