Revisiting non-Gaussianity from non-attractor inflation models

TL;DR

This paper investigates how the transition from non-attractor to slow-roll inflation affects the evolution and magnitude of local non-Gaussianity, revealing that the transition can significantly suppress or preserve the non-Gaussian signal depending on its nature.

Contribution

It provides a detailed analysis of the transition phase between non-attractor and slow-roll inflation, showing its crucial impact on the final non-Gaussianity amplitude, including for models with non-canonical kinetic terms.

Findings

Smooth or sharp transitions can erase large non-Gaussianity generated in the non-attractor phase.

In sharp transition cases, the original non-attractor non-Gaussianity is preserved.

Transition effects can introduce a suppression factor but still allow for large non-Gaussianity in non-canonical models.

Abstract

Non-attractor inflation is known as the only single field inflationary scenario that can violate non-Gaussianity consistency relation with the Bunch-Davies vacuum state and generate large local non-Gaussianity. However, it is also known that the non-attractor inflation by itself is incomplete and should be followed by a phase of slow-roll attractor. Moreover, there is a transition process between these two phases. In the past literature, this transition was approximated as instant and the evolution of non-Gaussianity in this phase was not fully studied. In this paper, we follow the detailed evolution of the non-Gaussianity through the transition phase into the slow-roll attractor phase, considering different types of transition. We find that the transition process has important effect on the size of the local non-Gaussianity. We first compute the net contribution of the non-Gaussianities at the end of inflation in canonical non-attractor models. If the curvature perturbations keep evolving during the transition - such as in the case of smooth transition or some sharp transition scenarios - the $\mathcal{O}(1)$ local non-Gaussianity generated in the non-attractor phase can be completely erased by the subsequent evolution, although the consistency relation remains violated. In extremal cases of sharp transition where the super-horizon modes freeze immediately right after the end of the non-attractor phase, the original non-attractor result can be recovered. We also study models with non-canonical kinetic terms, and find that the transition can typically contribute a suppression factor in the squeezed bispectrum, but the final local non-Gaussianity can still be made parametrically large.