Robustness of Inflation to Large Tensor Perturbations

TL;DR

This paper examines how large tensor perturbations affect inflation's robustness, revealing that initial reductions in e-folds can reverse at higher amplitudes due to increased Hubble friction.

Contribution

It extends previous work by analyzing the impact of tensor metric perturbations on inflation, highlighting a non-linear effect on the number of e-folds in small field models.

Findings

Tensor perturbations initially decrease e-folds with increasing amplitude.

Beyond a certain amplitude, the reduction in e-folds saturates and reverses.

Large metric inhomogeneities induce significant Hubble friction.

Abstract

Extending our previous work on the robustness of inflation to perturbations in the scalar field, we investigate the effects of perturbations in the transverse traceless part of the extrinsic curvature on the evolution of an inhomogeneous inflaton field. Focusing on small field models, we show that these additional metric inhomogeneities initially reduce the total number of e-folds as the amplitude increases, but that the reduction saturates and even reverses above a certain amplitude. We present an argument that this is due to the presence of a large initial Hubble friction when metric perturbations are large.