An inflationary model with small scalar and large tensor nongaussianities

TL;DR

This paper presents an inflationary model where scalar perturbations remain nearly gaussian, but tensor perturbations exhibit significant equilateral nongaussianity and chirality, with implications for CMB polarization patterns.

Contribution

The model introduces a pseudoscalar field coupled to the inflaton, generating large tensor nongaussianities and chirality while keeping scalar nongaussianity negligible.

Findings

Tensor nongaussianity is sizable and detectable.

Tensor perturbations are chiral, affecting CMB polarization.

Scalar perturbations remain nearly gaussian.

Abstract

We study a model of inflation where the scalar perturbations are almost gaussian while there is sizable (equilateral) nongaussianity in the tensor sector. In this model, a rolling pseudoscalar gravitationally coupled to the inflaton amplifies the vacuum fluctuations of a vector field. The vector sources both scalar and tensor metric perturbations. Both kinds of perturbations are nongaussian, but, due to helicity conservation, the tensors have a larger amplitude, so that nongaussianity in the scalar perturbations is negligible. Moreover, the tensors produced this way are chiral. We study, in the flat sky approximation, how constraints on tensor nongaussianities affect the detectability of parity violation in the Cosmic Microwave Background. We expect the model to feature interesting patterns on nongaussianities in the polarization spectra of the CMB.