Inflationary trispectrum from graviton exchange

TL;DR

This paper calculates the inflationary scalar trispectrum resulting from graviton exchange, revealing it to be larger than previous estimates and proportional to the tensor-to-scalar ratio, with implications for non-Gaussianity in the early universe.

Contribution

It provides the first detailed computation of the inflationary trispectrum from graviton exchange, showing it exceeds previous scalar-only estimates and relates directly to the tensor-to-scalar ratio.

Findings

The graviton exchange contribution to the trispectrum is about ten times larger than scalar-only calculations.

The non-linearity parameter tauNL is approximately equal to the tensor-to-scalar ratio r.

The relation simplifies when the exchanged graviton's momentum is much smaller than external momenta.

Abstract

We compute the connected four-point correlation function of the primordial curvature perturbation generated during inflation with standard kinetic terms, where the correlation is established via exchange of a graviton between two pairs of scalar fluctuations. Any such correlation yields a contribution to the scalar trispectrum of the order of the tensor to scalar ratio r. This contribution is numerically one order of magnitude larger than the one previously calculated on the basis of scalar perturbations interacting at a point and satisfies a simple relation in the limit where the momentum of the graviton which is exchanged becomes much smaller than the external momenta. We conclude that the total non-linearity parameter generated by single-field models of slow-roll inflation is at maximum tauNL ~ r.