Non-Gaussianity in multi-sound-speed disformally coupled inflation

TL;DR

This paper investigates non-Gaussianities in a two-field disformally coupled inflation model with novel kinetic interactions, finding typically small and negative non-Gaussian signals despite varied sound speeds.

Contribution

It introduces a first analysis of non-Gaussianities in disformally coupled inflation with a new kinetic interaction, including both canonical and DBI kinetic terms.

Findings

Non-Gaussianity is generally small and negative.

Perturbation modes have different sound speeds during inflation.

The model includes both canonical and DBI kinetic terms.

Abstract

Most, if not all, scalar-tensor theories are equivalent to General Relativity with a disformally coupled matter sector. In extra-dimensional theories such a coupling can be understood as a result of induction of the metric on a brane that matter is confined to. This article presents a first look at the non-Gaussianities in disformally coupled inflation, a simple two-field model that features a novel kinetic interaction. Cases with both canonical and Dirac-Born-Infeld (DBI) kinetic terms are taken into account, the latter motivated by the possible extra-dimensional origin of the disformality. The computations are carried out for the equilateral configuration in the slow-roll regime, wherein it is found that the non-Gaussianity is typically rather small and negative. This is despite the fact that the new kinetic interaction causes the perturbation modes to propagate with different sounds speeds, which may both significantly deviate from unity during inflation.