Inflationary non-Gaussianities in the most general second-order scalar-tensor theories

TL;DR

This paper calculates the non-Gaussianities in primordial fluctuations for the most general second-order scalar-tensor theories, showing they predominantly have an equilateral shape and deriving the non-linear parameter for various inflation models.

Contribution

It provides a comprehensive formula for the equilateral non-Gaussianity parameter in general second-order scalar-tensor inflation models, extending previous specific cases.

Findings

Non-Gaussianities are well approximated by the equilateral shape.

Derived a formula for the non-linear parameter f_NL^equil in these models.

Applied the formula to multiple inflation scenarios, including Galileon and Gauss-Bonnet models.

Abstract

For very general scalar-field theories in which the equations of motion are at second-order, we evaluate the three-point correlation function of primordial scalar perturbations generated during inflation. We show that the shape of non-Gaussianities is well approximated by the equilateral type. The equilateral non-linear parameter f_NL^equil is derived on the quasi de Sitter background where the slow-variation parameters are much smaller than unity. We apply our formula for f_NL^equil to a number of single-field models of inflation--such as k-inflation, k-inflation with Galileon terms, potential-driven Galileon inflation, nonminimal coupling models (including field-derivative coupling models), and Gauss-Bonnet gravity.