Gradient expansion of superhorizon perturbations in G-inflation

TL;DR

This paper develops a gradient expansion formalism for G-inflation models, providing a general solution up to second order and identifying a conserved curvature perturbation during inflation.

Contribution

It introduces a novel gradient expansion approach for shift-symmetric Galileon actions in inflationary backgrounds, extending solutions to second order without extra conditions.

Findings

Derived a general second-order solution in the gradient expansion formalism.

Identified a curvature perturbation conserved up to first order during inflation.

Simplified the solution during late stages of inflation.

Abstract

We develop the gradient expansion formalism for shift-symmetric Galileon-type actions. We focus on backgrounds that undergo inflation, work in the synchronous gauge, and obtain a general solution up to second order without imposing extra conditions at first order. The solution simplifies during the late stages of inflation. We also define a curvature perturbation conserved up to first order.