Curvature and Entropy Perturbations in Generalized Gravity

TL;DR

This paper explores how curvature and entropy perturbations evolve in generalized gravity models during inflation, providing analytical solutions for specific cases and enhancing understanding of early universe perturbations.

Contribution

It introduces a framework for analyzing scalar perturbations in generalized gravity with non-minimal coupling, including derivation of their evolution equations and application to inflation scenarios.

Findings

Derived evolution equations for curvature and entropy perturbations.

Identified conditions where entropy perturbations decay or remain negligible.

Provided analytical solutions for specific inflationary models.

Abstract

We investigate the cosmological perturbations in generalized gravity, where the Ricci scalar and a scalar field are non-minimally coupled via an arbitrary function. In the Friedmann-Lemaitre-Robertson-Walker background, by studying the linear perturbation theory, we separate the scalar type perturbations into the curvature perturbation and the entropy perturbation, whose evolution equations are derived. Then we apply this framework to inflation. We consider the generalized slow-roll conditions and the quantization initial condition. Under these conditions, two special examples are studied analytically. One example is the case with no entropy perturbation. The other example is a model with the entropy perturbation large initially but decaying significantly after crossing the horizon.