The non-adiabatic pressure in general scalar field systems

TL;DR

This paper investigates the non-adiabatic pressure perturbation in scalar field systems with non-canonical actions, clarifying its behavior and implications for curvature perturbation conservation.

Contribution

It provides a detailed analysis of the non-adiabatic pressure in non-canonical scalar fields and distinguishes between sound and phase speeds, highlighting conditions for curvature perturbation conservation.

Findings

Non-adiabatic pressure vanishes in single field systems.

Curvature perturbation remains conserved on large scales in these systems.

Differences between sound and phase speeds are clarified.

Abstract

We discuss the non-adiabatic or entropy perturbation, which controls the evolution of the curvature perturbation in the uniform density gauge, for a scalar field system minimally coupled to gravity with non-canonical action. We highlight the differences between the sound and the phase speed in these systems, and show that the non-adiabatic pressure perturbation vanishes in the single field case, resulting in the conservation of the curvature perturbation on large scales.