The non-adiabatic pressure in general scalar field systems

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.

6 pages, revtex4; v2: typos corrected, results clarified, version being submitted; v3: version accepted for publication