The non-adiabatic pressure in general scalar field systems

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 oil large scales. (C) 2009 Elsevier B.V. All rights reserved.