Superhorizon perturbations and preheating

We discuss the evolution of linear perturbations about a Friedmann–Robertson Walker background metric, using only the local conservation of energy–momentum. We show that on sufficiently large scales the curvature perturbation on spatial hypersurfaces of uniform-density is constant when the non-adiabatic pressure perturbation is negligible. We clarify the conditions under which super-horizon curvature perturbations may vary, using preheating as an example.