Super-horizon 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. 
Karim A. Malik, D. Wands, D. H. Lyth and A. R. Liddle