Non-Adiabatic Perturbations from Single-Field Inflation

If the inflaton decays into several components during reheating, and if the corresponding decay rates are functions of spacetime-dependent quantities, it is possible to generate entropy perturbations after a stage of single-field inflation. In this paper, I present a simple toy example that illustrates this possibility. In the example, the decay rates of the inflaton into matter'' and radiation’’ are different functions of the total energy density. In particular cases, one can exactly solve the equations of motion both for background and perturbations in the long-wavelength limit, and show that entropy perturbations do indeed arise. Beyond these specific examples, I attempt to identify what are the essential ingredients responsible for the generation of entropy perturbations after single-field inflation, and to what extent these elements are expected to be present in realistic models.