Primordial curvature fluctuation and its non-Gaussianity in models with modulated reheating

Abstract

We investigate non-Gaussianity in the modulated reheating scenario where fluctuations of the decay rate of the inflaton generate adiabatic perturbations, paying particular attention to the nonlinearity (NL) parameters ${f}_{\mathrm{NL}}$, ${\ensuremath{\tau}}_{\mathrm{NL}}$, and ${g}_{\mathrm{NL}}$ as well as the scalar spectral index and tensor-to-scalar ratio which characterize the nature of the primordial power spectrum. We also take into account the preexisting adiabatic perturbations produced from the inflaton fluctuations. It has been known that the nonlinearity between the curvature perturbations and the fluctuations of the decay rate can yield non-Gaussianity at the level of ${f}_{\mathrm{NL}}\ensuremath{\sim}\mathcal{O}(1)$, but we find that the nonlinearity between the decay rate and the modulus field which determines the decay rate can generate much greater non-Gaussianity. We also discuss a consistency relation among nonlinearity parameters which holds in the scenario and find that the modulated reheating yields a different one from that of the curvaton model. In particular, they both can yield a large positive ${f}_{\mathrm{NL}}$ but with a different sign of ${g}_{\mathrm{NL}}$. This provides a possibility to discriminate these two competitive models by looking at the sign of ${g}_{\mathrm{NL}}$. Furthermore, we work on some concrete inflation models and investigate in what cases models predict the spectral index and the tensor-to-scalar ratio allowed by the current data while generating large non-Gaussianity, which may have many implications for model buildings of the inflationary universe.