Spectator field models in light of spectral index after Planck

Abstract

We revisit spectator field models including curvaton and modulatedreheating scenarios, specifically focusing on their viability in the newPlanck era, based on the derived expression for the spectral index ingeneral spectator field models. Importantly, the recent Planckobservations give strong preference to a red-tilted power spectrum,while the spectator field models tend to predict a scale-invariant one.This implies that, during inflation, either (i) the Hubble parametervaries significantly as in chaotic inflation, or (ii) a scalar potentialfor the spectator field has a relatively large negative curvature.Combined with the tight constraint on the non-Gaussianity, the Planck data provides us with rich implications forvarious spectator field models.