Spectator field models in light of spectral index after Planck

TL;DR

This paper examines spectator field models like curvaton and modulated reheating in light of Planck data, highlighting their challenges in producing the observed red-tilted spectrum and implications for inflationary parameters.

Contribution

It derives a general expression for the spectral index in spectator field models and analyzes their viability considering recent Planck constraints.

Findings

Spectator models tend to predict scale-invariance, conflicting with Planck's red tilt.

Significant variation in the Hubble parameter during inflation can reconcile models with observations.

Large negative curvature in the spectator potential is favored to match spectral index constraints.

Abstract

We revisit spectator field models including curvaton and modulated reheating scenarios, specifically focusing on their viability in the new Planck era, based on the derived expression for the spectral index in general spectator field models. Importantly, the recent Planck observations 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 parameter varies significantly as in chaotic inflation, or (ii) a scalar potential for 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 for various spectator field models.