Spectator field models in light of spectral index after Planck
  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.
