Non-minimally coupled curvaton

TL;DR

This paper explores how nonminimal coupling of a spectator field in two-field inflation models can tune the spectral index of cosmological perturbations, making certain curvaton scenarios compatible with observations by producing a red spectrum.

Contribution

It demonstrates that nonminimal coupling can adjust the spectral index in curvaton models, potentially saving scenarios previously ruled out by spectral constraints.

Findings

Negative nonminimal coupling makes the spectrum redder.

Large nonminimal coupling is excluded in power law inflation.

Nonminimal coupling influences postinflationary perturbation growth.

Abstract

We investigate two-field inflationary models in which scalar cosmological pertubations are generated via a spectator field nonminimally coupled to gravity, with the particular emphasis on curvaton scenarios. The principal advantage of these models is in the possibility to tune the spectator spectral index via the nonminimal coupling. Our models naturally yield red spectrum of the adiabatic perturbation demanded by observations. We study how the nonminimal coupling affects the spectrum of the curvature perturbation generated in the curvaton scenarios. In particular we find that for small, negative nonminimal couplings the spectral index gets a contribution that is negative and linear in the nonminimal coupling. Since in this way the curvature spectrum becomes redder, some of curvaton scenarios can be saved, which would otherwise be ruled out. In the power law inflation we find that a large nonminimal coupling is excluded since it gives the principal slow roll parameter that is of the order of unity. Finally, we point out that nonminimal coupling can affect the postinflationary growth of the spectator perturbation, and in this way the effectiveness of the curvaton mechanism.