Does Current Data Prefer a Non-minimally Coupled Inflaton?

TL;DR

This paper investigates the effects of a non-minimal coupling of the inflaton to gravity on inflationary predictions, finding observational hints favoring such a coupling with specific implications for the tensor-to-scalar ratio.

Contribution

It provides the first observational analysis suggesting a non-zero non-minimal coupling in the simplest quadratic inflation model using recent CMB data.

Findings

Non-minimal coupling $\xi$ is favored at 99% CL assuming $V \\propto \\phi^2$.

The coupling $\xi$ differs from zero at 2σ level.

Predicted tensor-to-scalar ratio $r$ is approximately 0.038, testable by future observations.

Abstract

We examine the impact of a non-minimal coupling of the inflaton to the Ricci scalar, $\frac12 \xi R\phi^2$, on the inflationary predictions. Such a non-minimal coupling is expected to be present in the inflaton Lagrangian on fairly general grounds. As a case study, we focus on the simplest inflationary model governed by the potential $V\propto \phi^2$, using the latest combined 2015 analysis of Planck and BICEP2/Keck Array. We find that the presence of a coupling $\xi$ is favoured at a significance of $99\%$ CL, assuming that nature has chosen the potential $V\propto \phi^2$ to generate the primordial perturbations and a number of e-foldings $N=60$. Within the context of the same scenario, we find that the value of $\xi$ is different from zero at the $2\sigma$ level. When considering the cross-correlation polarization spectra from BICEP2/Keck Array and Planck, a value of $r=0.038_{-0.030}^{+0.039}$ is predicted in this particular non-minimally coupled scenario. Future cosmological observations may therefore test these values of $r$ and verify or falsify the non-minimally coupled model explored here.