Does Current Data Prefer a Non-minimally Coupled Inflaton?

We examine the impact of a non-minimal coupling of the inflaton to the Ricci scalar, $\frac12 ξRϕ^2$, 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 $V\propto ϕ^2$, using the latest combined 2015 analysis of Planck and BICEP2/Keck Array. We find that the presence of a coupling $ξ$ is favoured at a significance of $99\%$ CL, assuming that nature has chosen the potential $V\propto ϕ^2$ 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 $ξ$ is different from zero at the $2σ$ 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.

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