Does Current Data Prefer a Non-minimally Coupled Inflaton?

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We examine the impact of a non-minimal coupling of the inflaton to the Ricci scalar, 12 ξRϕ², 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∝ ϕ², 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∝ ϕ² 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.