Do current data prefer a nonminimally coupled inflaton?

Abstract

We examine the impact of a nonminimal coupling of the inflaton to the Ricci scalar, 1/2 ξ R ϕ², on the inflationary predictions. Such a nonminimal 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 the BICEP 2 /K e c k Array. We find that the presence of a coupling ξ is favored at a significance of 99% C.L., 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 2 σ level. When considering the cross-correlation polarization spectra from the BICEP 2 /K e c k Array and Planck, a value of r =0.03 8_-0.030^+0.039 is predicted in this particular nonminimally coupled scenario. Future cosmological observations may therefore test these values of r and verify or falsify the nonminimally coupled model explored here.