UV completion of the Starobinsky model, tensor-to-scalar ratio, and constraints on nonlocality

In this paper, we build upon the successes of the ultraviolet (UV) completion of the Starobinsky model of inflation. This involves an extension of the Einstein-Hilbert term by an infinite covariant derivative theory of gravity which is quadratic in curvature. It has been shown that such a theory can potentially resolve the cosmological singularity for a flat, homogeneous and isotropic geometry, and now it can also provide a successful cosmological inflation model, which in the infrared regime matches all the predictions of the Starobinsky model of inflation. The aim of this paper is to show that the tensor-to-scalar ratio is modified by the scale of nonlocality, and in general a wider range of tensor-to-scalar ratios can be obtained in this class of model, which can put a lower bound on the scale of nonlocality for the first time as large as the O (1 0¹⁴) GeV .