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

TL;DR

This paper extends the Starobinsky inflation model with a non-local gravity theory, showing that the tensor-to-scalar ratio depends on the non-locality scale, which can be constrained to be as high as 10^{14} GeV.

Contribution

It demonstrates how non-local modifications to gravity alter inflation predictions and establishes a lower bound on the non-locality scale based on tensor-to-scalar ratio measurements.

Findings

Tensor-to-scalar ratio is affected by the non-locality scale.

A wider range of tensor-to-scalar ratios is possible in the model.

Lower bound on non-locality scale is estimated at around 10^{14} GeV.

Abstract

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 matches all the predictions of the Starobinsky model of inflation. The aim of this note is to show that the tensor-to-scalar ratio is modified by the scale of non-locality, 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 non-locality for the first time as large as the O$(10^{14})$ GeV.