Suppressing the primordial tensor amplitude without changing the scalar sector in quadratic curvature gravity

TL;DR

This paper demonstrates how specific quadratic curvature corrections in gravity can suppress the primordial tensor amplitude without affecting scalar perturbations, aligning inflation models with observational data.

Contribution

The authors identify two quadratic curvature modifications that selectively reduce tensor amplitude without altering scalar sector predictions.

Findings

One correction reduces tensor amplitude to 65% of standard value.

Both corrections are free from instabilities and affect only tensor perturbations.

Even simple power-law inflation models can fit Planck data with these modifications.

Abstract

We address the question of how one can modify the inflationary tensor spectrum without changing at all the successful predictions on the curvature perturbation. We show that this is indeed possible, and determine the two quadratic curvature corrections that are free from instabilities and affect only the tensor sector at the level of linear cosmological perturbations. Both of the two corrections can reduce the tensor amplitude, though one of them generates large non-Gaussianity of the curvature perturbation. It turns out that the other one corresponds to so-called Lorentz-violating Weyl gravity. In this latter case one can obtain as small as 65% of the standard tensor amplitude. Utilizing this effect we demonstrate that even power-law inflation can be within the 2$\sigma$ contour of the Planck results.