Post-inflationary GW production in generic higher (infinite) derivative gravity

TL;DR

This paper investigates graviton production after inflation in a higher derivative gravity model that avoids ghosts by using entire functions, finding that graviton production remains negligible despite potential instabilities.

Contribution

It demonstrates that in a ghost-free higher derivative gravity model, graviton production after inflation is negligible, extending understanding of stability in such theories.

Findings

Graviton production after inflation is negligible in the model.

Higher derivative operators do not lead to significant instabilities.

The model successfully avoids ghosts while maintaining cosmological solutions.

Abstract

Gravity can be embedded into a renormalizable theory by means of adding quadratic in curvature terms. However, this at first leads to the presence of the Weyl ghost. It is possible to get rid of this ghost if the locality assumption is weakened and the propagator of the graviton is represented by an entire function of the d'Alembertian operator without new poles and zeros. Models of this type admit a cosmological solution describing the $R^2$, or Starobinsky, inflation. We study graviton production after inflation in this model and show that it is negligible despite the presence of the higher derivative operators which could potentially cause instabilities.