Perturbations in higher derivative gravity beyond maximally symmetric spacetimes

Understand

We study (covariant) scalar-vector-tensor (SVT) perturbations of infinite derivative gravity (IDG), at the quadratic level of the action, around conformally-flat, covariantly constant curvature backgrounds which are not maximally symmetric spacetimes (MSS).

• This extends a previous analysis of perturbations done around MSS, which were shown to be ghost-free.
• We motivate our choice of backgrounds which arise as solutions of IDG in the UV, avoiding big bang and black hole singularities.
• Contrary to MSS, in this paper we show that, generically, all SVT modes are coupled to each other at the quadratic level of the action.