Islands of Stability for Consistent Deformations of Einstein's Gravity
Kühnel, Florian (LMU Munich)

We study consistency of the most relevant extension to Einstein's gravity at quadratic order in the fluctuation around a general background. Based on unitarity requirements, we show that these deformation have unique symmetries. For cosmological backgrounds we demonstrate explicitly that there exists a region in parameter space where the quadratic system is always stable. Any sensible nonlinear completion must incorporate this self--stabilizing aspect already at lowest order in perturbation theory. Our findings include the possibility of consistently deforming general relativity solely on curved backgrounds. The latter might be relevant for issues related consistency arguments by Boulware and Deser.