A tensor instability in the Eddington inspired Born-Infeld Theory of Gravity

In this paper we consider an extension to Eddington’s proposal for the gravitational action. We study tensor perturbations of a homogeneous and isotropic space-time in the Eddington regime, where modifications to Einstein gravity are strong. We find that the tensor mode is linearly unstable deep in the Eddington regime and discuss its cosmological implications.