Causality Bounds in Quadratic Inflation from Purely Virtual Particles

Abstract: The "$\phi^2$" slow roll inflation combined with General Relativity is largely excluded by Planck data. In this paper, we consider the same potential combined with the $R+C^2$ gravity of purely virtual particles (or fakeons), where the would-be ghost introduced by the Weyl tensor term, $C^2$, is quantized with the fakeon prescription. We compute the tensor power spectrum in the full theory by means of the Cosmic Renormalization Group formalism and critically examine its physical meaning. In particular, we show that it is not possible to retrieve the power spectrum of the fakeon free-theory by considering the decoupling limit of the purely virtual particles. We provide a physical explanation in terms of the causal structure of the theory to infer that a model of quadratic inflation from purely virtual particles is also discarded from a phenomenological point of view.