Galilean genesis: an alternative to inflation

Abstract

We propose a novel cosmological scenario, in which standard inflation is\nreplaced by an expanding phase with a drastic violation of the Null Energy\nCondition (NEC): \\dot H >> H^2. The model is based on the recently introduced\nGalileon theories, that allow NEC violating solutions without instabilities.\nThe unperturbed solution describes a Universe that is asymptotically Minkowski\nin the past, expands with increasing energy density until it exits the regime\nof validity of the effective field theory and reheats. This solution is a\ndynamical attractor and the Universe is driven to it, even if it is initially\ncontracting. The study of perturbations of the Galileon field reveals some\nsubtleties, related to the gross violation of the NEC and it shows that\nadiabatic perturbations are cosmologically irrelevant. The model, however,\nsuggests a new way to produce a scale invariant spectrum of isocurvature\nperturbations, which can later be converted to adiabatic: the Galileon is\nforced by symmetry to couple to the other fields as a dilaton; the effective\nmetric it yields on the NEC violating solution is that of de Sitter space, so\nthat all light scalars will automatically acquire a nearly scale-invariant\nspectrum of perturbations.\n