Galilean Genesis: an alternative to inflation
Paolo Creminelli, Alberto Nicolis, Enrico Trincherini
TL;DR
The paper presents Galilean Genesis, a cosmological scenario in which a conformal Galileon sector violates the Null Energy Condition to drive an NEC-violating expansion that starts from asymptotically flat spacetime and evolves toward reheating. The background is an attractor that remains robust even for contracting initial conditions, and perturbations from the Galileon itself fail to generate observable adiabatic fluctuations. A second light scalar coupled via the conformal symmetry perceives a Jordan-frame de Sitter geometry, yielding a nearly scale-invariant isocurvature spectrum that can be converted to adiabatic perturbations by standard mechanisms. Gravitational waves are predicted to be strongly blue and effectively unobservable, while a central caveat is the generic presence of superluminal propagation in NEC-violating Galileon theories, motivating potential modifications or alternative symmetry structures for a UV-complete picture.
Abstract
We propose a novel cosmological scenario, in which standard inflation is replaced by an expanding phase with a drastic violation of the Null Energy Condition (NEC): \dot H >> H^2. The model is based on the recently introduced Galileon theories, that allow NEC violating solutions without instabilities. The unperturbed solution describes a Universe that is asymptotically Minkowski in the past, expands with increasing energy density until it exits the regime of validity of the effective field theory and reheats. This solution is a dynamical attractor and the Universe is driven to it, even if it is initially contracting. The study of perturbations of the Galileon field reveals some subtleties, related to the gross violation of the NEC and it shows that adiabatic perturbations are cosmologically irrelevant. The model, however, suggests a new way to produce a scale invariant spectrum of isocurvature perturbations, which can later be converted to adiabatic: the Galileon is forced by symmetry to couple to the other fields as a dilaton; the effective metric it yields on the NEC violating solution is that of de Sitter space, so that all light scalars will automatically acquire a nearly scale-invariant spectrum of perturbations.