Scale-invariant perturbations from NEC violation: A new variant of Galilean Genesis

TL;DR

This paper introduces a new variant of Galilean Genesis that starts from Minkowski space with NEC violation, producing diverse scale-invariant tensor spectra and nearly scale-invariant scalars, differing from traditional NEC-violating models.

Contribution

It presents a novel Galilean Genesis model within Horndeski theory with distinct primordial fluctuation characteristics and non-standard consistency relations.

Findings

Tensor perturbations can grow on superhorizon scales.

Tensor spectra can be red, blue, or scale invariant.

Scalar perturbations are nearly scale invariant.

Abstract

We propose a novel branch of the Galilean Genesis scenario as an alternative to inflation, in which the universe starts expanding from Minkowski in the asymptotic past with a gross violation of the null energy condition (NEC). This variant, described by several functions and parameters within the Horndeski scalar-tensor theory, shares the same background dynamics with the existing Genesis models, but the nature of primordial quantum fluctuations is quite distinct. In some cases, tensor perturbations grow on superhorizon scales. The tensor power spectrum can be red, blue, or scale invariant, depending on the model, while scalar perturbations are nearly scale invariant. This is in sharp contrast to typical NEC-violating cosmologies, in which a blue tensor tilt is generated. Though the primordial tensor and scalar spectra are both nearly scale invariant as in the inflationary scenario, the consistency relation in our variant of Galilean Genesis is non-standard.