Generalized Galilean Genesis

TL;DR

This paper presents a unified, generalized framework for galilean genesis within Horndeski theories, introducing a new parameter that controls the evolution of the universe and analyzing the resulting primordial perturbations.

Contribution

It extends previous galilean genesis models by incorporating a new parameter in Horndeski theory, providing a comprehensive description and analyzing the perturbation spectra.

Findings

The generalized model is an attractor solution.

Tensor perturbations are not amplified in this scenario.

The spectral index depends solely on the new parameter {}.

Abstract

The galilean genesis scenario is an alternative to inflation in which the universe starts expanding from Minkowski in the asymptotic past by violating the null energy condition stably. Several concrete models of galilean genesis have been constructed so far within the context of galileon-type scalar-field theories. We give a generic, unified description of the galilean genesis scenario in terms of the Horndeski theory, i.e., the most general scalar-tensor theory with second-order field equations. In doing so we generalize the previous models to have a new parameter (denoted by {\alpha}) which results in controlling the evolution of the Hubble rate. The background dynamics is investigated to show that the generalized galilean genesis solution is an attractor, similarly to the original model. We also study the nature of primordial perturbations in the generalized galilean genesis scenario. In all the models described by our generalized genesis Lagrangian, amplification of tensor perturbations does not occur as opposed to what happens in quasi-de Sitter inflation. We show that the spectral index of curvature perturbations is determined solely from the parameter {\alpha} and does not depend on the other details of the model. In contrast to the original model, a nearly scale-invariant spectrum of curvature perturbations is obtained for a specific choice of {\alpha}.