G-Bounce

TL;DR

This paper introduces a class of models within General Relativity that achieve a smooth cosmological bounce using noncanonical scalar fields with Galileon-like self-couplings, avoiding instabilities and enabling transitions to standard cosmological phases.

Contribution

It develops a framework for healthy bouncing models with specific scalar field Lagrangians, expanding the theoretical landscape of nonsingular cosmologies.

Findings

Models avoid ghosts and gradient instabilities at the bounce

The universe can transition smoothly from contraction to expansion

Phase portraits illustrate viable bouncing scenarios including Galilean Genesis

Abstract

We present a wide class of models which realise a bounce in a spatially flat Friedmann universe in standard General Relativity. The key ingredient of the theories we consider is a noncanonical, minimally coupled scalar field belonging to the class of theories with Kinetic Gravity Braiding / Galileon-like self-couplings. In these models, the universe smoothly evolves from contraction to expansion, suffering neither from ghosts nor gradient instabilities around the turning point. The end-point of the evolution can be a standard radiation-domination era or an inflationary phase. We formulate necessary restrictions for Lagrangians needed to obtain a healthy bounce and illustrate our results with phase portraits for simple systems including the recently proposed Galilean Genesis scenario.