Stability of nonsingular cosmologies in Galileon models with torsion. A no-go theorem for eternal subluminality

TL;DR

This paper extends a no-go theorem for stable, nonsingular cosmologies in Galileon models to spacetimes with torsion, showing that a brief superluminal phase of gravity can enable stable bouncing solutions.

Contribution

It generalizes the no-go theorem to torsionful spacetimes and demonstrates that a short superluminal phase of gravity allows for stable, nonsingular cosmological bounces.

Findings

A no-go theorem is extended to torsionful geometries.

A brief superluminal phase of gravity can produce stable bouncing cosmologies.

A concrete model with a short superluminal phase and asymptotic GR behavior is constructed.

Abstract

Generic models in Galileons or Horndeski theory do not have cosmological solutions that are free of instabilities and singularities in the entire time of evolution. We extend this No-Go theorem to a spacetime with torsion. On this more general geometry the No-Go argument now holds provided the additional hypothesis that the graviton is also subluminal throughout the entire evolution. Thus, critically different for Galileons' stability on a torsionful spacetime, an arguably unphysical although arbitrarily short (deep UV) phase occurring at an arbitrary time, when the speed of gravity $(c_g)$ is slightly higher than luminal $(c)$, and by at least an amount $c_g\geq \,\sqrt{2}\,c $, can lead to an all-time linearly stable and nonsingular cosmology. As a proof of principle we build a stable model for a cosmological bounce that is almost always subluminal, where the short-lived superluminal phase occurs before the bounce and that transits to General Relativity in the asymptotic past and future.