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

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 $c_g\ge \sqrt{2}c$, 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.

8 pages, 4 figures. Matches the version accepted for publication in PRD. Added clarifying comments in the proof of the No-Go, references added, results unchanged

References in corpus (7)

• Causality, Analyticity and an IR Obstruction to UV Completion
• Stability of Geodesically Complete Cosmologies
• Cosmological bounces and Lorentzian wormholes in Galileon theories with an extra scalar field
• Positivity bounds on effective field theories with spontaneously broken Lorentz invariance
• Nonsingular cosmological models with strong gravity in the past
• Stable cosmological solutions in Horndeski theory
• Quartic Horndeski-Cartan theories in a FLRW universe

Cited by in corpus (3)

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• Healthy Horndeski cosmologies with torsion