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

• Published in: Physical review. D 2024-02, Vol.109 (4), Article 044073
• Main Authors: Mironov, S., Valencia-Villegas, M.
• Format: Article
• Language: English
• ISSN: 2470-0010; 2470-0029
• DOI: 10.1103/PhysRevD.109.044073

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 ≥ 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.