Healthy Horndeski cosmologies with torsion

TL;DR

This paper demonstrates that incorporating torsion into the full Horndeski theory enables stable, nonsingular, and subluminal cosmological solutions, overcoming previous no-go theorems in curved spacetime.

Contribution

It shows that torsion inclusion in Horndeski gravity allows for stable, nonsingular cosmologies with subluminal propagation, especially emphasizing the importance of nonminimal derivative couplings.

Findings

Torsion enables stable, nonsingular cosmologies at all times.

The no-go theorem is avoided with torsion in Horndeski theory.

Nonminimal derivative couplings are essential for eternal subluminality.

Abstract

We show that the full Horndeski theory with both curvature and torsion can support nonsingular, stable and subluminal cosmological solutions at all times. Thus, with torsion, the usual No-Go theorem that holds in a curved spacetime is avoided. In particular, it is essential to include the nonminimal derivative couplings of the $\mathcal{L}_{5}$ part of the Horndeski action ($G^{\mu\nu}\,\nabla_\mu \nabla_\nu \phi,$ and $(\nabla^2 \phi)^3$). Without the latter a No-Go already impedes the eternal subluminality of nonsingular, stable cosmologies.