Nonsingular cosmological models with strong gravity in the past

TL;DR

This paper constructs stable nonsingular cosmological models within scalar-tensor Horndeski theories, addressing past instabilities by employing strong gravity conditions while ensuring classical validity and stability of perturbations.

Contribution

It introduces various stable nonsingular cosmological models with strong gravity in the past, overcoming ghost and gradient instability issues in Horndeski theories.

Findings

Models are stable and free of superluminal perturbations.

Strong gravity in the past does not lead to classical or stability problems.

Constructed models include bouncing and genesis scenarios with different evolutionary stages.

Abstract

In scalar-tensor Horndeski theories, nonsingular cosmological models - bounce and genesis - are problematic because of potential ghost and/or gradient instabilities. One way to get around this obstacle is to send the effective Planck mass to zero in the asymptotic past ("strong gravity in the past"). One may suspect that this feature is a signal of a strong coupling problem at early times. However, the classical treatment of the cosmological background is legitimate, provided that the strong coupling energy scale remains at all times much higher than the scale associated with the classical evolution. We construct various models of this sort, namely (i) bouncing Universe which proceeds through inflationary epoch to kination (expansion within general relativity, driven by massless scalar field); (ii) bouncing Universe with kination stage immediately after bounce; (iii) combination of genesis and bounce, with the Universe starting from flat space-time, then contracting and bouncing to the expansion epoch; (iv) "standard" genesis evading the strong coupling problem in the past. All these models are stable, and perturbations about the backgrounds are not superluminal.