Stability of self-accelerating Universe in modified gravity with dynamical torsion: the case of small background torsion

TL;DR

This paper investigates the stability of a self-accelerating universe within a modified gravity model with dynamical torsion, showing that when torsion is comparable to the cosmological constant, the model remains stable against scalar perturbations.

Contribution

The study demonstrates that in the case where torsion is of the order of the cosmological constant, the self-accelerating solution is free of exponential instabilities, extending previous stability analyses.

Findings

No exponential instabilities for scalar perturbations when torsion is comparable to the cosmological constant.

Stability depends on the relative magnitude of torsion and the effective cosmological constant.

Previous instability issues are resolved in this specific parameter regime.

Abstract

We consider the model of modified gravity with dynamical torsion. This model was found to have promising stability properties about various backgrounds. The model admits a self-accelerating solution. We have shown previously that if the parameters are adjusted in such a way that the torsion is much greater than the effective cosmological constant, the self-accelerating solution is unstable: there are exponentially growing modes. Here we study the scalar perturbations in the case when the torsion is of the order of the effective cosmological constant. We find that there are no exponential instabilities.