# On the stability of self-accelerating Universe in modified gravity with dynamical torsion

## TL;DR

This paper investigates the stability of self-accelerating solutions in a modified gravity model with dynamical torsion, revealing potential instabilities in the scalar sector depending on torsion magnitude.

## Contribution

It provides a detailed stability analysis of linear perturbations around self-accelerating solutions in a modified gravity model with dynamical torsion, focusing on scalar perturbations.

## Key findings

- Number of degrees of freedom is two, same as Minkowski background.
- Potential instability in scalar sector when background torsion is large.
- Stable self-acceleration possible with torsion comparable to the effective cosmological constant.

## Abstract

We consider the model of modified gravity with dynamical torsion. This model was previously found to have promising stability properties about various backgrounds. Here we study the stability of linear perturbations about the self-accelerating solution. We apply the $(3 + 1)$-decomposition and consider the scalar sector of perturbations. We find that the number of degrees of freedom is equal to 2, which is the same as in Minkowski background. However, there is at least one instability in the scalar sector, if the value of background torsion is large enough. This does not rule out the possibility of stable self-acceleration with torsion of the order of the effective cosmological constant.