The dynamics of Bianchi I universes in $R^n$ cosmologies with torsion

TL;DR

This paper investigates how torsion and spin influence the evolution of Bianchi I cosmologies within $f(R)=R^n$ gravity, revealing torsion's significant role and the minimal impact of spin, with implications for isotropization and bounce solutions.

Contribution

It provides a detailed phase space analysis of Bianchi I universes with torsion in $f(R)$ gravity, highlighting torsion's effect and the limited role of spin in cosmic dynamics.

Findings

Torsion modifies cosmological evolution compared to metric-only models.

Spin has little influence due to symmetry differences.

The model exhibits isotropization and exotic bounce solutions.

Abstract

We analyze the phase space of Bianchi I cosmologies filled by a spin fluid in the framework of $f(R)$-gravity with torsion using a combination of the dynamical systems approach and the 1+3 covariant formalism. In the simple case of $f(R)=R^n$ our results allow a quantification of the role of torsion and the spin of the cosmic fluid in the evolution of the cosmology. While torsion is able to modify the cosmological dynamics with respect to the purely metric case, the spin has little influence on the cosmology. We argue that this is due to the different symmetries of the tensor characterizing the anisotropies and the spin tensor. The cosmological model we analyzed presents isotropization for a wide set of initial conditions and values of the parameters and allows for two types of exotic bounce solutions.