Cosmology with a spin

TL;DR

This paper explores how spinor fields in Einstein-Cartan-Holst gravity can lead to bouncing cosmological models that avoid singularities, with self-interactions influencing the dynamics and potential parity violations.

Contribution

It provides a transparent framework for deriving general spinor dynamics in Einstein-Cartan-Holst gravity and demonstrates their implications for cosmological models with bounce solutions.

Findings

Self-interactions can invert the mass potential to an upside-down Mexican hat.

Cosmological models exhibit bounces with maximal energy density, avoiding singularities.

Solutions range from simple to complex, with some models showing no significant novelties.

Abstract

Using the chiral representation for spinors we present a particularly transparent way to generate the most general spinor dynamics in a theory where gravity is ruled by the Einstein-Cartan-Holst action. In such theories torsion need not vanish, but it can be re-interpreted as a 4-fermion self-interaction within a torsion-free theory. The self-interaction may or may not break parity invariance, and may contribute positively or negatively to the energy density, depending on the couplings considered. We then examine cosmological models ruled by a spinorial field within this theory. We find that while there are cases for which no significant cosmological novelties emerge, the self-interaction can also turn a mass potential into an upside-down Mexican hat potential. Then, as a general rule, the model leads to cosmologies with a bounce, for which there is a maximal energy density, and where the cosmic singularity has been removed. These solutions are stable, and range from the very simple to the very complex.