Spinor driven cosmic bounces and their cosmological perturbations

TL;DR

This paper explores how fermion-induced torsion can lead to non-singular cosmic bounces, analyzing their stability and unique perturbation features, including potential observational signatures in gravitational waves.

Contribution

It extends previous models by allowing general fermion couplings, considering both commuting and anti-commuting spinors, and demonstrates the feasibility of arbitrary equations of state for bounce scenarios.

Findings

Spinor bounces are potentially stable.

Scalar fluctuations can source gravitational waves at linear order.

First order dynamics show directional dependence.

Abstract

When coupling fermions to gravity, torsion is naturally induced. We consider the possibility that fermion bilinears can act as a source for torsion, altering the dynamics of the early universe such that the big bang gets replaced with a classical non-singular bounce. We extend previous studies in several ways: we allow more general fermion couplings, consider both commuting and anti-commuting spinors, and demonstrate that with an appropriate choice of potential one can easily obtain essentially arbitrary equations of state, including violations of the null energy condition, as required for a bounce. As an example, we construct a model of ekpyrotic contraction followed by a non-singular bounce into an expanding phase. We analyze cosmological fluctuations in these models, and show that the perturbations can be rewritten in real fluid form. We find indications that spinor bounces are stable, and exhibit several solutions for the perturbations. Interestingly, spinor models do not admit a scalar-vector-tensor decomposition, and consequently some types of scalar fluctuations can act as a source for gravitational waves already at linear order. We also find that the first order dynamics are directionally dependent, an effect which might lead to distinguished observational signatures.