Spinor driven cosmic bounces and their (in)stability

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Resolving the big bang singularity with a non-singular classical bounce usually requires the introduction of some sort exotic matter which violates the null-energy condition (NEC), such as a scalar field that undergoes ghost condensation, or models involving Galileon fields. In such models an NEC violating phase is not difficult to achieve on its own, but the situation becomes much more restrictive once observational and stability requirements are taken into consideration. In this talk I discuss whether a more desirable outcome might be achieved by making use of fermionic rather than scalar matter. In particular, I describe bouncing scenarios which arise naturally within the context of Einstein-Cartan-Holst gravity coupled to classical Dirac spinors. As I will show, it is relatively easy to construct backgrounds which not only undergo a bounce, but which also accommodate other interesting dynamics outside the bouncing phase, such as inflation or ekpyrosis. Unfortunately, things work less well when considering perturbations in such bouncing backgrounds as I explain within the context of the simplest models: the comoving curvature perturbation diverges as the moment of NEC violation is approached, and hence the models of greatest interest break down before reaching the bounce.