Spinor driven cosmic bounces and their cosmological perturbations
  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.
