Non-Singular Bouncing Cosmology: Consistency of the Effective
  Description
  We explicitly confirm that spatially flat non-singular bouncing cosmologies
make sense as effective theories. The presence of a non-singular bounce in a
spatially flat universe implies a temporary violation of the null energy
condition, which can be achieved through a phase of ghost condensation. We
calculate the scale of strong coupling and demonstrate that the
ghost-condensate bounce remains trustworthy throughout, and that all
perturbation modes within the regime of validity of the effective description
remain under control. For this purpose we require the perturbed action up to
third order in perturbations, which we calculate in both flat and co-moving
gauge -- since these two gauges allow us to highlight different physical
aspects. Our conclusion is that there exist healthy descriptions of
non-singular bouncing cosmologies providing a viable resolution of the big-bang
singularities in cosmological models. Our results also suggest a variant of
ekpyrotic cosmology, in which entropy perturbations are generated during the
contracting phase, but are only converted into curvature perturbations after
the bounce.
