  For bouncing cosmologies such as the ekpyrotic/cyclic scenarios we show that
it is possible to make predictions for density perturbations which are
independent of the details of the bouncing phase. This can be achieved, as in
inflationary cosmology, thanks to the existence of a dynamical attractor, which
makes local observables equal to the unperturbed solution up to exponentially
small terms. Assuming that the physics of the bounce is not extremely sensitive
to these corrections, perturbations can be evolved even at non-linear level.
The resulting spectrum is not scale invariant and thus incompatible with
experimental data. This can be explicitly shown in synchronous gauge where,
contrary to what happens in the commonly used Newtonian gauge, all
perturbations remain small going towards the bounce and the existence of the
attractor is manifest.

High Energy Physics - Theory;Astrophysics;General Relativity and Quantum Cosmology;High Energy Physics - Phenomenology