Fine-tuning challenges for the matter bounce scenario
  A bouncing universe with a long period of contraction during which the
average density is pressureless (the same equation of state as matter) as
cosmologically observable scales exit the Hubble horizon has been proposed as
an explanation for producing a nearly scale-invariant spectrum of adiabatic
scalar perturbations. A well-known problem with this scenario is that, unless
suppressed, the energy density associated with anisotropy grows faster than
that of the pressureless matter, so the matter-like phase is unstable. Previous
models introduce an ekpyrotic phase after the matter-like phase to prevent the
anisotropy from generating chaotic mixmaster behavior. In this work, though, we
point out that, unless the anisotropy is suppressed first, the matter-like
phase will never start and that suppressing the anisotropy requires
extraordinary, exponential fine-tuning.
