Anisotropic Pressures at Ultra-stiff Singularities and the Stability of Cyclic Universes

We show that the inclusion of simple anisotropic pressures stops the isotropic Friedmann universe being a stable attractor as an initial or final singularity is approached when pressures can exceed the energy density. This shows that the situation with isotropic pressures, studied earlier in the context of cyclic and ekpyrotic cosmologies, is not generic, and Kasner-like behaviour occurs when simple pressure anisotropies are present. We find all the asymptotic behaviours and determine the dynamics when the anisotropic principal pressures are proportional to the density. We expect distortions and anisotropies to be significantly amplified through a simple cosmological bounce in cyclic or ekpyrotic cosmologies when ultra-stiff pressures are present.