BULK VISCOSITY AND KALUZA-KLEIN MODELS

In this work we consider a Kaluza-Klein cosmological model with d non-compact and D compact space-like dimensions, an energy-momentum tensor depicting a fluid out of equilibrium, specifically we take a mixture of two gases, one is formed by relativistic particles while the other is a gas constituted by non-relativistic particles and they are not in thermodynamical equilibrium, such that a bulk viscosity term arises. Without actually solving the Einstein equations, we prove that the scale factor of the non-compact space is a monotonically increasing function of time, and that if the scale factor of the compact space reaches a maximum at a certain time, then the non-compact space is driven to expand rapidly. This suggests a possible way of solving the horizon problem.