Quasi-Isotropic Expansion for a Two-Fluid Cosmological Model Containing Radiation and String Gas

The quasi-isotropic expansion for a simple two-fluid cosmological model consisting of radiation and string gas is constructed. Expressions for the first-order gradient-dependent corrections to the space-time metric, energy densities, and velocities are presented explicitly. Their small- and large-time behavior is investigated. It is found that the anisotropic component of the metric grows faster than the isotropic (trace-proportional) one at late times. As a result, the behavior of the metric tensor does not have the standard quasi-isotropic form corresponding to the string gas equation of state.