Perturbations of an anisotropic spacetime: Radiation filled medium

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Abstract
We consider the evolution of a perturbed Bianchi type-I model universe filled with a radiation fluid. The background model shows a smooth transition from the shear-dominated early anisotropic universe to the radiation-dominated isotropic one. We have analytic solutions describing the background evolution. We numerically investigate the evolution of perturbations in the metric and the fluid variables through the transition. We consider the wave vector lying in a plane of two principal axes of the background anisotropy. In view of the well-studied case in the isotropic model we adopt the comoving gauge to treat the density perturbation in an ideal fluid. Because of the coupling between different modes in the anisotropic stage, the amplitude of resulting tensor perturbation in the isotropic stage is comparable to the one of scalar perturbation. We present the evolution characterizing the density perturbation and the gravitational waves in some selected parameter space. © 1996 The American Physical Society.

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