Model of two perfect fluids for an anisotropic and homogeneous universe.

Abstract

We consider a cosmological model in which two perfect fluids flow with distinct four-velocities. This is equivalent to a single anisotropic fluid flowing with a four-velocity that is an appropriate combination of the two fluid four-velocities. The energy density of the single fluid is larger than the sum of the energy densities of the two perfect fluids and it contains a correction due to the anisotropy. For a homogeneous anisotropic Bianchi type-I universe there is a big bang (initial singularity) and the shear can increase or decrease with time. We give a particular solution in which the spacetime begins as an isotropic spatially flat Friedmann-Lemaitre-Robertson-Walker universe, goes through a symmetry breaking during a ``collision time,'' and develops an anisotropy that reaches a maximum and asymptotically returns to zero.