Anisotropic Fluids and Cosmology

Applications of anisotropic fluids to cosmology, where the anisotropic pressure is generated by the presence of two noninteracting perfect fluids that are in relative motion with respect to each other are considered. The extent of anisotropic pressure that can occur with various equations of state is discussed, and a perturbative analysis, where the amount of anisotropy induced in the pressure ditribution by a small (spherically symmetric) deviation from the Friedmann metric is studied, is also presented. It is shown that such anisotropies could grow faster than the expansion of the Universe. Finally, three exact analytic models, one of which could be used to represent voids, are discussed. These models have anisotropic pressure, and they are spherically symmetric with respect to the origin of the coordinates.