Abstract
The evolution equations for small perturbations in the metric, energy density and material velocity for an anisotropic viscous Bianchi I universe are studied. It is found (whether or not viscosity is present) that, just as in the flat Friedmann-Robertson-Walker universe, the general solution of the perturbation equations can be split up into three non-coupled perturbations, namely gravitational waves ('tensor perturbations'), vortex motions ('vector perturbations') and density enhancements ('scalar perturbations'). This result is in contrast to that found by Perko, Matzner and Shepley, and by Tomita and Den, who find a coupling between the various perturbations.
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