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Some Gravity Waves in Isotropic Cosmologies

We construct metric perturbations of two families of isotropic expanding universes describing gravitational waves propagating through these universes. The waves are non--planar and owe their wave front expansion solely to the expansion of the universes. The presence of this radiation leads to a small perturbation of the perfect fluid matter content of the universes by the appearance of an anisotropic stress. We then construct exact models of gravity waves in these universes. In this case the matter content of the models consists of the perfect fluid matter supplemented by anisotropic stress and lightlike matter traveling with the waves. Under appropriate conditions of approximation the lightlike matter can be neglected and the exact models coincide with the perturbative models.