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Perturbations of a spatially homogeneous isotropic universe are investigated in terms of small variations of the curvature. It is found that rotational perturbations die away. Density perturbations grow relatively to the background, but galaxies cannot be formed by the growth of perturbations that were initially small. In the steady-state universe small rotational and density perturbations die away. The behavior of gravitational radiation in expanding universes is also investigated. Its "energy density" decreases at the same rate as that of electromagnetic radiation, although its active gravitational effect is only half as great. If a small amount of viscosity is present, gravitational radiation will be completely absorbed in the steady-state universe but not in an evolutionary universe.
S. W. Hawking (Mon,) studied this question.