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General formulae are derived for the bulk viscosity, shear viscosity, and heat transport due to radia tion, and for the damping rate of sound waves, in an imperfect relativistic fluid. These results are used to evaluate the cosmological entropy production associated with a nonvanishing mean free time of photons, neutrinos, or gravitons, and to calculate the rate of damping of protogalactic fluctuations in the period immediately prior to the recombination of hydrogen.
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Steven Weinberg (1971) studied this question.
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