Has there been any observation where a gas or a fluid expands following the Hubble flow (the expansion of the universe) and gets heated because it has a large viscosity (transforming its kinetic energy from the Hubble flow into heat)?
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$\begingroup$ would seem very unlikely. Expansion occurs in the void between galaxy clusters, regions of exceptional low density. If there is enough gas to make "viscosity" a factor, then the density would be high enough for local gravity to overcome Hubble flow. Perhaps things were different in the early universe, but generally the temperature of the universe got lower. $\endgroup$– James KJan 15 at 20:46
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