## Why Pressure does not fit into General Relativity

In 1934 Milne and McCrea [Quart. J. Math. 5 (1934) 73-80] made a startling discovery that still is not fully resolved. In Harrison's [Cosmology (Cambridge U. P., 2000), 2nd ed. p.326] words: "Why should Newtonian theory and general relativity, when applied to a uniform universe, yield identical results? At best, Newtonian theory is only approximately true, and yet in this most unlikely of all applications it gives the correct result." Milne and McCrea derived the Friedmann-Lemaitre equations for the expansion rate of the universe at zero pressure.

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