Matter in General Relativity and Cosmology

[mikehelland]

@Francois-Zinserling

I agree that on large scale the entirety of space approaches a perfect fluid. The granularity of matter might disappear in large enough samples but that still doesn't make matter a fluid.

Gases and liquids are matter and they're modeled as fluids in physics. I think dust is too.

I don't know enough about FLRW if this was one of their original postulates, or if it is an interpretation by the author.

https://en.wikipedia.org/wiki/Friedmann_equations

They were first derived by Alexander Friedmann in 1922 from Einstein's field equations of gravitation for the Friedmann–Lemaître–Robertson–Walker metric and a perfect fluid with a given mass density ρ and pressure p.[1]