To use Einstein fields equations you have to choose a coordonate systeme(c.s.). The FRW metric is the metric in a c.s. in wich the matter appears isotropic and homogeneouse. This c.s. is the comoving c.s. in wich galaxies keep their own time independent coordinate. The R-H-S of Einstein equations must be expressed in the same c.s. Now the energy-momentum tensor takes the form of a perfect fluid at rest. How are defined pressure and density? In a "comoving volume" the quantity of matter will remains the same...(adsbygoogle = window.adsbygoogle || []).push({});

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# Coordonate systemes and density

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