= Solution
The elementary Newtonian derivation of the <Friedmann equation> follows a test particle on the boundary of a finite expanding sphere. It therefore introduces a distinguished centre and can misleadingly suggest that all matter expands away from one origin.
The fluid derivation is local: the <Euler equations for an inviscid fluid>[Euler equation] and <Poisson equation> must be compatible in every comoving neighbourhood. It consequently describes a genuinely homogeneous infinite universe, in which every comoving observer sees the same <Hubble flow>, without selecting a physical centre. This is the improvement captured by the <Newtonian fluid derivation of the Raychaudhuri equation>.
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