Ive been reading papers on inhomogeneous cosmological models like the McVittie spacetime. The apparent horizons of such spacetimes are time dependent. In almost every paper, he authors always make it a point to show that the rate of change in position of the horizons is not equal to the cosmic flow parameter $\dot{R}/R \neq H(t)$ and declare that the horizons are not comoving with the cosmic fluid. My question is, why is it important to note that those surfaces are not comoving with the cosmic fluid?


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