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High Quality Content by WIKIPEDIA articles! In relativistic cosmology, Weyl's postulate stipulates that in a fluid cosmological model, the world lines of the fluid particles, which act as the source of the gravitational field and which are often taken to model galaxies, should be hypersurface orthogonal. THat is, the world lines should be everywhere orthogonal to a family of spatial hyperslices. SOmetimes, the additional hypothesis is added that the world lines form timelike geodesics. IN the ADM formalism we introduce a family of spatial hyperslices. THis allows us to think of as the geometry of "space" as evolving over "time". THis is an attractive viewpoint, but in general no such family of hyperslices will be physically preferred. THe Weyl hypothesis can be understood as the assumption that we should consider only cosmological models in which there is such a preferred slicing, namely the one given by taking the unique hyperslices orthogonal to the world lines of the fluid particles. ONe consequence of this hypothesis is that if it holds true, we can introduce a comoving chart such that the metric tensor contains no terms of form dt dx, dt dy, or dt dz.