Weyl's postulate
Weyl's postulate states that the world lines of the galaxies form a three-bundle of non-intersecting geodesics orthonormal to a series of space-like hypersurfaces.It is a fundamental assumption of the Big bang theory. See the wikipedia entries for precise definitions, but a rough translation, is "that a global time and space coordinate system can be defined, where time is, on average, in some sense locally perpendicular to space".
See shape of the universe, comoving coordinates and Friedman-Robertson-Walker for more.
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