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 and comoving coordinates for more.

External links:

• http://www.astr.ua.edu/keel/galaxies/friedman.html