Cosmological principle

Summary

The cosmological principle is the assumption that, on large enough scales, the universe looks the same at every place and in every direction. In general relativity it says that spacetime can be sliced into spatial sections that are homogeneous and isotropic. This symmetry assumption reduces the Einstein equations for the whole universe to a few ordinary differential equations. It is the starting point of the Friedmann–Lemaître–Robertson–Walker metric and of standard cosmology.