Cosmological Principle

A principle invoked in cosmology that severely restricts the large variety of possible cosmological theories: On large scales, the Universe is homogeneous and isotropic.

The universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places. Or, in other words, on a large scale the Universe looks the same in all directions for an observer at any place. This principle is consistent with observations of the universe. If one assumes these two properties to be true about the Universe, then the only possible cosmic evolution is a global expansion or contraction. Exerpt from Wikipedia. Information held under GNU Free Documentation License.