Cosmological Principle

The Cosmological Principle is an assumption invoked in cosmology that severely restricts the large variety of possible cosmological theories:

On large scales, the Universe is homogeneous and isotropic,

Or, in other words, the the universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places.

If one assumes these two properties to be true about the Universe, then the only possible cosmic evolution is a global expansion or contraction.

In this case, the velocity between two points must be proportional to their separation; this is Hubble's law.

This principle is tightly related to the more metaphysical Anthropic principle.