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.

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.