Cosmological Principle
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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.

The original article can be found at: http://en.wikipedia.org/wiki/Cosmological_Principle.
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