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The Cosmological Principle is a principle 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 universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places. 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.
In this case, at a given time, the velocity between two points must be proportional to their separation; this is Hubble's law.
An extension proposed by Fred Hoyle is the Steady-State Principle which is that the universe is homogenous aand isotropic both in space and in time. The original steady-state principle is inconsistent with the Big bang and is widely seen as having been disproved by the mid-1970s.
This principle is tightly related to the more metaphysical Anthropic principle.
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