Cosmological Principle
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The Cosmological Principle is not a principle, but rather a reasonable assumption or axiom that severely restricts the large variety of possible cosmological theories. It follows from the observation of the Universe on a large scale, and states that:
- On large scales, the Universe is homogeneous and isotropic.
Explanation
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, which would seem to imply a Universe infinite in extent. This principle is consistent with observations of the universe from the Earth. In regard to the observations, the principle states that Earth is nowhere special.
If one assumes these two properties to be true about the Universe, and assumes a finite Universe, then the only possible cosmic evolution is a global expansion or contraction.
In this case, at a time, the spectral redshift 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, also known as the Perfect Cosmological Principle, which is that the universe is homogeneous and 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.
Support
- Observed isotropy of the cosmic microwave background radiation (CMB), combined with the Copernican principle [1]ca:Principi cosmològic
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