The Cosmological Principle is a principle invoked in cosmology that severely restricts the large variety of possible cosmological theories: Cosmology, from the Greek: κοσμολογία (cosmologia, κόσμος (cosmos) world + λογια (logia) discourse) is the study of the universe in its totality and by extension mans place in it. ...

On large scales, the Universe is homogeneous and isotropic.

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. This principle is consistent with observations of the universe. Homogeneity in physics In physics, homogeneity is the quality of having all properties independent of the position. ... Isotropic means independent of direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. ...

If one assumes these two properties to be true about the Universe, then the only possible cosmic evolution is a global expansion or contraction. The deepest visible-light image of the cosmos. ...

In this case, at a time, the velocity between two points must be proportional to their separation; this is Hubble's law. Hubbles law is the statement in physical cosmology that the redshift in light coming from distant galaxies is proportional to their distance. ...

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. Sir Fred Hoyle Sir Fred Hoyle (June 24, 1915 in Yorkshire – August 20in Bournemouth, England, 2001) was a British astronomer, notable for a number of his theories that run counter to current astronomical opinion, and a writer of science fiction, including a number of books co-authored by his son... In cosmology, the steady state theory (also known as the Infinite Universe Theory) is a model developed in 1948 by Fred Hoyle, Thomas Gold, Hermann Bondi and others as a non-standard cosmology to the Big Bang theory (known, usually, as the standard cosmological model). ... According to the Big Bang theory, the universe originated in an extremely dense and hot state (bottom). ...

This principle may be compared to the more metaphysical Anthropic principle. Metaphysics (Greek words meta = after/beyond and physics = nature) is a branch of philosophy concerned with the study of first principles and being (ontology). ... In philosophy, the anthropic principle in its most basic form states that any valid theory of the universe must be consistent with our existence as carbon-based human beings at this particular time and place in the universe. ...