In modern physical cosmology, the cosmological principle is a prediction based on the idea that the universe is about the same in all places when viewed on a large scale.
Forces are expected to act uniformly throughout the universe. There should, therefore, be no observable irregularities in the large scale structure. The structure is the result of the evolution of the matter field after the Big Bang.
Astronomer William Keel explains:
The cosmological principle is usually stated formally as 'Viewed on a sufficiently large scale, the properties of the universe are the same for all observers.' This amounts to the strongly philosophical statement that the part of the universe which we can see is a fair sample, and that the same physical laws apply throughout.[1]
The two testable consequences of the cosmological principle are homogeneity and isotropy. Homogeneity means that the same observational evidence is available to observers at different locations in the universe ("the part of the universe which we can see is a fair sample"). Isotropy means that the same observational evidence is available by looking in any direction in the universe ("the same physical laws apply throughout"). The principles are closely related, because a universe that appears isotropic from any two (for a spherical geometry, three) locations must also be homogeneous.
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