Cosmological principle
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The cosmological principle (or cosmological postulate) summarizes two basic assumptions of scientific cosmology, which underlie their models of the universe as a whole. It was introduced in 1933 by the astrophysicist Edward A. Milne and states:
- The universe is homogeneous - i.e., it always appears the same to an observer regardless of his point of view in space (principle of homogeneity, also called Copernican principle).
- The universe is isotropic - i.e., it always presents itself to the observer in the same way, regardless of the direction of observation in space (principle of isotropy).
A stricter version of the cosmological principle, the perfect cosmological principle, requires not only spatial but also temporal homogeneity. This version, originally supported by representatives of steady-state cosmologies such as Hermann Bondi and Fred Hoyle, now plays only a minor role.
Since matter in the universe is not evenly distributed at "short distances", these assumptions are only valid under the condition that one considers areas of such extension in which matter is to be regarded as evenly distributed on average.
The cosmological principle, assuming some cosmologies, leads to Olber's paradox, so that it is consistent only with certain cosmologies, such as the Big Bang theory.
Questions and Answers
Q: What is the cosmological principle?
A: The cosmological principle is the idea that the universe is the same in all places when viewed on a large scale, and forces act uniformly throughout the universe, resulting in no observable irregularities in the large scale structure.
Q: What is the result of the evolution of matter field after the Big Bang?
A: The large scale structure of the universe is the result of the evolution of the matter field after the Big Bang.
Q: Who is William Keel, and what does he explain about the cosmological principle?
A: William Keel is an astronomer who explains that 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." He also states that the principle is a 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.
Q: What are the two testable consequences of the cosmological principle?
A: The two testable consequences of the cosmological principle are homogeneity and isotropy.
Q: What does homogeneity mean in the context of the cosmological principle?
A: Homogeneity means that the same observational evidence is available to observers at different locations in the universe.
Q: What does isotropy mean in the context of the cosmological principle?
A: Isotropy means that the same observational evidence is available by looking in any direction in the universe.
Q: How are homogeneity and isotropy related in the context of the cosmological principle?
A: Homogeneity and isotropy are closely related because a universe that appears isotropic from any two (for a spherical geometry, three) locations must also be homogeneous.
