Cosmological principle

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Under the cosmological principle (or cosmological postulate ) two basic assumptions are summarized in scientific cosmology , which are based on its models of the universe as a whole. It is closely related to the Copernican principle . The cosmological principle was introduced in 1933 by the astrophysicist Edward A. Milne .

  • The universe is homogeneous - that is, it always presents itself in the same way to an observer regardless of the point of the space in which he is (principle of homogeneity, also called the Copernican principle ).
  • The universe is isotropic - that is, it always appears the same to the observer regardless of the direction of observation in space (principle of isotropy).

A stricter version of the cosmological principle, the so-called 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 only plays a subordinate role.

Since matter in the universe is not evenly distributed over “short distances”, these assumptions only apply under the condition that areas of such extent are considered in which matter is to be regarded as being equally distributed on average.

However, assuming some cosmologies, the cosmological principle leads to the Olbers paradox , so that it is only consistent with certain cosmologies, such as the big bang theory .

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