|
In modern physical cosmology, the cosmological principle is the notion that the distribution of matter in the Universe is homogeneous and isotropic when viewed on a large enough scale, since the forces are expected to act uniformly throughout the Universe, and should, therefore, produce no observable irregularities in the large scale structuring over the course of evolution of the matter field that was initially laid down by 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. In essence, this in a sense says that the Universe is knowable and is playing fair with scientists.〔. p. 2.〕 The cosmological principle depends on a definition of "observer," and contains an implicit qualification and two testable consequences. "Observers" means any observer at any location in the Universe, not simply any human observer at any location on Earth: as Andrew Liddle puts it, "the cosmological principle (that ) the Universe looks the same whoever and wherever you are."〔. p. 2.〕 The qualification is that variation in physical structures can be overlooked, provided this does not imperil the uniformity of conclusions drawn from observation: the Sun is different from the Earth, our galaxy is different from a black hole, some galaxies advance toward rather than recede from us, and the Universe has a "foamy" texture of galaxy clusters and voids, but none of these different structures appears to violate the basic laws of physics. The two testable structural 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 distinct but closely related, because a Universe that appears isotropic from any two (for a spherical geometry, three) locations must also be homogeneous. == Origin == The cosmological principle is first clearly asserted in the ''Philosophiæ Naturalis Principia Mathematica'' (1687) of Isaac Newton. In contrast to earlier classical or medieval cosmologies, in which Earth rested at the center of Universe, Newton conceptualized the Earth as a sphere in orbital motion around the Sun within an empty space that extended uniformly in all directions to immeasurably large distances. He then showed, through a series of mathematical proofs on detailed observational data of the motions of planets and comets, that their motions could be explained by a single principle of "universal gravitation" that applied as well to the orbits of the Galilean moons around Jupiter, the Moon around the Earth, the Earth around the Sun, and to falling bodies on Earth. That is, he asserted the equivalent material nature of all bodies within the Solar System, the identical nature of the Sun and distant stars ("the light of the fixed stars is of the same nature with the light of the Sun, ... and lest the systems of the fixed stars should, by their gravity, fall on each other, () hath placed those systems at immense distances from one another"), and thus the uniform extension of the physical laws of motion to a great distance beyond the observational location of Earth itself. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Cosmological principle」の詳細全文を読む スポンサード リンク
|