Cosmological Principle

In modern physical cosmology, the cosmological principle is the working assumption that observers on Earth do not occupy an unusual or privileged location within the universe as a whole, judged as observers of the physical phenomena produced by uniform and universal laws of physics. As 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.

The cosmological principle contains three implicit qualifications and two testable consequences. The first implicit qualification is that "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 the universe looks the same whoever and wherever you are."

The second implicit qualification is that "looks the same" does not mean physical structures necessarily, but the effects of physical laws in observable phenomena. Thus, wavelength ratios observed for different ionic species in the absorption spectra of quasi stellar objects (QSO or quasars) place a limit on any variation in the fine-structure constant to less than 1 part in 1 million out to a distance in space (and time) of z = 3 (about 6500 megaparsecs or 11.5 billion light years); as the fine-structure constant is determined by the relation between the speed of light (c), Planck's constant (h) and the electron charge (e), these physical constants are constrained as well.

The third qualification, related to the second, 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.

The cosmological principle is consistent with the observed isotropy of: (i) the celestial distribution of radio galaxies, which are randomly distributed across the entire sky, (ii) the large scale spatial distribution of galaxies, which form a randomly tangled web of clusters and voids up to around 400 megaparsecs in width, (iii) the isotropic distribution of observed red shift in the spectra of distant galaxies, which implies a uniform expansion of space or Hubble flow in all directions, and (iv) the cosmic microwave background radiation, the relic radiation released by the expansion and cooling of the early universe, which is constant in all directions to within 1 part in 100,000. For example, deep sky galaxy surveys, such as the Sloan Digital Sky Survey or the 2dF Galaxy Redshift Survey, combine line of sight galaxy positions with red shift data to produce three dimensional maps of galaxy clustering across an estimated area over 4 billion light years wide (a red shift radius of z > 0.20); statistical tests applied to these maps confirm that isotropy applies to different viewpoints within them. The cosmic microwave background is the same from all parts of the sky, yet in cosmological theory these must have originated in completely different parts of the early universe.

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.