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| Universe · Big Bang Age of the Universe Timeline of the Big Bang Ultimate fate of the universe
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The cosmological principle is an assumption invoked in cosmology that, when applied, severely restricts the large variety of possible cosmological theories. It follows from the observation of the Universe on a large scale, and states that:
- On large spatial scales, the Universe is homogeneous and isotropic.
Or simply put, the universe is the same everywhere on a large scale.
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Implications
The properties of homogeneity and isotropy assumed by the Cosmological Principle suggest that Earth is not at a preferred place (see the Copernican principle), and that at very large scales the Universe is smooth (i.e. not fractal).
One implication of the cosmological principle is that the largest discrete structures in the universe are in mechanical equilibrium. Homogeneity and isotropy of matter at the largest scales would suggest that the largest discrete structures are parts of a single indiscrete form, like the crumbs which make up the interior of a cake. At extreme cosmological distances, the property of mechanical equilibrium in surfaces lateral to the line of sight can be empirically tested; however, under the assumption of the cosmological principle, it cannot be detected parallel to the line of sight (see timeline of the universe).
Observations of the cosmos reveal a higher density and lower metallicity in the population of galaxies at further distances with respect to Earth.1 To account for this scientists applying the cosmological principle suggest the unfalsifiable notion that a change in the population of galaxies along the line of sight translates into change of the homogeneous universe as a whole. Cosmologists agree that in accordance with observations of distant galaxies, a universe must be non-static if it follows the cosmological principle. To their benefit, a non-static universe is also implied, independent of these observations of distant galaxies, as the result of applying the cosmological principle to General Relativity.
A different view
A challenge to cosmological principle comes from the problem of induction:
- Empirical observations of patterns occurring within a limited scope can shed no light on the state of things outside that scope.
Heterogeneous spaces often contain (irregularly and unevenly distributed) homogeneous and isotropic masses. According to this view, Earth is situated in such a homogeneous and isotropic mass. In general, limited cosmological observations have shown greater energy density at greater luminosity distances. More/less dense regions in the heterogeneous distribution may be determined only if its structure remains stable over the time that light travels between different parts of the structure.
If a heterogeneous universe were detectable, some of the following may apply:
- Galaxies of the same diameter and angular size in the sky would have significantly different redshifts from which different values for the Hubble constant would be observed.
- Populations of the same proportion of blue irregular galaxies to regular galaxies may have different values of redshifts, leading to different Hubble constants which are heterogeneously distributed over large angular scales.
Challenging validity of the cosmologial principle
Standard assumption that the observed high-degree of isotropy of the cosmic microwave background radiation (CMB), combined with the Copernican principle, necessarily forces the universe to be homogeneous (i.e., the cosmological principle), is seriously undermined by some recent investigations. 2
In 2008, researchers studying fluctuations in the cosmic microwave background caused by the scattering of its microwave photons by hot X-ray-emitting gas inside clusters of galaxies found that the 700 clusters reaching out up to 6 billion light-years are all moving nearly 3.2 million km/h toward a 20-degree region in the sky between the constellations of Centaurus and Vela. This flow is difficult to explain by gravitation and may be indicative of a tilt exerted across the visible universe by far-away pre-inflationary inhomogeneities.3
See also
References
- ^ http://en.wikipedia.org/en/Image:CMB_Timeline75.jpg - NASA (public domain image)
- ^ Richard K Barrett and Chris A Clarkson, Undermining the cosmological principle: almost isotropic observations in inhomogeneous cosmologies, Quantum Grav. Retrieved 7 December 2006.
- ^ Kashlinsky, A., et al. (2008) "A measurement of large-scale peculiar velocities of clusters of galaxies: results and cosmological implications." Astrophysical Journal Letters (Science Daily, Space.com)
