Cosmological principle

Physical cosmology

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In physical cosmology, the cosmological principle is an assumption, or working hypothesis, about the large scale structure of the cosmos, stating that:

On large spatial scales, the Universe is homogeneous and isotropic.

In more detail:

The cosmological principle severely restricts the set of possible cosmological theories. The principle is consistent with the observed isotropy of the: (i) spatial distribution of galaxies, (ii) spatial distribution of radio sources, and (iii) cosmic microwave background radiation.^[2][3]

In 1923, Alexander Friedmann set out a variant of Einstein's equations of general relativity that describe the dynamics of a homogeneous isotropic universe. Friedmann's theory was applied a few years later by Eddington and Lemaître.^[4][5][6]

Implications

The homogeneity and isotropy of the universe the Cosmological Principle assumes, suggest that Earth does not occupy a privileged location in the Universe (this is the Copernican principle), and that at very large scales the Universe is smooth (i.e. not fractal). One might reasonably ask whether such properties can actually be verified, given our limited vantage point and the inaccessibility of much of the universe.^[7]

An interesting paradox resulting from this view is the horizon problem: the cosmological principle hypothesizes that similar conditions exist in different regions of the universe too widely separated to have any causal connection. Real evidence for such similar conditions, based upon observations from the vantage of Earth, are an isotropy of the density of galaxies and of the temperature of cosmic microwave background radiation.^[8]

A related 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 distant galaxies reveal that as the distance from the Earth increases, the density of galaxies rises and their "metal" content declines.^[9] 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 extrapolation:

Empirical observations of patterns occurring within a limited scope can shed no light on the state of things outside that scope.

One often hears that extrapolation beyond the sampled population is 'invalid'. We believe that this statement is too strong, and prefer saying that extrapolation of conclusions beyond the sampled population must be based upon additional evidence.^[10]

Causal generalizations from one context to another are a challenge when homogeneity cannot be assumed.^[11]

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.

Observable implications of a heterogeneous universe include:

• 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 would have different values of redshifts, leading to different Hubble constants which are heterogeneously distributed over large angular scales.

Recent findings inconsistent with the cosmological principle

The standard conclusion that the observed isotropy of the cosmic microwave background radiation (CMB), combined with the Copernican principle, requires a homogeneous universe (i.e., the cosmological principle), is called into question by some recent findings.^[12]

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.^[13]

See also

• Copernican principle
• Friedmann-Lemaître-Robertson-Walker metric
• Large scale structure of the cosmos
• Metric expansion of space
• Perfect cosmological principle
• Redshift

References