# cosmological models

## cosmological models

(world models) Possible representations of the Universe in simple terms. Models are an essential link between observation and theory and act as the basis for prediction. Complications are added only when necessary (Occam's razor). A simple model for a two-dimensional Universe is the surface of an expanding balloon, on which Hubble's law and the isotropy of the microwave background radiation may be demonstrated.

Most standard cosmological models of the Universe are mathematical and are based on the Friedmann Universe, derived by Aleksandr Friedmann in 1922 and independently by Georges Lemaître in 1927. They assume the homogeneity and isotropy of an expanding (or contracting) Universe in which the only force that need be considered is gravitation. The Big Bang theory is such a model. These models result from considerations of Einstein's field equations of general relativity. When the pressure is negligible the equations reduce to

(dR /dt )2/R 2 + kc 2/R 2 = (8π/3)G ρ

for energy conservation – this is known as the Friedmann equation – and

ρR 3 = constant

for mass conservation. R is the cosmic scale factor, ρ the mean density of matter, G the gravitational constant, and c the speed of light; k is the curvature index of space of value +1 (closed Universe), –1 (open Universe), or 0 (flat or Einstein–de Sitter Universe). See illustration.

Other models involving the cosmological constant, λ, have been proposed, such as the de Sitter model, in which no mass is present, the Lemaître model, which exhibits a coasting phase during which R is roughly constant, the steady-state theory, and those in which the gravitational constant, G, varies with time (see Brans–Dicke theory). The cosmological constant is an arbitrary constant. Although it is possible for it to have any value that does not conflict with observation, it is highly probable that it is close to zero. Cosmological models involving λ have recently come back into fashion. See also static Universe.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006
References in periodicals archive ?
According to &nbsp;Zhong-Zhi of Harvard University and the study's co-author, the key to testing out the theories is to check how the universe behaves within these cosmological models. Should there be a primordial universe before our own, then basic understanding of physics will tell us that there should be massive particles that oscillated at a regular frequency&nbsp;much like how the pendulum of a clock swings back and forth.
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van den Hoogen, "Dynamics of multiscalar-field cosmological models and assisted inflation," Physical Review D: Particles, Fields, Gravitation and Cosmology, vol.
GRBs with this redshift value usually are used for calibrating cosmological models as distance markers [22]..
Among his topics are observational overview, the geometry of the universe, simple cosmological models, the density of the universe and dark matter, the cosmic microwave background, and the initial singularity.
By correlating data on hydrogen clouds in the Milky Way with ongoing star formation, data from the new radio telescopes will support real numbers that can be entered into the cosmological models.
Abraham Zelmanov's profundity "sine qua non" is reflected in the singular creation of the theories of chronometric, kinemetric, and orthometric (monad) formalism in General Relativity, the Infinite Relativity Principle, the Anthropic Principle, the extensive classification of all possible cosmological models in the space-time of General Relativity (the Zelmanov Classification, including the possibility of absolute reference frames in a deforming, rotating, gravitating closed finite Universe), and many others (see the website of The Abraham Zelmanov Journal for details, and in particular the 2012 foreword to the book Particles Here and Beyond the Mirror).
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The team suspects such monster galaxy clusters are rare in the early universe, based on current cosmological models. The immense size of El Gordo was first reported in January 2012.
To determine the mass and size of planets found around other stars or to date stellar populations in order to limit the number of cosmological models, among other things, it is essential to know what goes on inside a star.

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