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.

References in periodicals archive ?
Deffner adds, If we want to understand all these cosmological models, and we want to understand the thermodynamics of the universe, we actually have no means to do that.
edu/2018/02/01/distant-galaxy-group-contradicts-common-cosmological-models-simulations/) another statement : "The significance of this finding is that it calls into question the validity of certain cosmological models and simulations as explanations for the distribution of host and satellite galaxies in the universe.
This observation had huge impacts for cosmological models and continues to serve to remind us that the universe works in strange ways.
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).
This is a big problem that contradicts our standard cosmological models.
The team suspects such monster galaxy clusters are rare in the early universe, based on current cosmological models.
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.
His topics include the basics of geometry and relativity, affine connection and covariant derivative, the geodesic equation and its applications, curvature tensor and Einstein's equation, black holes, and cosmological models and the big bang theory.