1,558 research outputs found
Friedmann universe with dust and scalar field
We study a spatially flat Friedmann model containing a pressureless perfect
fluid (dust) and a scalar field with an unbounded from below potential of the
form V(\fii)=W_0 - V_0\sinh(\sqrt{3/2}\kappa\fii), where the parameters
and are arbitrary and . The model is
integrable and all exact solutions describe the recollapsing universe. The
behavior of the model near both initial and final points of evolution is
analyzed. The model is consistent with the observational parameters. We single
out the exact solution with the present-day values of acceleration parameter
and dark matter density parameter describing
the evolution within the time approximately equal to .Comment: 11 pages, 10 figure
Models of G time variations in diverse dimensions
A review of different cosmological models in diverse dimensions leading to a
relatively small time variation of the effective gravitational constant G is
presented. Among them: 4-dimensional general scalar-tensor model,
multidimensional vacuum model with two curved Einstein spaces, multidimensional
model with multicomponent anisotropic "perfect fluid", S-brane model with
scalar fields and two form field etc. It is shown that there exist different
possible ways of explanation of relatively small time variation of the
effective gravitational constant G compatible with present cosmological data
(e.g. acceleration): 4-dimensional scalar-tensor theories or multidimensional
cosmological models with different matter sources. The experimental bounds on
G-dot may be satisfied ether in some restricted interval or for all allowed
values of the synchronous time variable.Comment: 27 pages, Late
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