155 research outputs found
Gravitational entropy of cosmic expansion
We apply a recent proposal to define "gravitational entropy" to the expansion
of cosmic voids within the framework of non-perturbative General Relativity. By
considering CDM void configurations compatible with basic observational
constraints, we show that this entropy grows from post-inflationary conditions
towards a final asymptotic value in a late time fully non-linear regime
described by the Lemaitre-Tolman-Bondi (LTB) dust models. A qualitatively
analogous behavior occurs if we assume a positive cosmological constant
consistent with a -CDM background model. However, the term
introduces a significant suppression of entropy growth with the terminal
equilibrium value reached at a much faster rate.Comment: 5 Pages, 2 figures, AN style. Contribution to Proceedings of the
Second Caribbean Symposium on Nuclear and Astroparticle Physics (STARS 2013),
La Habana, Cuba, 5-9 May 201
Quasi-local variables and scalar averaging in LTB dust models
We introduce quasi--local (QL) scalar variables in spherically symmetric LTB
models. If the QL scalars are defined as functionals, they become weighed
averages that generalize the standard proper volume averages on space slices
orthogonal to the 4-velocity. We examine the connection between QL functions
and functionals and the "back-reaction" term in the context of Buchert's
scalar averaging formalism. With the help of the QL scalars we provide rigorous
proof that back--reaction is positive for (i) all LTB models with negative and
asymptotically negative spatial curvature, and (ii) models with positive
curvature decaying to zero asymptotically in the radial direction. We show by
means of qualitative, but robust, arguments that generic LTB models exist,
either with clump or void profiles, for which an "effective" acceleration
associated with Buchert's formalism can mimic the effects of dark energy.Comment: Submitted contribution for the proceedings of the "Invisible Universe
Internationale Confernce", Palais de L'UNESCO, Paris, France, June 29-July 3,
2009. Jean-Michel ALIMI, editor. 10 pages, 2 figures. AIP macros: layoutstyle
8x11singl
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