278 research outputs found

    On the absence of the usual weak-field limit, and the impossibility of embedding some known solutions for isolated masses in cosmologies with f(R) dark energy

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    This version deposited at arxiv 02-10-12 arXiv:1210.0730v1. Subsequently published in Physical Review D as Phys. Rev. D 87, 063517 (2013) http://link.aps.org/doi/10.1103/PhysRevD.87.063517. Copyright American Physical Society (APS).11 pages11 pages11 pages11 pagesThe problem of matching different regions of spacetime in order to construct inhomogeneous cosmological models is investigated in the context of Lagrangian theories of gravity constructed from general analytic functions f(R), and from non-analytic theories with f(R)=R^n. In all of the cases studied, we find that it is impossible to satisfy the required junction conditions without the large-scale behaviour reducing to that expected from Einstein's equations with a cosmological constant. For theories with analytic f(R) this suggests that the usual treatment of weak-field systems may not be compatible with late-time acceleration driven by anything other than a constant term of the form f(0), which acts like a cosmological constant. For theories with f(R)=R^n we find that no known spherically symmetric vacuum solutions can be matched to an expanding FLRW background. This includes the absence of any Einstein-Straus-like embeddings of the Schwarzschild exterior solution in FLRW spacetimes

    Irrotational dust with Div H=0

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    For irrotational dust the shear tensor is consistently diagonalizable with its covariant time derivative: σab=0=σ˙ab,  a≠b\sigma_{ab}=0=\dot{\sigma}_{ab},\; a\neq b, if and only if the divergence of the magnetic part of the Weyl tensor vanishes: div H=0div~H = 0. We show here that in that case, the consistency of the Ricci constraints requires that the magnetic part of the Weyl tensor itself vanishes: Hab=0H_{ab}=0.Comment: 19 pages. Latex. Also avaliable at http://shiva.mth.uct.ac.za/preprints/text/lesame2.te

    Cosmological dynamics of Scalar--Tensor Gravity

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    We study the phase--space of FLRW models derived from Scalar--Tensor Gravity where the non--minimal coupling is F(ϕ)=ξϕ2F(\phi)=\xi\phi^2 and the effective potential is V(ϕ)=λϕnV(\phi)=\lambda \phi^n. Our analysis allows to unfold many feature of the cosmology of this class of theories. For example, the evolution mechanism towards states indistinguishable from GR is recovered and proved to depend critically on the form of the potential V(ϕ)V(\phi). Also, transient almost--Friedmann phases evolving towards accelerated expansion and unstable inflationary phases evolving towards stable ones are found. Some of our results are shown to hold also for the String-Dilaton action.Comment: 25 pages, 4 figures, 12 tables, submitted to CQ
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