7,405 research outputs found

    Radiating Collapse with Vanishing Weyl stresses

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    In a recent approach in modelling a radiating relativistic star undergoing gravitational collapse the role of the Weyl stresses was emphasised. It is possible to generate a model which is physically reasonable by approximately solving the junction conditions at the boundary of the star. In this paper we demonstrate that it is possible to solve the Einstein field equations and the junction conditions exactly. This exact solution contains the Friedmann dust solution as a limiting case. We briefly consider the radiative transfer within the framework of extended irreversible thermodynamics and show that relaxational effects significantly alter the temperature profiles.Comment: 10 pages, submitted to IJMP-

    Endocranial Morphology of the Extinct North American Lion (Panthera atrox)

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    The extinct North American lion (Panthera atrox) is one of the largest felids (Mammalia, Carnivora) to have ever lived, and it is known from a plethora of incredibly well-preserved remains. Despite this abundance of material, there has been little research into its endocranial anatomy. CT scans of a skull of P. atrox from the Pleistocene La Brea Tar pits were used to generate the first virtual endocranium for this species and to elucidate previously unknown details of its brain size and gross structure, cranial nerves, and inner-ear morphology. Results show that its gross brain anatomy is broadly similar to that of other pantherines, although P. atrox displays less cephalic flexure than either extant lions or tigers, instead showing a brain shape that is reminiscent of earlier felids. Despite this unusual reduction in flexure, the estimated absolute brain size for this specimen is one of the largest reported for any felid, living or extinct. Its encephalization quotient (brain size as a fraction of the expected brain mass for a given body mass) is also larger than that of extant lions but similar to that of the other pantherines. The advent of CT scans has allowed nondestructive sampling of anatomy that cannot otherwise be studied in these extinct lions, leading to a more accurate reconstruction of endocranial morphology and its evolution

    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

    Variations on Birkhoff's theorem

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    The relation between the expanding universe and local vacuum solutions, such as that for the Solar System, is crucially mediated by Birkhoff's theorem. Here we consider how that relation works, and give generalizations of Birkhoff's theorem when there are geometric and matter and perturbations. The issue of to what degree dark matter might influence the solar system emerges as a significant question.Comment: Conference proceeding for ERE 2012, submitted to GRG for ERE2012 special issue, based on arXiv:1005.1809, arXiv:1101.4520 and arXiv:1202.024

    Corrections to Tri-bimaximal Neutrino Mixing: Renormalization and Planck Scale Effects

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    We study corrections to tri-bimaximal (TBM) neutrino mixing from renormalization group (RG) running and from Planck scale effects. We show that while the RG effects are negligible in the standard model (SM), for quasi-degenerate neutrinos and large tanβ\tan\beta in the minimal supersymmetric standard model (MSSM) all three mixing angles may change significantly. In both these cases, the direction of the modification of θ12\theta_{12} is fixed, while that of θ23\theta_{23} is determined by the neutrino mass ordering. The Planck scale effects can also change θ12\theta_{12} up to a few degrees in either direction for quasi-degenerate neutrinos. These effects may dominate over the RG effects in the SM, and in the MSSM with small tanβ\tan \beta. The usual constraints on neutrino masses, Majorana phases or tanβ\tan \beta stemming from RG running arguments can then be relaxed. We quantify the extent of Planck effects on the mixing angles in terms of "mismatch phases" which break the symmetries leading to TBM. In particular, we show that when the mismatch phases vanish, the mixing angles are not affected in spite of the Planck scale contribution. Similar statements may be made for μ\mu-τ\tau symmetric mass matrices.Comment: 21 pages, 3 eps figures. Comments added, to appear in PR

    Effect of grain boundary microstructural features on the fracture behavior of Al-Li alloys

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    Commercial Al-Li-Cu alloys are highly desirable candidate for aircraft components because of their lower density, higher stiffness and high strength as compared to conventional Al alloys. These alloys, however, have low fracture toughness and exhibit brittle intergranular fracture. It is, therefore, important to understand the fracture behavior to design better Al-Li alloys for future aircraft systems. The critical issue is to understand the nature of grain boundary phases and their interfacial characteristics, as these characteristics dictate the fracture toughness of Al-Li alloys, and also to determine the localized dissolution characteristics of grain boundary intermetallic phases at nanoscale. Using transmission electron microscopy (TEM) and density functional theory (DFT) simulations, we recently demonstrated for Al-Cu-Li alloys the failure is likely to occur at the grain boundary T1/matrix interfaces, as these interfaces have low decohesion energies. It was reported that the toughness of the third generation Al-Li alloys has been significantly improved by reducing Li content and by adding small amounts of Ag and Mg. There are unresolved questions as to how Ag and Mg modify the interfacial characteristics of grain boundary/matrix interfaces. We will discuss the microstructure and interfacial characteristics of grain boundary precipitates/matrix interfaces, and correlate with the observed fracture behavior of third generation Al-Li alloys