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  • 1
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    Springer
    Publication Date: 2012-05-14
    Description:    Birkhoff’s theorem for spherically symmetric vacuum spacetimes is a key theorem in studying local systems in general relativity theory. However realistic local systems are only approximately spherically symmetric and only approximately vacuum. In a previous paper, we showed the theorem remains approximately true in an approximately spherically symmetric vacuum space time. In this paper we prove the other case: the theorem remains approximately true in a spherically symmetric, approximately vacuum space time. Content Type Journal Article Category Research Article Pages 1-14 DOI 10.1007/s10714-012-1376-x Authors Rituparno Goswami, ACGC and Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch, 7701 South Africa George F. R. Ellis, ACGC and Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch, 7701 South Africa Journal General Relativity and Gravitation Online ISSN 1572-9532 Print ISSN 0001-7701
    Print ISSN: 0001-7701
    Electronic ISSN: 1572-9532
    Topics: Physics
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  • 2
    Publication Date: 2012-02-16
    Description:    Explaining the well established observation that the expansion rate of the universe is apparently accelerating is one of the defining scientific problems of our age. Within the standard model of cosmology, the repulsive ‘dark energy’ supposedly responsible has no explanation at a fundamental level, despite many varied attempts. A further important dilemma in the standard model is the lithium problem, which is the substantial mismatch between the theoretical prediction for 7 Li from Big Bang Nucleosynthesis and the value that we observe today. This observation is one of the very few we have from along our past worldline as opposed to our past lightcone. By releasing the untested assumption that the universe is homogeneous on very large scales, both apparent acceleration and the lithium problem can be easily accounted for as different aspects of cosmic inhomogeneity, without causing problems for other cosmological phenomena such as the cosmic microwave background. We illustrate this in the context of a void model. Content Type Journal Article Category Editor’s Choice (Research Article) Pages 1-13 DOI 10.1007/s10714-011-1322-3 Authors Marco Regis, Centre for Astrophysics, Cosmology and Gravity, Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch, Cape Town, 7701 South Africa Chris Clarkson, Centre for Astrophysics, Cosmology and Gravity, Department of Mathematics and Applied Mathematics, University of Cape Town, Rondebosch, Cape Town, 7701 South Africa Journal General Relativity and Gravitation Online ISSN 1572-9532 Print ISSN 0001-7701
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    Electronic ISSN: 1572-9532
    Topics: Physics
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  • 3
    Publication Date: 2012-02-28
    Description:    The expansion of space, and other geometric properties of cosmological models, can be studied using geometrically defined notions of relative velocity. In this paper, we consider test particles undergoing radial motion relative to comoving (geodesic) observers in Robertson-Walker cosmologies, whose scale factors are increasing functions of cosmological time. Analytical and numerical comparisons of the Fermi, kinematic, astrometric, and the spectroscopic relative velocities of test particles are given under general circumstances. Examples include recessional comoving test particles in the de Sitter universe, the radiation-dominated universe, and the matter-dominated universe. Three distinct coordinate charts, each with different notions of simultaneity, are employed in the calculations. It is shown that the astrometric relative velocity of a radially receding test particle cannot be superluminal in any expanding Robertson-Walker spacetime. However, necessary and sufficient conditions are given for the existence of superluminal Fermi speeds, and it is shown how the four concepts of relative velocity determine geometric properties of the spacetime. Content Type Journal Article Category Research Article Pages 1-31 DOI 10.1007/s10714-012-1344-5 Authors Vicente J. Bolós, Dpto. Matemáticas para la Economía y la Empresa, Facultad de Economía, Universidad de Valencia, Avda. Tarongers s/n., 46022 Valencia, Spain David Klein, Department of Mathematics and Interdisciplinary Research Institute for the Sciences, California State University, Northridge, CA, USA Journal General Relativity and Gravitation Online ISSN 1572-9532 Print ISSN 0001-7701
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    Topics: Physics
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  • 4
    Publication Date: 2012-02-28
    Description:    A general class of holographic superconductor models in Schwarzschild AdS black hole with the Born–Infeld electrodynamics is investigated. It is found that the Born–Infeld coupling parameter affects the critical temperature, the order of phase transitions, the conductivity and the critical exponents near the second-order phase transition point. It is also noted that both the position of the conductivity coherence peak and the ratio of the gap frequency in conductivity to the critical temperature depend on the model parameters and the Born–Infeld coupling parameter. Content Type Journal Article Category Research Article Pages 1-11 DOI 10.1007/s10714-012-1341-8 Authors Liancheng Wang, Key Laboratory of Low Dimensional Quantum Structures and Quantum Control of Ministry of Education, Department of Physics, Hunan Normal University, Changsha, 410081 Hunan, People’s Republic of China Jiliang Jing, Key Laboratory of Low Dimensional Quantum Structures and Quantum Control of Ministry of Education, Department of Physics, Hunan Normal University, Changsha, 410081 Hunan, People’s Republic of China Journal General Relativity and Gravitation Online ISSN 1572-9532 Print ISSN 0001-7701
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  • 5
    Publication Date: 2012-02-16
    Description:    In this work we consider the entropy-corrected version of interacting holographic dark energy (HDE), in the non-flat universe enclosed by apparent horizon. Two corrections of entropy so-called logarithmic ‘LEC’ and power-law ‘PLEC’ in HDE model with apparent horizon as an IR-cutoff are studied. The ratio of dark matter to dark energy densities u , equation of state parameter w D and deceleration parameter q are obtained. We show that the cosmic coincidence problem is solved for interacting models. By studying the effect of interaction in EoS parameter of both models, we see that the phantom divide may be crossed and also understand that the interacting models can drive an acceleration expansion at the present and future, while in non-interacting case, this expansion can happen only at the early time. The graphs of deceleration parameter for interacting models, show that the present acceleration expansion is preceded by a sufficiently long period deceleration at past. Moreover, the thermodynamical interpretation of interaction between LECHDE and dark matter is described. We obtain a relation between the interaction term of dark components and thermal fluctuation in a non-flat universe, bounded by the apparent horizon. In limiting case, for ordinary HDE, the relation of interaction term versus thermal fluctuation is also calculated. Content Type Journal Article Category Research Article Pages 1-17 DOI 10.1007/s10714-012-1332-9 Authors A. Khodam-Mohammadi, Department of Physics, Faculty of Science, Bu-Ali Sina University, Hamedan, 65178 Iran M. Malekjani, Department of Physics, Faculty of Science, Bu-Ali Sina University, Hamedan, 65178 Iran Journal General Relativity and Gravitation Online ISSN 1572-9532 Print ISSN 0001-7701
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    Topics: Physics
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  • 6
    Publication Date: 2012-02-18
    Description:    We have studied here black hole entropy in the framework of quantum geometry. It is pointed out that the black hole radiation consistent with Hawking spectrum can be realized as an effect of quantum geometry using a dynamical formalism for diffeomorphism invariance which envisages a discretized unit of time in the Planck scale. This formalism suggests that torsion acts within a quantized area unit (area bit) associated with a loop and this eventually forbids the Hamiltonian constraint to be satisfied for a finite loop size. We assign a spin with torsion in each area bit and entanglement entropy of a black hole is computed in terms of the entanglement entropy of this spin system. We have derived the Bekenstein-Hawking entropy along with a logarithmic correction term with a specific coefficient. Also we have shown that the Bekenstein-Hawking entropy can be formulated in terms of the Noether charge associated with a diffeomorphism invariant Lagrangian. Content Type Journal Article Category Research Article Pages 1-7 DOI 10.1007/s10714-012-1334-7 Authors L. Mullick, Department of Mathematics, Hiralal Mazumdar Memorial College for Women, Dakshineswar, Kolkata, 700 035 India P. Bandyopadhyaya, Physics and Applied Mathematics Unit, Indian Statistical Institute, 203, B.T. Road, Kolkata, 700 108 India Journal General Relativity and Gravitation Online ISSN 1572-9532 Print ISSN 0001-7701
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  • 7
    Publication Date: 2012-02-18
    Description:    Logarithmic corrections to the extremal black hole entropy can be computed purely in terms of the low energy data—the spectrum of massless fields and their interaction. The demand of reproducing these corrections provides a strong constraint on any microscopic theory of quantum gravity that attempts to explain the black hole entropy. Using quantum entropy function formalism we compute logarithmic corrections to the entropy of half BPS black holes in N =2 supersymmetric string theories. Our results allow us to test various proposals for the measure in the OSV formula, and we find agreement with the measure proposed by Denef and Moore if we assume their result to be valid at weak topological string coupling. Our analysis also gives the logarithmic corrections to the entropy of extremal Reissner–Nordstrom black holes in ordinary Einstein–Maxwell theory. Content Type Journal Article Category Research Article Pages 1-60 DOI 10.1007/s10714-012-1336-5 Authors Ashoke Sen, Harish-Chandra Research Institute, Chhatnag Road, Jhusi, Allahabad, 211019 India Journal General Relativity and Gravitation Online ISSN 1572-9532 Print ISSN 0001-7701
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  • 8
    Publication Date: 2012-11-14
    Description:    The purpose of the paper is to define and study, in the presence of the electromagnetic potentials, the fifth force induced by the fifth dimension in the 4 D physics. A 4 D tensor calculus and the Riemannian horizontal connection on a 5 D general Kaluza–Klein space, are the main tools in our study. We prove that the fifth force does not violate the basic concepts in 4 D physics. Our approach is different from what is known so far in literature. Content Type Journal Article Category Research Article Pages 1-15 DOI 10.1007/s10714-012-1482-9 Authors Aurel Bejancu, Department of Mathematics, Kuwait University, Kuwait City, Kuwait Hani Reda Farran, Department of Mathematics, Kuwait University, Kuwait City, Kuwait Journal General Relativity and Gravitation Online ISSN 1572-9532 Print ISSN 0001-7701
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  • 9
    Publication Date: 2012-09-29
    Description:    It is shown for a spherically symmetric black hole of general type that it is impossible to observe the infinite future of the Universe external to the hole during the finite proper time interval of the free fall. Quantitative evaluations of the effect of time dilatation for circular orbits around the Kerr black hole are obtained and it is shown that the effect is essential for ultrarelativistic energies of the rotating particle. Content Type Journal Article Category Research Article Pages 1-9 DOI 10.1007/s10714-012-1453-1 Authors Yuri V. Pavlov, Institute of Problems in Mechanical Engineering, Russian Academy of Sciences, Bol’shoy pr. 61, V.O., Saint Petersburg, 199178 Russia Journal General Relativity and Gravitation Online ISSN 1572-9532 Print ISSN 0001-7701
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    Topics: Physics
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  • 10
    Publication Date: 2012-10-02
    Description:    Ordinary, macroscopic systems, naturally tend to a state of maximum entropy compatible with their constraints. However, this might not hold for gravity-dominated systems since their entropy may increase without bound unless this is precluded by the formation of a black hole. In this short note we suggest, based on the Hubble expansion history, that our Universe likely behaves as an ordinary system, i.e., that its entropy seems to tend to some maximum value. Content Type Journal Article Category Research Article Pages 1-6 DOI 10.1007/s10714-012-1457-x Authors Diego Pavón, Departamento de Física, Universidad Autónoma de Barcelona, 08193 Bellaterra, Barcelona, Spain Ninfa Radicella, Departamento de Física, Universidad Autónoma de Barcelona, 08193 Bellaterra, Barcelona, Spain Journal General Relativity and Gravitation Online ISSN 1572-9532 Print ISSN 0001-7701
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    Topics: Physics
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