402 research outputs found

    Conditions for low-redshift positive apparent acceleration in smooth inhomogeneous models

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    It is known that a smooth LTB model cannot have a positive apparent central acceleration. Using a local Taylor expansion method we study the low-redshift conditions to obtain an apparent negative deceleration parameter qapp(z)q^{app}(z) derived from the luminosity distance DL(z)D_L(z) for a central observer in a LTB space, confirming that central smoothness implies a positive central deceleration. Since observational data is only available at redshift greater than zero we find the critical values of the parameters defining a centrally smooth LTB model which give a positive apparent acceleration at z>0z>0, providing a graphical representation of the conditions in the q0app,q1appq_0^{app},q_1^{app} plane, which are respectively the zero and first order terms of the central Taylor expansion of qapp(z)q^{app}(z). We finally derive a coordinate independent expression for the apparent deceleration parameter based on the expansion of the relevant functions in red-shift rather than in the radial coordinate. We calculate qapp(z)q^{app}(z) with two different methods to solve the null geodesic equations, one based on a local central expansion of the solution in terms of cosmic time and the other one using the exact analytical solution in terms of generalized conformal time. %The expansion of the solution in terms of cosmic time is quite useful also for other applications requiring foliation %of space-time in space-like hyper-surfaces, such as spatial averaging, which is much more difficult to study using the %analytical solution in terms of the generalized conformal time coordinate.Comment: 18 pages, 3 figures, abstract, added section with coordinate independent conditions, version accepted for publication in GR

    Coulomb drag between two strange metals

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    We study the Coulomb drag between two strange-metal layers using the Einstein-Maxwell-Dilaton model from holography. We show that the low-temperature dependence of the drag resistivity is ρDT4\rho_D \propto T^4, which strongly deviates from the quadratic dependence of Fermi liquids. We also present numerical results at room temperature, using typical parameters of the cuprates, to provide an estimate of the magnitude of this effect for future experiments. We find that the drag resistivity is enhanced by the plasmons characteristic of the two-layer system.Comment: 5 pages, 3 figure

    Godel-type space-time metrics

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    A simple group theoretic derivation is given of the family of space-time metrics with isometry group SO(2,1) X SO(2) X R first described by Godel, of which the Godel stationary cosmological solution is the member with a perfect-fluid stress-energy tensor. Other members of the family are shown to be interpretable as cosmological solutions with a electrically charged perfect fluid and a magnetic field.Comment: Heavly rewritten respect to the orginal version, corrected some typos due to files transfer in the last submitted versio

    Vector and Tensor Contributions to the Luminosity Distance

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    We compute the vector and tensor contributions to the luminosity distance fluctuations in first order perturbation theory and we expand them in spherical harmonics. This work presents the formalism with a first application to a stochastic background of primordial gravitational waves.Comment: 14 pages, 3 figure

    Approximate Consistency Condition from Running Spectral Index in Slow-Roll Inflationary Models

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    Density perturbations generated from inflation almost always have a spectral index n_s which runs (varies with the wavelength). We explore a running spectral index scenario in which the scalar spectral index runs from blue (n_s >1) on large length scales to red (n_s<1) on short length scales. Specifically, we look for a correlation between the length scale at which n_s-1=0 and the length scale at which tensor to scalar ratio T/S reaches a minimum for single field slow roll inflationary models. By computing the distribution of length scale ratios, we conclude that there indeed is a new approximate consistency condition that is characteristic of running spectral index scenarios that run from blue to red. Specifically, with strong running, we expect 96% of the slow roll models to have the two length scales to be within a factor of 2, with the length scale at which the tensor to scalar ratio reaching a minimum longer than the wavelength at which n_s-1=0.Comment: 18 pages, 2 figure

    Redshift spherical shell energy in isotropic Universes

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    We introduce the redshift spherical shell energy (RSSE), which can be used to test in the redshift space the radial inhomogeneity of an isotropic universe, providing additional constraints for LTB models, and a more general test of cosmic homogeneity.Comment: 11 pages, 2 figures, Accepted by Physical Review D1

    Evolution of Coordination in Social Networks: A Numerical Study

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    Coordination games are important to explain efficient and desirable social behavior. Here we study these games by extensive numerical simulation on networked social structures using an evolutionary approach. We show that local network effects may promote selection of efficient equilibria in both pure and general coordination games and may explain social polarization. These results are put into perspective with respect to known theoretical results. The main insight we obtain is that clustering, and especially community structure in social networks has a positive role in promoting socially efficient outcomes.Comment: preprint submitted to IJMP

    Spatial averaging and apparent acceleration in inhomogeneous spaces

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    As an alternative to dark energy that explains the observed acceleration of the universe, it has been suggested that we may be at the center of an inhomogeneous isotropic universe described by a Lemaitre-Tolman-Bondi (LTB) solution of Einstein's field equations. To test this possibility, it is necessary to solve the null geodesics. In this paper we first give a detailed derivation of a fully analytical set of differential equations for the radial null geodesics as functions of the redshift in LTB models. As an application we use these equaions to show that a positive averaged acceleration aDa_D obtained in LTB models through spatial averaging can be incompatible with cosmological observations. We provide examples of LTB models with positive aDa_D which fail to reproduce the observed luminosity distance DL(z)D_L(z). Since the apparent cosmic acceleration aFLRWa^{FLRW} is obtained from fitting the observed luminosity distance to a FLRW model we conclude that in general a positive aDa_D in LTB models does not imply a positive aFLRWa^{FLRW}.Comment: 16 pages, 12 figures. Explicit derivation of the fully analytical null geodesic equations has been added. Published in GR

    Testing homogeneity with galaxy number counts : light-cone metric and general low-redshift expansion for a central observer in a matter dominated isotropic universe without cosmological constant

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    As an alternative to dark energy it has been suggested that we may be at the center of an inhomogeneous isotropic universe described by a Lemaitre-Tolman-Bondi (LTB) solution of Einstein's field equations. In order to test this hypothesis we calculate the general analytical formula to fifth order for the redshift spherical shell mass. Using the same analytical method we write the metric in the light-cone by introducing a gauge invariant quantity G(z)G(z) which together with the luminosity distance DL(z)D_L(z) completely determine the light-cone geometry of a LTB model.Comment: 13 page
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