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Arxiv (Free subscription) | yesterday
We discuss the noncommutative generalizations of polynomial algebras which after appropriate completions can be used as coordinate algebras in various noncommutative settings, (noncommutative differential geometry, noncommutative algebraic geometry, etc.). These algebras have finite presentations and are completely characterized and classified by their (noncommutative) volume forms.
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Arxiv (Free subscription) | yesterday
We compute the contribution of various gravitational instantons to the path integral in the standard formulation of unimodular gravity, an alternative theory of gravity where the metric has unit determinant. Following computations analogous to those in General Relativity, we derive the entropy/area ratio for cosmological and black hole horizons, finding in general disagreement with General Relativity...
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Arxiv (Free subscription) | yesterday
The field theoretic action for gravitational interactions in d+2 dimensions is constructed in the formalism of 2T-physics. General Relativity in d dimensions emerges as a shadow of this theory with one less time and one less space dimensions. The gravitational constant turns out to be a shadow of a dilaton field in d+2 dimensions that appears as a constant to observers stuck in d dimensions. If elementary...
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Arxiv (Free subscription) | 14/10/2008
We study the quantum-mechanical corrections to two point particles accelerated by a strut in a 2+1 D flat background. Since the particles are accelerating, we use finite temperature techniques to compute the Green's function of a conformally coupled scalar applying transparent and Dirichlet boundary conditions at the location of the strut. We find that the induced energy-momentum tensor diverges at...
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Arxiv (Free subscription) | 14/10/2008
We extend our previous calculation of the quasi-local contribution to the self-force on a scalar particle to general (not necessarily geodesic) motion in a general spacetime. In addition to the general case and the case of a particle at rest in a stationary spacetime, we consider as examples a particle held at rest in Reissner-Nordstrom and Kerr-Newman space-times. This allows us to most easily analyse...
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Arxiv (Free subscription) | 14/10/2008
Recently \cite{Rindler} it has been shown that despite previous claims the cosmological constant affects light bending. In the present article we study light bending and the advance of Mercury's perihelion in the context of the Newtonian limit of the Schwarzschild-de Sitter spacetime employing the special relativistic equivalence of mass and energy. In both cases, up to a constant factor, we find...
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Arxiv (Free subscription) | 14/10/2008
I review the definition of n-point functions in loop quantum gravity, discussing what has been done and what are the main open issues. Particular attention is dedicated to gauge aspects and renormalization.
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Arxiv (Free subscription) | 14/10/2008
An obstruction to the implementation of spatially flat Painleve-Gullstrand(PG) slicings is demonstrated, and explicitly discussed for Reissner-Nordstrom and Schwarzschild-anti-deSitter spacetimes. Generalizations of PG slicings which are not spatially flat but which remain regular at the horizons are introduced. These metrics can be obtained from standard spherically symmetric metrics by physical...
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Arxiv (Free subscription) | 14/10/2008
We model massive dark objects at centers of many galaxies as Schwarzschild black hole lenses and study gravitational lensing by them in detail. We show that the ratio of mass of a Schwarzschild lens to the differential time delay between outermost two relativistic images (both of them either on the primary or on the secondary image side) is extremely insensitive to changes in the angular source position...
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Arxiv (Free subscription) | 14/10/2008
The Osher-Chakrabarthy family of linear flux-modification schemes is considered. Improved lower bounds on the compression factors are provided, which suggest the viability of using the unlimited version. The LLF flux formula is combined with these schemes in order to obtain compact finite-difference algorithms. The resulting schemes are applied to a battery of numerical tests, going from advection...
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Arxiv (Free subscription) | 14/10/2008
In this paper, we study anisotropic spheres builded from known static spherical solutions. In particular, we are interested in the physical consequences caused by a "small" departure from a physically sensible configuration. The obtained solutions smoothly depend on free parameters. By setting these parameters to zero, the starting seed solution is regained. We apply our procedure in detail by taking...
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Arxiv (Free subscription) | 14/10/2008
Whenever and wherever one talks of dark matter, one does so when and where there is a luminous matter and a dynamical issue to be settled. We promote this observation to the status of an axiom and assume that there is a dark companion to every luminous matter and there are orders to this companionship. To pursue the proposition in a formal and quantitative manner, we consider the anomalous rotation...
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Arxiv (Free subscription) | 13/10/2008
We review the unification of early-time inflation with late-time acceleration in several local modified gravity models which pass Solar System and cosmological tests. It is also demonstrated that account of non-local gravitational corrections to the action does not destroy the possibility of such unification. Dark matter effect is caused by composite graviton degree of freedom in such models.
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Arxiv (Free subscription) | 13/10/2008
A classical result in Lorentzian geometry states that a strongly causal spacetime is globally hyperbolic if and only if the Lorentzian distance is finite valued for every metric choice in the conformal class. It is proven here that a non-total imprisoning spacetime is causally simple if and only if for every metric choice in the conformal class the Lorentzian distance is continuous wherever it vanishes....
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Arxiv (Free subscription) | 13/10/2008
I review the development of numerical evolution codes for general relativity based upon the characteristic initial value problem. Progress is traced from the early stage of 1D feasibility studies to 2D axisymmetric codes that accurately simulate the oscillations and gravitational collapse of relativistic stars and to current 3D codes that provide pieces of a binary black hole spacetime. Cauchy codes...
