The Experts below are selected from a list of 67857 Experts worldwide ranked by ideXlab platform
Alejandro Satz - One of the best experts on this subject based on the ideXlab platform.
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SemiClassical regime of Regge calculus and spin foams
Nuclear Physics B, 2009Co-Authors: Eugenio Bianchi, Alejandro SatzAbstract:Recent attempts to recover the graviton propagator from spin foam models involve the use of a boundary quantum state peaked on a Classical Geometry. The question arises whether beyond the case of a single simplex this suffices for peaking the interior Geometry in a semiClassical configuration. In this paper we explore this issue in the context of quantum Regge calculus with a general triangulation. Via a stationary phase approximation, we show that the boundary state succeeds in peaking the interior in the appropriate configuration, and that boundary correlations can be computed order by order in an asymptotic expansion. Further, we show that if we replace at each simplex the exponential of the Regge action by its cosine -- as expected from the semiClassical limit of spin foam models -- then the contribution from the sign-reversed terms is suppressed in the semiClassical regime and the results match those of conventional Regge calculus.
Namil Park - One of the best experts on this subject based on the ideXlab platform.
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Classical Geometry of De Sitter Spacetime : An Introductory Review
arXiv: High Energy Physics - Theory, 2002Co-Authors: Yoonbai Kim, Namil ParkAbstract:Classical Geometry of de Sitter spacetime is reviewed in arbitrary dimensions. Topics include coordinate systems, geodesic motions, and Penrose diagrams with detailed calculations.
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Classical Geometry of De Sitter Spacetime : An Introductory Review
Journal of the Korean Physical Society, 2002Co-Authors: Yoonbai Kim, Namil ParkAbstract:The cosmological constant Λ was originally introduced in general relativity with the motivation to allow a static homogeneous universe for Einstein’s equations in the presence of matter. Once the expansion of the universe was discovered, its role turned out to be unnecessary, so it has experienced a checkered history. Recent cosmological observations suggest some evidence for the existence of a positive cosmological constant [1]: From a variety of observational sources, its magnitude is (G~/c)Λ ∼ 10−123, and both matter, Ωmatter, and vacuum, ΩΛ, are of comparable magnitude, Ωmatter/ΩΛ ∼ 0.3/0.7 in a spatially flat universe. These two results are paraphrased as the old and the new cosmological constant problems [2, 3]. Inclusion of quantum field theory predicts naturally the energy density via quantum fluctuations even up to the Planck scale, so the old problem is to understand why the present observed cosmological constant is so small. It is the worst fine-tuning problem requiring a magic cancellation with an accuracy to 120 decimal places. The new problem is to understand why the vacuum energy density is comparable to the present mass density of the universe. Though there are several classes of efforts to solve the cosmological constant problem, e.g., cancellation of vacuum energy based on symmetry principles like supersymmetry, the idea of quintessence, and the anthropic principle, none of them is widely accepted as the solution. In the early universe, the existing cosmological constant also provides an intriguing idea, an inflationary cosmology, to fix long-standing cosmological
Eugenio Bianchi - One of the best experts on this subject based on the ideXlab platform.
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SemiClassical regime of Regge calculus and spin foams
Nuclear Physics B, 2009Co-Authors: Eugenio Bianchi, Alejandro SatzAbstract:Recent attempts to recover the graviton propagator from spin foam models involve the use of a boundary quantum state peaked on a Classical Geometry. The question arises whether beyond the case of a single simplex this suffices for peaking the interior Geometry in a semiClassical configuration. In this paper we explore this issue in the context of quantum Regge calculus with a general triangulation. Via a stationary phase approximation, we show that the boundary state succeeds in peaking the interior in the appropriate configuration, and that boundary correlations can be computed order by order in an asymptotic expansion. Further, we show that if we replace at each simplex the exponential of the Regge action by its cosine -- as expected from the semiClassical limit of spin foam models -- then the contribution from the sign-reversed terms is suppressed in the semiClassical regime and the results match those of conventional Regge calculus.
Donatello Dolce - One of the best experts on this subject based on the ideXlab platform.
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Classical Geometry to quantum behavior correspondence in a virtual extra dimension
Annals of Physics, 2012Co-Authors: Donatello DolceAbstract:Abstract In the Lorentz invariant formalism of compact space–time dimensions the assumption of periodic boundary conditions represents a consistent semi-Classical quantization condition for relativistic fields. In Dolce (2011) [18] we have shown, for instance, that the ordinary Feynman path integral is obtained from the interference between the Classical paths with different winding numbers associated with the cyclic dynamics of the field solutions. By means of the boundary conditions, the kinematical information of interactions can be encoded on the relativistic geometrodynamics of the boundary, see Dolce (2012) [8] . Furthermore, such a purely four-dimensional theory is manifestly dual to an extra-dimensional field theory. The resulting correspondence between extra-dimensional geometrodynamics and ordinary quantum behavior can be interpreted in terms of AdS/CFT correspondence. By applying this approach to a simple Quark–Gluon–Plasma freeze-out model we obtain fundamental analogies with basic aspects of AdS/QCD phenomenology.
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Classical Geometry to quantum behavior correspondence in a virtual extra dimension
arXiv: High Energy Physics - Theory, 2011Co-Authors: Donatello DolceAbstract:In the Lorentz invariant formalism of compact space-time dimensions the assumption of periodic boundary conditions represents a consistent semi-Classical quantization condition for relativistic fields. In [arXiv:0903.3680] we have shown, for instance, that the ordinary Feynman path integral is obtained from the interference between the Classical paths with different winding numbers associated with the cyclic dynamics of the field solutions. By means of the boundary conditions, the kinematics information of interactions can be encoded on the relativistic geometrodynamics of the boundary [arXiv:1110.0315]. Furthermore, such a purely four-dimensional theory is manifestly dual to an extra-dimensional field theory. The resulting correspondence between extra-dimensional geometrodynamics and ordinary quantum behavior can be interpreted in terms of AdS/CFT correspondence. By applying this approach to a simple Quark-Gluon-Plasma freeze-out model we obtain fundamental analogies with basic aspects of AdS/QCD phenomenology.
Edward Witten - One of the best experts on this subject based on the ideXlab platform.
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Heterotic String Conformal Field Theory And A-D-E Singularities
Journal of High Energy Physics, 2000Co-Authors: Edward WittenAbstract:We analyze the behavior of the heterotic string near an A-D-E singularity without small instantons. This problem is governed by a strongly coupled worldsheet conformal field theory, which, by a combination of Script O(α') corrections and worldsheet instantons, smooths out the singularities present in the Classical Geometry.
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anti de sitter space thermal phase transition and confinement in gauge theories
arXiv: High Energy Physics - Theory, 1998Co-Authors: Edward WittenAbstract:The correspondence between supergravity (and string theory) on $AdS$ space and boundary conformal field theory relates the thermodynamics of ${\cal N}=4$ super Yang-Mills theory in four dimensions to the thermodynamics of Schwarzschild black holes in Anti-de Sitter space. In this description, quantum phenomena such as the spontaneous breaking of the center of the gauge group, magnetic confinement, and the mass gap are coded in Classical Geometry. The correspondence makes it manifest that the entropy of a very large $AdS$ Schwarzschild black hole must scale ``holographically'' with the volume of its horizon. By similar methods, one can also make a speculative proposal for the description of large $N$ gauge theories in four dimensions without supersymmetry.
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On orbifolds with discrete torsion
Journal of Geometry and Physics, 1995Co-Authors: Cumrun Vafa, Edward WittenAbstract:Abstract We consider the interpretation in Classical Geometry of conformal field theories constructed from orbifolds with discrete torsion. In examples we can analyze, these spacetimes contain “tringy regions” that from a Classical point of view are singularities that are to be neither resolved nor blown up. Some of these models also give particularly simple and clear examples of mirror symmetry.