140,849 research outputs found

    Is classical flat Kasner spacetime flat in quantum gravity?

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    Quantum nature of classical flat Kasner spacetime is studied using effective spacetime description in loop quantum cosmology. We find that even though the spacetime curvature vanishes at the classical level, non-trivial quantum gravitational effects can arise. For the standard loop quantization of Bianchi-I spacetime, which uniquely yields universal bounds on expansion and shear scalars and results in a generic resolution of strong singularities, we find that a flat Kasner metric is not a physical solution of the effective spacetime description, except in a limit. The lack of a flat Kasner metric at the quantum level results from a novel feature of the loop quantum Bianchi-I spacetime: quantum geometry induces non-vanishing spacetime curvature components, making it not Ricci flat even when no matter is present. The non-curvature singularity of the classical flat Kasner spacetime is avoided, and the effective spacetime transits from a flat Kasner spacetime in asymptotic future, to a Minkowski spacetime in asymptotic past. Interestingly, for an alternate loop quantization which does not share some of the fine features of the standard quantization, flat Kasner spacetime with expected classical features exists. In this case, even with non-trivial quantum geometric effects, the spacetime curvature vanishes. These examples show that the character of even a flat classical vacuum spacetime can alter in a fundamental way in quantum gravity and is sensitive to the quantization procedure.Comment: 14 pages, 2 figures. Prepared for IJMPD special issue on Loop Quantum Cosmolog

    Uniform-Velocity Spacetime Crystals

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    We perform a comprehensive analysis of uniform-velocity bilayer spacetime crystals, combining concepts of conventional photonic crystallography and special relativity. Given that a spacetime crystal consists of a sequence of spacetime discontinuities, we do this by solving the following sequence of problems: 1) the spacetime interface, 2) the double spacetime interface, or spacetime slab, 3) the unbounded crystal, and 4) the truncated crystal. For these problems, we present the following respective new results: 1) an extension of the Stokes principle to spacetime interfaces, 2) an interference-based analysis of the interference phenomenology, 3) a quick linear approximation of the dispersion diagrams, a description of simultaneous wavenumber and frequency bandgaps, and 4) the explanation of the effects of different types of spacetime crystal truncations, and the corresponding scattering coefficients. This work may constitute the foundation for a virtually unlimited number of novel canonical spacetime media and metamaterial problems

    Point particle kinematics and light propagation in de Sitter spacetime

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    V t√©to pr√°ci se zab√Ĺv√°me de Sitterovou speci√°ln√≠ relativiou, kter√° se odehr√°v√° v de Sitterovńõ ńćasoprostoru nam√≠sto Minkowsk√©ho ńćasoprostoru. Zańć√≠n√°me studi√≠ grupy symetrie de Sitterova ńćasoprostoru. SnaŇĺ√≠me se pouŇĺ√≠t tyto poznatky k rozvinut√≠ kinematiky v de Sitterovńõ ńćasoprostoru. D√°le pŇôezkoum√°v√°me spojen√≠ mezi elektrodynamikou na pevnńõ zvolen√©m ńćasoprostoru a elektrodynamikou v makroskopick√©m m√©diu v ploch√©m ńćasoprostoru. Toto aplikujeme na de SitterŇĮv ńćasoprostor a hled√°me index lomu sdruŇĺen√©ho m√©dia.In this thesis we study de Sitterian special relativity, which takes place in de Sitter spacetime instead of Minkowski spacetime. We start with study of the symmetry group of de Sitter spacetime. We try to use these results to develop kinematics in this spacetime. We also review connection between electrodynamics on fixed background spacetime and electrodynamics in macroscopic media in flat spacetime. We apply this on de Sitter spacetime and find refractive index of associated macroscopic media.

    A spacetime dual to the NUT spacetime

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    By decomposing the Riemann curvature into electric and magnetic parts, a duality transformation, which involves interchange of active and passive electric parts, has recently been proposed. It was shown that the Schwarzschild solution is dual to the one that describes the Schwarzschild particle with cloud of string dust or a global monopole. Following the same procedure we obtain the solution dual to the NUT spacetime.Comment: 10 pages, Latex. Accepted for publication in Classical and Quantum Gravit
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