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244
Discrete differential forms for computational modeling
 In Discrete Differential Geometry, A. Bobenko
, 2007
"... Abstract. This chapter introduces the background needed to develop a geometrybased, principled approach to computational modeling. We show that the use of discrete differential forms often resolves the apparent mismatch between differential and discrete modeling, for applications varying from grap ..."
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Cited by 77 (14 self)
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Abstract. This chapter introduces the background needed to develop a geometrybased, principled approach to computational modeling. We show that the use of discrete differential forms often resolves the apparent mismatch between differential and discrete modeling, for applications varying from graphics to physical simulations. 1.
Modern tests of Lorentz invariance
 Living Rev. Rel
"... Motivated by ideas about quantum gravity, a tremendous amount of effort over the past decade has gone into testing Lorentz invariance in various regimes. This review summarizes both the theoretical frameworks for tests of Lorentz invariance and experimental advances that have made such high precisio ..."
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Cited by 55 (1 self)
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Motivated by ideas about quantum gravity, a tremendous amount of effort over the past decade has gone into testing Lorentz invariance in various regimes. This review summarizes both the theoretical frameworks for tests of Lorentz invariance and experimental advances that have made such high precision tests possible. The current constraints on Lorentz violating effects from both terrestrial experiments and astrophysical observations are presented.
Élie Cartan’s torsion in geometry and in field theory, an essay
 ANNALES DE LA FONDATION LOUIS DE BROGLIE, MANUSCRIT
, 2007
"... We review the application of torsion in field theory. First we show how the notion of torsion emerges in differential geometry. In the context of a Cartan circuit, torsion is related to translations similar as curvature to rotations. Cartan’s investigations started by analyzing Einsteins general rel ..."
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Cited by 16 (2 self)
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We review the application of torsion in field theory. First we show how the notion of torsion emerges in differential geometry. In the context of a Cartan circuit, torsion is related to translations similar as curvature to rotations. Cartan’s investigations started by analyzing Einsteins general relativity theory and by taking recourse to the theory of Cosserat continua. In these continua, the points of which carry independent translational and rotational degrees of freedom, there occur, besides ordinary (force) stresses, additionally spin moment stresses. In a 3dimensional continuized crystal with dislocation lines, a linear connection can be introduced that takes the crystal lattice structure as a basis for parallelism. Such a continuum has similar properties as a Cosserat continuum, and the dislocation density is equal to the torsion of this connection. Subsequently, these ideas are applied to 4dimensional spacetime. A translational gauge theory of gravity is displayed (in a Weitzenböck or teleparallel spacetime) as well as the viable EinsteinCartan theory (in a RiemannCartan spacetime). In both theories, the notion of torsion is contained in an essential
Boundary Terms, Variational Principles and Higher Derivative Modified Gravity,” Phys
 Rev. D
"... We discuss the criteria that must be satisfied by a wellposed variational principle. We clarify the role of GibbonsHawkingYork type boundary terms in the actions of higher derivative models of gravity, such as F (R) gravity, and argue that the correct boundary terms are the naive ones obtained th ..."
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We discuss the criteria that must be satisfied by a wellposed variational principle. We clarify the role of GibbonsHawkingYork type boundary terms in the actions of higher derivative models of gravity, such as F (R) gravity, and argue that the correct boundary terms are the naive ones obtained though the correspondence with scalartensor theory, despite the fact that variations of normal derivatives of the metric must be fixed on the boundary. We show in the case of F (R) gravity that these boundary terms reproduce the correct ADM energy in the hamiltonian formalism, and the correct entropy for black holes in the semiclassical approximation.
Natural selection for least action
 Proc. R. Soc. A. 2008
"... The second law of thermodynamics is a powerful imperative that has acquired several expressions during the past centuries. Connections between two of its most prominent forms, i.e. the evolutionary principle by natural selection and the principle of least action, are examined. Although no fundamenta ..."
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Cited by 15 (8 self)
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The second law of thermodynamics is a powerful imperative that has acquired several expressions during the past centuries. Connections between two of its most prominent forms, i.e. the evolutionary principle by natural selection and the principle of least action, are examined. Although no fundamentally new findings are provided, it is illuminating to see how the two principles rationalizing natural motions reconcile to one law. The second law, when written as a differential equation of motion, describes evolution along the steepest descents in energy and, when it is given in its integral form, the motion is pictured to take place along the shortest paths in energy. In general, evolution is a nonEuclidian energy density landscape in flattening motion.
Spin2 spectrum of defect theories
 JHEP 1106 (2011) 005
"... We study spin2 excitations in the background of the recentlydiscovered typeIIB solutions of D’Hoker et al. These are holographicallydual to defect conformal field theories, and they are also of interest in the context of the KarchRandall proposal for a stringtheory embedding of localized gravi ..."
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We study spin2 excitations in the background of the recentlydiscovered typeIIB solutions of D’Hoker et al. These are holographicallydual to defect conformal field theories, and they are also of interest in the context of the KarchRandall proposal for a stringtheory embedding of localized gravity. We first generalize an argument by Csaki et al to show that for any solution with fourdimensional antide Sitter, Poincare ́ or de Sitter invariance the spin2 excitations obey the massless scalar wave equation in ten dimensions. For the interface solutions at hand this reduces to a LaplaceBeltrami equation on a Riemann surface with disk topology, and in the simplest case of the supersymmetric Janus solution it further reduces to an ordinary differential equation known as Heun’s equation. We solve this equation numerically, and exhibit the spectrum as a function of the dilatonjump parameter ∆φ. In the limit of large ∆φ a nearlyflat lineardilaton dimension grows large, and the Janus geometry becomes effectively fivedimensional. We also discuss the difficulties of localizing fourdimensional gravity in the more general backgrounds with NS5brane or D5brane charge, which will be analyzed in detail in a companion paper. 1 ar
Relic High Frequency Gravitational waves from the Big Bang, and How to Detect Them,” http://arxiv.org/ftp/arxiv/papers/0809/0809.1454.pdf
 Relic Graviton Production?” in the proceedings of Space Technology and Applications International Forum (STAIF08), edited by M.S. ElGenk, AIP Conference Proceedings 969
, 2008
"... All intext references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately. ..."
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Cited by 10 (7 self)
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All intext references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately.
Trust but verify: The case for astrophysical black holes,” in
 Proceedings of 33rd SLAC Summer Institute on Particle Physics (SSI 2005): Gravity in the Quantum World and the Cosmos, Menlo Park, California, 25 Jul  5 Aug 2005, pp L006 [arXiv:hepph/0511217
"... This article is based on a pair of lectures given at the 2005 SLAC Summer Institute. Our goal is to motivate why most physicists and astrophysicists accept the hypothesis that the most massive, compact objects seen in many astrophysical systems are described by the black hole solutions of general re ..."
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This article is based on a pair of lectures given at the 2005 SLAC Summer Institute. Our goal is to motivate why most physicists and astrophysicists accept the hypothesis that the most massive, compact objects seen in many astrophysical systems are described by the black hole solutions of general relativity. We describe the nature of the most important black hole solutions, the Schwarzschild and the Kerr solutions. We discuss gravitational collapse and stability in order to motivate why such objects are the most likely outcome of realistic astrophysical collapse processes. Finally, we discuss some of the observations which — so far at least — are totally consistent with this viewpoint, and describe planned tests and observations which have the potential to falsify the black hole hypothesis, or sharpen still further the consistency of data with theory. 1. Background Black holes are among the most fascinating and counterintuitive objects predicted by modern physical theory. Their counterintuitive nature comes not from obtuse features of gravitation physics, but rather from the extreme limit of wellunderstood and wellobserved features — the bending of light and the redshifting of clocks by gravity. This limit is inarguably strange, and drives us to predictions that may reasonably be considered bothersome. Given the lack of direct observational or experimental data about gravity in the relevant regime, a certain skepticism about the black hole hypothesis is perhaps reasonable. Indeed, one can understand Eddington’s somewhat plaintive cry “I