Results 1 - 10 of 4520
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[en] M-theory effects prevent five-dimensional domain-wall and black-hole solutions from developing curvature singularities. While so far this analysis has been performed for particular models, we now present a model-independent proof that these solutions do not have naked singularities as long as the Kaehler moduli take values inside the extended Kaehler cone. As a by-product we obtain information on the regularity of the Kaehler-cone metric at boundaries of the Kaehler cone and derive relations between the geometry of moduli space and spacetime
[en] In gauge/gravity duality, points which are not causally related on the boundary cannot be causally related through the bulk; this is the statement of boundary causality. By the Gao–Wald theorem, the averaged null energy condition in the bulk is sufficient to ensure this property. Here we proceed in the converse direction: we derive a necessary as well as sufficient condition for the preservation of boundary causality under perturbative (quantum or stringy) corrections to the bulk. The condition that we find is a (background-dependent) constraint on the amount by which light cones can ‘open’ over all null bulk geodesics. We show that this constraint is weaker than the averaged null energy condition. (paper)
[en] The ''effective geometry'' formalism is used to study the perturbations of a perfect barotropic Newtonian self-gravitating rotating and compressible fluid coupled with gravitational backreaction. The case of a uniformly rotating polytrope with index n=1 is investigated, due to its analytical tractability. Special attention is devoted to the geometrical properties of the underlying background acoustic metric, focusing, in particular, on null geodesics as well as on the analog light cone structure.
[en] Non-perturbative approach which we develop is based on representation of the state vector as a set of Fock components. After truncation to a finite number of components, we come to a coupled-channel problem which is solved non-perturbatively, in light-front dynamics, without using any decomposition in degrees of the coupling constant. Previously [J.-F. Mathiot, V.A. Karmanov and A.V. Smirnov, Proc. of Sci., LC2008:024, 2008; (arXiv:0812.1100[hep-ph]).], in Yukawa model, we took into account the Fock sectors corresponding to single fermion, a fermion coupled to one boson, and a fermion coupled to two bosons. Now we extend the Fock space and incorporate also the fermion-antifermion pair, i.e., the Fock sector with two fermions and one antifermion. An appropriate non-perturbative renormalization procedure is developed.
[en] It is shown that unsteadiness of light velocity leads to a red shift effect analogous to Hubble effect due to the expanding Universe. The hypothesis of existence a new kind of physical phenomena influencing the light velocity is given. A more general law of energy conservation is formulated on this basis. (Author)
[en] Continuing work initiated in an earlier publication [Phys. Rev. D 69, 084007 (2004)], we construct a system of light-cone coordinates based at a geodesic world line of an arbitrary curved spacetime. The construction involves (i) an advanced-time or a retarded-time coordinate that labels past or future light cones centered on the world line (ii) a radial coordinate that is an affine parameter on the null generators of these light cones, and (iii) angular coordinates that are constant on each generator. The spacetime metric is calculated in the light-cone coordinates, and it is expressed as an expansion in powers of the radial coordinate in terms of the irreducible components of the Riemann tensor evaluated on the world line. The formalism is illustrated in two simple applications, the first involving a comoving world line of a spatially flat cosmology, the other featuring an observer placed on the axis of symmetry of Melvin's magnetic universe
[en] For gravity coupled to N scalar fields, with arbitrary potential V, it is shown that all flat (homogeneous and isotropic) cosmologies correspond to geodesics in an (N + 1)-dimensional 'augmented' target space of Lorentzian signature (1, N), timelike if V > 0, null if V = 0 and spacelike if V < 0. Accelerating cosmologies correspond to timelike geodesics that lie within an 'acceleration subcone' of the 'lightcone'. Non-flat (k = ±1) cosmologies are shown to evolve as projections of geodesic motion in a space of dimension N + 2, of signature (1, N + 1) for k = -1 and signature (2, N) for k = +1. This formalism is illustrated by cosmological solutions of models with an exponential potential, which are comprehensively analysed; the late-time behaviour for other potentials of current interest is deduced by comparison
[en] We discuss the interpretation of the angles in the geodesic light-cone (GLC) coordinates. In particular, we clarify the way in which these angles can be identified with the observed ones. We show that, although this identification is always possible in principle, one cannot implement it in the usual gauge-fixing way, i.e. through a set of conditions on the GLC metric. Rather, one needs to invoke a tetrad at the observer and a Cartesian-like coordinate system in order to obtain the desired map globally on the observed sky. (note)
[en] We show that the standard result for the Casimir force between conducting plates at rest in an inertial frame can be computed in light-front quantization. This is not the same as light-front analyses where the plates are at “rest” in an infinite momentum frame. In that case, Lenz and Steinbacher have shown that the result does not agree with the standard result for plates at rest. The two important ingredients in the present analysis are a careful treatment of the boundary conditions, inspired by the work of Almeida et al. on oblique light-front coordinates, and computation of the ordinary energy density, rather than the light-front energy density. (author)