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J. M. Salim

Publications and source records attributed to J. M. Salim.

At least 19 recordsLinked to original sources
arXiv2014

Weyl geometry and gauge-invariant gravitation

We provide a gauge-invariant theory of gravitation in the context of Weyl Integrable Space-Times. After making a brief review of the theory's postulates, we carefully define the observers' proper-time and point out its relation with space-time description. As a consequence of this relation and the theory's gauge symmetry we recover all predictions of General Relativity. This feature is made even clearer by a new exact solution we provide which reveals the importance of a well defined proper-time. The thermodynamical description of the source fields is given and we observe that each of the geometric fields have a certain physical significance, despite the gauge-invariance. This is shown by two examples, where one of them consists of a new cosmological constant solution. Our conclusions highlight the intimate relation among test particles trajectories, proper-time and space-time description which can also be applied in any other situation, whether or not it recovers General Relativity results and also in the absence of a gauge symmetry.

gr-qc↗
arXiv2014

Analogue black holes for light rays in static dielectrics

Propagation of light in nonlinear materials is here studied in the regime of the geometrical optics. It is shown that a spherically symmetric medium at rest with some specific dielectric properties can be used to produce an exact analogue model for a class of space-times which includes spherically symmetric and static black hole solutions. The optical model here presented can be a useful tool to reproduce in laboratory the behavior of optical null geodesics near a compact object with an observable gravitational Schwarzschild radius.

gr-qc↗
arXiv2013

Friedmann-like solutions with a non-vanishing Weyl tensor

We have solved the Einstein equations of general relativity for a class of metrics with constant spatial curvature and found a non-vanishing Weyl tensor in the presence of an energy-momentum tensor with an anisotropic pressure component. The time evolution of the spacetime is guided by the usual Friedmann equations and the constraints on the hypersurface comprise a separated system of equations that can be independently solved. Contrary to the apparent behavior induced by some choices of coordinates, the metric we have obtained is completely regular everywhere and is free of singularities (except the well-known Friedmann singularity at $t=0$). The physical features of this solution are elucidated by using the Quasi-Maxwellian equations (a set of third order differential equations describing the dynamics of the gravitational field in terms of the Weyl tensor). The motion of test particles is also analyzed in order to confirm the maximal extension of the manifold under consideration. These results indicate that the anisotropic pressure could mimic dark matter effects on certain geodesic congruences keeping the cosmic flow unchanged.

physics.gen-ph↗
arXiv2013

The Quasi-Maxwellian Equations of General Relativity: Applications to the Perturbation Theory

A comprehensive review of the equations of general relativity in the quasi-Maxwellian (QM) formalism introduced by Jordan, Ehlers and Kundt is made. Our main interest concerns its applications to the analysis of the perturbation of standard cosmology in the Friedman-Lemaître-Robertson-Walker framework. The major achievement of the QM scheme is its use of completely gauge independent quantities. We shall see that in the QM-scheme we deal directly with observable quantities. This reveals its advantage over the old method introduced by Lifshitz et al that deals with perturbation in the standard Einstein framework. For completeness, we compare the QM-scheme to the gauge-independent method of Bardeen, a procedure consisting on particular choices of the perturbed variables as a combination of gauge dependent quantities.

gr-qc↗
arXiv2013

Geometric scalar theory of gravity

We present a geometric scalar theory of gravity. Our proposal will be described using the "background field method" introduced by Gupta, Feynman and others as a field theory formulation of general relativity. We analyze previous criticisms against scalar gravity and show how the present proposal avoids these difficulties. This concerns not only the theoretical complaints but also those related to observations. In particular, we show that the widespread belief of the conjecture that the source of scalar gravity must be the trace of the energy-momentum tensor - which is one of the main difficulties to couple gravity with electromagnetic phenomenon in previous models - does not apply to our geometric scalar theory. Some consequences of the new scalar theory are explored.

gr-qc↗
arXiv2011

Weyl Geometry as Characterization of Space-Time

Motivated by an axiomatic approach to characterize space-time it is investigated a reformulation of Einstein's gravity where the pseudo-riemannian geometry is substituted by a Weyl one. It is presented the main properties of the Weyl geometry and it is shown that it gives extra contributions to the trajectories of test particles, serving as one more motivation to study general relativity in Weyl geometry. It is introduced its variational formalism and it is established the coupling with other physical fields in such a way that the theory acquires a gauge symmetry for the geometrical fields. It is shown that this symmetry is still present for the red-shift and it is concluded that for cosmological models it opens the possibility that observations can be fully described by the new geometrical scalar field. It is concluded then that this reformulation, although representing a theoretical advance, still needs a complete description of their objects.

gr-qc↗
arXiv2009

On a Geometrical Description of Quantum Mechanics

We show that Quantum Mechanics can be interpreted as a modification of the Euclidean nature of 3-d space into a particular Weyl affine space which we call Q-wis. This is proved using the Bohm-de Broglie causal formulation of Quantum Mechanics. In the Q-wis geometry, the length of extended objects changes from point to point. In our proposed geometrical formulation, deformation of the standard rulers used to measure physical distances are in the core of quantum effects.

gr-qc↗
arXiv2008

Bounce and wormholes

We investigate if theories yielding bouncing cosmological models also generate wormhole solutions. We show that two of them present sensible traversable static wormhole solutions, while for the third possibility such solutions are absent.

gr-qc↗
arXiv2008

Cyclic Magnetic Universe

Recent works have shown the important role Nonlinear Electrodynamics (NLED) can have in two crucial questions of Cosmology, concerning particular moments of its evolution for very large and for low-curvature regimes, that is for very condensed phase and at the present period of acceleration. We present here a toy model of a complete cosmological scenario in which the main factor responsible for the geometry is a nonlinear magnetic field which produces a FRW homogeneous and isotropic geometry. In this scenario we distinguish four distinct phases: a bouncing period, a radiation era, an acceleration era and a re-bouncing. It has already been shown that in NLED a strong magnetic field can overcome the inevitability of a singular region typical of linear Maxwell theory; on the other extreme situation, that is for very weak magnetic field it can accelerate the expansion. The present model goes one step further: after the acceleration phase the universe re-bounces and enter in a collapse era. This behavior is a manifestation of the invariance under the dual map of the scale factor $ a(t) \to 1/ a(t),$ a consequence of the corresponding inverse symmetry of the electromagnetic field ($ F \to 1/ F,$ where $F \equiv F^{μν}F_{μν}$) of the NLED theory presented here. Such sequence collapse-bouncing-expansion-acceleration-re-bouncing-collapse constitutes a basic unitary element for the structure of the universe that can be repeated indefinitely yielding what we call a Cyclic Magnetic Universe.

astro-ph↗
arXiv2006

Creation of cosmological magnetic fields in a bouncing cosmology

We show (in a completely analytical and exact manner) that an efficient magnetic field amplification method is operative during the bounce in a time-dependent gauge coupling model. The cosmological magnetic fields so generated have particular spectral features, and may be observed by future CMB measurements and by direct cluster measurements.

astro-ph↗
arXiv2006

Cosmological Effects of Nonlinear Electrodynamics

It will be shown that a given realization of nonlinear electrodynamics, used as source of Einstein's equations, generates a cosmological model with interesting features, namely a phase of current cosmic acceleration, and the absence of an initial singularity, thus pointing to a way to solve two important problems in cosmology.

gr-qc↗
arXiv2004

Production of photons in a bouncing universe

Using a new non-singular solution,it is shown that the production of photons in dilaton electrodynamics in a cosmological setting is increased if the effect of matter creation on the geometry is taken into account. This increment may be related to the problem of the origin of magnetic fields in the universe.

astro-ph↗
arXiv2003

The stability of a bouncing universe

We investigate the stability of a spatially homogeneous and isotropic non-singular cosmological model. We show that the complete set of independent perturbations (the electric part of the perturbed Weyl tensor and the perturbed shear) are regular and well behaved functions which have no divergences, contrary to previous claims in the literature.

hep-th↗
arXiv2002

Nonlinear electrodynamics and FRW cosmology

Maxwell electrodynamics, considered as a source of the classical Einstein field equations, leads to the singular isotropic Friedmann solutions. We show that this singular behavior does not occur for a class of nonlinear generalizations of the electromagnetic theory. A mathematical toy model is proposed for which the analytical nonsingular extension of FRW solutions is obtained.

gr-qc↗
arXiv2002

On the self-consistence of electrodynamics in the early universe

The issue of a self-consistent solution of Maxwell-Einstein equations achieves a very simple form when all quantum effects are neglected but a weak vacuum polarization due to an external magnetic field is taken into account. From a semi-classical point of view this means to deal with an appropriate limit of the one-loop effective Lagrangian for electrodynamics. When the corresponding stress-energy tensor is considered as a source of the gravitational field a surprisingly bouncing behavior is obtained. The present toy model leads to important new features which should have taken place in the early universe.

gr-qc↗
arXiv2001

A model for time-dependent cosmological constant

We present a model for a spacetime dependent cosmological {\it constant}. We make a realization of this model based on a possible quantum aspects of the initial stage of the universe and relate the cosmological constant with the chiral anomaly.

hep-th↗