73 research outputs found
On interrelations between Sibgatullin's and Alekseev's approaches to the construction of exact solutions of the Einstein-Maxwell equations
The integral equations involved in Alekseev's "monodromy transform" technique
are shown to be simple combinations of Sibgatullin's integral equations and
normalizing conditions. An additional complex conjugation introduced by
Alekseev in the integrands makes his scheme mathematically inconsistent;
besides, in the electrovac case all Alekseev's principal value integrals
contain an intrinsic error which has never been identified before. We also
explain how operates a non-trivial double-step algorithm devised by Alekseev
for rewriting, by purely algebraic manipulations and in a different (more
complicated) parameter set, any particular specialization of the known
analytically extended N-soliton electrovac solution obtained in 1995 with the
aid of Sibgatullin's method.Comment: 7 pages, no figures, section II extende
Is there a prescribed parameter's space for the adiabatic geometric phase?
The Aharonov-Anandan and Berry phases are determined for the cyclic motions
of a non-relativistic charged spinless particle evolving in the superposition
of the fields produced by a Penning trap and a rotating magnetic field.
Discussion about the selection of the parameter's space and the relationship
between the Berry phase and the symmetry of the binding potential is given.Comment: 7 pages, 2 figure
Extracellular ferritin contributes to neuronal injury in an in vitro model of ischemic stroke
Previous clinical and experimental studies have shown that neurological decline and poor functional outcome after acute ischemic stroke in humans are associated with high ferritin levels in serum and cerebrospinal fluid (CSF) within 24 hours of ischemic stroke onset. The aim of the present study was to find out if and how high extracellular ferritin concentrations can increase the excitotoxicity effect in a neuronal cortical culture model of stroke. Extracellular ferritin (100 ng/ml) significantly increased the excitotoxic effect caused by excessive exogenous glutamate (50 µM and 100 µM) by leading to an increase in lipid peroxidation, a reduction in mitochondrial membrane potential and a decrease in neuron viability. Extracellular apoferritin (100 ng/ml), the iron-free form of the protein, does not increase the excitotoxicity of glutamate, which proves that iron was responsible for the neurotoxic effect of the exogenous ferritin. We present evidence that extracellular ferritin iron exacerbate the neurotoxic effect induced by glutamate excitotoxicity and that the effect of ferritin iron is dependent of glutamate excitotoxicity. Our results support the idea that body iron overload is involved in the severity of the brain damage caused by stroke and reveal the need to control systemic iron homeostasis
Charged Dual String Vacua from Interacting Rotating Black Holes Via Discrete and Nonlinear Symmetries
Using the stationary formulation of the toroidally compactified heterotic
string theory in terms of a pair of matrix Ernst potentials we consider the
four-dimensional truncation of this theory with no U(1) vector fields excited.
Imposing one time-like Killing vector permits us to express the stationary
effective action as a model in which gravity is coupled to a matrix Ernst
potential which, under certain parametrization, allows us to interpret the
matter sector of this theory as a double Ernst system. We generate a web of
string vacua which are related to each other via a set of discrete symmetries
of the effective action (some of them involve S-duality transformations and
possess non-perturbative character). Some physical implications of these
discrete symmetries are analyzed and we find that, in some particular cases,
they relate rotating black holes coupled to a dilaton with no Kalb--Ramond
field, static black holes with non-trivial dilaton and antisymmetric tensor
fields, and rotating and static naked singularities. Further, by applying a
nonlinear symmetry, namely, the so-called normalized Harrison transformation,
on the seed field configurations corresponding to these neutral backgrounds, we
recover the U(1)^n Abelian vector sector of the four-dimensional action of the
heterotic string, charging in this way the double Ernst system which
corresponds to each one of the neutral string vacua, i.e., the stationary and
the static black holes and the naked singularities.Comment: 19 pages in latex, added referenc
Cosmological simulations using a static scalar-tensor theory
We present CDM -body cosmological simulations in the framework of
a static general scalar-tensor theory of gravity. Due to the influence of the
non-minimally coupled scalar field, the gravitational potential is modified by
a Yukawa type term, yielding a new structure formation dynamics. We present
some preliminary results and, in particular, we compute the density and
velocity profiles of the most massive group.Comment: 4 pages, 6 figures, to appear in Journal of Physics: Conference
Series: VII Mexican School on Gravitation and Mathematical Physics. 26
November to 1 December 2006, Playa del Carmen, Quintana Roo, Mexic
How can exact and approximate solutions of Einstein's field equations be compared?
The problem of comparison of the stationary axisymmetric vacuum solutions
obtained within the framework of exact and approximate approaches for the
description of the same general relativistic systems is considered. We suggest
two ways of carrying out such comparison: (i) through the calculation of the
Ernst complex potential associated with the approximate solution whose form on
the symmetry axis is subsequently used for the identification of the exact
solution possessing the same multipole structure, and (ii) the generation of
approximate solutions from exact ones by expanding the latter in series of
powers of a small parameter. The central result of our paper is the derivation
of the correct approximate analogues of the double-Kerr solution possessing the
physically meaningful equilibrium configurations. We also show that the
interpretation of an approximate solution originally attributed to it on the
basis of some general physical suppositions may not coincide with its true
nature established with the aid of a more accurate technique.Comment: 32 pages, 5 figure
Colliding Axion-Dilaton Plane Waves from Black Holes
The colliding plane wave metric discovered by Ferrari and Iba\~{n}ez to be
locally isometric to the interior of a Schwarzschild black hole is extended to
the case of general axion-dilaton black holes. Because the transformation maps
either black hole horizon to the focal plane of the colliding waves, this
entire class of colliding plane wave spacetimes only suffers from the formation
of spacetime singularities in the limits where the inner horizon itself is
singular, which occur in the Schwarzschild and dilaton black hole limits. The
supersymmetric limit corresponding to the extreme axion-dilaton black hole
yields the Bertotti-Robinson metric with the axion and dilaton fields flowing
to fixed constant values. The maximal analytic extension of this metric across
the Cauchy horizon yields a spacetime in which two sandwich waves in a
cylindrical universe collide to produce a semi-infinite chain of
Reissner-Nordstrom-like wormholes. The focussing of particle and string
geodesics in this spacetime is explored.Comment: 19 pages, 6 figure
Determinación de la condición de riesgo para el proceso de reinyección de recortes de perforación en pozos agotados en dos plataformas costa fuera en el suroeste de la Sonda de Campeche
Potential risks associated with the cuttings re-injection process as an alternative method for drilling wastes disposal were identi- fied and assessed in two offshore platforms located at the southwest of the Campeche Sonda in Mexico. Risks were evaluated using the HAZOP methodology, to identifying hazards in 11 nodes. In addition, deviations in the process were analyzed, and each risk analyzed was categorized as tolerable or non-tolerable and providing some recommendations. Consequences derived from accidental spills of slurry and/or cuttings were analyzed using the mathematical model YAXUM 3D to simulate the pollu- tants dispersion in seawater in three different climatic periods. The results of the consequences analysis shows that even the concentrations in the discharge point exceeded the recommended criteria for protection of the marine life and seawater quality, the spill is rapidly dispersed complying with permitted levels in 8 hr as a maximum.Los riesgos potenciales asociados al proceso de reinyección de recortes como un método alternativo para la disposición de los desechos de perforación fueron identificados y calculados en dos plataformas costa fuera situadas en el suroeste de la Sonda de Campeche en México. Los riesgos fueron evaluados usando la metodología Hazop, para identificar peligros en once nodos dentro del sistema de reinyección de recortes de perforación. Además, fueron analizadas las desviaciones en el proceso y cada riesgo analizado fue categorizado como tolerable o no-tolerable; asimismo, se propusieron algunas recomendaciones. Las consecuencias derivadas de derrames accidentales de lechada y/o recortes de perforación fueron analizados usando el modelo matemático YAXUM 3D para simular la dispersión de los agentes contaminantes en agua de mar en tres periodos climáticos. Los resultados del análisis de consecuencias demuestran que aun cuando las concentraciones en el punto de descarga excedieron los criterios recomendados para la protección de la vida de marina y de la calidad del agua de mar, el derrame es rápidamente dispersado cumpliendo con los niveles permitidos en ocho horas como máxim
Intermittent Hypobaric Hypoxic Preconditioning Provides Neuroprotection by Increasing Antioxidant Activity, Erythropoietin Expression and Preventing Apoptosis and Astrogliosis in the Brain of Adult Rats Exposed to Acute Severe Hypoxia
Background: Exposure to intermittent hypoxia has been demonstrated to be an efficienttool for hypoxic preconditioning, preventing damage to cells and demonstrating therapeutic benefits.We aimed to evaluate the effects of respiratory intermittent hypobaric hypoxia (IHH) to avoid braininjury caused by exposure to acute severe hypoxia (ASH). Methods: biomarkers of oxidative damage,mitochondrial apoptosis, and transcriptional factors in response to hypoxia were assessed by Westernblot and immunohistochemistry in brain tissue. Four groups of rats were used: (1) normoxic (NOR),(2) exposed to ASH (FiO27% for 6 h), (3) exposed to IHH for 3 h per day over 8 days at 460 mmHg,and (4) ASH preconditioned after IHH. Results: ASH animals underwent increased oxidative-stress-related parameters, an upregulation in apoptotic proteins and had astrocytes with phenotype formscompatible with severe diffuse reactive astrogliosis. These effects were attenuated and even preventedwhen the animals were preconditioned with IHH. These changes paralleled the inhibition of NF-κBexpression and the increase of erythropoietin (EPO) levels in the brain. Conclusions: IHH exertedneuroprotection against ASH-induced oxidative injury by preventing oxidative stress and inhibitingthe apoptotic cascade, which was associated with NF-κB downregulation and EPO upregulatio
Thin-shell wormholes with a generalized Chaplygin gas in Einstein-Born-Infeld theory
We construct spherically symmetric thin-shell wormholes supported by a
generalized Chaplygin gas in Born-Infeld electrodynamics coupled to Einstein
gravity, and we analyze their stability under radial perturbations. For
different values of the Born-Infeld parameter and the charge, we compare the
results with those obtained in a previous work for Maxwell electrodynamics. The
stability region in the parameter space reduces and then disappears as the
value of the Born-Infeld parameter is modified in the sense of a larger
departure from Maxwell theory.Comment: 9 pages, 6 figures; v2: improved versio
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