http://www.saber.ula.ve/handle/123456789/544 Articulos, Pre-prints del Centro de Física Fundamental (CFF) Wed, 22 Apr 2026 10:07:10 GMT 2026-04-22T10:07:10Z http://www.saber.ula.ve/handle/123456789/27197 Colapso gravitacional radiativo esféricamente simétrico en la aproximación poscuasiestática Navarro, A. A.; Núñez, Luis; Rueda H., Jorge A.; Sanabria Gómez, J. D. Se describe la evolución del radio de un objeto esféricamente simétrico en fase de colapso gravitacional radiante en el marco de la relatividad general, usando el factor de flujo de radiación, el factor variable de Eddington y la relación de clausura de Lorentz-Eddington para modelar los efectos de la disipación de radiación en la materia. Se hace uso de la aproximación pos-cuasiestática propuesta por Herrera et al [Phys. Rev D 65,104004(2002)] y de la ecuación de estado Tolman VI. La densidad y presión de materia y radiación en una capa interior del objeto son también mostrados. Las condiciones de energía débil, fuerte y dominante se satisfacen plenamente. Tue, 01 Apr 2008 00:00:00 GMT http://www.saber.ula.ve/handle/123456789/27197 2008-04-01T00:00:00Z http://www.saber.ula.ve/handle/123456789/16047 E-science perspectives in Venezuela. Díaz, Gilberto; Flórez López, Julio; Hamar, Vanessa; Hoeger, Herbert; Mendoza, Claudio; Méndez P., Zulay V.; Núñez, Luis; Ruiz, Nicolás; Torréns Heeren, Rodrigo J.; Uzcátegui, M. E-science perspectives in Venezuela. (Díaz, Gilberto; Flórez López, Julio; Hamar, Vanessa; Hoeger, Herbert; Mendoza, Claudio; Mendez, Z.; Núñez, Luis; Ruiz, Nicolás; Torréns Heeren, Rodrigo J.; Uzcátegui, M.) Abstract We describe the e-Science strategy in Venezuela, in particular initiatives by the Centro Nacional de Cálculo Científico Universidad de Los Andes (CECALCULA), Mérida, the Universidad de Los Andes (ULA), Mérida, and the Instituto Venezolano de Investigaciones Científicas (IVIC), Caracas. We present the plans for the Venezuelan Academic Grid and the current status of Grid ULA supported by Internet2. We show different web-based scientific applications that are being developed in quantum chemistry, atomic physics, structural damage analysis, biomedicine and bioclimate within the framework of the E-Infrastructure shared between Europe and Latin America (EELA). Fri, 29 Aug 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/16047 2008-08-29T09:00:00Z http://www.saber.ula.ve/handle/123456789/16923 Red bioclimática del estado Mérida: repositorio institucional de datos ambientales Contreras Zambrano, Hilda Yelitza; Méndez P., Zulay V.; Torréns Heeren, Rodrigo J.; Núñez, Luis Red bioclimática del estado Mérida: repositorio institucional de datos ambientales. (Contreras Zambrano, Hilda Yelitza; Méndez, Zulay V.; Torréns Heeren, Rodrigo J. y Núñez, Luis) Resumen Presentamos la experiencia en la construcción y desarrollo de la Red Bioclimática del Estado Mérida (Venezuela) como prueba de concepto en la utilización de las técnicas y estrategias para la captura, manejo y preservación de datos ambientales provenientes de la e-investigación. Se describen los elementos éticos, conceptuales y técnicos para la preservación del patrimonio de datos, haciendo énfasis en los conceptos datos libres (Open Data), metadatos y sus estándares. Igualmente el testimonio de la construcción del repositorio de datos bioclimáticos http://www.cecalc.ula.ve/redbc/, sus productos de información asociados y algunos resultados e impactos preliminares de su utilización. Finalizamos con algunas reflexiones y recomendaciones las cuales, quizá, permitan hacer sustentable y replicable esta iniciativa. Fri, 29 Aug 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/16923 2008-08-29T09:00:00Z http://www.saber.ula.ve/handle/123456789/16976 Efecto de las variaciones de la presión térmica sobre el estado del gas interestelar Mehrer Terradillos, Mariana; Parravano, Antonio Efecto de las variaciones de la presión térmica sobre el estado del gas interestelar. (Mehrer Terradillos, Mariana y Parravano, Antonio) Resumen Se estudió el cambio del estado termoquímico del gas interestelar cuando es sometido a fluctuaciones periódicas de presión analizando las trayectorias evolutivas. Se calculó el trabajo hecho sobre el gas en una trayectoria cíclica y se determinó su dependencia con el período T de la presión. Para periodos de oscilación entre 103 y 108 años se varió periódicamente la presión durante 100 ciclos y se calculó el trabajo en cada ciclo, lo cual permitió analizar la capacidad que posee el gas interestelar para radiar la energía mecánica asociada al proceso de compresión y descompresión. Los resultados obtenidos indican que el gas neutro de la Galaxia es capaz de disipar la energía mecánica que las supernovas depositan en el gas. También se encontró un comportamiento caótico en el rango de periodos T = [4,85 - 4,97] Maños, [5,04 - 5,10] Maños y [5,87 - 5,91] Maños. Abstract We study the evolution of the thermo-chemical state of the interstellar gas subject to periodic fluctuations of pressure with period T. Analyzing the evolutives trajectories we calculate the work done on the gas by the external pressure in a cycle and found its dependence with in the range 103 and 108 yr. The results indicate that the neutral gas in the Galaxy is able to dissipate the mechanical energy injected by SNe. We also observe that the work per cycle display a chaotic behavior for periods in the ranges [4,85 - 4,97] Myrs, [5,04 - 5,10] Myrs y [5,87 - 5,91] Myrs. Fri, 25 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/16976 2008-07-25T09:00:00Z http://www.saber.ula.ve/handle/123456789/16981 Lie Groups of Conformal Motions acting on Null Orbits Carot, Jaume; Coley, A. A.; Sintes, A. M. Lie Groups of Conformal Motions acting on Null Orbits (Carot, Jaume; Coley, A. A.; Sintes, A. M.) Abstract Space-times admitting a 3-dimensional Lie group of conformal motions acting on null orbits containing a 2-dimensional Abelian subgroup of isometries are studied. Coordinate expressions for the metric and the conformal Killing vectors (ckv) are provided (irrespective of the matter content) and then all possible perfect fluid solutions are found, although none of these verify the weak and dominant energy conditions over the whole space-time manifold. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/16981 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/16958 Conformal symmetries in warped manifolds. Apostolopoulos, Pantelis S.; Carot, Jaume Conformal symmetries in warped manifolds (P. S., Apostolopoulos; Carot, Jaume.) Abstract The existence of a Conformal Vector Field (CVF) is studied in the important class of warped manifolds of arbitrary dimension generalizing in this way the corresponding results of the four dimensional geometries. As a concrete example we apply the geometric results in the case of brane-world scenarios when the bulk geometry admits a hypersurface orthogonal Killing Vector Field (KVF) and is filled with a perfect fluid matter content. This article was published in Journal of Physics: Conference Series 8 (2005) 28-33 Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/16958 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/16971 Scalar field cosmologies: a dynamical systems study Carot, Jaume; Collinge, Magdalena M. Scalar field cosmologies: a dynamical systems study (Carot, Jaume; Collinge, Magdalena M.) Abstract Inhomogeneous G2 solutions whose material content can be modelled by a scalar field are considered with a view towards their application as cosmological models. Some generic features of these spacetimes are reviewed and discussed, and dynamical systems theory is used to analyse the qualitative behaviour of this class of spacetimes; giving special attention to the equilibrium (fixed) points of the dynamical system and their associated metrics, and discussing their inflationary behaviour. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/16971 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/17024 Scalar field spacetimes Carot, Jaume; Collinge, Magdalena M. Scalar field spacetimes (Carot, Jaume; Collinge, Magdalena M.) Abstract Scalar field spacetimes are considered with a view towards their applications in cosmology. Some results existing in the literature are reviewed and some new results are proved concerning scalar field spacetimes in general and inhomogeneous G2 cosmological models in particular. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17024 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/17048 On the geometry of warped spacetimes Carot, Jaume; da Costa, J. On the geometry of warped spacetimes (Carot, Jaume; da Costa, J.) Abstract An invariant characterization of warped spacetimes is given and a classification scheme for them is proposed. Some results on the curvature structure (Petrov and Segre types of the Weyl and Ricci tensors) are given and a thorough study of the isometry group that each class of warped spacetime may admit is carried out. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17048 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/17045 Energy-momentum types of warped spacetimes Carot, Jaume; Haddow, Barry M. Energy-momentum types of warped spacetimes (Carot, Jaume; Haddow, Barry M. Abstract The possible energy–momentum types of warped product pacetime are investigated using the classification given by Carot and da Costa. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17045 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/16984 Some developments on axial symmetry Carot, Jaume Some developments on axial symmetry (Carot, Jaume) Abstract The definition of axial symmetry in general relativity is reviewed, and some results concerning the geometry in a neighbourhood of the axis are derived. Expressions for the metric are given in different coordinate systems, and emphasis is placed on how the metric coefficients tend to zero when approaching the axis. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/16984 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/16993 Conformal symmetry inheritance in null fluid spacetimes Carot, Jaume; Tupper, Brian O. J.; Hall, J. S.; Keane, A. J.; Coley, A. A. Conformal symmetry inheritance in null fluid spacetimes (Carot, Jaume; Keane, A. J.; Coley, A. A.; Hall, J. S.; Tupper, Brian O. J.) Abstract We define inheriting conformal Killing vectors for null fluid spacetimes and find the maximum dimension of the associated inheriting Lie algebra. We show that for non-conformally flat null fluid spacetimes, the maximum dimension of the inheriting algebra is seven and for conformally flat null fluid spacetimes the maximum dimension is eight. In addition, it is shown that there are two distinct classes of non-conformally flat generalized plane wave spacetimes which possess the maximum dimension, and one class in the conformally flat case. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/16993 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/16991 Conformal symmetries in null Einstein-Maxwell fields Carot, Jaume; Hall, J. S. Conformal symmetries in null Einstein-Maxwell fields (Carot, Jaume; Hall, J. S.) Abstract It is shown that if a null Einstein-Maxwell spacetime admifs a proper conformal vector field it must either be a (generalized type N) pp-wave or a (generalized type III) Goldberg-Kerr metric. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/16991 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/17066 Conformally reducible 2 + 2 spacetimes Carot, Jaume; Tupper, Brian O. J. Conformally reducible 2 + 2 spacetimes (Carot, Jaume; Tupper, Brian O. J.) Abstract Spacetimes which are conformal to 2 + 2 reducible spacetimes are considered. We classify them according to their conformal algebra, giving in each case explicit expressions for the metric and conformal Killing vectors, and providing physically meaningful examples. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17066 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/17058 Homothetic perfect fluid spacetimes Carot, Jaume; Sintes, A. M. Homothetic perfect fluid spacetimes (Carot, Jaume; Sintes, A. M.) Abstract A brief summary of results on homotheties in general relativity is given, including general information about spacetimes admitting an r-parameter group of homothetic transformations for r > 2, as well as some specific results on perfect fluids. Attention is then focused on inhomogeneous models, in particular on those with a homothetic group H4 (acting multiply transitively) and H3. A classification of all possible Lie algebra structures along with (local) coordinate expressions for the metric and homothetic vectors is then provided (irrespectively of the matter content) and some new perfect fluid solutions are given and briefly discussed. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17058 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/17056 Matter collineations: The inverse "symmetry inheritance" problem. Carot, Jaume; da Costa, J.; Vaz, E. G. L. R. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17056 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/17013 Generalised soliton solutions of the Weyl class Verdaguer, Enric; Carot, Jaume Generalised soliton solutions of the Weyl class (Verdaguer, Enric ; Carot, Jaume) Abstract The whole family of generalised soliton solutions of the Weyl class for Einstein's equations in vacuum is considered. A classification scheme for the distinct types of metrics belonging to this family is given and emphasis is placed on the asymptotically flat solutions. The physical interpretation of the occurring parameters is also briefly considered. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17013 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/17009 On the definition of cylindrical symmetry Carot, Jaume; Senovilla, José M. M.; Vera, Raül On the definition of cylindrical symmetry (Carot, Jaume; Senovilla, José M. M.; Vera, Raül) Abstract The standard definition of cylindrical symmetry in general relativity is reviewed. Taking the view that axial symmetry is an essential prerequisite for cylindrical symmetry, it is argued that the requirement of orthogonal transitivity of the isometry group should be dropped, this leading to a new, more general definition of cylindrical symmetry. Stationarity and staticity in cylindrically symmetric spacetimes are then defined, and these issues are analysed in connection with orthogonal transitivity, thus proving some new results on the structure of the isometry group for this class of spacetimes. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17009 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/17015 Self-similar static solutions admitting a 2-space of constant curvature Carot, Jaume; Sintes, A. M. Self-similar static solutions admitting a 2-space of constant curvature (Carot, Jaume; Sintes, A. M.) Abstract A recent result by Haggag and Hajj-Boutros is reviewed within the framework of self-similar spacetimes, extending, in some sense, their results and presenting a family of metrics consisting of all the static spherically symmetric perfect fluid solutions admitting a homothety. Wed, 02 Jul 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17015 2008-07-02T09:00:00Z http://www.saber.ula.ve/handle/123456789/16051 Clusters de PCs. Núñez, Luis; Díaz, Gilberto; Hoeger, Herbert Clusters de PCs. (Núñez, Luis; Díaz, Gilberto; Hoeger, Herbert.) Resumen Los Clusters de PCs se hicieron populares con el proyecto Beowulf. La idea consiste en armar un máquina, con gran poder de computo, interconectando PCs y usando software gratuito como Linux, MPI o PVM, a un costo considerablemente menor que el de supercomputadores comparables. En este trabajo se presenta los distintos aspectos que se deben tomar en cuenta a la hora de implementar un cluster Linux. Mon, 09 Jun 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/16051 2008-06-09T09:00:00Z http://www.saber.ula.ve/handle/123456789/16044 E-science initiatives in Venezuela. Chaves, Juan Luis; Díaz, Gilberto; Hamar, Vanessa; Isea, Raúl; Rojas, Freddy; Ruiz, Nicolás; Torréns Heeren, Rodrigo J.; Uzcátegui, M.; Flórez López, Julio; Hoeger, Herbert; Mendoza, Claudio; Núñez, Luis E-science initiatives in Venezuela. (Chaves, Juan Luis; Díaz, Gilberto; Hamar, Vanessa; Isea, Raúl; Rojas, Freddy; Ruiz, Nicolás; Torréns Heeren, Rodrigo J.; Uzcátegui, M.; Flórez López, Julio; Hoeger, Herbert; Mendoza, Claudio y Núñez, Luis) Abstract Within the context of the nascent e-Science infrastructure in Venezuela, we describe several webbased scientific applications developed at the Centro Nacional de Cálculo Científico Universidad de Los Andes (CECALCULA), Mérida, and at the Instituto Venezolano de Investigaciones Científicas (IVIC), Caracas. The different strategies that have been followed for implementing quantum chemistry and atomic physics applications are presented. We also briefly discuss a damage portal based on dynamic, nonlinear, finite elements of lumped damage mechanics and a biomedical portal developed within the framework of the E-Infrastructure shared between Europe and Latin America (EELA) initiative for searching common sequences and inferring their functions in parasitic diseases such as leishmaniasis, chagas and malaria. Mon, 09 Jun 2008 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/16044 2008-06-09T09:00:00Z http://www.saber.ula.ve/handle/123456789/17072 E-investigación: una nueva manera de producir conocimiento. Núñez, Luis E-investigación: una nueva manera de producir conocimiento. (Núñez, Luis) Resumen Se describen algunas experiencias en e-investigación de en Física de Altas Energías, Astrofísica y Química. Luego se hace una descripción de algunas iniciativas desarrolladas por el Centro Nacional de Cálculo Científico, Universidad de Los Andes (CeCalCULA) en las áreas de e-ingeniería, e-biomedicina y eambiente. Abstract This work displays a brief description of two initiatives concerning e-research in High Energy Physics, Astrophysics and Chemistry. Later on we also describe other initiatives develop at our Centro Nacinal de Calculo Cientifico Universidad de los Andes concerning e-engineering, e-biomedicine and eenvironmental Sciences. Tue, 03 Jul 2007 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17072 2007-07-03T09:00:00Z http://www.saber.ula.ve/handle/123456789/17094 Sound speeds, cracking and stability of self-gravitating anisotropic compact objects. Abreu, Henso; Núñez, Luis; Hernández G., Héctor Sound speeds, cracking and stability of self-gravitating anisotropic compact objects. Abstract Using the the concept of cracking we explore the influence of density fluctuations and local anisotropy have on the stability of local and non-local anisotropic matter configurations in general relativity. This concept, conceived to describe the behaviour of a fluid distribution just after its departure from equilibrium, provides an alternative approach to consider the stability of selfgravitating compact objects. We show that potentially unstable regions within a configuration can be identify as a function of the difference of propagations of sound along tangential and radial directions. In fact, it is found that these regions could occur when, at particular point within the distribution, the tangential speed of sound is greater than radial one. Mon, 25 Jun 2007 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17094 2007-06-25T09:00:00Z http://www.saber.ula.ve/handle/123456789/17030 Crucial test of relativity theory: open currents and magnetic model of light. Spavieri, Gianfranco; Erazo, Jesús; Sánchez, Arturo; Ayazo, José; Rodriguez, M. Crucial test of relativity theory: open currents and magnetic model of light. Spavieri, Gianfranco; Erazo, J.; Sanchez, A.; Ayazo, José and Rodriguez, M. Abstract Fundamental tests of special theory of relativity (STR) are revised. Among crucial tests of the Faraday law we consider the experiment with a nonshielded Trouton-Noble capacitor and the nonconservation of mechanical angular momentum of an isolated charged system. As a new crucial tests of Faraday’s law for open currents, the induction on a coil moving with respect to a static electric charge is outlined. The magnetic model of light propagation in moving media is considered. According to this model the phase velocity of the light wave is affected by the flow but the momentum of particles (photons) is not. Thus, the Fizeau experiment does not corroborate the addition of velocities of STR. A non-interferometric experiment of the Fizeau type represents a crucial test of the magnetic model and of the speed of photons in moving media. Fri, 13 Apr 2007 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17030 2007-04-13T09:00:00Z http://www.saber.ula.ve/handle/123456789/17051 Locality and electromagnetic momentum in critical tests of special relativity. Spavieri, Gianfranco; Quintero M., Jesús A.; Sánchez, Arturo; Ayazo, José; Gillies, George Locality and electromagnetic momentum in critical tests of special relativity. Spavieri, Gianfranco; Quintero, Jesús; Sánchez, Arturo; Ayazo, José y Gillies, George Abstract In this review of recent tests of special relativity it is shown that the electromagnetic momentum plays a relevant role in various areas of classical and quantum physics. Crucial tests on the locality of Faraday's law for "open" currents, on a modified Trouton-Noble experiment, on nonconservation of mechanical angular momentum, on the force on the magnetic dipole, and on a reciprocal Rowland's experiment are outlined. Electromagnetic momentum provides a link also between quantum nonlocal effects and light propagation in moving media. Since light waves in moving media behave as matter waves in nonlocal quantum effects, the ?ow of the medium does a¤ect the phase velocity of light, but not necessarily the momentum of photons. Thus, Fizeau's experiment is not suitable for testing the addition of velocities of special relativity. A crucial, non-interferometric experiment for the speed of photons in moving media, is described. Fri, 13 Apr 2007 09:00:00 GMT http://www.saber.ula.ve/handle/123456789/17051 2007-04-13T09:00:00Z