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Search Results: 1 - 10 of 9043 matches for " Santiago;Perez "
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Inflationary solutions and inhomogeneous Kaluza-Klein cosmology in 4+n dimensions
Santiago E. Perez Bergliaffa
Physics , 1997,
Abstract: We analyze the existence of inflationary solutions in an inhomogeneous Kaluza-Klein cosmological model in 4+n dimensions. It is shown that the 5-dimensional case is the exception rather than the rule, in the sense that the system is integrable (under the assumption of the equation of state $\rho= kp$) for any value of k. It is also shown that the cases k=0 and k=1/3 are integrable if and only if n=1.
Propagation of perturbations in non-linear spin-2 theories
Santiago E. Perez Bergliaffa
Physics , 2003,
Abstract: In this communication I analyze the problem of complete exceptionality of wave propagation in a class of spin 2 field theories. I show that, under the imposition of the good weak-field limit, only two Lagrangians are completely exceptional. These are the linear Fierz Lagrangian, and a Born-Infeld-like Lagrangian. As a byproduct, I reobtain the result that in a nonlinear theory, spin 2 particles follow an effective metric that depends on the nonlinearities of the Lagrangian.
An overview of f(R) theories
Santiago Esteban Perez Bergliaffa
Physics , 2011,
Abstract: A brief introduction to theories of the gravitational field with a Lagrangian that is a function of the scalar curvature is given. The emphasis will be placed in formal developments, while comparison to observation will be discussed in the chapter by S. Jor\'as in this volume.
Inhomogeneous Multidimensional Cosmologies
Santiago E. Perez Bergliaffa
Physics , 1999,
Abstract: Einstein's equations for a 4+n-dimensional inhomogeneous space-time are presented, and a special family of solutions is exhibited for an arbitrary n. The solutions depend on two arbitrary functions of time. The time development of a particular member of this family is studied. This solution exhibits a singularity at t=0 and dynamical compactification of the n dimensions. It is shown that the behaviour of the system in the 4-dimensional i.e. post-compactification phase is constrained by the way in which the compactified dimensions are stabilized. The fluid that generates the solution is analyzed by means of the energy conditions.
Nonsingular cosmological models
Santiago Esteban Perez Bergliaffa
Physics , 2011,
Abstract: A short introduction to cosmological models that go from an era of accelerated collapse to an expanding era without displaying a singularity is presented.
Constraining f(R) theories with the energy conditions
Santiago Esteban Perez Bergliaffa
Physics , 2006, DOI: 10.1016/j.physletb.2006.10.003
Abstract: A new method to constrain gravitational theories depending on the Ricci scalar is presented. It is based on the weak energy condition and yields limits on the parameters of a given theory through the current values of the derivatives of the scale factor of the Friedmann-Robertson-Walker geometry. A further constraint depending on the current value of the snap is also given. Actual constraints (and the corresponding error propagation analysis) are calculated for two examples, which show that the method is useful in limiting the possible $f(R)$ theories.
Effective metric in nonlinear scalar field theories
E. Goulart,Santiago Esteban Perez Bergliaffa
Physics , 2011, DOI: 10.1103/PhysRevD.84.105027
Abstract: We discuss several features of the propagation of perturbations in nonlinear scalar field theories using the effective metric. It is shown that the effective metric can be classified according to whether the gradient of the scalar field is timelike, null, or spacelike, and this classification is illustrated with two examples. We shall also show that different signatures for the effective metric are allowed.
The world is not enough
Luis A. Anchordoqui,Santiago E. Perez Bergliaffa
Physics , 2000, DOI: 10.1103/PhysRevD.62.067502
Abstract: We show that the 5-dimensional model introduced by Randall and Sundrum is (half of) a wormhole, and that this is a general result in models of the RS type. We also discuss the gravitational trapping of a scalar particle in 5-d spacetimes. Finally, we present a simple model of brane-world cosmology in which the background is a static anti-de Sitter manifold, and the location of the two 3-branes is determined by the technique of ``surgical grafting''.
The Transcervical Extended Access, a Feasible Approach for the Surgical Treatment of Benign Tumors of the Posterior Mediastinum?  [PDF]
David Perez, Francisco Hernandez, Jose Ramon Cano, Wolker Tavarez, Gara Torrent, Santiago Quevedo, Luis Lopez
Surgical Science (SS) , 2018, DOI: 10.4236/ss.2018.93014
Abstract: Transcervical approach for tumors of the posterior mediastinum is traditionally thought not to be indicated. Hereby we report on a case of a patient with a huge neurogenic tumor of the posterior mediastinum which was successfully excised through a transcervical extended approach and, additionally, the variety of surgical approaches used to remove tumors of the posterior mediastinum is discussed. The new refined techniques of transcervical extended mediastinal operations, which are recently gaining popularity among surgeons, allowed for a safe dissection of the tumor, thus patient could benefit from a short painless postoperative course. The authors suggest that surgeons trained in these particular techniques should consider the choice of the transcervical extended access in selected cases of benign tumors of the posterior mediastinum.
Heavy metals in rocks and stream sediments from the northwestern part of Baja California, México
Leticia Rosales Hoz,Arturo Carranza Edwards,Susana Santiago Perez
Revista internacional de contaminación ambiental , 1994,
Abstract: Con el objeto de evaluar la cantidad de metales pesados suministrados por procesos naturales de intemperismo en el área costera del noroeste de Baja California, México, se estudiaron rocas que doran en las cuencas y fragmentos de rocas y sedimentos de los ríos y arroyos que drenan hacia esta área. Se evaluó la concentración de elementos mayores (Si, Al, Na, K, Ca, Mg, P, Fe, Mn y Ti) así como algunos elementos traza (Cu, Cd, Zn, Ni, Cr y Pb). Los sedimentos del Río Guadalupe son los que muestran el mayor grado de alteración, en tanto que los sedimentos del arroyo de San Antonio el menor grado. La concentración de metales pesados en los sedimentos estudiados fue de: 0.02 a 0.17% de MnO%, 0.07 a 1.49% de Ti02, 10 a 20 ppm de Cu, 10 pprn de Cu, 10 pprn de Cd, 30 a 140 pprn de Zn, 30 a 40 pprn de Ni, no detectado a 150 ppm de Cr. Las concentraciones evaluadas en los sedimentos están dentro de los límites establecidos para suelos no contaminados, con excepción del Cr cuyo límite máximo permitido en Suiza es de 75 pprn y de 100 pprn en Alemania.
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