Past Seminars

(beginning Sept. 2006)
Seminarios de Física Teórica
Theoretical Physics Seminar
Fisika Teorikoa: Hitzaldiak
Simulación y Modelización en la Enseñanza de la Física
Francisco Esquembre
Universidad de Murcia
Date: October 4, 2006.
Consistent theories for spin two gravitons
Enric Verdaguer
Universitat de Barcelona
Date: Thursday, November 16, 2006
Abstract: We consider some flat space theories for spin 2 gravitons. For the massless case, classical stability and absence of ghosts require invariance under transverse diffeomorphisms (TDiff). Generic TDiff invariant theories contain a propagating scalar, which disappears if the symmetry is enhanced in one of two ways. One possibility is to consider full diffeomorphisms (Diff). The other (which we denote WTDiff) adds a Weyl symmetry, by which the Lagrangian becomes independent of the trace. For massive gravitons, the situation is more restrictive. Whereas the non-linear extension of Diff leads to General Relativity, the non-linear extension of WTDiff leads to Einstein's unimodular theory. Non-linear extensions of the TDiff theory corresponds to scalar-tensor theories with an integration constant. It is at present unclear whether Diff, rather than TDiff or WTDiff, is a fundamental symmetry of Nature.
Observational constraints on phantom-like braneworld cosmologies
Elisabetta Majerotto
Institute of Cosmology and Gravitation, Portsmouth
Date: Tuesday, November 21, 2006
Abstract: We investigate a simple braneworld model in which the universe contains only cold dark matter and a cosmological constant, but the effective dark energy is phantom-like because of extra-dimensional gravity effects. Modified gravity screens the cosmological constant Λ, allowing for a larger Lambda. In practice, observations do not favour any significant screening. We use supernova data, the cosmic microwave background shift parameter, and the baryon oscillation peak in the galaxy distribution to constrain the model. We find the mean value of Ωm with 68% confidence limits, and an upper limit on ΩΛ at the 68% confidence level. The best-fit model is very close to a standard LCDM model, but the LCDM model provides a better fit since it has one less parameter.
La Lagrangiana de la interacción de dos partículas de Dirac
Martín Rivas Pérez
UPV-EHU
Date: Wednesday, November 29, 2006
Abstract:
Se ha demostrado recientemente que el grupo de simetrías espacio-temporales de una partícula de Dirac es mas amplio que el grupo de Poincaré. Contiene, entre otras, dilataciones espacio-temporales y rotaciones locales del sistema de referencia ligado al punto. Se analizará la Lagrangiana de interacción entre dos partículas de Dirac, que es invariante bajo el nuevo grupo de simetrías espacio-temporales. En el límite de suprimir el contenido del espín de las partículas, nos lleva a la interacción Coulombiana instantánea. Se analizarán varios ejemplos de interacción de dos partículas y, entre otros, la posibilidad de formación de pares ligados de dos partículas de la misma carga.
Initial data sets for the Schwarzschild spacetime
Alfonso García-Parrado Gómez-Lobo
Linköping University
Date: Wednesday, December 13, 2006
Abstract: In this work a characterisation of initial data sets whose development is a portion of Schwarzschild spacetime is provided. This characterisation is obtained by performing a 3+1 decomposition of a certain invariant characterisation of the Schwarzschild spacetime given in terms of concomitants of the Weyl tensor. This procedure supplies a set of necessary conditions ---which can be written in terms of the electric and magnetic parts of the Weyl tensor and their concomitants--- for an initial data set to be a Schwarzschild initial data set. Sufficient conditions can be obtained by appending conditions ensuring the existence of a stationary Killing vector field in the initial data development. Such conditions are known and we write them down in a manner adapted to our problem. In this way, we obtain an algorithmic procedure to check whether the development of a given initial data set is a patch of Schwarzschild spacetime. An explicit example involving time-symmetric initial data is presented.
Preparándonos para la Física del LHC con el Experimento CMS
Jorge Fernández de Trocóniz
Universidad Autónoma de Madrid
Date: Wednesday, December 13, 2006
Abstract: The Compact Muon Solenoid (CMS) is one of the huge experiments preparing for data taking at CERN Large Hadron Collider (LHC), starting in 2007. Our main goal is to reveal the Electroweak Symmetry Breaking mechanism (Higgs boson discovery) and/or other signatures of Physics beyond the Standard Model of Elementary Particles and their Interactions (supersymmetry, etc). In the last years, the CMS Collaboration has been working in building the detector. In particular the UAM-CMS Group is responsible for the design, production, and operation of the Level-1 Muon Trigger. The talk will describe the experiment on the "eve" of data taking (motivation, status, and prospects for the immediate future). It will be dedicated to physicists and Physics students, but not necessarily specialists in High Energy Physics.
Causalidad y Geometría Lorentziana Conforme Global en Relatividad General
Miguel Sánchez Caja
Universidad de Granada
Date: Thursday, December 14, 2006
Abstract: En Relatividad General, cualquier problema global parte de fuertes suposiciones sobre la causalidad del espacio-tiempo. Históricamente, la Teoría Causal se ha identificado con la estructura conforme de los espacio-tiempos, lo que contituye el objeto de estudio de una rama de la Geometría Lorentziana. Los fundamentos de esa Teoría se asentaron a finales de los setenta, pero dos hechos recientes han suscitado un nuevo interés. De un lado, la solución de algunos problemas "folclóricos" que habían quedado abiertos en la etapa fundacional. De otro, una nueva propuesta matemática sobre qué significa realmente la causalidad de un espacio-tiempo. El propósito de la presente charla es dar una breve revisión de tales ideas.
A problem of unusual gravity
Alan Barnes
Aston University
Date: Wednesday, December 20, 2006
Abstract: Two well-known properties of the Newtonian potential GM/r are considered: i) the attraction outside a spherically symmetrical distribution of matter is the same as that produced by a point mass located at the centre; ii) the attraction inside a spherically symmetrical shell is zero. Some years ago Kilmister posed the problem of whether any other potentials satisfy property (i). In this paper all potentials satisfying this property are found explicitly. All potentials satisfying property (ii) are also listed. For cylindrically symmetrical systems the logarithmic potential has analogous properties: iii) the attraction outside a cylindrically symmetrical distribution of matter is the same as that produced by a line source located at the centre; iv) the attraction inside a cylindrically symmetrical shell is zero. All two-dimensional potentials satisfying properties (iii) and (iv) are also found explicitly. The possibility of generalising these results to relativistic gravitational theories is discussed.
Quantum Kaluza-Klein
Enrique Álvarez
Instituto de Física Teórica, UAM
Date: Wednesday, January 17, 2007

For more information, contact I.L. Egusquiza (wtpegegi (at) lg.ehu.es)
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Last update: Oct 29 2007