Física Teórica, UPV-EHU
Date: Wednesday, November 9, 2005.
Abstract:The notion of causal structure has been used in general relativity most of the times in connection with "conformal structure" or "conformal equivalence". Although these concepts of causal structure have proven themselves quite useful, there are cases in which two spacetimes have similar causal properties but no conformal relation between each other exists. In this work we elaborate on the definition of causal structure arising from the concept of causal relationship which was introduced two years ago. This is a generalization of conformal equivalence and it can be applied to a wide range of examples in a straightforward way. Causal structures can be sorted by means of a partial order and we show the extent as to what this ordering generalises the classification of spacetimes according to the "standard hierarchy of causality conditions". We also put forward the concept of stability and instability of the causal structure and explain how it can be applied to examples such as relevant as Minkowski or de Sitter spacetimes. We also study how our methods enable us to give conditions under which two Mp-waves (generalization of pp-waves) have the same causal structure.
Quantum effective actions from nonperturbative worldline dynamics.
Joaquín Sánchez Guillén
Física de Partículas, Universidade de Santiago de Compostela
Date: Monday, November 14, 2005.
Abstract: We demonstrate the feasibility of a nonperturbative analysis of quantum field theory in the worldline formalism with the help of an efficient numerical algorithm. In particular, we compute the effective action for a super-renormalizable field theory with cubic scalar interaction in four dimensions in quenched approximation to all orders in the coupling. We observe that nonperturbative effects exert a strong influence on the infrared behavior, rendering the massless limit well defined in contrast to the perturbative expectation. An interesting application to non-perturbative pair production in inhomogeneous fields will be also presented. (Based on hep-th/0505275 (JHEP 0508 (2005) 067)
Novel electronic properties of graphite compounds.
M. Ángeles Hernández Vozmediano
Instituto de Ciencia de Materiales de Madrid
Date: Wednesday, November 30, 2005.
Abstract: El grafito es bien conocido desde hace tiempo por sus aplicaciones como lubricante, minas de lápices, fabricación de acero o aislante térmico. Bajo un punto de vista más fundamental, los compuestos de intercalación son superconductores de baja temperatura. La síntesis de los fullerenos en los años ochenta supuso una revolución en el estudio de los derivados del grafito y abrió la puerta a la nanotecnología. Una nueva revolución se ha iniciando en los últimos meses tras el descubrmiento de propiedades ferromagnéticas, efecto campo y efecto Hall cuántico en grafito. En este seminario se hará un resumen de la situación y se revisarán las principales ideas teóricas en torno a estas nuevas propiedades. Se describirá brevemente un modelo para explicar el ferromagnetismo en grafito.
Gravedad cuántica con simetría cilíndrica.
Iñaki Garay
Instituto de Estructura de la Materia, CSIC
Date: Monday, December 19, 2005.
Stability properties of marginally outer trapped surfaces
Walter Simon
Instituto de Física Fundamental y Matemáticas, Universidad de Salamanca
Date: Tuesday, December 20, 2005.
Abstract: I discuss stability properties of marginally outer trapped surfaces (MOTS) within some spacelike slice of a spacetime. In particular, stability of a MOTS is shown to be related to the property of being a boundary of the region containing trapped surfaces. Moreover, given a spacetime with a smooth foliation and a strictly stable MOTS on some initial leaf, we show that there is a smooth "horizon" through the MOTS whose marginally outer trapped slices lie in the leaves of the given foliation.
Spin en el formalismo de la worldline
Ricardo Vázquez
Física de Partículas, Universidade de Santiago de Compostela
Date: Tuesday, December 20, 2005.
Semilocal String Networks.
Jon Urrestilla
University of Sussex
Date: Thursday, December 22, 2005.
String Decay.
Roberto Iengo
SISSA
Date: Wednesday, March 22, 2006.
Abstract: We describe general methods for computing the decay rate of very massive SuperStrings and obtain exact numerical results leading to simple rules which provide a consistent picture of the decay of various relevant kinds of strings in the possible channels. We also develop a formalism to investigate the average spectrum of the massless radiation channel and use it to reconstruct the shape of a typical string.
Cosmological significance of one-loop effective gravity
Domènec Espriu
Universitat de Barcelona
Date: Thursday, May 4, 2006.
Abstract: We study the one-loop effective action for gravity in a cosmological setup to determine possible cosmological effects of quantum corrections to Einstein theory. By considering the effect of the universal non-local terms in a toy model, we show that they can play an important role in the very early universe. We find that during inflation, the non-local terms are significant, leading to deviations from the standard inflationary expansion. [based on gr-qc/0503033]
Cosmological inflation on a brane
David Wands
University of Portsmouth
Date: Thursday, June 15, 2006.
Abstract: I will discuss aspects of cosmology on a lower-dimensional brane embedded in a higher-dimensional bulk spacetime. In particular I will discuss novel aspects of inflation in the very early universe driven by a scalar field living on a brane. Higher-dimensional gravity affects both the classical background evolution equations, and the spectrum of quantum fluctuations that gives rise to structure formation after inflation. In the extreme slow-roll limit one can recover suprisingly simple modifications of four-dimensional results, but the problem becomes more difficult when one tries to consistently take into account a finite coupling between scalar field fluctuations on the brane and the bulk gravitational field.
For more information, contact I.L. Egusquiza (wtpegegi (at) lg.ehu.es)
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