# Past Seminars

*(beginning Sept. 2005)*

**Seminarios
de Física Teórica**

**Theoretical Physics Seminar**

**Fisika Teorikoa: Hitzaldiak**

**Vortices in Bose Condensates**

*Eugene Zaremba*

Queen's University, Canada, and DIPC

Date: Wednesday, September 28, 2005.

**The Space-Time Symmetry Group Of A spin 1/2 Elementary Particle**

*Martín Rivas*

Física Teórica, UPV-EHU

Date: Wednesday, November 2, 2005.

**Abstract**:
The space-time symmetry group of a model of a relativistic spin 1/2
elementary particle, which satisfies Dirac's equation when quantized, is analysed.
It is shown that this group, larger than the
Poincar\'e group, also contains space-time dilations and local rotations. Its
similarities with the standard model are discussed. It has two Casimir operators,
one is the spin and the other can be interpreted as the isospin.
The elementary particle is a massive system of arbitray charge, spin 1/2 and isospin
1/2 and can be in a coloured or colourless state, where the colour observable is
also an internal spin contribution related to the zitterbewegung.

**Causal structure: a new viewpoint.**

Alfonso García Parrado

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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**Last update: Oct 2 2006**