Laboratoire de Physique Theorique d'Orsay
 Bâtiment 210 Univ. Paris-Sud 11 91405 Orsay Cedex France T. 01 69 15 63 53 F. 01 69 15 82 87

 Agenda > Séminaires / Seminars > Cosmologie Dernier ajout : jeudi 2 octobre 2014.

Séminaires Cosmologie 2005-2006

 Les séminaires se tiennent en salle 110 au 1er étage du bâtiment 210, généralement le mardi à 10h30, sauf indication contraire. Contact pour les séminaires : Christos Charmousis.   Jeudi 6 Juillet à 11h, au LPT, salle 110 SEMINAIRE ANNULE   Mercredi 14 juin à 10h 30, au LPT, salle 110 Anne Davis (DAMTP, University of Cambridge) : Brane Inflation, D-strings and fermion zero modes Recent developments in string theory suggest that inflation could arise in brane anti-brane systems or in a theory containing a D3/D7 brane. The interbrane distance plays the role of the inflaton. D-strings seem to arise naturally at the end of inflation in most models. I will discuss recent progress in brane inflation, concentrating on the D3/D7 system. I will analyse cosmic strings arising from an effective supergravity theory based on the D3/D7 brane model and calculate the corresponding zero modes. Physical constraints arising will be discussed   Mardi 13 juin à 10h 30, au LPT, salle 110 George Chapline (Lawrence Livermore National Laboratory) : Why Doesn’t the Universe Rotate ? Given the ubiquity of rotation and magnetic fields in the observable universe, it is a bit puzzling as to why the universe as a whole doesn’t rotate. Actually cosmological solutions of Einstein’s equations with rotation are inconsistent with quantum mechanics. However, in the condensate vacuum model for space-time rotation is carried by quantized vortices. These vortices are massless and can easily become tangled. The resulting quantum turbulence provides a much more natural explanation for the pattern of fluctuations observed in the CMB than that provided by inflation.   Mardi 6 juin à 10h 30, au LPT, salle 110 Filipe Moura (Polytechnique-Saclay) : Perturbative Stability and Absorption Cross-Section in Higher-Derivative Heterotic String Black Holes   Mardi 11 Avril à 10h 30 au LPT, salle 110 Ray Rivers (Imperial College, London) : Brane dynamics of Josephson junctions   Mardi 4 Avril à 11h au LPT, salle 110 George Chapline (Lawrence Livermore National Laboratory) : Gossamer Superconductivity and Emergent Space-time The physical nature of gossamer superconductivity (i.e., superconductivity associated with antiferromagnetism) has long been a puzzle. Recently, it has been suggested that the physical origin of gossamer superconductivity is the localization of charge carriers due to spin orbit effects that occur when there is a strong electron-phonon coupling. These localized charge carriers are magnetic monopole-like objects that are naturally paired because spin up and spin down carriers have opposite monopole charge. Remarkably such a condensate of monopole pairs can also be interpreted as a quantum model for space-time.   Mardi 7 Mars à 10h 30 au LPT, salle 110 Yann Mambrini (LPT, Orsay) : Direct and indirect detection of SUSY dark matter   Jeudi 23 Février à 11h au LPT, salle 110 Martin Kunz (Université de Genève) : Why we live in 3+1 dimensions ?   Mardi 21 Février à 11h au LPT, salle 110 Martin Kunz (Université de Genève) : The Dark Side of the Energy(-momentum) Tensor Although we do not know what the energy is, we have some experimental results as to what it is not. I will introduce the dark energy problem and proposed solutions. I then discuss how we try to use experiments to weed out those solutions that don’t work, including a few words on how not to do it.   Mardi 14 Février à 11h à l’IAS, salle 1-2-3 Jochen Weller (University College London) : Constraining Inverse Curvature Gravity with Supernovae We show that the current accelerated expansion of the Universe can be explained without resorting to dark energy. Models of generalized modified gravity, with inverse powers of the curvature can have late time accelerating attractors without conflicting with solar system experiments. We have solved the Friedman equations for the full dynamical range of the evolution of the Universe. This allows us to perform a detailed analysis of Supernovae data in the context of such models that results in an excellent fit. Hence, inverse curvature gravity models represent an example of phenomenologically viable models in which the current acceleration of the Universe is driven by curvature instead of dark energy. If we further include constraints on the current expansion rate of the Universe from the Hubble Space Telescope and on the age of the Universe from globular clusters, we obtain that the matter content of the Universe is 0.07 <= omega_m <= 0.21 (95% confidence). Hence the inverse curvature gravity models considered can not explain the dynamics of the Universe just with a baryonic matter component.   Mardi 31 Janvier à 11h au LPT Julien Lesgourgues (Annecy) : CMB lensing extraction It was realized in the past years that future CMB experiments can deliver, as a bonus, a very usefull observable, beyond temperature and polarization anisotropies : namely, the map of the CMB weak lensing deflection field. I will review the methods scheduled for CMB lensing extraction, and concentrate on their impact for a precise neutrino mass determination from CMB experiments alone.   Jeudi 19 Janvier à 11h au LPT (Séminaire exceptionnel) Bartjan Van Tent (Cambridge) : Large non-Gaussianity from inflation.   Mardi 17 Janvier à 11h à l’IAS (salle 1-2-3) (Séminaire de cosmologie IAS/LAL/LPT) C. Caprini (Genève) : Primordial magnetic fields and gravitational waves A primordial stochastic magnetic field in the early universe induces tensor perturbations in the FRW metric. I will first demonstrate that the spectrum of a causally created magnetic field is always blue, and then analyze the generation of tensor perturbations by a magnetic field with a helical component. By applying the nucleosynthesis limit on the energy density of the gravitational waves induced by the field, I will derive a strong upper bound on its amplitude. Finally I analyze the CMB polarization signal due to the helical component of the field. The interesting effect is the generation of parity odd cross-correlations between temperature anisotropy and polarization. This effect can be potentially observable only if the magnetic field has a scale invariant spectrum.   Lundi 16 Janvier à 10h 30 au LPT A. Papazoglou (Lausanne) : Six-dimensional conical brane-worlds   Mercredi 30 Novembre à 10h30 au LPT J. Gray (IAP/APC) : Some aspects of M-theory cosmology   Mardi 15 Novembre à 11h à l’IAS (Salle 4) (Séminaire de Cosmologie IAS/LAL/LPT) C. Boehm (LAPP Annecy) : Origine des positrons de basse énergie et implications pour la physique des particules La détection d’une raie à 511 keV dans le centre de notre galaxie pose depuis plusieurs décennies le problème de l’origine des positrons de basse énergie au sein de la voie lactée. Un scénario possible est qu’ils proviennent de l’annihilation de particules de matière noire. Je discuterai des conséquences d’un tel scénario pour la physique des particules et la cosmologie.   Mardi 8 Novembre à 10h30 au LPT F. Nitti (CPhT Ecole Polytechnique) : Applications of a Boundary Effective Field Theory Approach to Cosmologial Models Boundary Effective Field Theory (BEFT) is a useful approach to parametrize high energy effects and unknown phases in cosmological history. In this talk I will discuss applications of BEFT to inflationary initial conditions, to the description of cyclic cosmologies, and to the study of non-Gaussianities in the primordial spectrum of gravitational fluctuations.   Mercredi 2 Novembre à 11h au LPT R. Parentani (LPT Orsay) : Dissipation in QFT and mode creation in expanding universes (II)   Mardi 25 Octobre à 10h30 au LPT G. Niz (DAMTP Cambridge) : Classical propagation of strings across the Big-Crunch/Big-Bang singularity Based on a simple M-theory model of the Big-Crunch/Big-Bang transition, we show how the lowest modes can cross the singularity. The lightest modes in this setup are winding M2-branes stretching between two colliding orbifold planes. When the planes are very close to each other, these winding membranes look like weakly coupled fundamental strings. If the background space-time is analytically continued in a natural manner, the classical evolution for the winding membranes is completely regular across the singularity. From the ten dimensional point of view, one has fundamental strings living in a FRW background with a time-dependent dilaton. As suggested by the M-theory description the string equations are regular for all values of time, leading to a well-defined evolution across the singularity. We study the consequences of this classical evolution, showing how higher $\alpha’$ modes are excited during the transition. Moreover, we use these results to compute the quantum production and transmutation of string modes to leading order in $\hbar$.   Mardi 18 Octobre à 10h30 au LPT R. Parentani (LPT Orsay) : Mode creation in expanding universes   Mercredi 5 Octobre à 10h30 au LPT V. Rubakov (Institut de Recherche Nucléaire de l’Académie des Sciences Russe) : Lorentz Violation and Cosmological Perturbations : A Braneworld Model   Lundi 19 Septembre à 16h au LAL (Séminaire commun LAL/IAS/LPT) M. Landriau (Texas-Austin) : Fluctuations du CMB et weak lensing causés par des cordes cosmiques Les cordes cosmiques ont connu un regain d’intérêt durant les dernières années, pour des raisons théoriques et observationelles. Je discuterai de ces aspects et présenterai des travaux calculant les fluctuations du CMB causées par des cordes cosmiques dans le contexte de futures expériences, notamment Planck, AMI et ACT. Je terminerai en discutant de travaux en cours concernant le weak lensing par des cordes cosmiques.