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.