Lundi 16 Juin à 10h30, salle 110
Nan Li (Bielefeld Univ.) : Cosmological Backreaction
ATTENTION : Jour et horaire modifiés
Jeudi 12 Juin à 16h, salle 110
Ted Jacobson (Maryland) : Einstein Aether theory
Jeudi 17 Avril à 10h30, salle 110
Valery Rubakov : Can one make sense of phantom dark energy ?
Phantom is a hypothetical type of dark energy whose equation of
state is $p=w \rho$ with $w <-1$. For phantom matter, weak energy
condition is violated, signaling instabilities and possibly other
pathologies. We propose a phenomenological model in which
instabilities occur at very low spatial momenta only, so that
their time scale may be roughly comparable to (though somewhat
smaller than) the present age of the Universe. The model has
interesting cosmological behavior, and also potentially observable
properties related precisely to the unstable character of low
momentum modes.
Mercredi 9 Avril à 11h30, salle 114
A. Riotto (CERN) : The Quest for Non-Gaussianity
Non-Gaussianity emerges as a key observable to discriminate among
competing scenarios for the generation of cosmological perturbations and
is one of the primary targets of present and future Cosmic Microwave
Background satellite missions. We discuss the state-of-the-art of the
subject of non-Gaussianity both from the theoertical and the observational
point of view.
Lundi 7 Avril à 10h30, salle 110
Michele Trapletti (LPT, Orsay) : Gauge coupling unification in Heterotic string model building
Mercredi 2 Avril à 10h30, salle 110
Sandro Fabbri (Valencia) : Holographic description of evaporating black holes
Jeudi 27 Mars à 10h30, salle 110
Jean Macher (LPT, Orsay) : Signatures of trans-Planckian dispersion in inflationary spectra
Mercredi 19 Mars à 14h, salle 110
Lefteris Papantonopoulos (Ecole polytechnique, Athènes) : Black holes on the brane and their extension into the bulk in codimension-2 braneworlds
Mardi 11 Mars à 11h, salle 110
Sébastien Renaux-Petel (APC) : Perturbations in generalized multi-field inflation
I study the linear perturbations of multi-field inflationary models
governed by a Lagrangian which is a general function of the scalar fields
and of a global kinetic term combining their spacetime gradients with an
arbitrary field space metric. This analysis thus includes k-inflation, DBI
inflation and its multi-field extensions which have been recently studied.
For this general class of models, I calculate the action to second order
in the linear perturbations. I decompose the perturbations into an
(instantaneous) adiabatic mode, parallel to the background trajectory, and
entropy modes. I show that all the entropy modes propagate with the speed
of light whereas the adiabatic mode propagates with an effective speed of
sound. I also identify the specific combination of entropy modes which
sources the curvature perturbation on large scales. I then study in some
detail the case of two scalar fields : I write explicitly the equations
of motion for the adiabatic and entropy modes in a compact form and
discuss their quantum fluctuations and primordial power spectra.
Mercredi 27 Février à 16h, salle 110
Franscesco Nitti (CPhT) : Exploring 5D holographic duals to
QCD
Vendredi 22 Février à 11h, salle 110
Renée Hlozek (niversity of Capetown, South Africa) : Hunting in the dark : is the dynamics of tracking dark energy detectable ?
We highlight the unexpected impact of nucleosynthesis and other early
universe constraints on the
detectability of scaling quintessence dynamics at late times, showing
that such dynamics may well be
invisible until the unveiling of the Stage-IV dark energy experiments
(DUNE, JDEM, LSST, SKA).
Nucleosynthesis strongly limits potential deviations from LambdaCDM.
Surprisingly, the standard CPL
parametrisation, w(z) = w0 + waz=(1 + z), cannot match the
nucleosynthesis bound for minimally
coupled scalar fields. Given that such models are arguably the
best-motivated alternatives to a
cosmological constant these results may signifcantly impact future
cosmological survey design and
imply that dark energy may well be dynamical even if we do not detect
any dynamics in the next
decade.
Mercredi 13 Février à 11h, salle 110
David Mota (Université de Heidelberg) : Evading Equivalence Principle Violations, Cosmological and other
Experimental Constraints in Scalar Field Theories with a Strong Coupling
to Matter
I will show that, as a result of non-linear self-interactions,
it is feasible, at least in light of the bounds coming from terrestrial
tests of gravity, measurements of the Casimir force, optical experiments,
and those constraints imposed by astrophysical and cosmological, for there
to exist, in our Universe, one or more scalar fields that couple to matter
much more strongly than gravity does. These scalar fields behave like
chameleons : changing their properties to fit their surroundings. As a
result these scalar fields can be not only very strongly coupled to
matter, but also remain relatively light over solar system scales. These
fields could also be detected by a number of future experiments provided
they are properly designed to do so.
Mercredi 6 Février à 16h, salle 114
Michele Liguori (DAMTP, Cambridge Univ.) : CMB primordial non-Gaussianity : results from WMAP and prospects for Planck
Small, but non-vanishing non-Gaussianity (NG) of primordial cosmological
perturbations is a general
prediction of inflation. The amplitude of the expected non-Gaussian
signal is model-dependent and
can vary by many orders of magnitude from one scenario to another. For
this reason experimental
bounds on primordial NG allow us to significantly constrain different
scenarios for the generation of
perturbations in the context of inflation. In this talk I will firstly
review and discuss recent claims of
detection of a primordial non-Gaussian signal in the WMAP temperature
data. I will then show how the
accuracy and the robustness of these results can be significantly
improved and tested using the future
temperature and polarization datasets from the Planck satellite.
Mardi 5 Février à 10h30, salle 110
Daniele Steer (APC) : Collisions of strings with Y-junctions
Mercredi 30 Janvier à 11h, salle 110
Dmitry Podolsky (Helsinki) : Infrared divergences in de Sitter space
Analytic continuation of AdS/CFT correspondence into dS/CFT
shows on the field theory side that the corresponding ? should be
non-unitary. This allows one to argue that de Sitter space is
intrinsically unstable, and I discuss what does this instability mean
on the gravity side.
Mardi 29 Janvier à 11h, salle 110
Julien Grain (APC, Université Paris VII) : Quelques aspects de phénoménologie quantique en espace courbe
Les phénomènes quantiques en espace courbe font certainement partie des
processus les plus fascinants de la physique moderne, dont les plus belles
illustrations sont l’’évaporation des trous noirs et l’’amplification
paramétrique des fluctuations du vide dans les espaces-temps
inflationnaires. Aux limites de la physique moderne, ces processus
permettent, du moins au niveau de l’’expérience de pensée, d’’aller sonder
les propositions théoriques tentant d’’unifier mécanique quantique et
relativité générale.
Dans ce séminaire, je montrerai tout d’abord comment la dynamique
d’’évaporation des trous noirs est modifiée par rapport au résultat
originel de S. Hawking, lorsque la gravitation est décrite par la théorie
de Gauss-Bonnet, une extension prometteuse de la relativité générale. Ce
sera aussi l’’occasion d’’aborder quelques phénomènes purement quantiques
comme les interférences induites par la courbure.
Dans une deuxième partie, je présenterai quelques pistes de réflexions
quant aux phénomènes quantiques qui ont lieu dans l’’univers primordial.
Nous verrons par exemple comment la théorie quantique des systèmes
dépendant du temps, développée à la fin des années soixante, peut être
facilement applicable au cas des champs scalaires se propageant en espace
Friedman-Lemaître-Robertson-Walker. Cette approche peut permettre en
particulier d’’apporter un regard nouveau sur la transition quantique vers
classique lors de la phase inflationnaire primordiale.
Mercredi 23 Janvier à 14h, salle 110
Ruth Gregory (Durham) : Branes and black holes
Mardi 15 Janvier à 14h, salle 110
Alberto Romagnoni : Moduli stabilisation and new low-energy spectra
After an introduction to the problems of the moduli
stabilisation and vanishing cosmological constant for phenomenological
KKLT-like models in supergravity setup, I will talk about the case where
the uplift to a Minkowski vacuum is provided by a Fayet-Iliopoulos sector.
The modulus contribution to supersymmetry breaking is larger than in the
models already present in the literature and a new mixed gauge-gravity
mediation appears in some explicit examples. This provides an original
phenomenology in principle testable at LHC, where the whole superpartner
spectrum can be efficiently compressed at low-energy and sleptons are
generically hevier than squarks.
Mercredi 19 Décembre à 14h30, salle 110
Nemanja Kaloper (UC Davis) : Charting the Landscape of Modified Gravity
We explore brane induced gravity on a 3-brane in six locally flat
dimensions. To regulate the short distance singularities in the brane
core, we resolve the thin brane by a cylindrical 4-brane, with the
geometry of 4D Minkowski x a circle, which has an axion flux to
cancel the vacuum pressure in the compact direction. We discover a large
diversity of possible solutions controlled by the axion flux, as governed
by its boundary conditions. Hence brane induced gravity models really give
rise to a landscape of vacua, at least semiclassically. The vacuum
energy problem is different in brane induced gravity : instead of tuning
the 4D curvature, generically one must tune the crossover scale. The most
interesting case is the near-critical limit, branes live inside very deep
throats which efficiently compactify the angular dimension. In there, 4D
gravity first changes to 5D, and only later to 6D. The crossover scale
saturates at the gravitational see-saw scale, independent of the tension,
but the 5D to 6D transition is still sensitive to it. Using the fields
of static loops on a wrapped brane, we check the perturbative description
of long range gravity below the crossover scale. Near the critical limit,
linearized perturbation theory remains under control below the crossover
scale, and we find that linearized gravity around the vacuum looks like a
scalar-tensor theory.
Vendredi 7 Décembre à 10h15, salle 110
Filippo Vernizzi (Trieste-CEA) : Primordial non-Gaussianity and the evolution of second order
perturbations
Mardi 27 Novembre à 10h30, salle 110
Cyril Pitrou (IAP-Greco) : Gauge invariant perturbation theory and the kinetic description
I will review the work done in the past ten years on the gauge invariant
perturbation theory for tensors beyond the first order, in the
cosmological context. I will then present the extension that I have
performed for the distribution function using the tetrad formalism. This
is necessary to compute the Boltzmann equation and thus the transfer
function beyond the linear order, and eventually to understand the
non-linear effects in the CMB such as Non-Gaussianity. I will discuss the
fluid approximation which enables to extract the main features and discuss
briefly the observational signatures predictions. Finally I will present
how the (first-order) gauge invariant perturbation theory can be extended
to handle non-isotropic universes such as Bianchi I. I will also present
briefly the ongoing work on anisotropic inflation as an application of
this formalism.
Séminaire reporté à une date ultérieure
Tony H. Padilla (Nottingham) : No resonant tunnelling in the landscape ?
Mardi 13 Novembre à 10h30, salle 110
Konstantinos Skordis (Perimeter Inst., Canada) : Cosmology of Bekenstein-Sanders theory of modified gravity
Vendredi 9 Novembre à 11h, salle 110
Stephen Davis : Hybrid Inflation vs. Moduli Stabilisation
Mardi 23 Octobre à 14h30, salle 110
Carla Sofia Carvalho (LPT & IST, Lisbon) : Non-minimal Coupling as a Mechanism for Spontaneous Symmetry Breaking on
the Brane
Motivated by the dimensional asymmetry characteristic of braneworlds, we
populate the bulk spacetime with matter scalar fields, both real and
complex, and couple them non-minimally to gravity. We derive the effective
equations of motion on the brane and realize the case when the fields
acquire a non-vanishing vacuum expectation value. This entails a change in
the effective cosmological constant and in the effective mass of the
scalar fields. We find that the non-minimal coupling provides a mechanism
for generating spontaneous symmetry breaking at very high energies on the
brane.
Mardi 9 octobre à 14h, salle 110
Luca Amendola (Roma) : Dark energy as modified gravity.
Accès / Find us
Séminaires Cosmologie 2007-2008