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Theorique d'Orsay

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

Séminaires Cosmologie 2007-2008

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.

-  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.