Colloquium APC

Abstract : During its first years of operation, the H.E.S.S. array of imaging atmospheric Cherenkov telescopes has revealed a sky full of sources of very high energy gamma rays, has demonstrated the great power of the Cherenkov technique, and has motivated work towards a future, even more powerful facility : the Cherenkov Telescope Array CTA. The talk will illustrate some of the key results from H.E.S.S., mention some of the open questions, and present the concept and goals of CTA.

Colloquium APC

Abstract : QUaD is a bolometric Cosmic Microwave Background polarimeter sited at the South Pole, operating at frequencies of 100 and 150 GHz. I will report results from the first season of operation (austral winter 2005). It has detected anisotropies in the polarization of the cosmic microwave background and has begun mapping the power spectrum of the polarization on angular scales complimentary to those measured by WMAP.
This data is a fraction of the data which will ultimately be acquired. The results are consistent with the Lambda-CDM cosmological model.

Colloquium APC

Abstract : The deepest hole that has ever been dug is about 12 km deep, a mere dent in planetary terms. Geochemists analyze samples from the Earth ?s crust and from the top of the mantle. Seismology can reconstruct the density profile throughout all Earth, but not its composition. In this respect, our planet is mainly unexplored.
Geo-neutrinos, the antineutrinos from the progenies of U, Th and 40K decays in the Earth, bring to the surface information from the whole planet, concerning its content of radioactive elements.
Their detection can shed light on the sources of the terrestrial heat flow, on the present composition and on the origin of the Earth.
This approach is now becoming practical as a consequence of two fundamental advances that occurred in the last few years : the development of extremely low background neutrino detectors and the progress on understanding neutrino propagation.
I shall discuss the potential of this new probe of our planet, the first observational results from the KamLAND experiment and the

Colloquium APC

Abstract : The NASA/ESA Laser Interferometer Space Antenna (LISA) mission is expected to measure, with exquisite precision, the gravitational waves from tens of super-massive merging black holes in galactic nuclei, hundreds of compact objects captured by black holes in galactic nuclei, and tens of thousands of Galactic white dwarf binaries. It may also discover more fundamental sources : bursts from cosmic superstrings, and cosmic backgrounds from phase or dimensionality transitions. After a brief review of LISA’s principles and status, I will describe how these precision measurements of physically (fairly) simple systems will test black hole physics and strong-field general relativity in unprecedented ways. I will also describe how both on their own, and by enabling and informing new electromagnetic observations, LISA’s measurements will give profoundly new insights into the astrophysics of interacting white dwarfs, the formation and growth of the first giant black holes in the universe, the structure and stellar composition of galactic nuclei, and the physics of accretion disks.

Colloquium APC

Résumé : Pourquoi étudier le coeur des étoiles ?
Comment accéder à celui-ci observationnellement ?
Ce fut une longue quête de plusieurs communautés pendant près de 30 ans avec de remarquables succès et une grande cohérence entre physique fondamentale, neutrinos et astrophysique.
La route aujourd’hui passe par la détection de premiers modes de gravité solaires qui ouvrent une nouvelle voie vers la dynamique profonde si importante pour comprendre l’origine du système solaire, la relation Soleil-Terre, mais surtout les phases avancées des étoiles, les presupernovae et probablement les bouffées de rayons gamma. Tout tourne dans l’Univers, et l’histoire du champ magnétique commence avec la compréhension du champ magnétique solaire interne, ceci conduit à de nouveaux projets spatiaux, comme SDO, PICARD et DynaMICCS et probablement bien d’autres projets comme CLOUD au CERN nous ramenant sans cesse à partager notre expertise entre plusieurs disciplines.

Colloquium APC

Abstract : Life is chiral. Amino acids that make up biomolecules are left-handed, while all sugars are right-handed. And yet, when synthesized in the laboratory, the solutions come out 50-50. Is life’s chirality simply an accident, or is it the result of dynamical processes that occurred in early Earth, during prebiotic times ? If life exists elsewhere in the Universe, will it choose the same chirality ? In this talk, I will address these and other questions of interest in astrobiology, using techniques from nonequilibrium statistical mechanics and field theory.
In particular, I will argue that life’s chirality might be due to a symmetry-breaking phase transition. Exploring this possibility, I will obtain bounds on possible processes that may have selected life’s chirality here and possibly in other life-bearing planetary platforms.

Colloquium APC

Abstract : The magnetic field within a few hundred parsecs of the center of the Galaxy is an essential component of any description of that region. Among the numerous observational manifestations of the field, I will discuss three : the nonthermal radio filaments (NTFs) — magnetic flux tubes illuminated by synchrotron emission from relativistic electrons, 2) a large-scale, helically wound structure interpreted as a torsional Alfvén wave, and 3) synchrotron emission from cosmic rays. Because most of the NTFs are roughly perpendicular to the Galactic plane, the implied large-scale geometry of the magnetic field is dipolar. morphological arguments suggest that the magnetic field strength may be on the order of a milligauss on scales of a few hundred parsecs, in which case the magnetic field is able to exert a strong influence on the dynamics of molecular clouds, on the collimation of a Galactic wind, and on the lifetimes and bulk motions of relativistic particles. Related to the field strength is the question of whether the field is pervasive throughout the central zone of the Galaxy, or whether the NTFs are predominantly localized field structures. I’ll explain why I think current evidence favors the pervasive model.

Colloquium APC

Abstract : A large fraction of the accretion energy in luminous BH systems is dissipated in the innermost regions of the accretion flow, a flat and dense disc orbiting the BH. Irradiation of the dense disc by hard X-rays gives rise to a characteristic reflection spectrum characterized, in the X-ray band, by a fluorescent narrow iron emission line at 6.4keV. Relativistic effects affect the appearance of the reflection spectrum and the iron line profile through Doppler, aberration, gravitational redshift and light bending. The recent observational results will be reviewed and discussed with a focus on the X-ray spectra and variability properties of accreting BH in both Galactic X-ray binaries and AGN.

Colloquium APC

Abstract : L’expérience Borexino a publié ses premiers résultats cet été, montrant la première détecteur en temps réel des neutrinos de basse énergie du soleil (neutrinos du Be7). Le détecteur est en fonctionnement depuis Mai 07, et ses performances s’avèrent conformes au cahier des charges.
Une mesure précise de ces neutrinos va permettre de confirmer les scénariis d’oscillation, et aussi d’améliorer la connaissance du soleil. Ce détecteur fournira également une mesure des neutrinos issus de la terre. Enfin, en cas de supernova galactique, sa très basse radioactivité permettra la mesure des protons de recul, et donc du spectre total.

Colloquium APC

Résumé : Quelle est l’origine du champ magnétique des objets astrophysiques qui nous entourent ? Nous présentons la mise en évidence de l’effet dynamo dans l’expérience VKS réalisée au CEA/Cadarache, i.e. de la génération spontanée d’un champ magnétique dans un écoulement pleinement turbulent de sodium liquide. Nous rapportons également la première observation des renversements erratiques et des basculements périodiques du champ magnétique créé par la dynamo qui présentent des similitudes étonnantes avec les dynamos naturelles. Nous montrons enfin que les comportements dynamiques du champ peuvent être décrits par des systèmes dynamiques de basse dimensionnalité.

Colloquium APC

Abstract : MINOS is a long baseline neutrino oscillation experiment, which measures neutrino oscillation parameters using the NuMI beam generated at Fermilab. The beam mainly consists of muon neutrinos and is measured by two functionally identical detectors. The near detector is located on the Fermilab site around 1 km away from the neutrino production target, while the far detector is 734 km further in the Soudan Underground Laboratory. MINOS has started to take accelerator neutrino data in 2005 and has now collected neutrino interactions from more than 3x10^20 protons on target. New results from the experiment will be presented.

Colloquium APC

Abstract : I will review the model independent predictions for the spectrum of the cosmological perturbations and gravity wave from inflation. The current observations of the CMB fluctuations and the prospectives for the future experiments will be discussed.

Colloquium APC

Résumé : L’Observatoire Pierre Auger à récemment mis en évidence une corrélation entre les directions d’arrivée des rayons cosmiques ultra-énergétiques avec les positions sur la sphère céleste des noyaux actifs de galaxies relativement proches. Le signal observé est compatible avec l’hypothèse que la composante la plus énergétiques des rayons cosmiques est originaire de sources extragalactiques suffisamment proches pour que leur flux ne soit pas atténué de manière significative par interaction avec le fond diffus cosmologique. Après une brève présentation de l’Observatoire Pierre Auger et des problématiques associées à la physique des rayons cosmiques ultra-énergétiques, je reviendrai sur le résultat annoncé par la collaboration Pierre Auger.

Colloquium APC

Abstract : Oscillograms are ``neutrino portraits’’ of the Earth. They encode information about the Earth interior and provide a comprehensive description of neutrino oscillation phenomena. I will present physics effects involved and explain structures of the oscillograms. Dependencies of oscillograms on neutrino parameters, in particular, on the unknown yet 1-3 mixing and CP-violation phase will be considered. The oscillograms, and consequently the neutrino parameters, can be measured by HAND’s - Huge Atmospheric Neutrino Detectors.

Colloquium APC

Abstract : Supernova outbursts and accretion-merging processes in clusters of galaxies have a common feature — they produce large-scale shocks. The plasma shocks are expected to be collisionless. We review the collective processes responsible for the microscopic structure of the collisionless shocks. The shocks, being the main gas-heating agent, generate turbulent magnetic fields and accelerate energetic particles via collisionless multi-fluid plasma relaxation processes. A tiny fraction of incoming particles is injected into non-thermal component that could get are substantial part of shock dissipated ram pressure power. The non-thermal particles penetrate into the shock upstream producing an extended precursor and could strongly amplify the magnetic field in the shock precursor. Simple scaling relations for postshock ion temperature and entropy as functions of shock velocity in strong collisionless multi-fluid shocks are discussed. We show that the multi-fluid nature of the collisionless shocks results in high gas compression, reduced entropy production and can seriously modify the standard observed scalings.

Colloquium APC

Abstract : The inflationary universe scenario is the current paradigm of early universe cosmology. However, current realizations of the inflationary scenario suffer from various conceptual problems. Superstring theory provides the framework for developing a new theory of the very early universe. I will discuss a new cosmological model which is based on a number of key ingredients of superstring theory. According to this model, the universe began not with a phase of inflation, but as a hot string soup. Thermal fluctuations of this string soup yield an alternative to inflation for explaining the origin of structure in the universe. The model also leads to testable predictions for upcoming experiments.

Colloquium APC

Résumé : Depuis quelques siècles, les activités humaines modifient de façon sensible la composition de l’atmosphère : l’utilisation des combustibles fossiles (charbon, pétrole, gaz naturel) est largement responsable de l’augmentation de la concentration en gaz carbonique depuis le début de l’ére industrielle (augmentation de 35 % depuis 1850) et, sur cette même période, les concentrations en méthane ont plus que doublé. Bien qu’il s’agisse là de constituants mineurs de l’atmosphère, de tels changements sont susceptibles de modifier notre climat car ils conduisent à une augmentation de l’effet de serre atmosphérique.

Notre communauté scientifique est désormais de plus en plus convaincue que le réchauffement observé au cours des dernières décennies est lié à cette modification. Elle affirme, qu’à défaut de mesures efficaces visant à maîtriser les émissions de ces gaz à effet de serre, le réchauffement lié aux activités humaines va s’accentuer d’ici la fin du siècle et au-delà. Quelles sont les certitudes des scientifiques dans ce domaine du réchauffement climatique ? Quelles incertitudes subsistent ? En quoi l’étude du climat passé est-elle pertinente vis à vis de son évolution future ? Quels seront les impacts du réchauffement climatique lié à l’action de l’homme ? Quelles mesures peuvent être envisagées ? Comment s’organise le dialogue entre cette communauté scientifique et les décideurs politiques ? Ces différents points seront abordés à la fois à travers les travaux conduits à l’IPSL, les rapports du GIEC (le Groupe Intergouvernemental d’Experts sur l"Evolution du Climat) et le Grenelle de l’Environnement.

Colloquium APC exceptionnel

Abstract : The standard cosmological model seems to be able to account for all cosmological observations. But in this model the present composition of the universe is 95% dark : 25% dark matter and 70% dark energy. This is either a feature, and we are presented with the opportunity of discovering the nature of dark matter and dark energy, or it is a bug, and nature might be different than described by the standard model. In the colloquium I will focus on issues involving dark energy.

Colloquium APC

Abstract : Cosmologists now believe that 75% of the energy density of the Universe could be in the form of "dark energy" ; a new form of matter with an effective negative pressure. The key question now is the true physical form of this dark energy and does it change with space and/or time in the Universe. Furthermore, is it simply a manifestation of our ignorance about gravity on large scales. These questions to date can only be address through detailed and careful observations of the Universe. In this talk, I will review the my recent observations of dark energy and look forward to the next set of cosmological measurements.

Colloquium APC

Abstract : In the minimal cosmological scenario, and given that neutrinos have a mass, dark matter (DM) is composed of two species : a dominant cold DM component, and a small hot DM fraction. The real universe might be more complicated : most of DM might instead be warm, or consist in a mixture of various species with different velocity dispersions. I will summarize theobservable implications of such scenarios, and review current constraintsarising mainly from CMB and large scale structure observations.

Colloquium APC

Résumé : La courbure de Ricci a été introduite très tardivement (1904) dans le développement de la géométrie riemannienne. C’est pourtant elle qui est au c ?ur des équations d’Einstein suivant les propositions d’Einstein et d’Hilbert en 1915. Cet aller-retour entre mathématiques et physique théorique va continuer au cours du XXème siècle.
C’est à ces aller-retours qu’est consacré cet exposé, notamment les conséquences spectaculaires de la suggestion de voir la courbure de Ricci comme champ de vecteurs sur l’espace des métriques venue de la communauté des relativistes dans les années 60-70 puis prise au sérieux par les mathématiciens jusqu’à conduire Grisha Perelman à la solution de la fameuse conjecture de Poincaré. D’autres avenues méritent d’être explorées comme l’étude des variétés de Calabi-Yau dans le cadre de la géométrie kählérienne et les liens entre supersymétries et métriques d’Einstein.

Colloquium APC

Abstract : The discovery that the expansion of the Universe is accelerating has stimulated enormous interest not only among astrophysicists and high energy physicists, but also in the broader scientifically interested public. We do not yet understand this phenomenon but many hope that is a reflection of new physics, perhaps extra dimensions or a higher level of unification. According to present ideas, only precise astronomical observations of the cosmic expansion and of the growth of structure can give additional information about the "Dark Energy" which may drive this acceleration. As a result, large groups of high energy physicists and astronomers have teamed up to plan the very large surveys needed. In my view this is a drunk+lampost phenomenon. We search for the key to the accelerated expansion in this way, not because we believe we have a good chance of finding it, but rather because we don’t know how to look elsewhere. In addition, I believe that the different outlooks, traditions and working styles of the two communities can lead to inappropriate risk assessment when planning such surveys/experiments, and to the further expansion of a "Big Science" approach to research which is antagonistic to and dangerous for the more individualistic approach which underlies the current vitality of astrophysics.

Colloquium APC

Abstract : one of the most pressing challenges in physics is to understand the origin of the apparent acceleration of the universe today. The community is exploring both theoretical and observational schemes to help answering such a question. In this talk I will first describe some new theoretical ideas on how we can probe the growth of structure and expansion factor of the universe as a function of redshift using the next generation of CMB SZ probes like the Planck satellite. I will then discuss some ideas on how to probe string theory using polarization measurements of the CMB. Finally I will describe a new Baryonic Acoustic Oscillation experiment (PAU) being developed in Spain that will survey the full sky with the aim of providing a spectroscopic-like survey up to redshift of z 1.2, with the aim of measuring BAOs and the dark energy equation of state to a  1% accuracy. This survey, besides aiming at dark energy studies, has a more broad scope in astrophysics, ranging from neutrino mass determinations to studying the origin and evolution of galaxies.

Colloquium APC exceptionnel

Colloquium APC

Abstract : Motivated by X-ray observations of Galactic Black Hole Binaries (GBHBs), I will present a detailed analysis of hydrodynamic and magnetohydrodynamic (MHD) simulations of geometrically-thin (h/r 0.05) black hole accretion disks. I will focus on the temporal properties of the simulated disks, and use the results to judge the likely relevance of two widely discussed models for the high-frequency quasi-periodic oscillations (HFQPOs) seen in many GBHBs. We shall see that the origin of HFQPOs remains a mystery, although our results suggest new avenues to explore. However, the simulated accretion disks do successfully capture important aspects of the aperiodic X-ray variability seen in Cygnus X-1 and suggest that the hard X-ray flux may not originate from a classical accretion disk corona as normally thought.

Colloquium APC

Abstract : Fermilab, the US’s primary laboratory for particle physics, proposes a plan to maintain leadership for the laboratory and U.S. particle physics in the quest to discover the fundamental nature of the physical universe in the decades ahead. Discoveries of the physics of the Quantum Universe would come from powerful next generation particle accelerators. Fermilab’s Tevatron, currently the world ?s most powerful particle accelerator, will shut down by the end of this decade after the LHC at CERN begins operations. At the LHC, U.S. physicists will join scientists from around the world in the exploration of the physics of the Terascale. To follow the LHC, physicists propose the International Linear Collider, a globally funded and operated accelerator to build on LHC results and illuminate Terascale science. Fermilab will work to host the proposed ILC in the U.S. as soon as possible, maintaining the nation ?s historic leadership of frontier particle physics. Should events postpone the start of the ILC, Fermilab would build an intensity-frontier accelerator at one percent of the ILC ?s length and combine it with existing accelerators to create Project X. Project X’s intense beams would give Fermilab’s scientific users a new way into the world of neutrinos and precision physics. With its ILC technology, Project X would spur U.S. industrialization and reduce costs of ILC components while advancing accelerator science for future applications in particle physics and beyond. In addition, Project X would drive forward the technology for still higher-energy accelerators of the future, such as a muon collider. Fermilab’s plan would maintain the nation’s leadership in particle physics, keeping the laboratory and U.S. particle physics on the pathway to discovery both at the Terascale with the ILC and beyond, and in the domain of neutrinos and precision physics at the intensity frontier.

Colloquium APC

Abstract : I will discuss the physics motivation and current status of two major new efforts in neutrino physics :
(1) Our current main emphasis is on the Daya Bay experiment, which has the goal of detecting oscillations of antineutrinos emitted by the Daya Bay nuclear reactors in China, in order to make a high-precision measurement of the unknown mixing angle Theta13.
(2) We are also participating in SNO+, which plans to use the existing SNO experimental setup, including the 12-meter acrylic vessel, the 104 PMT’s, and the DAQ, to search for neutrinoless double beta-decay in 150Nd. A common link between these two experiments is our development of new metal-loaded liquid scintillators (M-LS) as the neutrino detectors, Gd-LS for Daya Bay and Nd-LS for SNO+. We are also working on the detection and removal of low-levels of radioactive contaminants, such as U, Th, Ra, in the M-LS and other components of the detector.

Colloquium APC

Résumé : Virgo a effectué sa première prise de données scientifiques entre mai et octobre 2007, en concertation avec les détecteurs LIGO et GEO. Après avoir rappelé les caractéristiques principales de l’interféromètre Virgo, nous présenterons les étapes majeures dans sa mise en fonctionnement et la sensibilité atteinte. Nous décrirons ensuite le projet Advanced Virgo, qui vise une amélioration de la sensibilité d’un facteur dix et son potentiel scientifique. Nous dresserons finalement un bilan des résultats obtenus avec les données collectées par Virgo, LIGO et GEO, et de leurs implications sur les modèles des sources et de population. Nous examinerons en particulier les analyses combinant d’autres observations dans le spectre électromagnétiques (gamma et X) et des détecteurs de neutrinos.

Colloquium APC exceptionnel

Colloquium APC

Résumé : Les expériences scientifiques embarquées dans l’espace doivent atteindre des performances ultimes tout en respectant de sévères contraintes de masse, d’encombrement et de consommation. La réalisation d’un système embarqué, soumis à des sollicitations mécaniques et thermiques sévères pendant la mise en orbite, puis opérant sous vide, exposé aux rayonnements et au flux de particules solaires pendant plusieurs années, constitue un véritable défi technologique qui conduit à reconsidérer en détails la définition des expériences développées dans les laboratoires scientifiques. En nous appuyant sur l’exemple du projet d’horloge à atomes refroidis en orbite, PHARAO, nous passerons en revue les principales difficultés rencontrées dans le développement d’un système embarqué et nous donnerons un aperçu de la démarche industrielle mise en oeuvre pour garantir un niveau de fiabilité extrême.

Colloquium APC

Abstract : Weakly-interacting massive particle (WIMP) dark matter has been invoked to explain excess GeV gamma-rays, high-energy positrons, and even excess microwaves in the inner Milky Way. Until now, however, there has been no connection between these weak scale (mass of 300 GeV) WIMPs and the 511 keV e+e- annihilation radiation from the inner Milky Way.

Neal Weiner and I have proposed a WIMP candidate with an "excited state" 1-2 MeV above the ground state (known as "exciting" dark matter, XDM). This state may be collisionally excited and decays by $e^+e^-$ pair emission. Such a particle converts WIMP kinetic energy into pairs, which in turn could produce a substantial fraction of the 511 keV line observed by INTEGRAL/SPI in the inner Milky Way. Only a small fraction of the WIMPs are above threshold, and that fraction drops sharply with Galactocentric radius, naturally yielding a radial cutoff, as observed. Even if the scattering probability in the inner kpc is << 1% per Hubble time, enough power is available to produce the number of pairs per second observed. I will specify the parameters required to explain the INTEGRAL result, and discuss other observational consequences of this model for galaxy clusters, high-z 21cm observations, and the CMB.

Colloquium APC

Abstract : Dark matter, which constitutes a dominant component of our Galaxy, may be disclosed by many astrophysical signals, which range from gamma rays to neutrinos and antimatter in cosmic rays. In this talk I will discuss recent advancements in the theoretical calculations of these indirect detection signals, with special emphasis on the astrophysical uncertainties which affect them and on the capabilities offered by the different tenchniques. Theoretical predictions on the signals will be confronted with the current and foreseen experimental sensitivites for various dark matter candidates in different extensions of the Standard Model.

Colloquium APC

Abstract : Dark energy - a homogeneously distributed cosmic energy density - seems to explain various cosmological observations. The anisotropies in the microwave background radiation, the formation of structure and the late time acceleration of the Hubble expansion fit into a consistent picture. The role and origin of dark energy are among the greatest mysteries in fundamental physics, touching the question of unification of gravity with the fundamental quantum interactions.

We discuss quintessence - a dynamical form of dark energy - and possible signatures distinctive from a cosmological constant. In particular, we address the possibility that the time evolution of a cosmological scalar field can be stopped by an increasing mass of the neutrinos. This leads to a transition from a cosmological scaling solution with dynamical dark energy at early time to a cosmological constant dominated universe at late time. The trigger for the transition is set at the time when the neutrinos become non-relativistic , corresponding to a redshift z 5 , explaining why dark energy becomes important now. The dark energy density is related to the mass of the neutrinos.

Quintessence could be related to a new "fundamental" macroscopic force and induce a small time variation of fundamental constants.

Colloquium APC

Abstract : The LSST is a proposed 8m-class telescope that will survey over 20,000 square degrees of the sky to magnitude 25 in a single g-band visit of 30 seconds. Each location on the sky will be imaged 1000 times over the 10 year survey. The LSST will be located in northern Chile. The project currently includes 24 member institutions. Science applications include mapping the solar system to probes for dark energy. The survey will generate the largest database of astronomical data.

Colloquium APC

Abstract : The Arcminute Cosmology Bolometer Array Receiver (ACBAR) is a multi-frequency 16-element bolometer array which observed from the 2m Viper telescope at the South Pole during the 2001, 2002, 2004 and 2005 Austral winters. ACBAR’s small (5’) beams allow it to probe the damping tail of the cosmic microwave background (CMB) power spectrum, making it highly complementary to experiments at larger angular scales such as WMAP and Boomerang. I will present recent results from the complete set of ACBAR’s CMB temperature anisotropy observations. We include new data from the final 2005 observing season, expanding the number of detector-hours by 210% and the sky coverage by 490% over the data set used in the previous ACBAR release. As a result, the band-power uncertainties have been reduced by more than a factor of two on angular scales encompassing the third to fifth acoustic peaks as well as the damping tail of the CMB power spectrum. The calibration uncertainty has been reduced from 6% to 2.1% in temperature through a direct comparison of the CMB anisotropy measured by ACBAR with that of the dipole-calibrated WMAP5 experiment. The measured power spectrum is consistent with a spatially flat, LambdaCDM cosmological model. We include the effects of weak lensing in the power spectrum model computations and find that this significantly improves the fits of the models to the combined ACBAR+WMAP5 power spectrum. When marginalized over other cosmic parameters, the preferred strength of the lensing is found to be consistent with theoretical expectations. On fine angular scales, there is weak evidence (1.1 sigma) for excess power above the level expected from primary anisotropies. We expect this excess power to be dominated by the combination of emission from dusty protogalaxies and the Sunyaev-Zel’dovich effect. However, the excess observed by ACBAR is significantly smaller than the excess power at ell >2000 reported by the CBI experiment operating at 30 GHz. Therefore, while it is unlikely that the CBI excess has a primordial origin ; the combined ACBAR and CBI results are consistent with the source of the CBI excess being either the Sunyaev-Zel’dovich effect or radio source contamination.

Colloquium APC

Abstract : L’observation du milieu intergalactique (MIG) à travers les spectres d’absorption de quasar permet l’étude d’un grand nombre de sujets. Au niveau de la physique fondamentale (expansion de l’Univers, variation des constantes fondamentales), de la cosmologie (géométrie de l’Univers, évolution des grandes structures), de la réionisation (état e l’Univers a z > 6), jusqu’à de l’astronomie plus « traditionnelle » avec les interactions entre galaxies et MIG (métaux, vents...) et l’étude du milieu interstellaire dans les galaxies à grand z, avec les molécules. Cependant, le caractère uni-dimensionnelle des spectres d’absorption rend ces études complexes, avec de nombreuses dégénérescences. La détermination du continu du quasar rendant les choses encore plus difficile à grand z. Après avoir décrit les spectres d’absorption en toute généralité, je reviendrais sur ces différentes thématiques pour vous donner un aperçu de l’éventail des possibilités et difficultés de ce domaine de recherche.

Colloquium APC

Abstract : Une ère de la SAGA Planck s’achève en ce début 2009 puisque, 15 ans après le première proposition, le satellite Planck est prêt à être lancé.

Les essais effectués sur le satellite Planck complet l’été dernier dans la grande cuve cryogénique du Centre Spatial de Liège (CSL) ont permis de tester pour la première fois toute la chaîne cryongènique et les deux instruments ensemble. Les excellentes performances qui avaient été observées durant les calibrations à Orsay deux ans plus tôt ont été retrouvées et confirmées dans ce contexte très représentatif des conditions en vol.

Les objestifs scientifiques de Planck seront rappelés. On suivra ensuite le chemein qui va de l’idée initiale à un satellite et deux instruments construits par une vingtaine d’instituts et de nombreux indurstriels dans le monde, sous l’égide de six agences spatiales. Les tests finaux à CSL et leur résultats seront montrés.

On évoquera aussi les éléments qui tombent en panne quand on s’y attend le moins , les causes, la façon de résoudre les crises qui en découlent (dont la dernière en date !)

Et puis on finira sur l’ère qui va s’ouvrir avec le lancement à Kourou en Avril prochain, les opérations de l’instrument depuis l’IAS, le traitement de données jusqu’à leur distribution à la communauté et la sortie des premiers résultats scientifiques.

Colloquium APC

vendredi 16 janvier 2009

à 14h30, salle 454A

Colloquium Vendredi APC

Abstract :

14 h 30 : X-Shooter / Paolo Goldoni (10 + 5)

14 h 45 : Où en est Auger au moment de l’inauguration / Cyril Lachaud (10 + 5)

15 h 00 : Double Chooz bientôt couvert / Frédéric Dorigo (10 + 5)

15 h 15 : Les voeux du Directeur / Pierre Binetruy (10+5)

16 h 00 Galette des Rois !

Colloquium APC

mardi 27 janvier 2009

à 11h en salle 454A à l’APC

What the inflaton might tell us about RHIC/LHC

Juergen Berges (Darmstadt)

Abstract : Topical phenomena in high-energy physics related to collision experiments of heavy nuclei (’Little Bang’) and early universe cosmology (’Big Bang’) involve far-from-equilibrium dynamics described by quantum field theory. One example concerns the role of plasma instabilities for the process of thermalization in heavy-ion collisions. The reheating of the early universe after inflation may exhibit rather similar phenomena following a tachyonic or parametric resonance instability.

Certain universal aspects associated to nonthermal fixed points even quantitatively agree, and considering these phenomena from a common perspective can be fruitful.

Colloquium APC

mardi 3 février 2009

à 11h en salle 454A

Le télescope à neutrinos ANTARES. Premiers résultats et perspectives.

Bruny Baret (APC)

Abstract : L’astronomie neutrino de haute énergie est aujourd’hui sur le point d’ouvrir sur l’Univers une nouvelle fenêtre d’observation pour étudier les phénomènes astrophysiques les plus violents. En effet, l’interaction très faible du neutrino avec la matière lui permet de s’échapper des régions denses et de parcourir des distances cosmologiques. Cette particularité implique cependant la construction d’instruments gigantesques pour détecter le sillage de lumière produite dans la glace ou dans l’eau par les particules créées lors de l’interaction du neutrino avec la matière environnante. La construction du télescope Antares, installé à 2475m de profondeur dans la mer Mediterranée, s’est achevée en mai 2008. Il est aujourd’hui , avec ses 12 lignes et ses quelques 900 photo-détecteurs, le plus grand instrument de ce type en fonctionnement permettant de scruter le ciel de l’hémisphère sud. Nous présenterons les performances du détecteur ainsi que les résultats des premières analyses réalisées grâce aux donnéees prises depuis 2007. Nous évoquerons aussi les perspectives d’une approche multi-messagère ainsi que le projet de son successeur de taille kilométrique, KM3NeT.

Colloquium APC

mardi 24 février 2009

11h en salle 454A

Stochastic Background of Gravitational Waves : sources and detection

Giancarlo Cella (INFN, Pisa - Virgo)

Abstract : One of the target of Virgo and LIGO detectors is the stochastic background of gravitational waves, which is expected to be connected both to astrophysical and cosmological physics. In this talk the status of this search is presented. Current and expected future sensitivities of earth bound and space detectors are compared with theoretical predictions, discussing the physical implications of the results which are available now or that could be obtained in the near future.

Colloquium APC

lundi 2 mars 2009

à 14h salle Klee (454A)

WMAP polarization observations : seeing through the Galaxy

Joanna Dunkley (Universite d’Oxford)

Abstract : Current polarization measurements from WMAP provide information about the reionization epoch. However, WMAP observes significant levels of polarized radiation from our Galaxy. I will present a sampling method for estimating the polarized CMB maps, marginalizing over Galactic contributions. I will show the application of this method to the five year WMAP data, where we find that the large scale polarized CMB signal is consistent with the standard ‘template-cleaned’ result. The synchrotron emission is found to have mean index -3.0 in regions of high signal to noise. The large scale polarized CMB is also used to search for a background of gravitational waves from inflation in the early universe, so I’ll discuss the application to future estimation of this primordial signal.

Colloquium APC

mardi 3 mars 2009

à 11h, salle 454A

νs from the South Pole : Perspectives en astronomie neutrino avec le télescope IceCube

Cécile Roucelle (MPI-K, Heidelberg)

Abstract : L’objectif principal d’IceCube est d’ouvrir la voie à un nouveau type d’astronomie reposant sur la détection de neutrinos de haute énergie d’origine astrophysique. De telles observations mettraient en lumière les mécanismes d’accélération des rayons cosmiques de haute énergie et pourraient permettre d’explorer leurs sources. Le télescope neutrino IceCube est en cours de déploiement dans la glace du pôle Sud. Il atteint aujourd’hui plus les deux tiers de sa taille finale et accumule déjà 3 années de données dans des configurations partielles avec 9 (en 2006), 22 (en 2007) et 40 lignes (en 2008) et entre maintenant dans une ère propice aux découvertes. Nous présenterons lors de ce séminaire les derniers résultats de l’expérience IceCube et discuterons également des perspectives ouvertes dans un avenir proche par IceCube et son extension à basse énergie IceCube DeepCore pour l’étude des sources de rayons cosmiques.

Colloquium APC

mardi 10 mars 2009

à 11h, salle 454A

Gamma ray emission from molecular clouds

Stefano GABICI (Institute for Advanced Studies, Dublin, Irlande)

Abstract : Molecular clouds are expected to emit non-thermal radiation due to cosmic ray interactions in the dense magnetized gas. Such emission is amplified if a cloud is located close to an accelerator of cosmic rays and if energetic particles can leave the accelerator site and diffusively reach the cloud. I will discuss the situation in which a molecular cloud is located in the proximity of a supernova remnant which is efficiently accelerating cosmic rays and gradually releasing them in the interstellar medium. I will show the expected multiwavelength spectrum from radio to gamma rays which is emerging from the cloud as the result of cosmic ray interactions. The total energy output is dominated by the gamma ray emission, which can exceed the emission in other bands by an order of magnitude or more. This suggests that some of the unidentified TeV sources detected so far, with no obvious or very weak counterparts in other wavelengths, might be in fact associated with clouds illuminated by cosmic rays coming from a nearby source. Moreover, under certain conditions, the gamma ray spectrum exhibit a concave shape, being steep at low energies and hard at high energies. This fact might have important implications for the studies of the spectral compatibility of GeV and TeV gamma ray sources. I will also discuss the role of molecular clouds in the more general problem of the diffuse gamma ray emission from the galactic disk.

Colloquium APC

mardi 17 mars 2009

à 11h en salle 454A

Phenomenology of Neutrino Mixing and Oscillations

Carlo Giunti (INFN, Torino)

Abstract : The basic aspects of the theory of neutrino masses and mixing are briefly explained. The results of neutrino oscillation experiments are reviewed and interpreted as evidence of three-neutrino mixing. Implications for the mixing parameters and the neutrino masses are discussed, with emphasis on the connection with the measurements of the absolute values of neutrino masses in beta decay and neutrinoless double-beta decay experiments and through cosmological observations.

Colloquium APC

mercredi 18 mars 2009

à 11 heures, salle 455 A (Klee)

Vendredi APC

Abstract :

11 h 00 La nouvelle salle propre d’APC Eric Bréelle 10 mn + 5

11 h 15 Le travail d’un doctorant-théoricien Sébastien Renaux-Patel 10 mn + 5

11 h 30 Première observation conjointe d’un objet (blazar) par le satellite Fermi et l’expérience HESS Lucie Gérard 10 mn + 5

11 h 45 Le mot du Directeur Pierre Binetruy 20 mn + 10

Colloquium APC

mardi 31 mars 2009

à 11h en salle 454A

Les premiers résultats du satellite Fermi

Jean Ballet, Service d’Astrophysique du CEA à Saclay

Le satellite GLAST de la NASA, maintenant rebaptisé Fermi, a été lancé le 11 juin 2008 et les opérations scientifiques ont commencé le 11 août. Son instrument principal, le LAT, couvre la gamme 30 MeV - 300 GeV. Il est composé d’un détecteur de traces qui mesure la direction d’arrivée des photons et d’un calorimètre qui mesure leur énergie. La réjection des particules chargées est effectuée par le dispositif d’anti coïncidence ainsi que l’analyse des traces. Grâce à son très grand champ de vue, sa grande surface efficace et sa meilleure résolution spatiale, Fermi réalise un saut qualitatif par rapport à son prédécesseur EGRET lancé 17 ans plus tôt. Il balaye le ciel en permanence, ce qui permet à la fois d’effectuer un relevé complet et de surveiller l’activité des sources variables du ciel. Un détecteur de sursauts gamma lui est associé. Je présenterai l’instrument ainsi qu’une revue des résultats majeurs des premiers mois d’opérations. Parmi les 205 sources qui forment le premier échantillon Fermi de sources brillantes, la majorité sont des noyaux actifs de galaxies dont beaucoup ont pu être identifiés. Une première étude statistique est donc possible, reliant les propriétés gamma à celles aux autres longueurs d’onde. L’autre grande famille de sources Fermi est celle des pulsars Galactiques. Fermi en a détecté une trentaine, parmi lesquels une fraction importante de nouveaux pulsars non détectés en radio, et de pulsars milliseconde. J’aborderai plus brièvement les autres catégories de sources (binaires, sursauts gamma, nébuleuses de pulsars, restes de supernova) ainsi que l’émission diffuse Galactique elle-même.

Colloquium APC

mardi 7 avril 2009

à 11h en salle 454A

Fingerprints of the early universe

Hiranya Peiris, IOA cambridge, UK

I will review recent progress in testing with cosmological data the inflationary hypothesis for describing the very early universe. I will present snapshots of different aspects of confronting the theory with data, including a "bottom-up" approach : the latest results from a systematic reconstruction of the inflationary dynamics ; and a "top-down" approach : testing specific string theoretic constructions that attempt to implement inflation, while predicting distinctive observables not found in simple field-theory models. I will discuss the ambiguities inherent in attempting to quantify generic predictions of the inflationary "paradigm" (as opposed to the predictions of specific models). Finally, I will discuss the astrophysical complexities underlying an observational program to look for primordial tensor modes that will discriminate between inflation and alternative theories.

Colloquium APC

mardi 14 avril 2009

à 14h en salle 454A

Strong Lensing by Optically-Selected Galaxy Clusters

Mike Gladders (KICP, Chicago)

Gravitational lensing by galaxy clusters was predicted in the 1930s, and finally discovered in 1986. Since these initial discoveries, several dozen significant cluster lenses have been discovered in a variety of ways. Lensing clusters probe the distribution of massive haloes in the universe ; the expected arc production frequency can be predicted from simulations and compared to existing data. Massive lensing clusters act as ’natural telescopes’, providing highly magnified images of background sources which cannot otherwise be studied using the current generation of telescopes. The details of the observed lensing in clusters also probes the internal properties of these massive haloes. Most cluster strong lens studies to date have been rather limited by the small number and heterogeneous nature of the sample of known lenses (most of which are one-off discoveries). I will report on efforts to take the study of strong lensing clusters to a new statistical regime, by identifying and studying two new samples of strong lenses within large catalogs of optically selected galaxy clusters from the RCS-2 and SDSS surveys. In total we expect to find hundreds of new giant arcs. These efforts are now approximately half-complete ; in this mid-course report I will describe some of the successes of these studies, and the remaining challenges. Time permitting, I will also discuss a recently commissioned instrument at the Magellan telescopes which was designed specifically for studying these new lens samples.

Colloquium APC

lundi 25 mai 2009

à 14h en salle 366A

Nus and Anti-Nus from MiniBooNE

Geoffrey Mills (Subatomic Physics Group, Los Alamos National Laboratory, USA)

Abstract : The MiniBooNE experiment, a short baseline neutrino oscillation experiment currently running at Fermilab, has spent the last two years building up its supply of anti-neutrino data. A sizable fraction of the data has now been studied along with the already substantial neutrino data set. The intriguing results will be explored along with future possibilities for short baseline programs at Fermilab and elsewhere.

Colloquium APC

mardi 16 juin 2009

à 11h en salle 454A

Résultats de l’expérience ballon BLAST : sondage des galaxies submillimétriques entre 250 et 500 microns

Guillaume Patanchon, APC

Abstract : L’expérience BLAST (Balloon Borne Large Aperture Submillimeter Telescope) a pour objectif la mesure des galaxies submillimétriques lointaines à fort taux de formation d’étoiles. BLAST est un télescope embarqué dans un ballon stratosphérique observant dans trois bandes centrées à 250, 350 et 500 microns, au maximum de l’émission du fond de rayonnement infrarouge. BLAST a effectué un vol de 11 jours en Antarctique dans le courant de l’hivers 2006 et a observé une région très profonde du ciel centrée sur GOODS-S. Je présenterai les résultats de BLAST, qui ont été publiés très récemment. Un des résultats principaux a été de montrer que le rayonnement infrarouge détecté par COBE est constitué de galaxies à fort taux de formation d’étoiles dont plus de la moitié est située à des redshifts supérieurs à 1.2. Ce résultat a été obtenu grâce à la comparaison avec un catalogue de sources à 24 microns obtenu avec le satellite SPITZER. Je montrerai les contraintes apportées sur le taux de comptage des galaxies submillimétriques, l’évolution du taux de formation d’étoile dans l’Univers, et les corrélations angulaires des galaxies. Je présenterai par ailleurs les résultats obtenus sur un champ galactique de 50 degrés carrés.

Colloquium APC

mardi 30 juin 2009

à 11h, salle 454A

Searching for gravity waves from the ground and from space

Gregory Tucker (Brown University, Providence, RI, USA)

Abstract : Measurements of the cosmic microwave background (CMB) temperature anisotropies have yielded a wealth of information about the universe. Now one of the current goals is to measure the polarization of the CMB, which should reveal the presence of gravity waves and improve our understanding of the early universe. This talk will concentrate on two experimental approaches to measuring this polarization.

Colloquium APC

mercredi 1 juillet 2009

à 14h en salle 454A

Some Vignettes of Cosmic Ray History : 19th century to 1953

Jim Cronin (KICP, University of Chicago, IL, USA)

Abstract : The discovery of cosmic ray history began serendipidously. The subsequent cosmic ray research began with the study of phenomena that had never been observed in any scientific context. Its progress involved experiments of daring nature. It involved many outstanding scientists and conflicts among them. My story ends in 1953 with the famous conference at Bagneres de Bigorre.

Colloquium APC

jeudi 22 octobre 2009

en salle Paul Klee (454A) à 11h

Large-angle anomalies in the cosmic microwave background : Are they real, and if so, what do they mean ?

Ted Bunn, University of Richmond

Abstract : In recent years, a standard model has emerged in cosmology. Central to this model is the idea that the Universe is statistically homogeneous and isotropic — that is, that there are no globally preferred locations or directions. Multiple analyses of the all-sky cosmic microwave background (CMB) maps from WMAP have claimed to find anomalous patterns, such as the so-called ``axis of evil,’’ that appear inconsistent with statistical isotropy. Moreover, the maps show a lack of fluctuation power on large angular scales that may conflict with the predictions of inflation. Assessing the significance of these anomalies is difficult, largely because the statistics used to quantify them are chosen a posteriori. Nonetheless, because of their potential for revolutionary changes to the standard model, they are worth examining closely. I will review the evidence for several of the apparently most significant anomalies. Assuming that the anomalies are real, I will examine what form an explanation for them might take.

 

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