Séminaires sur les aspects théoriques et expérimentaux de la gravitation
 2013 | ||
| 5 décembre 2013 | Jérémie Szeftel Laboratoire Jacques-Louis Lions & Université Pierre et Marie Curie | The resolution of the bounded L2 curvature conjecture in general relativity In order to control locally a space-time which satisfies the Einstein equations, what are the minimal assumptions one should make on its curvature tensor? The bounded L2 curvature conjecture roughly asserts that one should only need L2 bound on the curvature tensor on a given space-like hypersuface. I will present the proof of this conjecture, which sheds light on the specific nonlinear structure of the Einstein equations. This is joint work with S. Klainerman and I. Rodnianski. Transparents de l'exposé PDF |
| 21 novembre 2013 | Ted Jacobson University of Maryland | Spacetime approach to force-free magnetospheres Neutron star and black hole magnetospheres, converting rotational energy to Poynting flux, are often modeled using force-free electrodynamics, since the energy of the field dominates that of the charged matter. In this peculiar scheme, the requirement that the 4-force on the current vanish plays the role of a non-linear field equation, with the current defined by the field via Maxwell's equations. This talk will show how the theory becomes remarkably simple and elegant when treated relativistically with the help of differential forms, and some applications and new exact solutions will be discussed. Transparents de l'exposé PDF |
| 12 novembre 2013 | Slava Mukhanov Munich University Jour exceptionel | Does cosmology require a new physics beyond the standard model? |
| 7 novembre 2013 | Karim Noui Université de Tours | Black Holes microstates in Canonical Quantum Gravity In the context of Loop Quantum Gravity, Black Holes are closely related to Chern-Simons theory on a punctured 2-sphere with SU(2) gauge group. Using this link, one can describe precisely the space of microstates for the Black Holes and compute the corresponding statistical entropy. However, it turns out that the entropy depends on the unphysical Immirzi parameter $\gamma$. But, using a suitable analytic continuation of $\gamma$ to complex values, we show that the entropy reproduces the expected Bekenstein-Hawking expression when $\gamma=\pm i$ at the semi-classical limit. This remarkable result has a nice and clear geometric interpretation and many very interesting physical consequences. In particular, we show that, at the semi-classical limit, the Black Hole microstates (at the vicinity of the horizon) are particles in equilibrium at the Unruh temperature. Transparents de l'exposé PDF |
| 10 octobre 2013 | David Hilditch University of Jena | Hyperbolicity of constrained Hamiltonian systems Motivated by the question, "what coordinates choices are 'good' for general relativity?" I will discuss gauge theories from the from the free evolution point of view, in which initial data satisfying constraints of a theory are given, and because the constraints are compatible with the field equations they remain so. I will present a model constrained Hamiltonian theory and identify a particular structure in the equations of motion from which statements can be made about the status of the initial value problem, just by examining a subset of the equations, called the pure gauge subsystem. I will then present the application of these ideas to GR, which results in a five parameter generalization of the 'good' gauge used in Choquet-Bruhat's original treatment of the initial value problem. Transparents de l'exposé PDF |
| 10 octobre 2013 | Yacine ALI-HAIMOUD Institute for Advanced Study, Princeton horaire exceptionel 14h45 | Massive neutrinos in the non-linear regime of cosmological structure formation Massive neutrinos make up a fraction of the dark matter, but due to their large thermal velocities, cluster significantly less than cold dark matter (CDM) on small scales. An accurate theoretical modelling of their effect during the non-linear regime of structure formation is required in order to properly analyse current and upcoming high-precision large-scale structure data, and constrain the neutrino mass. Taking advantage of the fact that massive neutrinos remain linearly clustered up to late times, we treat the linear growth of neutrino overdensities in a non-linear CDM background. The evolution of the CDM component is obtained via N-body computations. The smooth neutrino component is evaluated from that background by solving the Boltzmann equation linearised with respect to the neutrino overdensity. CDM and neutrinos are simultaneously evolved in time, consistently accounting for their mutual gravitational influence. This method avoids the issue of shot-noise inherent to particle-based neutrino simulations, and, in contrast with standard Fourier-space methods, properly accounts for the non-linear potential wells in which the neutrinos evolve. Inside the most massive late-time clusters, where the escape velocity is larger than the neutrino thermal velocity, neutrinos can clump non-linearly, causing the method to formally break down. It is shown that this does not affect the total matter power spectrum, which can be very accurately computed on all relevant scales up to the present time. Transparents de l'exposé PDF |
| 19 septembre 2013 | François Bouchet IAP | First cosmological results from the Planck satellite Sketched out in 1992, selected by ESA in 1996, launched in 2009, Planck delivered on March 21st its first full sky maps of the millimetric emission at 9 frequencies, as well as those which follow from them, and in particular Planck map of the anisotropies of the Cosmic Microwave Background (CMB). The later displays minuscule variations as a function of the observing direction of the temperature of the fossile radiation around its mean temperature of 2.725K. I will briefly describe how these high resolution maps with a precision of a few parts in a million have been obtained, from collection to analysis of the first 500 billion samples of our HFI instrument. CMB anisotropies reveal the imprint of the primordial fluctuations which initiate the growth of the large scale structures of the Universe, as transformed by their evolution, in particular during the first 370 000 years, i.e. till the Universe became transparent and the forming of the image we record today. The statistical characteristics of these anisotropies allow constraining jointly the physics of the creation of the primordial fluctuations and that of their evolution. They teach us the possible value of the parameters of the models which we confront to data. I will describe Planck estimates of the density of the constituents of the Universe (usual matter, cold dark matter or CDM, dark energy...), and their implication in terms of derived quantities like the expansion rate or the spatial curvature. I will review what we learnt on the generation of the fluctuation, and wil discuss extensions of the standard cosmological model, so called "Lambda-CDM", both in term of non minimal physical models -- multi-field inflation for instance, or additional constituents - like cosmic strings or a fourth neutrino. Finally, it will briefly describe other promising results on the matter distribution which is travelled through by the CMB image on its long 13.7 billion years trip towards us. I will mention in particular what we can learn on the dark matter distribution - which is detected through its distorting effet of the CMB image by gravitationnal lensing, or that of hot gaz, which is revealed by the spectral distortion it induces. Transparents de l'exposé PDF |
| 19 septembre 2013 | Nicholas Warner USC et IPhT CEA-Saclay horaire exceptionel 14h45 | Resolving the Structure of Black Holes I show how massless supergravity theories can evade the theorems that try to establish that there are "No Solitons without Horizons.'' The explicit construction of smooth, horizonless BPS solitons will be reviewed and some of their physical properties will be discussed. The possible role of such solitons in describing black-hole microstates will be examined. Transparents de l'exposé PDF |
| 27 juin 2013 | Abed Bounemoura IHÉS | Autour du théorème de nekhoroshev et de la stabilité du problème planétaire Le théorème de Nekhoroshev garantit la stabilité, pendant un intervalle de temps exponentiellement long, des systèmes hamiltoniens qui sont proches des systèmes intégrables. C'est un résultat théorique, qui est hélas difficilement applicable à des systèmes hamiltoniens concrets. Le but de l'exposé est d'expliquer une variante de ce théorème de Nekhoroshev, plus flexible, que l'on espère pouvoir appliquer à des systèmes concrets tels que le problème à trois corps Soleil-Jupiter-Saturne. Transparents de l'exposé PDF |
| 27 juin 2013 | Reiko Toriumi University of California, Irvine horaire exceptionel 14h45 | Discrete Wheeler DeWitt equation The infrared structure of quantum gravity is explored by solving a lattice version of the Wheeler-DeWitt equations. For this talk, first, the case of 2+1 dimensions is presented. The wavefunction solutions only depend on the geometric quantities indicating preservation of diffeomorphism. Properties of the lattice vacuum are consistent with the existence of an ultraviolet fixed point in $G$ located at the origin, thus precluding the existence of a weak coupling perturbative phase. The correlation length exponent is determined exactly and found to be $\nu={6\over11}$. The results obtained lend support to the claim that the Lorentzian and Euclidean formulations belong to the same field-theoretic universality class. I then discuss some results in 3+1 dimensions. Investigations of the vacuum wave functional further indicate that for weak enough coupling, $G < G_c$, a pathological ground state with no continuum limit appears, where configurations with small curvature have vanishingly small probability Transparents de l'exposé PDF |
| 13 juin 2013 | Walter Freyn Technischen Universität Darmstadt and IHÉS | Affine Kac-Moody symmetric spaces We construct affine Kac-Moody symmetric spaces. These spaces are associated to affine Kac-Moody algebras in a similar way as finite dimensional Riemannian symmetric spaces are associated to finite dimensional simple Lie algebras. Affine Kac-Moody symmetric spaces are constructed as tame Fréchet manifolds and equipped with a weak Lorentzian metric. We describe their classification and explain their geometry. Transparents de l'exposé PDF |
| 30 mai 2013 | Ralf Köhl Giessen University | Generalized spin representations Motivated by the 32-dimensional extension of the spin representation of the compact Lie algebra $so(10)$ to the 'maximal compact' subalgebra of the real Kac-Moody Lie algebra of type $E_{10}$ described by Damour et al. and Henneaux et al. Hainke and myself introduced the concept of a generalized spin representation that allows similar constructions for the 'maximal compact' subalgebras of real Kac-Moody Lie algebras of arbitrary simply laced type. By work of Ghatei, Horn, Weiss and myself, integration of these representations leads to two-fold spin covers of the 'maximal compact' subgroups of the corresponding split real Kac-Moody groups. The problem that semisimple elements generally do not have a locally finite action and therefore obstruct integration is circumvented by an amalgamation method using the Iwasawa decomposition and the theory of buildings. The existence of these spin covers has been conjectured by Damour and Hillmann; it contains an extended Weyl group, which in the $E_{10}$ case is relevant to fermionic billards. Transparents de l'exposé PDF |
| 23 mai 2013 | Marcos Marino Université de Genève | Quantum gravity at one-loop and AdS/CFT In the AdS/CFT correspondence, gauge theory calculations beyond the planar approximation correspond to quantum corrections in gravity or in string theory. Recently, the partition function on the three-sphere of Chern-Simons-matter theories has been computed at all orders in the 1/N expansion, and this leads to predictions for quantum corrections in M-theory/string theory. Using the ideas of effective field theory, we show that some of these corrections can be calculated reliably by doing one-loop calculations in supergravity. A similar reasoning has been used recently to calculate logarithmic corrections to black hole entropy, and we use it here to perform a successful test of $AdS_4/CFT_3$ beyond the leading, planar approximation. Transparents de l'exposé PDF |
| 2 mai 2013 | Henning Samtleben Laboratoire de Physique ENS de Lyon | Affine symmetries in supergravity I review the construction of maximal supergravities in two dimensions. These are parametrized by an embedding tensor transforming in the basic representation of the affine algebra $E_9$. Among the examples is the $SO(9)$ theory describing the low-energy effective action upon reduction of the type IIA theory on the D0-brane near-horizon geometry. Transparents de l'exposé PDF |
| 14 mars 2013 | Mikhail Volkov Université de Tours & IHES | Ghost-free bigravity theories and their cosmological solutions Transparents de l'exposé PDF |
| 7 mars 2013 | Jürgen Renn Max Planck Institute for the History of Science | Einstein's path to general relativity The talk will review the birth of GR on the background of historical documents, and in particular on the basis of Einstein's extant manuscripts. It will show how Einstein's work emerged from a transformation of the knowledge of classical physics in a process that involved an interaction between the development of mathematical formalism and its physical interpretation. It will also emphasize the role of philosophical reflections for both the heuristics and the interpretation of the theory. Transparents de l'exposé PDF |
| 7 mars 2013 | Ugo Moschella University of Insubria & IHES horaire exceptionel 14h45 | Problèmes infrarouges dans l'espace-temps de de Sitter (d'après les travaux d'A.M. Polyakov) |
| 31 janvier 2013 | Frank Ferrari Université libre de Bruxelles | On Explicit Models of Emergent Space I will describe general ideas that lead to the explicit constructions of models of emergent space. In particular, I will solve a pre-geometric quantum mechanical model for D-particles in a presence of a large number of background D4 branes, and show that it is equivalent to the classical motion of the particles in the ten dimensional geometry sourced by the D4 branes. If time allows, other cases will be briefly discussed as well. Transparents de l'exposé PDF |
| 24 janvier 2013 | Kirill Krasnov University of Nottingham | Gravity as a Gauge Theory I will explain how General Relativity in four space-time dimensions (with cosmological constant) can be reformulated as an SU(2) gauge theory of a certain type. The action is a (diffeomorphism invariant) functional of an SU(2) connection; no metric is present in the formulation of the theory. This formulation of GR is in many ways analogous to the one proposed many years ago by Eddington (Eddington's Lagrangian is a function of just the affine connection and is given by the square root of the determinant of the Ricci tensor). The new formulation has some remarkable properties. First, on a 4-manifold, the space of SU(2) connections modulo gauge transformations has just 9 components per space-time point, as compared to 10 components of a metric tensor. Correspondingly, the action of the new formulation can be interpreted as a functional on the space of conformal classes of metrics. Thus, the conformal factor is not free to propagate in this formulation even off-shell. This has the effect that the action functional is (at least locally) convex - there is no conformal mode problem of the usual metric formulation of GR. Another remarkable property of the new formulation is that diffeomorphisms are very easy to deal with. The most natural gauge-fixing of these does not make the corresponding components of the connection propagate. As a result, in (linearized) quantum theory only 8 components of the connection propagate as compared to 10 metric components in the usual formulation. All these 8 components fit into a single irreducible representation of the Lorentz group, which makes the propagator very simple. There are also some remarkable simplifications in the structure of the interaction vertices. The full off-shell 3-vertex (in de Sitter space) contains just 3 terms as compared to a couple of dozen terms in the metric formulation. The 4-vertex is a couple of lines as compared to a couple of pages in the standard description. As an illustration of the formalism I will describe how the graviton scattering amplitudes are computed in this approach. The new gauge-theoretic reformulation of GR also leads to (an infinite-parameter) family of modified gravitational theories, all propagating just two polarizations of the graviton, as GR. This leads to a rather strong claim that, in spite of the standard GR uniqueness theorems, General Relativity is not the only interacting theory of massless spin two particles. However, GR appears to be the only parity-invariant gravitational theory, as all the "deformations of GR" can be shown to be parity violating. I will also describe how matter is coupled in this approach, and give some speculations as to a possible UV completion of gravity. Transparents de l'exposé pdf |