Séminaires sur les aspects théoriques et expérimentaux de la gravitation
 2014 | ||
| 18 décembre 2014 | Ben
Freivogel University of Amsterdam Horaire exceptionel 14h30 | Does quantum gravity predict observable causality violation? The black hole firewall problem is a conflict between three important physical principles: causality, unitarity, and the equivalence principle. I will describe how the conflict arises in the description of Hawking radiation from a black hole, and explain why resolving the problem is crucially important for our understanding of quantum gravity. I will briefly describe possible resolutions of the conflict. Solving the firewall problem is likely to teach us something important about quantum gravity. |
| 5 Novembre 2014 | Dimitri Skliros University of Nottingham and IHES | Highly Excited Strings A large amount of work in perturbative superstring theory is associated to certain limits of string amplitudes with massless (or very low mass) asymptotic states, in simple string backgrounds where computations are feasible. In this talk I will focus on a much less explored regime, that of amplitudes with 'highly excited' asymptotic string states, and will give an overview of a formalism (based on covariant coherent state vertex operators) which is particularly efficient for explicit computations involving highly excited strings. I will discuss generic features of 'all' 2-point 1-loop amplitudes, eventually focusing on a simple explicit example to illustrate the effectiveness of this new approach, while also making contact with low energy effective field theory results. Transparents de l'exposé PDF |
| 5 Novembre 2014 | Abhay Shah University of Southampton and IHES Horaire exceptionel 14h30 | Ultra-high-accuracy computation of gravitational self-force observables
in binary black holes Transparents de l'exposé PDF |
| 30 octobre 2014 | Alessandro Nagar
IHES | The effective-one-body (EOB) modelling of coalescing compact binaries I will review the state-of-the-art analytic effective one body (EOB) approach to the general-relativistic two-body dynamics and its completion using numerical relativity (NR) simulations. I will discuss in detail three cases: i) coalescence of (spinning) black hole binaries (BBHs); ii) tidal effects in coalescing neutron star binaries and the EOS-universality at merger; iii) the BBH scattering problem and the determination of the scattering angle in EOB and NR. I will present several examples about how NR simulations can inform the EOB model so to build a comprehensive EOBNR model of the two-body dynamics and waveforms in general relativity. Implications for gravitational wave data analysis are discussed. Transparents de l'exposé PDF |
| 30 octobre 2014 | Eric Poisson
University of Guelph Horaire exceptionel 11h00 | Tidal deformation and dynamics of black holes I will present an overview of the work that has gone into formulating a theory of tidal deformations and dynamics of black holes in general relativity. This includes a description of the tidal environment around a black hole, a description of the tidal deformation in terms of the intrinsic geometry of the event horizon, and a description of how the black hole can exchange energy and angular momentum with its tidal environment. Transparents de l'exposé PDF |
| 23 octobre 2014 | Alexandre
Le Tiec Observatoire de Meudon | Thermodynamics of a Black Hole with Moon Much of black hole thermodynamics is limited to systems with a high degree of symmetry. In this talk, I will discuss a non-stationary, non-axisymmetric black hole spacetime that nevertheless admits a standard thermodynamics: a black hole corotating with an orbiting moon. More precisely, we consider a Kerr black hole perturbed by a particle on the circular orbit whose frequency matches that of the event horizon. The key point is that the spacetime has a helical Killing vector that generates the event horizon, allowing the surface gravity to be defined in the standard way. The surface gravity is uniform on the horizon and should correspond to the Hawking temperature of the black hole. We calculate the change in surface gravity/temperature, finding it negative: the moon has a cooling effect on the black hole. We also calculate the area/entropy of the perturbed black hole, finding no change from the background Kerr value. Transparents de l'exposé PDF |
| 1 octobre 2014 | Gabriele
Veneziano CERN & Collège de France & New-York University Jour exceptionel | An adapted coordinate system for light-signal-based cosmology The "geodetic light-cone gauge" is a convenient coordinate system for the observation of light-like signals by a geodetic observer. After introducing it I will give two examples on how it can be applied to physically interesting problems: i) the precision determination of dark-energy parameters; ii) the description of strong gravitational lensing. If time allows I will also mention some very recent work on gravitational bremsstrahlung from massless particle collisions. Transparents de l'exposé PDF |
| 25 septembre 2014 | Kellogg
Stelle Imperial & IHÉS | Localizing gravity on branes in hyperbolic spaces The construction of braneworlds localizing massless gravity on subsurfaces of spacetimes with infinite transverse spaces has remained an open problem. There have even been some "no-go" theorems claiming to show that such constructions are not possible. The talk will present a resolution of this problem simply based upon a hyperbolic solution of type IIA theory with a superposed Neveu-Schwarz 5-brane. Gravity is bound to the brane surface owing to the existence of a single normalizable bound state of the transverse space Laplacian. |
| 6 mai 2014 | Sergey
Alexandrov Laboratoire Charles Coulomb, Université Montpellier Jour exceptionel | First order gravity on the light front After a brief review of unusual features of the light front canonical formulations of field theories, I'll present the analysis of the canonical structure of the first order formulation of general relativity on a lightlike foliation. It appears to be quite different from the usual spacelike case leading, for instance, to the presence of tertiary constraints. Besides, I'll discuss the issue of the zero modes and argue that there might be some hidden correspondence with two-dimensional theories. Transparents de l'exposé PDF |
| 6 mai 2014 | Eric d'Hoker
UCLA Jour exceptionel et horaire exceptionel 14h30 | Recent advances in two-loop superstrings Recent advances in two-loop superstring theory will be discussed, including the structure of supermoduli space, the spontaneous supersymmetry breaking on Calabi-Yau orbifolds, and the matching of the $D^6 R^4$ effective low energy corrections to supergravity with predictions from supersymmetry and duality. Transparents de l'exposé PDF |
| 10 avril 2014 | Marios
Petropoulos CPhT, École Polytechnique | The Geroch group in Einstein spaces I will discuss the extension of Geroch's solution-generating method to the case of Einstein spaces. This will include the reduction to a three-dimensional coset space, the description of the dynamics in terms of a sigma-model and its transformation properties under the SL(2,R) group, and the reconstruction of new four-dimensional Einstein spaces. The detailed analysis of the space of solutions will be presented in a minisuperspace reduction, and will be performed using the Hamilton-Jacobi method. The cosmological constant will appear in this framework as a constant of motion. Transparents de l'exposé PDF |
| 10 avril 2014 | Agnès Fienga Observatoire de la Côte d'Azur Horaire exceptionel 14h30 | Testing General Relativity in the solar system with planetary ephemerides Current knowledge of Mercury orbit is mainly brought by the direct radar ranging obtained from the 60s to 1998 and five Mercury flybys made by Mariner 10 in the 70s, and MESSENGER made in 2008 and 2009. On March 18, 2011, MESSENGER became the first spacecraft orbiting Mercury. The radioscience observations acquired during the orbital phase of MESSENGER drastically improved our knowledge of the Mercury orbit. An accurate MESSENGER orbit is obtained by fitting one-and-half years of tracking data using GINS orbit determination software. The systematic error in the Earth-Mercury geometric positions, also called range bias, obtained from GINS are then used to fit the INPOP dynamical modeling of the planet motions. An improved ephemeris of the planets is then obtained, INPOP13a, and used to perform general relativity test of PPN-formalism. Monte Carlo simulations will be introduced for estimating the most significant levels of GR violations in terms of PPN parameters and their correlated parameter (oblateness of the sun) and of time-varying Gravitational constant G. Transparents de l'exposé PDF |
| 6 mars 2014 | Pierre Teyssandier Observatoire de Paris | Enhanced terms in the time delay and the direction of light propagation.
Discussion for some solar system experiments It is generally stated that the determination of the light travel time up to the order $G^2$ is sufficient for modelling space missions in project---such as LATOR, ASTROD, SAGAS, ODYSSEY or GAME--- designed to reach an accuracy of $10^{-7}$ to $5\times 10^{-9}$ in measuring the post-Newtonian parameter $\gamma$. However, this statement has to be re-examined in light of our recent demonstration that a so-called `enhanced' term of order $G^3$ in the time delay may become comparable to some of the terms of order $G^2$ when the emitter and the receiver are almost on opposite sides of the central body --- a configuration of crucial importance in experimental gravitation. This talk yields an overview of the new methods enabling us to carry out the calculation of the light travel time as a function of the positions of the emitter and the receiver (time transfer function) at any order of approximation for a large class of static, spherically symmetric metrics generalizing the Schwarzschild solution. The time transfer function is explicitly determined up to order $G^3$. The enhanced terms are inferred from this solution and their significance for some tests of general relativity in the solar system is analyzed. To finish, the appearance of enhanced terms in the direction of light propagation is discussed. Transparents de l'exposé PDF |
| 6 mars 2014 | Richard Woodard University of Florida horaire exceptionel 11h00 | Nonlocal Cosmology I review a class of nonlocally modified gravity models which were proposed to explain the current phase of cosmic acceleration without dark energy. Among the topics considered are deriving causal and conserved field equations, adjusting the model to make it support a given expansion history, why these models do not require an elaborate screening mechanism to evade solar system tests, degrees of freedom and kinetic stability, and the negative verdict of structure formation. Although these simple models are not consistent with data on the growth of cosmic structures many of their features are likely to carry over to more complicated models which are in better agreement with the data. The talk is be based on 1401.0254. Transparents de l'exposé PDF |
| 6 février 2014 | Jean-Pierre Lasota Institut d'Astrophysique de Paris et N. Copernicus Astronomical Center, Warsaw, Poland | Extracting black-hole rotational energy: The generalized Penrose process Relativistic jets are often launched from the vicinity of accreting black holes. They are observed to be produced in stellar-mass black-hole binary systems and are believed to be the fundamental part of the gamma-ray burst phenomenon. Powerful relativistic jets are also ejected by accreting supermassive black holes in some active galactic nuclei. There is no doubt that the jet-launching mechanism is related to accretion onto black holes, but there has been no general agreement as to the ultimate source of energy of these spectacular high energy phenomena. In principle, relativistic jets can be powered either by the black hole gravitational pull or by its rotation (spin), with large-scale magnetic fields invoked as energy extractors in both cases. In the context of strongly magnetized jets Blandford & Znajek (1977) proposed a model of electromagnetic extraction of black hole's rotational energy based on the analogy with the classical unipolar induction phenomenon. The physical meaning of this process has been subject to a long controversy. I will show that the Blanford-Znajek process is a Penrose process of black-hole energy extraction. I will first consider the case of arbitrary fields or matter described by an unspecified, general energy-momentum tensor and show that the necessary and sufficient condition for extraction of a black hole's rotational energy is analogous to that in the mechanical Penrose process: absorption of negative energy and negative angular momentum. I will show that a necessary condition for the Penrose process to occur is for the Noether current to be spacelike or past directed (timelike or null) on some part of the horizon. In the particle ("mechanical") case, the general criterion for the occurrence of a Penrose process reproduces the standard result. For stationary, force-free electro-magnetic field one recovers the condition obtained by Blandford and Znajek in their original article. In the case of relativistic jet-producing ``magnetically arrested disks'' I will show that the negative energy and angular-momentum absorption condition is obeyed when the Blandford-Znajek mechanism is at work, and hence the high energy extraction efficiency up to $\sim 300\%$ found in recent numerical simulations of such accretion flows results from tapping the black hole's rotational energy through the Penrose process. I will show how black-hole rotational energy extraction works in this case by describing the Penrose process in terms of the Noether current. Transparents de l'exposé PDF |
| 30 janvier 2014 | Tristan Dennen Niels Bohr International Academy | The Perturbative Ultraviolet Structure of N=4 Supergravity I will present recent calculations of the ultraviolet limit of multi-loop scattering amplitudes in $N=4$ supergravity. These calculations are performed using a squaring relationship between integrands for Yang-Mills amplitudes and for gravity amplitudes. I will discuss in detail the procedure we use to extract ultraviolet divergences from Feynman integrals, and I will present the three- and four-loop ultraviolet divergences in four-graviton scattering. I will also show how the four-loop divergence might be interpreted in terms of the $U(1)$ duality anomaly of the theory. Transparents de l'exposé PDF |
| 30 janvier 2014 | Shigenori Seki CQUeST, Seoul Horaire exceptionel 14h30 | Holographic entanglement and interaction Entanglement is an intriguing property in quantum mechanics. Maldacena and Susskind have recently conjectured that the entanglement of EPR pair is interpreted to an ER bridge or a wormhole. This conjecture is called "EPR = ER". Indeed, it is known that, from the holographic point of view, there is a wormhole on the world-sheet minimal surface corresponding to a (EPR) pair of accelerating quark and anti-quark. Therefore we discuss the causal structure on the world-sheet minimal surface of other scattering particles. Transparents de l'exposé PDF |
| 23 janvier 2014 | Elie During Université de Paris Ouest Nanterre | La querelle des temps : Einstein, Bergson,
Whitehead, Bachelard Article Langevin Ou Le Paradoxe Introuvable Transparents de l'exposé PDF |