Séminaires sur les aspects théoriques et expérimentaux de la gravitation
 2015 | ||
| Seth Hopper Instituto Superior Técnico, Lisboa & IHES | Finding self-force quantities in a post-Newtonian expansion: Eccentric orbits on a Schwarzschild background Small compact objects orbiting supermassive back holes are an important potential source of gravitational radiation. Detection of such waves and the parameter estimation of their sources will require accurate waveform templates. To this eventual end, I present work on bound eccentric motion around a static black hole. In two separate approaches, I examine solutions to the first order (in mass-ratio) field equations. First, I consider solving the field equations entirely analytically in a double post-Newtonian/small-eccentricity expansion. Then I show numeric work wherein we use the MST formalism to solve the field equations to 200 digits. We use this extreme accuracy to fit for previously unknown PN energy flux parameters, extending the previous state of the art from 3PN to 7PN. | |
| 4 novembre 2015 | V.D. Ivashchuk
Center for Gravitation and FM, VNIIMS and Peoples' Friendship University of Russia (Moscow) Horaire exceptionel 14h45 | Exact solutions in multidimensional gravitational models with fields of forms Transparents de l'exposé PDF |
| 28 octobre 2015 | Vincent Moncrief
Yale University | Euclidean-Signature Semi-Classical Methods for Bosonic Quantum Field Theories It has long been realized by microlocal analysts that an alternative, ´Euclidean-signature´ semi-classical ansatz for solutions to Schrödinger´s equation is much more natural, from a mathematical point of view, than the classical, physics textbook, W.K.B. ansatz, even though the latter has the conceptual advantage of emphasizing the classical correspondence principle. For technical reasons though this microlocal approach has seemed to be limited to quantum mechanical problems and not to be applicable to quantum field theories. In this seminar I will review this method and discuss an expansive, ongoing project with A. Marini (Yeshiva, L´Aquila) and R. Maitra (Wentworth I.T.) to extend these influential ideas to bosonic quantum field theories including φp type scalar and Yang-Mills fields. Transparents de l'exposé PDF |
| 15 octobre 2015 | Lionel Mason
University of Oxford | Ambitwistor strings and the scattering equations at one loop Ambitwistor strings are holomorphic string theories whose target space is the space of complex null geoedesics in a complexified space-times. I will explain how these theories explain the origin of the scattering equations in twistor strings and the CHY formulae in arbitrary dimensions and provide a reformulation of standard gauge, gravity and other theories in a holomorphic infinite tension analogue of conventional string theories. I will show how these results extend to 1-loop both on a torus and on a nodal Riemann sphere. Transparents de l'exposé PDF |
| 29 septembre 2015 | Diederik Roest University of Groningen | The Palatini Formalism for Galileons In this blackboard presentation, we will outline a new Palatini formalism for Galileon scalar field theories. After a pedagogical review of the Palatini formalism for GR, we will outline how this carries over to the spin-0 case. This will uniquely single out the Galileon symmetry and provides a first-order formalism for such theories. Finally, we present possible extensions including the coupling to gravity. |
| 3 juillet 2015 | Massimo Porrati
New York University | Single-field Inflationary Models in Supergravity After reviewing the well known fact that bosonic "Starobinsky" $R + f(R)$ models of inflation are equivalent to standard single-scalar slow-roll theories, we show how to embed $R + f(R)$ models in supergravity, in an efficient way, using supergravity's "new minimal" formulation. We also discuss how supergravity can improve the naturalness of such models and conclude presenting a rather general class of $R + f(R)$ models coupled to matter. Transparents de l'exposé PDF |
| 18 juin 2015 | Valery Rubakov
Institute for Nuclear Research, Russian Academy of Sciences, Moscow | Conformal Universe as false vacuum decay We point out that the (pseudo-)conformal Universe scenario may be realized as decay of conformally invariant, metastable vacuum, which proceeds via spontaneous nucleation and subsequent growth of a bubble of a putative new phase. We study perturbations about the bubble and show that their leading late-time properties coincide with those inherent in the original models with homogeneously rolling backgrounds. In particular, the perturbations of a spectator dimension-zero field have flat power spectrum. (arXiv:1502.05897) |
| 20 mai 2015 | Jnan Maharana
Institute Of Physics, Bhubaneswar | Scattering of Stringy States and T-duality I shall briefly review the salient features of target space duality (T-duality) and recall some essential results. I shall outline a prescription to derive the S-matrix for the scattering of massless stringy states that arise in the compactification of closed bosonic strings on a torus at the tree level. It will be shown that the S-matrix elements can be expressed in a T-duality invariant form. The Kawai-Lewellen-Tye formalism plays an important role in our approach. |
| 15 janvier 2015 | Michele Levi
Université Pierre et Marie Curie | An EFT for gravitating spinning objects in the PN scheme Over the last decade considerable worldwide efforts have been invested in order to detect gravitational radiation. Inspiralling binary systems of black holes, which can be described analytically by the post-Newtonian (PN) approximation of GR, are promising candidate sources of gravitational waves signals. For a successful detection of such signals, PN corrections are required to high orders. We will present the main ideas of the Effective Field Theory (EFT) approach to the PN formalism of the binary inspiral problem. We then formulate an EFT for gravitating spinning objects. We aim at an effective action, where all field modes below the orbital scale are integrated out. We point out the relevant degrees of freedom, in particular the rotational ones, and the associated symmetries. Using these symmetries, we construct the minimal coupling part of the point particle action in terms of gauge rotational variables. We then proceed to construct the spin- induced nonminimal couplings, where we obtain the LO couplings to all orders in spin. We specify to a gauge for the rotational variables, where the unphysical degrees of freedom are eliminated already from the Feynman rules, and all the orbital field modes are conveniently integrated out. The EOM of spin are then obtained from an appropriate variation of the action, and a simple form is obtained. Finally, the corresponding Hamiltonians are also straightforwardly obtained from the potentials derived via this formulation. Thus, the formulation is ideal for the treatment of further higher order spin- dependent sectors. New results are underway. Publication 1501.04956 |
| 8 janvier 2015 | Christos Charmousis
University of Orsay Horaire exceptionel 14h30 | Black holes in higher order scalar tensor theories We will review modified gravity theories and in particular scalar tensor theories, where we have an additional scalar field coupling to the metric tensor. By means of a theorem given by Horndeski back in 1974 we will briefly discuss the most general of these theories. We will examine a particular sub-class of Horndeski theory which has interesting properties with respect to the cosmological constant problem. We will then find black hole solutions of this subclass which in some cases will be identical to GR solutions. The novel ingredient will be the presence of a time and space dependent scalar field. As we will see time dependence and higher order Galileon terms will bifurcate no hair theorems and provide scalar tensor black holes with a non trivial scalar field. Transparents de l'exposé PDF |