Séminaires Cosmologie 2011-2012

  Jeudi 21 juin à 16h, salle 110

Sergey Sibiryakov (Institute for Nuclear Research, Moscow) : Emergent Lorentz invariance : the holographic description

  Mercredi 23 mai à 16h, salle 110

Michele Levi (Ben-Gurion University and Weizmann Institute, Israel) : Higher order Post-Newtonian corrections via Effective Field Theory

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, which has enabled the efficient obtainment of new higher order PN corrections. We will illustrate how the effective action is constructed for each scale in the problem, introduce the non-relativistic gravitational fields, and also review the treatment of spinning objects. We will demonstrate how to obtain the leading order PN corrections, in both the non- spinning and spinning cases, in terms of Feynman rules and diagrams. Recent and prospective applications of the EFT approach will be reviewed, from higher order PN corrections for spinning and non-spinning compact binaries, which are at the focus of my recent works, to such that involve other physical settings.

  Mercredi 4 avril à 16h, salle 110

Mokhtar Hassaine (Universidad de Talca (Chile) : Towards on the Uniqueness of Lifshitz Black Holes in New Massive Gravity (continued).

  Mercredi 28 mars à 16h, salle 110

Filippo Vernizzi (CEA, IPhT) : Squeezing the CMB bispectrum

  Mercredi 21 mars à 16h, salle 110

Mokhtar Hassaine (Universidad de Talca (Chile) : Towards on the Uniqueness of Lifshitz Black Holes in New Massive Gravity

  Mercredi 14 mars à 16h, salle 110

Jean-Francois Dufaux (APC) : Gravitational Waves and Kaluza-Klein Modes from Cosmic Super-Strings

Cosmic super-strings are fundamental objects of string theory that have a cosmological size and evolve as a network of cosmic strings. They could lead to observational signatures of string theory, notably through gravitational wave (GW) astronomy, if they can be distinguished to some degree from their traditional field theory counterparts. In the first part of this talk, I will revisit the computation of the GW background produced by cosmic (super-)string networks and discuss its essential and distinctive properties. I will then address the prospects for its detection with ESA’s space-based interferometer eLISA/NGO, as well as pulsar timing observations and ground-based interferometers. In the second part of the talk, I will study one specific aspect of cosmic super-strings that may help to distinguish them from traditional field theory cosmic strings : their production of Kaluza-Klein (KK) modes associated with the extra dimensions. I will show that KK emission by cosmic super-strings is largely independent of the geometry of the internal space and that it is constrained by Big Bang Nucleosynthesis and observations of the diffuse gamma-ray backgrounds. I will discuss the resulting constraints in the parameter space of cosmic super-strings and highlight their complementarity with the regions that can be probed with GW experiments.

  Mercredi 15 février à 17h, salle 110

Marc Geiller (APC) : The classical structure of loop quantum gravity - PART II -

  Mercredi 8 février à 17h, salle 110

Marc Geiller (APC) : The classical structure of loop quantum gravity

Canonical loop quantum gravity and spin foam models are attempts to define a non-perturbative and background independent theory of quantum general relativity. In this talk we will focus on the classical structure of the theory, and in particular introduce a new way to understand its discrete phase space in terms of a certain type of continuous geometries. This will be the occasion to review some of the foundational aspects of loop gravity as well as more recent developments in the field.

  Mardi 7 février à 11h, salle 110

Stefano Finazzi (Trento University) : Instability of superluminal warp drives

Warp drives are very interesting configurations in General Relativity. First, at least theoretically, they provide a way to travel at superluminal speeds, albeit at the cost of requiring exotic matter to exist as solutions of Einstein’s equations. Second, they are potentially dangerous for causality, since it is known that a "time machine" can be realized by appropriately combining two superluminal travels. In this talk, I shall address the issue of their semiclassical stability in both Lorentz-invariant and Lorentz-violating quantum field theories. In the former case, warp drives are unstable because they produce a Hawking-like radiation which is blue shifted at their front wall without any bound. In the latter one, even if this ultraviolet divergence is regulated, warp drives are still unstable, but the type of instability is different whether one uses a subluminal or a superluminal dispersion relation. Interestingly, these results suggest that chronology could be protected in both cases.

  Mercredi 1er février à 16h, salle 110

Oscar Dias (IPhT) :Gravitational Turbulent Instability of Ant-de Sitter

Bizon and Rostworowski have recently suggested that anti-de Sitter spacetime might be nonlinearly unstable to transfering energy to smaller and smaller scales and eventually forming a small black hole. We consider pure gravity with a negative cosmological constant and find strong support for this idea. While one can start with a single linearized mode and add higher order corrections to construct a nonlinear geon, this is not possible starting with a linear combination of two or more modes. One is forced to add higher frequency modes with growing amplitude ; a process very similar to what happens with turbulence. The implications of this turbulent instability for the dual field theory are discussed.

  Mercredi 25 janvier à 16h, salle 110

Dietmar Klemm (Milano University) : Supersymmetric black holes in N=2, D=4 gauged supergravity and attractors

We review the classification of all supersymmetric backgrounds of N=2, D=4 gauged supergravity coupled to abelian vector multiplets. It is then shown how this can be used to construct genuine BPS black holes in AdS with nontrivial scalar fields turned on. These solutions permit to study the BPS attractor flow in AdS. We finally show that there is an intriguing and misterious relationship between the equations that determine BPS geometries of this supergravity theory, and the gravitational Chern-Simons action in three dimensions.

  Mercredi 18 Janvier à 11h, salle 110

Paul Saffin : Self-tuning the cosmological constant with the fab-four

  Mercredi 14 décembre à 11h, salle 110

Gonzalo J. Olmo (University of Valencia & IFIC, Valencia, Spain) : Can Planck scale physics cure black hole singularities ?

It is usually claimed that quantum gravitational effects should be non-negligible at the Planck scale. However, since lengths and energies are not relativistic invariants it is not clear how the Planck length or energy could be consistently introduced in field theories to explore the effects of this new physics. I will show that an invariant an universal length scale can be consistently introduced in a generally covariant theory through the gravitational sector using the Palatini approach. As an application, I consider and discuss in detail how black hole structure is modified in this theory.

  Mercredi 7 décembre à 11h, salle 110

Julian Adamek (U. Wuerzburg) : Dynamical Isotropization in General Relativity

Differential expansion of the Universe, caused by a single anisotropic source in Einstein’s equations, can dynamically generate additional anisotropic terms which may tend to counteract their cause. As a physical example, I will consider the case of a homogeneous magnetic field mode leading to a Bianchi type I background model in cosmology. I will demonstrate that a fluid composed of free-streaming relativistic particles (e.g. cosmic neutrinos) acquires an anisotropic stress-energy tensor due to differential expansion. The system evolves to an equilibrium state where the various sources of anisotropy cancel each other, resulting in dynamical isotropization of the background.

  Mercredi 21 septembre à 11h, salle 110

Shuntaro Mizuno (LPT Orsay and APC Paris) : Primordial fluctuations and non-Gaussianities from DBI Galileon inflation

  Mercredi 28 septembre à 11h, salle 110

Shuntaro Mizuno (LPT Orsay and APC Paris) : Primordial fluctuations and non-Gaussianities from DBI Galileon inflation (Continued).