Centre de Physique Théorique


Octobre 2017

Mercredi 4 octobre 2017 14:00-15:00, Amphi 5 du CPT

On the simultaneous identification of scattering parameters for classical waves

Andrea Mantile (LMR,Université de Reims)


We prove uniqueness in inverse acoustic scattering in the case
the density of the medium has an unbounded gradient across Σ⊆∂Ω, where Ω
is a 3D-Lipschitz domain. The corresponding direct problem is related to
the stationary waves scattering for 3D Schrödinger operators with δ-type
singular perturbations supported on ∂Ω and of strength α∈L^p(∂Ω), p>2.
This is a multiple scattering problem from obstacles and potentials
whose solutions depend on the obstacles locations and shapes, the
related transmission impedances and the background potentials. The
inverse problem then consists in determining these scattering parameters
from a complete set of far-field data at a fixed energy. In this
framework, we show that the acoustic far-field pattern can be defined in
terms of the scattering amplitude for the corresponding Schrödinger
operator. A uniqueness result is then obtained by using new estimates
for complex geometrical optics solutions (recently provided by B.
Haberman for the Calderon’s problem).

Jeudi 5 octobre 2017 16:00-17:00, Amphi 6, Bât A, Campus de Luminy

The role of black holes in our understanding of the physical world

Gerard ’t Hooft

Vendredi 6 octobre 2017 10:30-11:30, Amphi 6, Bât A, Campus de Luminy

Black holes, quantum mechanics, and the topology of space and time.

Gerard ’t Hooft

Standard laws of physics seem to lead to contradictions when applied to the quantum states a black hole can be in. This implies that black holes must be important clues to help us improving the formulation of these laws. On the one hand, space and time seem to become discrete, on the other they become topologically non-trivial. To gain control over the way black holes process information, it is advised to use spherical harmonics, in terms of which the problems reduce to simple partial differential equations that we can solve, just as in the hydrogen atom.

Lundi 23 octobre 2017 11:00-13:00, Amphi 5 du CPT

On the road to covariant statistical mechanics

Thibaut Josset

Soutenance de thèse de Thibaut Josset.

Lundi 23 octobre 2017 14:00-16:00, Amphi 5 du CPT

Geometry transition in covariant loop quantum gravity

Marios Christodoulou

Soutenance de thèse de Marios Christodoulou

Lundi 23 octobre 2017 15:00-16:00, Amphi 4 du CPT (4eme étage)

Towards new constraints in alternative theories of gravity : cosmography, extended g-evolution, neutron stars and gravitational waves

Alvaro de la Cruz-Dombriz (Gravity & Cosmology Group, University of Cape Town)

Model-independent methods in cosmology, N-body simulations and theoretical predictions in strong-gravity regimes have become essential tool in order to deal with an increasing number of theoretical alternatives for explaining several open issues in the cosmic history. In principle, this provides a way of testing the ΛCDM cosmological paradigm under different assumptions and to rule out whole classes of competing theories. In this talk I will present some of the latest progress and shortcomings in the cosmographic approaches for several modified gravity theories using supernovae catalogues, baryonic acoustic oscillation data and H(z) differential age compilations. I will also sketch a recent line of full N-body simulation technique modifying g-evolution to accommodate effects from higher-order theories and finally, I will provide a flavour on the phenomenology of neutron stars and gravitational waves constraints once the Einsteinian paradigm is abandoned.

Mardi 24 octobre 2017 14:00-16:00, Amphi 5 du CPT

Elements of Phenomenology of Dark Energy

Louis Perenon

Soutenance de thèse - Louis Perenon

A little over a hundred years ago a revolution in modern physics occurred. The description of spacetime provided by General Relativity changed the way physics describes our universe. The Newtonian gravity force is promoted to a gravitational field paving spacetime and links intimately the energy content of the universe with its geometry. Mass curves spacetime which, in turn, dictates how bodies move in relation to one another. The foundation provided by General Relativity has allowed cosmologists to establish a well defined cosmological paradigm. Its large scale time evolution is understood to be the direct consequence of the type of energy it contains while the lumpiness of the large-scale structures we observe today, such as galaxies and clusters of galaxies, is the result of gravitational instabilities developing on the evolving frame. The increasing precision of several cosmological probes enabled the possibility of encoding our theoretical and observational knowledge within a standard model of cosmology ; the LambdaCDM paradigm. This model was able to account for the recent discovery that the expansion of the universe is accelerating ; a milestone of modern cosmology. In the LambdaCDM picture, the universe is constituted today for the major part by Cold Dark Matter along with the Cosmological Constant Lambda that drives cosmic acceleration. However, this standard model is not fully complete and further breakthroughs in modern physics can be expected within this century. These will arise from an accurate description of high energy scales of gravity through a quantum theory of gravity ---extremely small lengths--- or through a new description of gravity at low energy scales ---on cosmological distances. These are essentially motivated on two grounds : using the Cosmological Constant introduces theoretical issues in a quantum field theory description and tentative observational evidences suggests our large scale description of the universe should be refined. Therefore, we devote the first chapter of this thesis to an overview of today’s cosmological paradigm ; starting from its founding principles, up to its shortcomings.

Finding alternatives to the standard model is of crucial importance for two reasons. On the one hand, a general theory providing a universal description of all the stages the universe has gone through, and of all physical interactions it contains, still evades our grasp. On the other hand, the second reason is more closely related to the subject of the thesis. A given cosmological model can indeed be tested against many observational probes so as to gauge its viability. However, the soundness and the precision of a viable model can only be assessed once it is compared to another model. This is why, the LambdaCDM model stands as the most faithful model ; describing most of the universe’s evolution according to the observations gathered up to now. This is also why one has to explore alternatives to the standard model. An incredible amount of alternative theories have been put forward and most of them are based on the addition of degrees of freedom to the standard picture given by General Relativity. These additional freedoms can manifest in the form of a new energy component of the universe ---the dark energy picture--- or through fundamental modifications of the gravitational interaction ---the modified gravity landscape. Such a profusion of research material, as much on the observational side as on the theoretical, has substantially increased our understanding of the universe. However, this is also at the expense of our efficiency in doing so, as it is rather cumbersome to study theories one by one and confront each theoretical proposal to observations. Aiming to study or create common formulations, enabling the description of large classes of alternative theories within the same framework, is an efficient path to overcoming this obstacle. This is the reason why, in chapter 2, after reviewing some alternative models to LambdaCDM, we will focus on presenting a promising unifying framework : the effective field theory of dark energy. This approach to modified gravity has established a common formalism virtually describing all alternative theories which add a single extra scalar degree of freedom to Einstein’s equation. A large class of these models fall under Horndeski theories.

The increasing amount of theoretical knowledge, and the growing amount of data, is also a cry for phenomenological studies. Future surveys such as EUCLID, SKA, WFIRST and DESI, to name but a few, will provide ever more precise constraints on deviations from the standard model. In light of this, understanding the phenomenology behind alternative theories, extracting and testing observables which would characterise measurable deviations from standard gravity are thus a crucial step cosmological studies must go through today. Therefore, we consider the effective field theory of dark energy and attempt to provide answers to the following three guiding questions : What is the cosmological portrait of gravity that emerges in Horndeski theories ? Are there any universal behaviours and, if yes, where do they stand with respect to the standard model ? Can we identify observables that will, in the future, enable us to discriminate between theories ? To do so, in chapter 3, we show how one can parametrise the effective field theory of dark energy framework in order to extract predictions on a set of large-scale structure observables and how the effective field theory of dark energy framework can embed early modifications of gravity in Horndeski theories. Following this, we adopt a Monte Carlo procedure to explore Horndeski models, paying a significant attention to the viability of the models we obtain. This procedure enables us to identify some definite and less definite observational features Horndeski theories yield. This corresponds, for the major part, to what was explained in [1]. In the second half of chapter 3, we use the Monte Carlo approach to further synthesise the previous conclusions, and more, into an observable diagnostic. The goal of this diagnostic is to assess how Horndeski theories could be strongly disfavoured in an observable space given where future measurements will point. This corresponds to the results developed in [2].

The effective field theory of dark energy framework having allowed the exploration of the phenomenology of a large number of dark energy and modified gravity models we provide a review of the results it has produced in chapter 4. The goal of this final is twofold. It allows us to give a presentation of the landscape this framework can be applied to. In particular, we show the novel predictions it has brought up, the constraints that were derived with observations, but also how information on modified gravity can be extracted in an astrophysical context. We also discuss further theoretical an numerical developments without forgetting the caveats the effective field theory of dark energy presents. This chapter also has the purpose of discussing the results presented in chapter 3 with respect to other studies, and suggest paths needing to be explored in the future.

[1] Louis Perenon, Federico Piazza, Christian Marinoni, and Lam Hui. “Phenomenology of dark energy : general features of large-scale perturbations.” In : JCAP 1511.11 (2015), p. 029. DOI : 10.1088/1475-7516/2015/11/029 . arXiv : 1506.03047 [astro-ph.CO] .+
[2] Louis Perenon, Christian Marinoni, and Federico Piazza. “Diagnostic of Horndeski Theories.” In : JCAP 1701.01 (2017),p.035.DOI :10.1088/1475 -7516/2017/01/035 . arXiv:1609.09197 [astro-ph.CO].

Vendredi 27 octobre 2017 14:00-15:00, Amphi 5 du CPT

Finite quantum gravity and spacetime singularities

Leonardo Modesto

I present a class of weakly nonlocal gravitational theories finite and unitarity at quantum level. Moreover, such class of theories is actually a range of anomaly-free conformally invariant quantum theories in the spontaneously broken phase of the Weyl symmetry. At classical level I show how conformal invariance is likely able to tame the spacetime singularities that plague not only Einstein gravity, but also local and weakly non-local higher derivative theories. This latter statement is rigorously proved by a simple singularity theorem that applies to a large class of weakly non-local theories. Following the seminal paper by Narlikar and Kembhavi, I provide examples of exact solutions for singularity-free black holes.

Du 30 octobre 2017 au 3 novembre 2017, CIRM, Luminy

Collisionless Boltzmann (Vlasov) Equation and Modeling of Self-Gravitating Systems and Plasmas

Conférence internationale co-organisée par R. Triay (CPT)

The modelling of the matter interactions at long range is a fertile problem for Mathematics, a major issue in Physics and a challenge for Analysis and Numerical Simulations. The aim of this meeting is to conjugate all these aspects, with a special attention to the simulations, and try to infuse a stimulating working atmosphere, such as in a workshop, with few key lectures (part of these will be chosen when finalizing the program).


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Rien pour ce mois

janvier 2019 | mars 2019