Chercheur CEA, directeur de l'Institut d'Astrophysique de Paris et
Professeur Chargé de Cours à l'école polytechnique
Institut de Physique Théorique,
CEA, IPhT, F-91191 Gif-sur-Yvette, France et CNRS, URA 2306, F-91191 Gif-sur-Yvette, France
Institut d'Astrophysique de Paris
CNRS et UPMC, UMR 7095, 98 bis Bd Arago, 75014 Paris, France
33-1 4432 8160
Research activities: theoretical coslology including the exploration of inflationary models, the origin and evolution of the large scale structure of the universe, gravitational lenses and cosmic microwave background physics.
Dynamics of gravitational instabilities: my early works during my PhD thesis followed the invesigations of R. Schaeffer and R. Balian in which so-called hierarchical models for the non-linear stages of the gravitational dynamics of a pressureless fluid were proposed. The quasilinear regime however appeared to be much easier to investigate thanks to ressumation techniques. It was then possible to show that,
- the general properties of the local density probability distribution function and the behavior of its cumulants can be derived exactly at the so-called tree order,Astroph. J. 392, 1 (1992), Astroph. J. 427, 51 (1994); Astron. & Astroph. 291, 697 (1994) and in collaboration with L. Kofman (CITA), Astroph. J. 443, 479 (1995);
- the extension of those results to joint PDFs, Astron. & Astroph. 312, 11 (1996), and to projection effects Astron. & Astroph. 301, 309 (1995), to the cosmic velocity fields, etc.;
Those efforts culminated with the publication of a long and exhaustive review paper, Physics Reports, 367, 1-128 (2002) written in collaboration with S. Colombi, E. Gaztanaga and R. Scoccimarro.
These rather formal approaches recently got new interests with the development of new resummation techniques that allow precision calculations. The basis of these methods can be found in papers by R. Scoccimarro and M. Crocce, Phys. Rev. D 73, 063519 (2006) , Phys. Rev. D 73, 063520 (2006). I am now actively participating in the development of those techniques, with the Γ-resummation method, the computation of propagators with the eikonal approximation, in collaboration with P. Valageas (of IPhT), R. Scoccimarro and M. Crocce, Phys. Rev. D 78, 103521 (2008), Phys. Rev. D 81, 043516 (2010), Phys. Rev. D 82, 083507 (2010).
Cosmic Shear observations: the previous works on the large-scale structure of the universe led to rich studies on the phenomenology of weak gravitational lensing induced by the large-scale structure of the universe, e.g. the cosmic shear,
- In collaboration with L. van Waerbeke and Y. Mellier (IAP) we embarked in extensive studies of the cosmic shear, Astron. & Astroph. 322, 1 (1997), Astron. & Astroph. 342, 15 (1999).
- These studies have been extended to the search of non-Gaussian features in shear surveys, including theoretical and numerical analysis, Astron. & Astroph. 397, 405 (2003), as well as an explicit detection in the data, Astron. & Astroph. Letter 389 (2002) L28.
- The development of this theme, in particular due to the many projects of wide field surveys, eventually led us to examine the importance of the non-linear couplings for the statistical properties of large-scale cosmic shear, Phys. Rev. D 81, 083002 (2010). The observational consequences of those couplings are under investigations.
Mode coupling effects in the statistical properties of the cosmic microwave background: another field where perturbation theory appraoches can be used is naturally the physics of the CMB. In this case mode coupling effects are a priori small but ubiquitous, either through the physics of the recombinaison, metric couplings among which the effects of gravitational lensing.
- With my student K. Benabed we initially investigated the lens effets on the Cosmic Microwave Background statistical properties, Astroph. J. 540, 14 (2000), Phys. Rev. D, 61, 123510, Phys. Rev. D, 63, 043501.It is now clear that those effects are detectable.
- In collaboration with Jean-Philippe Uzan and his student Cyril Pitrou, I finally completed a long lasting project with the computation of the bispectrum of the temperature anisotropies of the CMB. We first identified the leading mecanism at small scales, e.g. mode coupling effects in the gravitational potential field induced by the dark matter dynamics, Phys. Rev. D 78, 063526 (2008). Then, with the help of a dedicated 2nd order Boltzmann code, we finally predicted the expected amplitude and shape of the temperature bispectrum, JCAP07 (2010) 003.
The search for new physics: the concordant model leaves a number of issues unanswered. Through various collaboration I explored over the years mechanisms at play during the inflationary phase; observational signature of possible relics, such as cosmic strings, and the effect the dark energy component on the large-scale structure growth.
- The nature of dark energy is a crucial issue regarding both high energy physics and the observational cosmology. With my student Karim Benabed we explored the effect of so called quintessence models on the growth of large-scale structure, Phys. Rev. D, 64, 083501, including non-linear coupling effects. In a brane cosmology context, I have also explored the implication of a modification of the gravity law at cosmological scales, Phys. Rev. D, 64, 083004, and finally Ph. Brax, we recently completed an exhaustive study of the impact of a dilatonic field on the nonlinear growth of structure, ArXiv/1102.1907.
- One possible key to unveil the nature of inflationary models could come from their non-Gaussian signature. In collaboration with J.-P. Uzan and my student Tristan Brunier we identified a class of models of inflation that produce large primordial metric fluctuations with large non-Gaussian mode couplings, Phys. Rev. D 66, 103506 (2002) and Phys. Rev. D 67, 121301 (2002). Among other things, those investigations led us to examine the expected properties of a test self-interacting scalar field in a de Sitter, Phys. Rev. D 69, 063520 (2004), or close to a de Sitter background, JCAP02 (2011) 017. Those constructions also led to the formulation of a model of inflation where primordial non-Gaussianities can be naturally generated, Phys. Rev. D 76, 043526 (2007), and arise through non-gravity couplings, as described in, Class. Quantum Grav. 27 124004.