Jean-Michel Alimi - Academia.edu (original) (raw)

Papers by Jean-Michel Alimi

Classical and Quantum Gravity, 2002

HAL (Le Centre pour la Communication Scientifique Directe), Jul 2, 2017

In the context of the ESA M5 (medium mission) call we proposed a new satellite mission, Theia, ba... more In the context of the ESA M5 (medium mission) call we proposed a new satellite mission, Theia, based on relative astrometry and extreme precision to study the motion of very faint objects in the Universe. Theia is primarily designed to study the local dark matter properties, the existence of Earth-like exoplanets in our nearest star systems and the physics of compact objects. Furthermore, about 15 % of the mission time was dedicated to an open observatory for the wider community to propose complementary science cases. With its unique metrology system and "point and stare" strategy, Theia's precision would have reached the sub micro-arcsecond level. This is about 1000 times better than ESA/Gaia's accuracy for the brightest objects and represents a factor 10-30 improvement for the faintest stars (depending on the exact observational program). In the version submitted to ESA, we proposed an optical (350-1000nm) on-axis TMA telescope. Due to ESA Technology readiness level, the camera's focal plane would have been made of CCD detectors but we anticipated an upgrade with CMOS detectors. Photometric measurements would have been performed during slew time and stabilisation phases needed for reaching the required astrometric precision.

Astronomy from Wide-Field Imaging, 1994

We present preliminary results of a galaxy redshift survey we are accomplishing as an ESO Key Pro... more We present preliminary results of a galaxy redshift survey we are accomplishing as an ESO Key Project over about 40 square degrees in a region near the South Galactic Pole, to a limiting magnitude bj = 19.4. Up to now ~ 50% of the survey has been completed, providing about 2000 galaxy redshifts.

The Eleventh Marcel Grossmann Meeting, 2008

International Journal of Modern Physics D, 2007

We investigate the possibility that dark energy does not couple to gravitation in the same way as... more We investigate the possibility that dark energy does not couple to gravitation in the same way as ordinary matter, yielding a violation of the weak and strong equivalence principles on cosmological scales. We build a transient mechanism in which gravitation is pushed away from general relativity (GR) by a Born–Infeld (BI) gauge interaction acting as an "abnormally weighting (dark) energy" (AWE). This mechanism accounts for the Hubble diagram of far-away supernovae by cosmic acceleration and time variation of the gravitational constant while accounting naturally for the present tests on GR.

We build a mechanism of gravitational symmetry breaking (GSB) of a global U(1) symmetry based on ... more We build a mechanism of gravitational symmetry breaking (GSB) of a global U(1) symmetry based on the relaxation of the equivalence principle due to the mass variation of pseudo Nambu-Goldstone dark matter (DM) particles. This GSB process is described by the modified cosmological convergence mechanism of the Abnormally Weighting Energy (AWE) Hypothesis previously introduced by the authors. Several remarkable constraints from the Hubble diagram of faraway supernovae are derived, notably on the explicit and gravitational symmetry breaking energy scales of the model. We then briefly present some consequences on neutrino masses when this mechanism is applied to the particular case of the breaking of lepton number symmetry.

2012 International Conference for High Performance Computing, Networking, Storage and Analysis, 2012

We have performed the first-ever numerical Nbody simulation of the full observable universe (DEUS... more We have performed the first-ever numerical Nbody simulation of the full observable universe (DEUS "Dark Energy Universe Simulation" FUR "Full Universe Run"). This has evolved 550 billion particles on an Adaptive Mesh Refinement grid with more than two trillion computing points along the entire evolutionary history of the universe and across 6 order of magnitudes length scales, from the size of the Milky Way to that of the whole observable Universe. To date, this is the largest and most advanced cosmological simulation ever run. It provides unique information on the formation and evolution of the largest structure in the universe and an exceptional support to future observational programs dedicated to mapping the distribution of matter and galaxies in the universe. The simulation has run on 4752 (of 5040) thin nodes of BULL supercomputer CURIE, using more than 300 TB of memory for 10 million hours of computing time. About 50 PBytes of data were generated throughout the run. Using an advanced and innovative reduction workflow the amount of useful stored data has been reduced to 500 TBytes.

The Astrophysical Journal, 1998

The 2nd Electronic Conference on Universe

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

The Fourteenth Marcel Grossmann Meeting, 2017

International audienceWe discuss numerical investigations of the effects of cosmological inhomoge... more International audienceWe discuss numerical investigations of the effects of cosmological inhomogeneities on light propagation. A new algorithm directly integrating the geodesics equations has been developed and has been run on very large scale cosmological simulations up to a redshift of z = 30. Comparisons have been made for several dark energy models and for several observers, revealing percent-level discrepancies between the angular diameter distance interpreted in a friedmannian background, and the angular diameter distance computed from geodesics integration. Here, we focus on the methodology that has been deployed to obtain quantitative results in cosmological simulations

The International Journal of High Performance Computing Applications, 2015

We report the realization of the first cosmological simulations on the scale of the whole observa... more We report the realization of the first cosmological simulations on the scale of the whole observable universe. These simulations have been carried out on 4752 nodes of the Curie supercomputer as a part of the Dark Energy Universe Simulation: Full Universe Runs (DEUS-FUR) project which aims at establishing new probes to put constraints on the nature of dark energy by comparing the growth of large-scale structures, the characteristics of extreme statistical events and the matter distribution in redshift space. The numerical challenge of the first DEUS-FUR simulation associated with the concordance ΛCDM (Λ Cold Dark Matter) model was already presented during the 2012 supercomputing conference (Alimi et al., 2012, in The international conference for high performance computing, networking, storage and analysis). Here we first focus on the numerical aspects of the two new simulations. In practice, each one of these simulations has evolved 550 billion dark matter particles in an adaptive m...

Journal of Cosmology and Astroparticle Physics, 2008

We generalize tensor-scalar theories of gravitation by the introduction of an abnormally weightin... more We generalize tensor-scalar theories of gravitation by the introduction of an abnormally weighting type of energy. This theory of tensor-scalar anomalous gravity is based on a relaxation of the weak equivalence principle that is now restricted to ordinary visible matter only. As a consequence, the convergence mechanism toward general relativity is modified and produces naturally cosmic acceleration as an inescapable gravitational feedback induced by the mass-variation of some invisible sector. The cosmological implications of this new theoretical framework are studied. From the Hubble diagram cosmological test alone, this theory provides an estimation of the amount of baryons and dark matter in the Universe that is consistent with the independent cosmological tests of Cosmic Microwave Background (CMB) and Big Bang Nucleosynthesis (BBN). Cosmic coincidence is naturally achieved from a equally natural assumption on the amplitude of the scalar coupling strength. Finally, from the adequacy to supernovae data, we derive a new intriguing relation between the spacetime dependences of the gravitational coupling and the dark matter mass, providing an example of crucial constraint on microphysics from cosmology. This glimpses at an enticing new symmetry between the visible and invisible sectors, namely that the scalar charges of visible and invisible matter are exactly opposite.

The Astrophysical Journal, 1997

Regionally averaged relativistic cosmologies have recently been considered as a possible explanat... more Regionally averaged relativistic cosmologies have recently been considered as a possible explanation for the apparent late time acceleration of the Universe. This contribution reports on a mean field description of the backreaction in terms of a minimally coupled regionally homogeneous scalar field evolving in a potential, then giving a physical origin to the various phenomenological scalar fields generically called quintessence fields. As an example, the correspondence is then applied to scaling solutions.

Quintessence has been proposed to account for dark energy in the Universe. This component causes ... more Quintessence has been proposed to account for dark energy in the Universe. This component causes a typical modification of the background cosmic expansion, which in addition to its clustering properties, can leave a potentially distinctive signature on large scale structures. Many previous studies have investigated this topic, particularly in relation to the non-linear regime of structure formation. However, no careful pre-selection of viable quintessence mod-els with high precision cosmological data was performed. Here we show that this has led to a misinterpretation (and underestimation) of the imprint of quintessence on the distribution of large scale structures. To this purpose we perform a likelihood analysis of the combined Su-pernova Ia UNION dataset and WMAP5-years data to identify realistic quintessence models. These are specified by different model parameter values, but still statistically indistinguish-able from the vanilla ΛCDM. Differences are especially manifest in the...

Analyse de la fonction de correlation spatiale des galaxies, obtenue a partir de catalogues de ga... more Analyse de la fonction de correlation spatiale des galaxies, obtenue a partir de catalogues de galaxies, selon differentes procedures. La distribution des galaxies isolees dans le catalogue cfa est etudiee. Etude du collapse gravitationnel non lineaire de particules sans collision, du point de vue analytique et numerique. On montre l'importance du traitement tridimensionnel de ces processus gravitationnels et quelles peuvent en etre les consequences sur la formation des galaxies

Directeur de Recherche au CNRS, specialiste de Cosmologie, Jean-Michel Alimi a ete president des ... more Directeur de Recherche au CNRS, specialiste de Cosmologie, Jean-Michel Alimi a ete president des manifestations « Le siecle d'Albert Einstein » et « l'Univers Invisible » durant l'Annee Mondiale de la Physique en 2005 et de l'Astronomie en 2009. Cet ouvrage est issu de l’exposition eponyme inauguree dans le cadre des manifestations [i]Le Siecle d’Albert Einstein[/i], le 7 juillet 2005 au Palais de L’Unesco a Paris. – L’A. entend, au-dela de la caricature du physicien theoricien, brosser le portrait de l’homme «rive a quelque point de l’espace-temps. Il est toujours de quelque part, dans un certain lieu, a une certaine epoque, et son environnement ne cesse d’influer sur lui». L’experience de la pensee modifie les manieres d’etre au monde, tout comme les manieres d’etre au monde formatent la pensee. Ce travail participe ainsi de la culture scientifique la plus aboutie, culture qui n’est pas seulement celle des principes, des equations ou des resultats, mais aussi celle...