Laboratoire Univers et Théories

LUTH

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Université Paris Diderot, CNRS

Présentation

Le Laboratoire Univers et Théories (LUTH) est une unité mixte de recherche du CNRS, de l’Observatoire de Paris et de l’Université Paris Diderot. Le laboratoire regroupe plus de 80 personnes. Son activité de recherche concerne la théorie et la modélisation avancées des systèmes astrophysiques. Notre laboratoire met l’accent sur deux aspects de l’activité de recherche astrophysique : le calcul numérique intensif et la pluridisciplinarité.

La simulation numérique joue un rôle fondamental en astrophysique et a été à l’origine de progrès spectaculaires. Complément indispensable des approches purement observationnelles et théoriques, elle constitue une troisième voie de recherche dont la spécificité est l’utilisation heuristique de l’ordinateur.

La complexité des systèmes astrophysiques étudiés nécessite, une expertise sur des processus physiques multiples et sur de nombreuses méthodes en mathématiques, statistiques et numérique. Le LUTH est un lieu d’échange afin de permettre l’élaboration et le suivi de projets de modélisation ambitieux.

Les thématiques de recherches vont de la cosmologie, aux planètes extra solaires, en passant par l’étude des trous noirs et objets compacts, des phénomènes de hautes énergies, des phénomènes hydrodynamiques et de plasmas présents au sein des objets astrophysiques.

Le LUTH participe aux grands projets instrumentaux et observationnels de notre discipline tels que : ALMA, COROT, CTA, Euclid, Fermi, DEUS, GAIA, Herschel, Hess, Horizon, LMJ, Planck, TPF, XMIVI, etc.

Le LUTH participe à plusieurs expériences d’intérêt astrophysique comme l’étude des chocs radiatifs en utilisant les grands lasers comme le LULI.

L’enseignement et la formation par la recherche sont parmi les principaux objectifs du LUTH en participant à la formation des étudiants en astrophysique, en physique et en mathématiques tant au niveau du master d’astrophysique d’lle de France, que dans des cursus universitaires ou de grandes écoles.

Thèmes de recherche

Astrophysique Multi-Echelle (AME)

L’équipe AME a pour objectif principal la compréhension des échanges et des mélanges de matière dans l’Univers. Les recherches qui y sont menées vont de la physique fondamentale à l’expérimentation de laboratoire.

Phénomènes aux Hautes Energies (Equipe PHE)

L’équipe PHE se consacre à l’étude des sources astrophysiques aux hautes énergies et de la physique des milieux moléculaires hors équilibre thermodynamique.

Relativité et Objets Compacts (Equipe ROC)

Les thèmes de recherche de l'équipe ROC concernent la théorie et les tests de la gravitation, les ondes gravitationnelles, la formation et les propriétés des astres compacts (étoiles à neutrons, trous noirs). Le développement d'outils numériques ouverts et originaux y tient une place importante.

Cosmologie : structures et origines (Equipe COS)

L'activité de l'équipe COS couvre plusieurs sujets de recherche en cosmologie parmi lesquels l'étude de l'Energie Noire et ses empreintes sur la formation et évolution des grandes structures cosmiques, travaux qui sont réalisé à l'aide de simulations numériques a haute-performance.

[hal-02981237] Sensitivity of the Cherenkov Telescope Array for probing cosmology and fundamental physics with gamma-ray propagation

Date: 29 oct 2020 - 03:30

Desc: The Cherenkov Telescope Array (CTA), the new-generation ground-based observatory for $\gamma$-ray astronomy, provides unique capabilities to address significant open questions in astrophysics, cosmology, and fundamental physics. We study some of the salient areas of $\gamma$-ray cosmology that can be explored as part of the Key Science Projects of CTA, through simulated observations of active galactic nuclei (AGN) and of their relativistic jets. Observations of AGN with CTA will enable a measurement of $\gamma$-ray absorption on the extragalactic background light with a statistical uncertainty below 15% up to a redshift $z=2$ and to constrain or detect $\gamma$-ray halos up to intergalactic-magnetic-field strengths of at least 0.3pG. Extragalactic observations with CTA also show promising potential to probe physics beyond the Standard Model. The best limits on Lorentz invariance violation from $\gamma$-ray astronomy will be improved by a factor of at least two to three. CTA will also probe the parameter space in which axion-like particles could constitute a significant fraction, if not all, of dark matter. We conclude on the synergies between CTA and other upcoming facilities that will foster the growth of $\gamma$-ray cosmology.

[hal-02188268] Dipolar tidal effects in scalar-tensor theories

Date: 28 oct 2020 - 22:15

Desc: The inclusion of finite-size effects in the gravitational waveform templates allows one not only to constrain the internal structure of compact objects, but to test deviations from general relativity. Here, we address the problem of tidal effects in massless scalar-tensor theories. We introduce the scalar-type tidal Love numbers that relate the time-varying scalar dipole moment to the induced scalar tidal field. We compute the leading-order scalar tidal contribution in the conservative dynamics and for the first time in the wave generation for quasicircular orbits. Importantly, we show that, in a system dominated by dipolar emission, such tidal effects may be detectable by third generation detectors such as the Einstein Telescope.

[hal-01827985] Horizon Surface Gravity in Corotating Black Hole Binaries

Date: 28 oct 2020 - 22:14

Desc: For binary systems of corotating black holes, the zeroth law of black hole mechanics states that the surface gravity is constant over each component of the horizon. Using the approximation of a conformally flat spatial metric, we compute sequences of quasi-equilibrium initial data for corotating black hole binaries with irreducible mass ratios in the range . For orbits outside the innermost stable one, the surface gravity is found to be constant on each component of the apparent horizon at the sub-percent level. We compare those numerical results to the analytical predictions from post-Newtonian theory at the fourth (4PN) order and from black hole perturbation theory to linear order in the mass ratio. We find a remarkably good agreement for all mass ratios considered, even in the strong-field regime. In particular, our findings confirm that the domain of validity of black hole perturbative calculations appears to extend well beyond the extreme mass-ratio limit.

[hal-01554830] Neutral gas heating by X-rays in primitive galaxies: Infrared observations of the blue compact dwarf I Zw 18 with Herschel

Date: 28 oct 2020 - 22:08

Desc: Context. The neutral interstellar medium of galaxies acts as a reservoir to fuel star formation. The dominant heating and cooling mechanisms in this phase are uncertain in extremely metal-poor star-forming galaxies. The low dust-to-gas mass ratio and low polycyclic aromatic hydrocarbon abundance in such objects suggest that the traditional photoelectric effect heating may not be effective. Aims. Our objective is to identify the dominant thermal mechanisms in one such galaxy, I Zw 18 (1/30Z⊙), assess the diagnostic value of fine-structure cooling lines, and estimate the molecular gas content. Even though molecular gas is an important catalyst and tracer of star formation, constraints on the molecular gas mass remain elusive in the most metal-poor galaxies. Methods. Building on a previous photoionization model describing the giant H ii region of I Zw 18-NW within a multi-sector topology, we provide additional constraints using, in particular, the [C ii] 157 μm and [O i] 63 μm lines and the dust mass recently measured with the Herschel Space Telescope. Results. The heating of the H i region appears to be mainly due to photoionization by radiation from a bright X-ray binary source, while the photoelectric effect is negligible. Significant cosmic ray heating is not excluded. Inasmuch as X-ray heating dominates in the H i gas, the infrared fine-structure lines provide an average X-ray luminosity of order 4 × 1040 erg s-1 over the last few 104 yr in the galaxy. The upper limits to the [Ne v] lines provide strong constraints on the soft X-ray flux arising from the binary. A negligible mass of H2 is predicted. Nonetheless, up to ~107 M⊙ of H2 may be hidden in a few sufficiently dense clouds of order ≲5 pc (≲0.05′′) in size. Regardless of the presence of significant amounts of H2 gas, [C ii] and [O i] do not trace the so-called “CO-dark gas”, but they trace the almost purely atomic medium. Although the [C ii]+[O i] to total infrared ratio in I Zw 18 is similar to values in more metal-rich sources (~1%), it cannot be safely used as a photoelectric heating efficiency proxy. This ratio seems to be kept stable owing to a correlation between the X-ray luminosity and the star formation rate. Conclusions. X-ray heating could be an important process in extremely metal-poor sources. The lack of photoelectric heating due to the low dust-to-gas ratio tends to be compensated for by the larger occurrence and power of X-ray binaries in low-metallicity galaxies. We speculate that X-ray heating may quench star formation. Key words: X-rays: binaries / infrared: ISM / photon-dominated region (PDR) / galaxies: ISM / galaxies: individual: I Zw 18 / galaxies: star formation⋆ Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

[hal-02065881] An Illustrated History of Black Hole Imaging : Personal Recollections (1972-2002)

Date: 28 oct 2020 - 21:53

Desc: The Event Horizon Telescope Consortium is on the verge to provide the first telescopic image of massive black holes SgrA* and M87* surrounded by accretion disks, at a resolution scale comparable to the size of their event horizons. Well before this remarkable achievement made possible by VLBI radio astronomy, many researchers used the computer to reconstruct how a black hole surrounded by luminous material would look from close-up views. The images must experience extraordinary optical deformations due to the deflection of light rays produced by the strong curvature of the space-time in the vicinity. General relativity allows the calculation of such effects, both on a surrounding accretion disk and on the background star field. This article is an exhaustive and illustrated review of the numerical work on black hole imaging done during the first thirty years of its history.