Laboratoire Univers et Théories
Présentation
Le Laboratoire Univers et Théories (LUTH) est une unité mixte de recherche (UMR 8102) du CNRS, de l’Observatoire de Paris et de l’Université de Paris. Le laboratoire regroupe une cinquantaine de personnes dont une petite moitié de chercheurs statutaires (CNRS, Universités, CNAP). L’activité scientifique du laboratoire se concentre essentiellement sur l’étude théorique des systèmes astrophysiques et sur leur modélisation. Une part des activités concerne également le traitement des données des observations à hautes énergies.
Le LUTH est actuellement organisé autour de trois thématiques. Le groupe Cosmologie étudie la formation des grandes structures de l’Univers et en particulier l’influence de la matière noire sur ce processus. L’équipe Phénomènes aux hautes énergies se consacre à la modélisation et à l’observation de objets comme les pulsars ou les noyaux actifs de galaxies. Une partie des activités concerne également la préparation des futurs instruments et la gestion des bases de données liées aux observations. La thématique Relativité et Objets Compacts se propose de travailler sur les différentes situations astrophysiques où la gravité est intense et décrite par la théorie d’Einstein. On pense principalement aux supernovae, aux étoiles à neutrons et aux trous noirs.
Par la diversité des sujets abordés, le LUTH est un laboratoire faisant la part belle à la pluridisciplinarité. Il regroupe des chercheurs aux profils variés venant de l’astronomie, de la physique théorique ou encore de la physique nucléaire. Cette richesse est illustrée par le fait que le laboratoire soit rattaché à trois instituts du CNRS (INSU, INP et IN2P3).
Le laboratoire a une forte composante numérique. Il s’agit non seulement de réaliser des simulations ou des calculs par l’outil informatique mais également de développer des outils performants, le plus souvent mis à la disposition de la communauté scientifique. Cette tâche bénéficie du soutien de l’équipe informatique du laboratoire qui comprend des ingénieurs spécialisés dans ce domaine.
Le LUTH, tout en étant fidèle à son ADN de laboratoire dédié à la modélisation et à la théorie, n’est pas déconnecté des grandes avancées observationnelles de l’astrophysique. Ses membres sont actifs dans de nombreux projets sol ou spatial, aussi bien dans les phases de préparation que d’exploitation des données. Ces activités peuvent prendre la forme de participation officielles aux projets (CTA, Euclid, HESS, LISA) ou d’échanges scientifiques moins formels (Gravity, Planck, PTA, SKA, Virgo...)
L’enseignement et la formation par la recherche font partie intégrante des missions de LUTH. Les chercheurs sont impliqués dans l’enseignement de leur spécialités à presque tous les niveaux des cursus universitaires ou des grandes écoles. Une dizaine de doctorants effectuent leur thèse au sein du laboratoire.
Les chercheurs du LUTH sont conscients de l’importance de la diffusion de la connaissance scientifique en direction du grand public. Cela peut prendre la forme de rencontres avec des scolaires, de participation à des conférences, en passant par des interventions dans les médias pour commenter les nouvelles scientifiques du moment.
Thèmes de recherche
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-01646052] Multi-messenger Observations of a Binary Neutron Star Merger
Date: 14 Apr 2020 - 11:52
Desc: On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of $\sim 1.7\,{\rm{s}}$ with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg(2) at a luminosity distance of ${40}_{-8}^{+8}$ Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 $\,{M}_{\odot }$. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at $\sim 40\,{\rm{Mpc}}$) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position $\sim 9$ and $\sim 16$ days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.
[hal-03791759] Accreting neutron stars from the nuclear energy-density functional theory. II. Equation of state and global properties
Date: 31 Mar 2023 - 17:43
Desc: The accretion of matter onto the surface of a neutron star in a low-mass X-ray binary triggers X-ray bursts, whose ashes are buried and further processed thus altering the composition and the properties of the stellar crust. In this second paper of a series, the impact of accretion on the equation of state and on the global properties of neutron stars is studied in the framework of the nuclear energy-density functional theory. Considering ashes made of $^{56}$Fe, we calculated the equations of state using the same Brussels-Montreal nuclear energy-density functionals BSk19, BSk20, and BSk21, as those already employed for determining the crustal heating in our previous study for the same ashes. All regions of accreting neutron stars were treated in a unified and thermodynamically consistent way. With these equations of state, we determined the mass, radius, moment of inertia, and tidal deformability of accreted neutron stars and compared with catalyzed neutron stars for which unified equations of state based on the same functionals are available. The equation of state of accreted neutron stars is found to be significantly stiffer than that of catalyzed matter, with an adiabatic index $\Gamma \approx 4/3$ throughout the crust. For this reason, accreting neutron stars have larger radii. However, their crustal moment of inertia and their tidal deformability are hardly changed provided density discontinuities at the interface between adjacent crustal layers are properly taken into account. The enhancement of the stiffness of the equation of state of accreting neutron stars is mainly a consequence of nuclear shell effects, thus confirming the importance of a quantum treatment as stressed in our first study. With our previous calculations of crustal heating using the same functionals, we have thus obtained consistent microscopic inputs for simulations of accreting neutron stars.
[hal-03726443] Double flows anchored in a Kerr black hole horizon – I. Meridionally self-similar MHD models with loading terms
Date: 31 Mar 2023 - 17:22
Desc: Recent observations of supermassive black holes have brought us new information on their magnetospheres. In this study, we attempt a theoretical modelling of the coupling of black holes with their jets and discs, via three innovations. First, we propose a semi-analytical MHD description of a steady relativistic inflow–outflow structure characteristic to the extraction of the hole rotational energy. The mass-loading is ensured in a thin layer, the stagnation surface, by a two-photon pair production originating to a gamma-ray emission from the surrounding disc. The double flow is described near the polar axis by an axisymmetric meridionally self-similar MHD model. Secondly, the inflow and outflow solutions are crossing the MHD critical points and are matched at the stagnation surface. Knowledge of the MHD field on the horizon gives us the angular momentum and energy extracted from the black hole. Finally, we illustrate the model with three specific examples of double-flow solutions by varying the energetic interaction between the MHD field and the rotating black hole. When the isorotation frequency is half of the black hole one, the extracted Poynting flux is comparable to the one obtained using the force-free assumption. In two of the presented solutions, the Penrose process dominates at large colatitudes, while the third is Poynting flux dominated at mid-colatitudes. Mass injection rate estimations, from disc luminosity and inner radius, give an upper limit just above the values obtained for two solutions. This model is pertinent to describe the flows near the polar axis, where pair production is more efficient.
[hal-03515399] Gravitational waveforms for compact binaries from second-order self-force theory
Date: 6 Jan 2022 - 16:55
Desc: We produce gravitational waveforms for nonspinning compact binaries undergoing a quasicircular inspiral. Our approach is based on a two-timescale expansion of the Einstein equations in second-order self-force theory, which allows first-principles waveform production in milliseconds. Although the approach is designed for extreme mass ratios, our waveforms agree remarkably well with those from full numerical relativity, even for comparable-mass systems. Our results will be invaluable in accurately modelling extreme-mass-ratio inspirals for the LISA mission and intermediate-mass-ratio systems currently being observed by the LIGO-Virgo-KAGRA Collaboration.
[hal-04051063] Peeling at extreme black hole horizons
Date: 29 Mar 2023 - 17:38
Desc: The starting point of this work was an intriguing similarity between the behaviour of fields near a degenerate horizon and near the infinity of an asymptotically flat spacetime, as revealed by the scattering theory for Dirac fields in the ``exterior'' region of the extreme Kerr - de Sitter black hole, developed by one of the authors (JB). However, in that situation, the comparison was somewhat clouded by some of the analytical techniques used in intermediate steps of the proof. The aim of the present work is to clarify the comparison further by studying instead the peeling behaviour of solutions to the wave equation at an extremal horizon. We focus first on the extreme Reissner-Nordstr\"om black hole, for which the Couch-Torrence inversion (a global conformal isometry that exchanges the horizon and infinity) makes the analogy explicit. Then, we explore more general spherically symmetric situations using the Couch-Torrence inversion outside of its natural context.
Autres contacts
Section de Meudon
Bâtiment du LAM (n°18)
5, place Jules Janssen
92190 MEUDON CEDEX
