Laboratoire Univers et Théories

LUTH

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Observatoire de Paris

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UFR Physique

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Université Paris Diderot, CNRS

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-03164443] Vortex pinning in the superfluid core of relativistic neutron stars

Date: 26 juil 2021 - 14:23

Desc: Our recent Newtonian treatment of the smooth-averaged mutual-friction force acting on the neutron superfluid and locally induced by the pinning of quantized neutron vortices to proton fluxoids in the outer core of superfluid neutron stars is here adapted to the general-relativistic framework. We show how the local non-relativistic motion of individual vortices can be matched to the global dynamics of the star using the fully 4D covariant Newtonian formalism of Carter & Chamel. We derive all the necessary dynamical equations for carrying out realistic simulations of superfluid rotating neutron stars in full general relativity, as required for the interpretation of pulsar frequency glitches. The role of vortex pinning on the global dynamics appears to be non-trivial.

[hal-01802098] Constraints on particle acceleration in SS433/W50 from MAGIC and H.E.S.S. observations

Date: 24 juil 2021 - 04:48

Desc: Context. The large jet kinetic power and non-thermal processes occurring in the microquasar SS 433 make this source a good candidate for a very high-energy (VHE) gamma-ray emitter. Gamma-ray fluxes above the sensitivity limits of current Cherenkov telescopes have been predicted for both the central X-ray binary system and the interaction regions of SS 433 jets with the surrounding W50 nebula. Non-thermal emission at lower energies has been previously reported, indicating that efficient particle acceleration is taking place in the system.Aim. We explore the capability of SS 433 to emit VHE gamma rays during periods in which the expected flux attenuation due to periodic eclipses (Porb~ 13.1 days) and precession of the circumstellar disk (Ppre ~ 162 days) periodically covering the central binary system is expected to be at its minimum. The eastern and western SS 433/W50 interaction regions are also examined using the whole data set available. We aim to constrain some theoretical models previously developed for this system with our observations.Methods. We made use of dedicated observations from the Major Atmospheric Gamma Imaging Cherenkov telescopes (MAGIC) and High Energy Spectroscopic System (H.E.S.S.) of SS 433 taken from 2006 to 2011. These observation were combined for the first time and accounted for a total effective observation time of 16.5 h, which were scheduled considering the expected phases of minimum absorption of the putative VHE emission. Gamma-ray attenuation does not affect the jet/medium interaction regions. In this case, the analysis of a larger data set amounting to ~40–80 h, depending on the region, was employed.Results. No evidence of VHE gamma-ray emission either from the central binary system or from the eastern/western interaction regions was found. Upper limits were computed for the combined data set. Differential fluxes from the central system are found to be ≲ 10−12–10−13 TeV−1 cm−2 s−1 in an energy interval ranging from ~few × 100 GeV to ~few TeV. Integral flux limits down to ~ 10−12–10−13 ph cm−2 s−1 and ~ 10−13–10−14 ph cm−2 s−1 are obtainedat 300 and 800 GeV, respectively. Our results are used to place constraints on the particle acceleration fraction at the inner jetregions and on the physics of the jet/medium interactions.Conclusions. Our findings suggest that the fraction of the jet kinetic power that is transferred to relativistic protons must be relatively small in SS 433, qp ≤ 2.5 × 10−5, to explain the lack of TeV and neutrino emission from the central system. At the SS 433/W50 interface, the presence of magnetic fields ≳10 μG is derived assuming a synchrotron origin for the observed X-ray emission. This also implies the presence of high-energy electrons with Ee− up to 50 TeV, preventing an efficient production of gamma-ray fluxes in these interaction regions.Key words: gamma rays: general / stars: black holes / X-rays: binaries / ISM: jets and outflows

[hal-02371616] H.E.S.S. observations of the flaring gravitationally lensed galaxy PKS 1830–211

Date: 24 juil 2021 - 04:48

Desc: PKS 1830–211 is a known macrolensed quasar located at a redshift of z = 2.5. Its high-energy gamma-ray emission has been detected with the Fermi-Large Area Telescope (LAT) instrument and evidence for lensing was obtained by several authors from its high-energy data. Observations of PKS 1830–211 were taken with the High Energy Stereoscopic System (H.E.S.S.) array of Imaging Atmospheric Cherenkov Telescopes in 2014 August, following a flare alert by the Fermi-LAT Collaboration. The H.E.S.S observations were aimed at detecting a gamma-ray flare delayed by 20–27 d from the alert flare, as expected from observations at other wavelengths. More than 12 h of good-quality data were taken with an analysis threshold of ∼67 GeV. The significance of a potential signal is computed as a function of the date and the average significance over the whole period. Data are compared to simultaneous observations by Fermi-LAT. No photon excess or significant signal is detected. An upper limit on PKS 1830–211 flux above 67 GeV is computed and compared to the extrapolation of the Fermi-LAT flare spectrum.

[hal-01797874] Extended VHE $\gamma$-ray emission towards SGR1806−20, LBV 1806−20, and stellar cluster Cl* 1806−20

Date: 24 juil 2021 - 04:48

Desc: Using the High Energy Spectroscopic System (H.E.S.S.) telescopes we have discovered a steady and extended very high-energy (VHE) γ-ray source towards the luminous blue variable candidate LBV 1806−20, massive stellar cluster Cl* 1806−20, and magnetar SGR 1806−20. The new VHE source, HESS J1808−204, was detected at a statistical significance of >6σ (post-trial) with a photon flux normalisation (2.9 ± 0.4stat ± 0.5sys) × 10−13 ph cm−2 s−1 TeV−1 at 1 TeV and a power-law photon index of 2.3 ± 0.2stat ± 0.3sys. The luminosity of this source (0.2 to 10 TeV; scaled to distance d = 8.7 kpc) is LVHE ~ 1.6 × 1034(d/8.7 kpc)2 erg s−1. The VHE γ-ray emission is extended and is well fit by a single Gaussian with statistical standard deviation of 0.095° ± 0.015°. This extension is similar to that of the synchrotron radio nebula G10.0−0.3, which is thought to be powered by LBV 1806−20. The VHE γ-ray luminosity could be provided by the stellar wind luminosity of LBV 1806−20 by itself and/or the massive star members of Cl* 1806−20. Alternatively, magnetic dissipation (e.g. via reconnection) from SGR 1806−20 can potentially account for the VHE luminosity. The origin and hadronic and/or leptonic nature of the accelerated particles responsible for HESS J1808−204 is not yet clear. If associated with SGR 1806−20, the potentially young age of the magnetar (650 yr) can be used to infer the transport limits of these particles to match the VHE source size. This discovery provides new interest in the potential for high-energy particle acceleration from magnetars, massive stars, and/or stellar clusters.Key words: gamma rays: general / stars: magnetars / stars: massive★ Wallenberg Academy Fellow.† Deceased.

[hal-02542819] Progress in integrative systems biology, physiology and medicine: towards a scale-relative biology

Date: 23 juil 2021 - 12:22

Desc: In this paper we present a review of progress in addressing the challenge to understand and describe the vast complexity and multi-level organisation associated with biological systems. We begin with a review of past and current approaches, key lessons, and unresolved challenges, which require a new conceptual framework to address them. After summarizing the core of the problem, which is linked to computational complexity, we review recent developments within the theoretical framework of scale relativity, which offers new insights into the emergence of structure and function (at multiple scales), providing a new integrative approach to biological systems. The theoretical framework describes the critical role of thermodynamics and quantum vacuum fluctuations in the emergence of charge-induced macroscopic quantum fields (effectively a new quantum field theory) at multiple scales, which underpin a macroscopic quantum description of biological systems as a complex exemplar of condensed matter. The theory is validated through a new biomimetic experimental approach, which leads to the emergence of plant and individual cell-like structures with the intrinsic capacity to divide, differentiate and form multicellular structures. We discuss how this theoretical framework could be applied to extend our understanding of cardiac systems biology and physiology, and challenges such as cancer and neurodegenerative disease. We also consider the potential of these new insights to support a new approach to the development of emerging quantum technologies.