LUTH - Observatoire de Paris

Testing the presence of a fifth force at the Galactic Center

Bernard Laura — 2024-11-15T08:42:39Z

The presence of stars orbiting at the Galactic Center, all placed within an arcsecond distance from the supermassive black hole Sagittarius A*, allow us to test the current theory of gravity outside the Solar System, in an completely different environment from those tested so far.
In this talk I'll show how the orbital motion of star S2, and specifically the astrometric data collected by GRAVITY, places quite strong constraints on the presence of an hypothetical fifth force, which appears as a Yukawa-like modification to the Newtonian potential. The strongest constraint is found for lambda \sim 10^14 m, for which we found |alpha| < 0.0017 at 95% confidence level, improving by one order of magnitude previous estimates of the same quantity (see Hees et al. 2017). Some future prospects for this work will be also shown.

Black Holes beyond General Relativity : Uniqueness & Dynamics

Bernard Laura — 2024-11-07T09:37:20Z

Recent astrophysical observations of electromagnetic and gravitational emission from close to black-hole horizons offer novel tests of General Relativity (GR) in the strong-gravity regime. Constraining potential deviations requires reliable predictions not just in but also beyond GR. To set the stage, I discuss different assumptions about physics beyond GR and resulting expectations about where to look for deviations.
In the first part of my talk, I will focus on black-hole uniqueness and how to leverage VLBI observations of the photon ring to constrain the underlying spacetime. In particular, I will discuss how to systematically parameterize potential deviations and demonstrate a systematic lensing-band framework to obtain robust constraints.
In the second part of my talk, I will focus on the dynamical strong-gravity regime and how to leverage observed binary waveforms to constrain the effective field theory (EFT) of gravity. In particular, I will summarise recent progress on well-posedness that place numerical relativity in the EFT of gravity on the same solid mathematical footing as in GR.

Contrasting neutron star heating mechanisms with Hubble Space Telescope observations

seminaire — 2024-10-04T16:45:53Z

Passively cooling neutron stars would reach very low surface temperatures $T_s < 10^4$ K within $<10^7$ yr. However, likely thermal UV emission has been detected in HST observations of 4 much older neutrons stars (2 “classical” and 2 millisecond pulsars), implying $T_s ∼ 10^5$ K.
We computed their evolution with different heating mechanisms, finding that the relevant ones are rotochemical heating and vortex creep. The former consists of non-equilibrium beta reactions induced by the continuous spin-down of the NS. If there are superfluid nucleons in its core, chemical energy is stored until a threshold is reached. Then, part of the energy is rapidly released, raising the temperature. If the protons in the core are superconducting, the magnetic flux is concentrated in quantized flux tubes. Outside the flux tube cores, protons are superconducting, while inside they remain non-superconducting, so reactions occur mostly inside. Vortex creep is the friction of the quantized neutron vortices moving through the NS crust.
We find that all the observations can be explained by rotochemical heating with superconducting protons and suitable internal magnetic fields, or by a combination of vortex creep and rotochemical heating with superfluid neutrons.

Marie-Noelle Celerier

seminaire — 2024-09-30T08:19:03Z

La solution homogène et isotrope des équations d'Einstein dite LambdaCDM, assortie de ses perturbations linéaires et d'ordres supérieurs, forme le socle de la cosmologie contemporaine permettant de comprendre, dans ses grandes lignes, l'évolution et la géométrie de notre Univers. Les réflexions suivantes nous incitent à aller plus loin. Tout d'abord, l'homogénéité et l'isotropie de l'Univers ne sont que des propriétés moyennes valables à des échelles qui reculent au fur et à mesure de la découverte de structures à des décalages vers le rouge de plus en plus grands. L'avènement d'une cosmologie de précision conduit ainsi à l'émergence ou à l'amplification de tensions, voire d'anomalies, dues au décalage entre prédictions extrapolées depuis l'Univers lointain et observations à précision accrue de l'Univers proche. La Relativité Générale nous offre une manière de résoudre ces incompatibilités. Le modèle Szekeres est une solution exacte des équations d'Einstein, inhomogène et sans symétrie, capable de représenter la région de notre Univers dominée par la matière et/ou la constante cosmologique, puisque sa source gravitationnelle est un fluide de pression nulle et que la constante cosmologique peut y être incorporée. Son atout majeur est d'inclure la solution homogène et isotrope de Friedmann comme cas limite. Elle est donc susceptible de reproduire naturellemnt la transition homogène/inhomogène
à l'échelle où les fonctions qui la décrivent deviennent analogues aux paramètres cosmologiques standards. Les prédictions les plus robustes de la cosmologie de l'Univers lointain restent ainsi conservées. Au cours de ce séminaire, la solution Szekeres sera présentée avec ses principales propriétés nécessaires à une utilisation dans un contexte cosmologique. Les différentes expériences qui devront être utilisées pour contraindre les fonctions-paramètres qui déterminent le modèle Szekeres d'Univers valable à précision donnée seront décrites et commentées. Les perspectives ouvertes par les réseaux de neurones pour réaliser ce travail d'analyse de grandes quantités de données seront discutées.

Black Hole Jet Launching in Polarized Light

Bernard Laura — 2024-09-06T08:24:34Z

Extragalactic jets throughout the universe transport energy from small scales near a galaxy's central supermassive black hole to extragalactic distances.
These jets may be launched via the Blandford-Znajek (BZ) mechanism, where magnetic fields extract the black hole's spin energy ; however, BZ energy extraction has not yet been confirmed observationally.
In this talk, I will discuss what polarized images of synchrotron radiation from close to the event horizon of the supermassive black hole M87* can tell us about black hole magnetic fields, jet launching, and black hole spin.
Near-horizon Event Horizon Telescope (EHT) images in linear and circular polarization strongly suggest that the accretion disk in M87* is magnetically arrested with coherent and dynamically important magnetic fields.
I will show that the pattern of linear polarization in EHT images directly probes the direction of electromagnetic energy flux, and that the EHT images indicate that electromagnetic energy flows outward on horizon scales around M87*.
The spiral pattern of polarization vectors in EHT images is directly connected to the underlying magnetic field structure ; if the fields are wound up by the black hole, these images will allow precise measurements of black hole spin.
Future EHT observations of M87 will be sensitive enough to detect faint emission both closer to the event horizon and farther downstream in the jet launching region.
These observations will enable a definitive test of the Blandford-Znajek mechanism for powering extragalactic jets.

Internal hydrodynamical shocks as a mechanism for GRB prompt emission

seminaire — 2024-08-29T12:36:39Z

In the wake of recent analytical work highlighting the role as synchrotron emission sources of both the forward and reverse shock emerging from the collision of two ultra relativistic shells, we perform a numerical study of such a system and its observed flux. We precise the hydrodynamical structure, properties, and resulting emission timings by extending the approach to spherical geometry. We confirm the reproduction of GRB spectral features explained by the simpler analytical approach such as the sub-dominant low-energy spectral component and the doubly-broken power-law spectrum without any fine-tuning of the physical conditions, and lay the foundation to explore emission from internal shocks of more complex outflows.

Radiation-mediated shocks in gamma-ray bursts

Meliani Zakaria — 2024-08-20T11:30:34Z

The origin of the prompt emission in gamma-ray bursts (GRBs) remains highly debated despite decades of research. The GRB jet is initially optically thick and the trapped radiation is released when the jet transitions to the optically thin regime at the photospheric radius. Due to the high radiation pressure close to the central engine, shocks that occur before this transition are radiation mediated. In this seminar, I give an introduction to the theory of radiation-mediated shocks (RMSs) in the context of GRBs. I show that such shocks are expected in GRB jets and that the energized radiation downstream of the shock exhibits several promising features with regard to the observations. However, these shocks are computationally challenging to model from first principles. With this motivation, we have developed an approximate simulation code that is both quick and accurate. The simulation code is unique in its ability to fit an RMS model to the observational data.

Chaos and Stability in the Long-term Dynamics of the Inner Planets in the Solar System

seminaire — 2024-04-04T10:12:52Z

The orbits of the inner planets within the Solar System exhibit chaotic behavior, forbidding deterministic predictions of their positions and velocities beyond a few tens of millions of years. Despite this chaos, the planetary orbits demonstrate remarkable dynamical stability over timescales comparable to the age of the Solar System. The likelihood that Mercury's eccentricity exceeds 0.7, potentially resulting in catastrophic events such as close encounters or collisions, is estimated to be only about 1% over the next 5 billion years. In this talk, I will cover recent advancements in isolating the planet interactions that lead to random changes in the orbits over long times. Furthermore, I will present a global picture in which the intriguing stability of the planetary orbits over the Solar System's lifetime emerges quite naturally.

On the integrability of extended body dynamics around black holes

Bernard Laura — 2024-02-08T14:42:33Z

In general relativity, freely-falling objects follow geodesics of the background spacetime in which they live. In a sense, this feature is a mere rephrasing of Einstein's equivalence principle.
In 1968, Brandon Carter showed that the geodesic motion of objects orbiting a Kerr black hole was integrable, in the sense of Hamiltonian mechanics, by discovering a fourth constant of motion that now bears his name. This “universality” of geodesic free fall, however, is but an approximation : In general, two different bodies will follow two distinct paths, depending on how they spin and deform. In this talk, I will show how, and to which extent, Carter's integrability of Kerr geodesics can be extended to the motion of not just mere point masses, but also extended bodies that can spin and deform.

Euclid

seminaire — 2024-02-08T10:32:57Z

TBD

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