Journées Scientifiques de l'Action Thématique GRAM

26 nov. 2026, 09:00 → 27 nov. 2026, 14:30 Europe/Paris

Amphi Pierre-Gilles de Gennes (Université Paris Cité)

L’Action Thématique Gravitation, Références, Astronomie, Métrologie ATGRAM a été créé en 2024, succédant au Programme Nationale GRAM qui avait été mise en place en 2016. Il est financé par trois instituts du CNRS (INSU, INP et IN2P3) ainsi que par le CNES et le CEA.

L’ATGRAM a vocation à coordonner et développer des actions dans trois grands domaines thématiques :

• La physique fondamentale, notamment en vue de tester les théories de la gravitation, la relativité générale et les théories des interactions fondamentales ;
• La métrologie de l’espace et du temps (astrométrie, géodésie spatiale, horloges), les systèmes de référence célestes et terrestres, les échelles de temps, le géopotentiel et la rotation de la Terre ;
• La mécanique céleste et la mécanique spatiale (trajectoires des corps du système solaire, des satellites et des sondes interplanétaires), avec sous-jacentes les lois de la dynamique.

Ces actions peuvent être de nature théorique, procéder de simulations ou d’analyse de données, et inclure le développement d’instrumentation au sol ou embarquée ; elles peuvent aussi viser à animer et fédérer la communauté sur certains sujets. D’un point de vue observationnel, l’ATGRAM est en prise avec les grands moyens sol/espace sur lesquels s’appuie la recherche dans ces domaines.

L’ATGRAM organise des journées scientifiques qui sont l’occasion de réunir la communauté française relevant de son périmètre scientifique.

Les journées scientifiques 2026 se tiendront à Paris, à l’Université Paris-Cité, du 26 au 27 novembre 2026, et en parallèle par visioconférence.

jeu. 26 novembre

09:00 → 09:30 Bienvenue

Mots de bienvenue et organisation des Journées

09:30 → 12:30 Physique fondamentale

09:30 Variation of Planck's quantum of action in Entangled Relativity and its impact on thermal radiation

Entangled Relativity is a non-linear reformulation of General Relativity that precludes the definition of a universe devoid of matter. In this framework, matter and spacetime are inseparably linked, in the literal sense that the gravitational dynamics cannot be defined independently of matter fields.

Despite its non-standard formulation, the phenomenology of General Relativity can be recovered through an “intrinsic decoupling”, originally identified in scalar-tensor theories, such that General Relativity emerges as a limiting regime of the theory.

A key prediction of Entangled Relativity is a variation of the Planck quantum of action, ℏ. Although this variation is extremely small in amplitude, it generically leaves an imprint on thermal radiation, such as the Cosmic Microwave Background.

In this talk, we review recent numerical estimates of the expected variation of ℏ, with particular emphasis on high-density environments such as white dwarfs, neutron stars, and binary systems composed of both. These environments provide unique natural laboratories for constraining a varying ℏ through observations of electromagnetic signals and gravitational waves.

The talk is based on 10.1088/1361-6382/ae30c7 and 10.1103/n927-nrbn.

Orateur: Thomas Chehab (Observatoire de la côte d'Azur, Laboratoire ARTEMIS)

10:00 Probing spin-0 and spin-1 ultralight dark matter with binary pulsar timing

Ultralight bosons are well-motivated dark-matter candidates that can behave as coherent oscillating fields on Galactic scales. At the lower end of the mass spectrum, these oscillations can remain coherent for decades and leave detectable signatures in high-precision astrophysical observations. Binary pulsars are particularly powerful probes of such effects.

We investigate quadratically coupled scalar dark matter and spin-1 vector dark matter. We show how these fields perturb binary orbits, inducing variations in pulse arrival times, and use a Bayesian framework to combine information from different systems while accounting for orbital uncertainties. For scalar dark matter, we probe masses between $2\times10^{-22}$ and $2\times10^{-21} \, \, \mathrm{eV}$, a region inaccessible to other current searches. For vector dark matter, our sensitivity estimates are competitive with laboratory and space-based experiments near orbital resonances. These results highlight binary pulsar timing as a complementary probe of ultralight dark matter.

10:30 Pause

11:00 Searching for short-range fifth-forces in space with MICROSCOPE and STEIN

General Relativity (GR) is the best description of gravity so far. In 2018, the MICROSCOPE mission tested its cornerstone, the Equivalence Principle (EP), with unprecedented precision, setting the current state-of-the-art upper limit on its validity. Yet, keeping on testing gravity at better precision is of crucial interest in the quest of a model that could explain Dark Matter, Dark Energy or unify forces.

MICROSCOPE could give more information than just testing the EP: by measuring the gravitational interaction between its Test Masses (TM), it is, in principle, possible to look for a deviation to GR at small scales (~). Nevertheless, beside MICROSCOPE not being optimized for this, looking for a small-scale fifth-forces was limited by the use of simplifying assumptions about MICROSCOPE’s geometry.

In this talk, I will show how Finite Elements Method simulations can help overcome such simplifications. I will also discuss how STEIN (MICROSCOPE successor) may allow us to look for fifth-forces between its TM, showing the first results made by simulating Yukawa interaction in a STEIN-like satellite.

Orateur: Matthieu Dellavalle (ONERA - Office Nationale d'Études et de Recherches Aérospatiales)

11:30 Probing Casimir-Polder and Fifth Forces with Nanograting Atom Interferometry.

A fifth fundamental force (as a deviation from Newtonian gravity) is predicted by several theories addressing, for instance, questions relative to the dark sector or the lack of a unified framework for General Relativity and the Standard Model. Probing the existence of a fifth force at the submicron scale is challenging because electromagnetic interactions (e.g., the Casimir-Polder potential acting on an atom near a macroscopic body) dominate and may shield new physics. We propose to constrain representative fifth-force models (chameleon and symmetron fields, axion-nucleon coupling, and Yukawa parametrization) using the diffraction of slow atoms by material transmission nanogratings. The diffraction patterns are strongly modified by both the Casimir-Polder and putative non-Newtonian potentials arising inside the slits. To minimize the effects of the Casimir-Polder potential while preserving the effects of the putative fifth force on the analyzed observable, we are working on a differential measurement comparing diffraction patterns across nanograting geometries. Preliminary theoretical constraints await experimental confirmation. Thus, prospects include the fabrication of the nanogratings, a metrological measurement of the Casimir-Polder potential, and experimental tests of fifth-force models.

12:00 Fundamental physics and impact on astrophysics of Extended Theories of Electro-Magnetism (ETEM)

Although compatible with General Relativity (GR), the dark Universe lacks experimental confirmation and support by the Standard Model (SM). Opposing darkness, extensions of GR face the successes of the latter. But photons, main messengers, are still read with the XIX Maxwell theory, although Quantum Electro-Dynamics (QED) requires non-linear corrections, e.g., photon-photon. Extended Theories of Electro-Magnetism (ETEM) induce a reinterpretation of the observations. In presence of a background, the SM Extension (SME) or Non-Linear Electro-Magnetism dress the photon, the only SM free massless particle, with an effective mass [1-3], compatible with the upper limits from Fast Radio Bursts [4-6] and solar wind [7,8]. Birefringence, dispersion, second-order QED are tested or searched ETEM effects at, e.g., BMV Toulouse, ATLAS CERN, DeLLight Paris. All photons either massive - ab initio as in the de Broglie-Proca theory or through the SME or the Born-Infeld, Heisenberg-Euler non-linear type of theories - undergo a frequency shift in presence of an electromagnetic and/or a Lorentz Symmetry Violation background [9,10]. This small additional shift, towards the red or the blue, added to the expansion redshift, determines new cosmological scenarios, e.g., without dark energy and matter [11-14]. Interferometry guarantees testing perspectives [15,16] below 3x10-18 in Δν/ν per metre for an Earth-Moon-like optical length. We aim to determine a dissipative effect free from blurring due to the interaction of the photon energy with the curvature produced by an electromagnetic or gravitational field [17-20]. In parallel, we aim to study photon-photon interactions in the frame of ETEM. Finally, we implement the Heisenberg principle at cosmological distances, where the Hubble tension appears as quantum measurement limit [21,22].
References
[1] Bonetti L., dos Santos Filho L.R., Helayël-Neto J.A., Spallicci A.D.A.M., Phys. Lett. B, 764, 203 (2017)
[2] Bonetti L., dos Santos Filho L.R., Helayël-Neto J.A., Spallicci A.D.A.M., Eur. Phys. J. C, 78, 811 (2018)
[3] Dib A., Helayël-Neto J.A., Spallicci A.D.A.M., Eur. Phys. J. C, 86, 3 (2026)
[4] Bonetti L., Ellis J., Mavromatos N.E., Sakharov A.S., Sarkisyan-Grinbaum E.K.G., Spallicci A.D.A.M., Phys. Lett. B, 757, 548 (2016)
[5] Bonetti L., Ellis J., Mavromatos N.E., Sakharov A.S., Sarkisyan-Grinbaum E.K.G., Spallicci A.D.A.M., Phys. Lett. B, 768, 326 (2017)
[6] Bentum M.J., Bonetti L., Spallicci A.D.A.M., Adv. Space Res., 59, 736 (2017)
[7] Retinò A., Spallicci A.D.A.M., Vaivads A., Astropart. Phys., 82, 49 (2016)
[8] Spallicci A.D.A.M., Sarracino G., Randriamboarison O., Helayël-Neto J.A., Dib A., Eur. Phys. J. Plus, 139, 551 (2024)
[9] Helayël-Neto J.A., Spallicci A.D.A.M., Eur. Phys. J. C, 79, 590 (2019)
[10] Spallicci A.D.A.M., Dib A., Helayël-Neto J.A., Phys. Lett. B, 885, 138873 (2024).
[11] Spallicci A.D.A.M., Helayël-Neto J.A., López-Corredoira M., Capozziello S., Eur. Phys. J. C, 81, 4 (2021)
[12] Spallicci A.D.A.M., Sarracino G., Capozziello S., Eur. Phys. J. Plus, 137, 253 (2022)
[13] Sarracino G., Spallicci A.D.A.M., Capozziello S., Eur. Phys. J. Plus, 137, 1386 (2022)
[14] Dib A., Djeghloul N., Spallicci A.D.A.M., submitted
[15] Abend S. et al., AVS Quantum Sci., 6, 024701 (2024)
[16] Abdalla A. et al., Eur. Phys. J. Quantum Techn., 12, 42 (2025)
[17] Blanchet L., Spallicci A., Whiting B., Mass and motion in general relativity, Fundamental Theory of Physics 162 (Springer, 2011)
[18] Spallicci A.D.A.M., Ritter P., Aoudia S., Int. J. Geom. Meth. Mod. Phys., 11, 1450072 (2014)
[19] Ritter P., Aoudia S., Spallicci A.D.A.M., Cordier S., Int. J. Geom. Meth. Mod. Phys., 13, 1650019 (2016)
[20] Dib A., Garnier A., Spallicci A.D.A.M., Int. J. Geom. Meth. Mod. Phys., 23, 2550278 (2026)
[21] Capozziello S., Benetti M., Spallicci A.D.A.M., Found. Phys. 50, 893 (2020)
[22] Spallicci A.D.A.M., Benetti M., Capozziello S., Found. Phys. 52, 23 (2022)

12:30 → 13:30 Buffet

13:30 → 16:00 Métrologie Temps-Espace

13:30 Probing Ultralight Dark Matter with Molecular Spectroscopy Referenced to an Optical Network

Ultralight bosonic dark matter may induce oscillations of fundamental constants such as the fine-structure constant α and the proton-to-electron mass ratio μ, which can be searched for through precision spectroscopy. We present a molecular spectroscopy platform developed to search for such signals using the REFIMEVE optical frequency dissemination network. An ultrastable optical reference at 1542 nm, disseminated from the Paris Observatory to the University of Lille through an actively noise-compensated fiber link, is directly used as the frequency reference for the experiment.
The setup combines optical amplification, single-sideband frequency synthesis and active intensity stabilization to probe the P(27) transition of the v₁+v₃ band of ¹²C₂H₂ near 1542 nm. Differential balanced photodetection provides efficient suppression of common-mode intensity noise and enables high-sensitivity monitoring of the molecular absorption. The transition was characterized with an experimental linewidth of 768(33) MHz. The detector signal was further converted into an equivalent fractional frequency-noise spectrum over a bandwidth extending to the GHz range, providing a quantitative assessment of the sensitivity to oscillating variations of α and μ.
Although the present sensitivity does not yet reach that of the most competitive atomic-clock searches, these results demonstrate the feasibility of molecular dark-matter searches directly referenced to a distributed ultrastable optical network. The approach provides a scalable basis for future multi-site, correlated measurements using the REFIMEVE infrastructure.

14:00 Construction of an 171Yb Optical Lattice Clock for Geodetic Exporation

Optical lattice clocks have progressed swiftly over the past decade, now outperforming state-of-the-art microwave clocks by more than two orders of magnitude in both stability and accuracy, reaching a level of control of the frequency of 18 digits or better. This opens up new applications in fundamental physics, astronomy and Earth science. In this context, LTE initiated the development of a transportable 171Yb lattice clock in 2021 to explore the coupling of the atoms to the Earth’s gravitational potential. The scientific objective is to build an instrument that can be connected to some of the ~60 outputs of the French research infrastructure REFIMEVE, which disseminates a 1542 nm ultrastable carrier throughout the country. This will allow remote comparisons to the ~12 operational European optical clocks and therefore give access to geopotential variations (in space and time), especially in places where abrupt changes occur (coastal/mountainous areas). As the control over 18 digits allows resolving 1 cm height differences, our approach will complement existing geodesy techniques based on spirit levelling and satellites, and help further constrain models describing the Geoid.
We present the status of the transportable optical lattice clock ROYMAGE. Its design aims at trapping a large number of atoms (>104) in less than 100ms, to reduce the dead-time of the clock and thereby improve its stability. We present the successive bricks we have developed to allow this ultrafast loading: a 399 nm source providing 1 W of power, an ytterbium oven featuring a flux of 4e11 atoms/s, a transverse molasses collimating the atomic beam so that the flux is preserved down the trapping region, a Zeeman slower bringing the atoms from 300 m/s to 30 m/s, a speed at which they are captured in a 3D Magneto-Optical Trap (MOT). We also introduce the atomic loader RAZPOUTYNE, which combines a 2D MOT and a Zeeman slower in a compact geometry, and has already demonstrated the trapping of 7e8 ytterbium atoms.
The next steps are a second cooling stage MOT at 556 nm, then loading the atoms into an optical lattice deep enough (Lamb-Dicke regime) to cancel the first-order Doppler effect and the recoil shift. This will pave the road towards transportation of the instrument along the REFIMEVE network and towards participation in a refined realization of the Geoid.

14:30 Low and high repetition rate two-colors satellite laser ranging capabilities at the Grasse optical ground station

The Grasse optical ground station (GRSM-7845) contributes to the International Laser Ranging Service (ILRS) by providing Satellite and Lunar Laser Ranging observations since 2009 with almost the same instrumental setup. If the current setup is still relevant for Lunar Laser Ranging (LLR), it does not allow to answer to the millimeter challenge required for the scientific applications in space geodesy. In this presentation, we will present a new setup which has been implemented in parallel to the operational one, allowing a continuity of observations with the old configuration and SLR observations at high count rate in two-colors and with sub-millimetric precision.

15:00 Pause

15:30 REFIMEVE+

TBW

16:00 → 17:00 Gravitation

16:00 Detectability of the spin parameters of SgrA* using multiple S stars

The orbits of S stars around the Galactic Center black hole, Sgr~A, offer a unique opportunity to probe the relativistic regime of gravity and test the no-hair theorem. While the Schwarzschild precession has been detected with S2, the detection of spin-induced (Kerr) effects—Lense–Thirring and quadrupole moment terms—remains a major observational challenge. We aim to quantify the time needed to detect the spin parameters of Sgr~A—magnitude and orientation—by exploiting astrometric and spectroscopic monitoring of multiple S stars with current (GRAVITY+, ERIS) and future facilities (MICADO). Using the post-Newtonian integrator, we simulate long-term astrometric observations of individual and combined S-star systems, including S2, S29, S38 and S55, and the recently discovered S301. We quantify spin detectability through $\chi^2$ minimization and forecast the evolution of spin-parameter uncertainties over the coming decades. We show that S301 has spin sensitivity comparable to a star identical to S2 but ten-times closer to Sgr A, and that combining its data with those of other stars of diverse orientations (S2, S29, S38, S55) reduces parameter degeneracies and accelerates spin detection. Quantitative forecasts indicate that the spin magnitude can be constrained to $\sigma_\chi \simeq$ 0.1 by 2032, 0.05 by 2034, and 0.03 by 2036 when only fitting the spin magnitude $\chi$, with comparable results when fitting all spin parameters. Astrometry provides the dominant leverage, while spectroscopy adds substantial value only if radial-velocity precision reaches ~2 km/s, as expected with MICADO. Overall, spin detectability is robust given sufficient monitoring and knowledge of orbital parameters. The black hole orientation is a critical factor, along with multi-star, multi-instrument datasets, which together offer the most promising path forward. The joint monitoring of multiple S stars with GRAVITY+ and MICADO will enable robust measurement of the spin of Sgr~A within the next decade. Astrometry remains indispensable for achieving this goal, while high-precision spectroscopy will serve as a critical complement once the required velocity accuracy is reached.

Orateur: Karim Abd El Dayem (Observatoire de Paris (LESIA))

16:30 Probing gravitational waves using the orbital dynamics of binary systems

The internal dynamics of a binary system can be affected by the passage of gravitational waves (GWs). This effect is significantly enhanced when the GW frequency is in resonance with the orbital frequency of the binary system. Recently, this phenomenon has been proposed as a novel approach to detecting GWs in the largely unexplored microhertz frequency band, using precise measurements of the orbital dynamics of the Moon and artificial satellites.

In this contribution, we will discuss the signatures induced by GWs in the orbital dynamics of a binary system, with particular emphasis on the resonant case. We will then show how these signatures could be detected through laser ranging measurements of the Moon and artificial satellites. We will present sensitivity estimates for such measurements and briefly discuss the concept of the GUEST space mission, recently submitted to the ESA F-3 call.

ven. 27 novembre

09:00 → 10:00 Gravitation

09:00 Quantum modified inertia: an application to galaxy rotation curves

The newly introduced Quantum Modified Inertia (QMI) framework is a relativistic theory of motion incorporating both upper and lower bounds on proper time. A mathematical consequence of these bounds is the emergence of corresponding upper and lower acceleration limits.
Focusing on the lower bound, a characteristic minimum acceleration naturally emerges from the analysis. Applied to galactic dynamics, the resulting modified-inertia model accounts for the rotation curves of systems such as the Milky Way (MW) and the dwarf galaxy DDO 52 while incorporating realistic baryonic contributions. Although the model differs significantly from MOND, it analytically recovers the radial acceleration relation and the baryonic Tully–Fisher relation, while remaining consistent with local Solar System constraints from the Cassini mission. These results suggest that bounding inertial dynamics can significantly reduce the need for cold dark matter, at least on galactic scales.
Building on this work, in collaboration with the UTINAM laboratory, we aim to refine in QMI the stellar mass-to-light ratio and the modelling of the MW rotation curve by accounting for several structural features of the Galaxy, including the thickness of the disk, the central bar, and the disk warp. The implications for celestial mechanics, as well as model-specific signatures that could enable the detection of the minimum-acceleration bound using metrological or interferometric techniques, will also be discussed.
Link to the original publication: https://rdcu.be/fuT30

09:30 Single-bounce quantum gravimeter to measure the free-fall of anti-hydrogen

We propose an innovative concept for a quantum gravimeter, where atoms prepared in a Heisenberg-limited state perform a single bounce on a mirror followed by a free fall. This quantum gravimeter produces a simple and robust interference pattern which should allow to measure the free-fall acceleration of atoms. The expected accuracy of the measurement is estimated using simulations and estimation techniques based on Cramer-Rao law and Fisher information. The proposal opens the door to free fall measurements on rare or exotic atomic species, especially in situations where experimental time or detection events are limited by intrinsic physical reasons.

10:00 → 12:30 Mécanique Céleste

10:00 Meteor Clusters

Meteors are streaks of light appearing in the sky (a.k.a. shooting stars), produced by the high entry velocity of a meteoroid (rock in the interplanetary space) into our atmosphere. A meteor cluster involves several meteors following the same orbit, appearing at the same time. About 10 have been reported so far, although continuous meteor monitoring has been going on for more than 25 years now. We will show how our current work aims to define a meteor cluster, partly by finding and characterizing new ones. This work is supported by AT-GRAM and PNP.

Orateur: Jérémie Vaubaillon (LTE, Observatoire de Paris)

10:30 Pause

11:00 The Internation Lunar Reference System ILuRS

The ILuRS is the new Moon references system proposed by the IAG/IAU WG 1.1.3 and adopted as a new product center by IERS in May 2026. I will introduce here the concepts and the associated definitions of ILuRs.

11:30 NAROO program

The New Astrometric Reduction of Old Observations (NAROO) program is dedicated to the measurement of astrophotographic plates and the analysis of old observations for scientific purposes. One of the main objectives is to provide accurate positional measurements of planets and satellites to improve our knowledge of their orbits and dynamics, and to infer the accuracy of the planet and satellite ephemerides.
We present the result of five years of exploitation through different scientific programs, from the precovery of Potentially Hazardous Asteroids to the measure of the priceless Greenwich historical observations. We also discuss the availability of digitized images and future collaborative programs.

12:00 Rotation du satellite géodésique Ajisai : détermination, évolution, applications

La connaissance de la rotation des satellites géodésiques est nécessaire ou utile pour de nombreuses applications, par exemple la modélisation de certaines perturbations du mouvement orbital (e.g. l’effet Yarkovsky) ou la synchronisation d’horloges distantes par lien laser via un satellite passif. L’étude de la dynamique de la rotation est également intéressante en soit. Nous avons développé deux techniques pour déterminer la rotation, du satellite Ajisai dans ce cas : l’une s’appuie sur des observations photométriques obtenues grâce à un dispositif installé sur la station MéO du plateau de Calern ; la seconde utilise des observations de télémétrie laser à très haute cadence. Ces deux techniques permettent par exemple de déterminer la période de rotation du satellite Ajisai avec une précision et une cohérence inter-techniques de quelques micros-secondes. L’évolution temporelle de la période de rotation montre une croissance exponentielle attendue, mais aussi des petites fluctuations corrélées à la durée moyenne d’éclairement du satellite.

12:30 → 13:30 Buffet

13:30 → 14:30 Discussion : futur du GRAM et projet de réforme des instituts