GDR CoPhy Episode 4

1 juin 2026, 13:30 → 3 juin 2026, 14:30 Europe/Paris

Clermont Ferrand

The General Assembly of the GdR CoPhy brings together the French community working in cosmological physics. It provides an opportunity to review the activities of the GdR, discuss ongoing and future scientific programs, and exchange on strategic directions, community initiatives, and upcoming projects. The meeting will include scientific presentations, thematic discussions, and dedicated sessions for coordination and planning within the network.

lun. 1 juin

13:30 → 14:00 Registration and Welcoming Time

14:00 → 16:00 Monday afternoon: BLOCK 1

14:00 welcome

Orateur: Prof. Jean Orloff

14:15 Introduction to the GDR COPHY episode 4

Orateur: Johann COHEN-TANUGI (LPCA, Université Clermont-Auvergne & CNRS)

14:30 GDR COPHY Status report

Orateur: Josquin Errard

15:00 WG TUG

Orateur: Vivian POULIN (LUPM, CNRS & U. de Montpellier, France)

15:30 Cosmology with strong lensing images

Massive galaxies, acting as gravitational lenses, can form greatly distorted images of background galaxies in so called strong lensing events. When background galaxy and lens are almost aligned along the line of sight, the image take the shape of an Einstein ring whose shape encodes basic properties of the lens.
However, since lenses are not isolated objects in an otherwise perfectly homogeneous universe, these Einstein rings are slightly altered by further weak lensing along the line of sight coming from the large scale matter distribution in the universe in front and behind the lens. This coupling between strong and weak lensing offers a novel opportunity to probe cosmic shear, in a slightly different regime than standard weak lensing shear.
We will present this new observable and, in particular, its recent measurements in the SLACS sample of gravitational lenses. We will also highlight its potential for cosmological parameter constraints when combined with standard cosmological observables such as cosmic shear and number counts.

Orateur: Julien Larena (LUPM, Université de Montpellier)

16:00 → 16:30 Coffee Break

16:30 → 18:30 Monday afternoon: BLOCK 2

16:30 Rubin-LSST : status and plans

I will review where the Rubin project stands one year after the first on-sky images, describing the observatory performances and some highlights of the passed year.
I will also discuss the future plans, focusing particularly on the content of the Data Preview 2, a Science Validation dataset planned to be released this Summer.

Orateur: M. Thibault Guillemin (LAPP)

17:00 The cosmic matter dipole problem

The Ellis and Baldwin formula provides a model-independant way to measure our own velocity with respect to matter at large scale, by measuring the kinematic impact on the dipole in the number count of far away light sources. However, this measurement doesn't coincide with the same measurement of our velocity with respect to the CMB. This discrepancy puts into question the existence of the Cosmological Rest Frame [Secrest et al. 2021]. Here, I will present this cosmic matter dipole problem in more details, and explain how we plan to tackle this issue with future large scale surveys, in particular using LSST and Euclid.

Orateur: Albert Bonnefous-Abiven (Institut d'Astrophysique de Paris)

17:30 An inhomogeneous cosmological model: the axially symmetric Szekeres GR solution

Due to the current precision reached by the cosmological surveys, the standard $\Lambda$CDM model, despite its successes, has become an insufficient average to describe our actual Universe. Given the tensions arising between theory and observations, it appears that a more precise representation of the local universe is mandatory. Since General Relativity (GR) is the best theory currently available to describe gravitation, the new model should agree with its specifications. The Szekeres solution of GR possesses all the nice properties allowing it to represent an inhomogeneous small scale universe becoming homogeneous (FLRW) at some large transition scale. Moreover, a number of observations shows that the distribution of matter and its expansion are anisotropic in roughly the same direction. While most of the community has been reluctant up to now to use the general Szekeres model, due to its complexity, its axially symmetric version, more simple to deal with, might be more appealing. Therefore, taking advantage of both its compatibility with observations and its relative simplicity, we show how it can be used as a new standard cosmological model.

Orateur: Marie-Noëlle Celerier (Observatoire de Paris)

18:00 WG ADE

Orateur: Alain Blanchard

mar. 2 juin

09:00 → 10:30 Tuesday Morning: BLOCK 1

09:00 WG CMB

09:30 The Simons Observatory: Early Data from the Small Aperture Telescopes

The Simons Observatory (SO) is a cosmic microwave background (CMB) experiment located in the Atacama Desert in Chile, comprising multiple Small Aperture Telescopes (SATs) and a Large Aperture Telescope (LAT). The SATs are specifically designed to target primordial B-mode polarization sourced by tensor perturbations in the early Universe, conducting a focused small-area survey with high sensitivity. The SO mid-frequency SATs saw first light in late 2023 and have since been conducting Initial Science Observations, a dedicated campaign of measurements obtained following commissioning that will form the basis of a series of forthcoming papers, spanning telescope performance characterization, map-making, and cosmological parameter estimation. In this talk, I will present an overview of the current status of these observations and early data from the SATs, along with the planned next steps toward SO's key science goals.

Orateur: Amalia VILLARRUBIA AGUILAR (APC CNRS / UPC)

10:00 KAIROS : Proposal for a French SAT observing at high frequencies

The proposal to build a high-frequency Small Aperture Telescope (SAT) as a new component of the Simons Observatory has gained increasing momentum over the recent years. This initiative, named KAIROS, is under evaluation for funding through the CNRS RI² program (“Recherche à risque et à impact”), with the backing of three CNRS institutes: IN2P3, INSU, and INP.
KAIROS aims to deploy a focal plane of approximately 30,000 LEKID detectors, covering two polarization-sensitive frequency bands above 200GHz. The French collaboration will take responsibility for the full development of all instrumental subsystems.
By extending the Simons Observatory’s frequency coverage into the high-frequency regime, KAIROS will significantly enhance our ability to map the polarized emission from interstellar dust, one of the main foregrounds obscuring the primordial CMB B-mode signal. This improved characterization of galactic dust contamination is essential for pushing the limits of inflationary cosmology.
We will present an overview of the proposed instrument concept and its anticipated scientific impact within the broader Simons Observatory framework.

Orateur: Benjamin BERINGUE (APC - CNRS)

10:30 → 11:00 Coffee Break

11:00 → 12:30 Tuesday Morning: BLOCK 2

11:00 Testing Entropic Explanations of Cosmic Acceleration with DESI and Supernovae

The accelerated expansion of the Universe is usually described by a cosmological constant, $\Lambda$. An interesting alternative idea is entropic cosmology, where the entropy associated with the cosmic horizon effectively contributes to (or mimics) dark energy. In this talk, I will compare two commonly discussed routes: (i) holographic dark energy (HDE), where the dark-energy density is tied to a horizon length scale, and (ii) gravity-thermodynamics (GT) models, where the expansion equations are modified by applying the first law of thermodynamics at the horizon.

Using late-time observations of Type Ia supernovae (Pantheon+ and DESy5) together with baryon acoustic oscillation measurements from DESI, we carry out Bayesian parameter estimation and model comparison for generalized entropy deformations motivated by non-standard (Barrow/Tsallis-type) horizon entropies. We find that HDE provides an excellent fit and is statistically competitive with $\Lambda$CDM across data combinations, with an effective equation of state consistent with a mild, quintessence-like evolution.

In contrast, the GT implementations are strongly disfavoured by Bayesian evidence for essentially all data combinations. We then extend the GT framework to broader three- and four-parameter generalized entropy families (which include several popular non-extensive entropies as special limits) and update the analysis with the latest DESI release. The data prefer the standard Bekenstein-Hawking area law, yielding a late-time expansion that is close to $\Lambda$CDM. We also find that the three-parameter entropy description is sufficient and is preferred over the four-parameter extension.

Orateur: Soumen Basak (IISER Thiruvananthapuram, Kerala, India)

11:30 The Secret Life of Voids: Unifying Density Profiles and their Evolution

Cosmic voids occupy the majority of the Universe’s volume and are dominated by dark energy at earlier times, making them one of the most promising probes for current and upcoming surveys. However, fully exploiting this potential requires a robust understanding of their evolution and modeling. We track the history of halo-defined voids from z = 5 to the present day using hydrodynamical simulations across multiple scales. We demonstrate that relying on traditional metrics, such as absolute size, can be misleading and may obscure the true nature of voids. Instead, our analysis reveals a fundamental property driving their evolution, leading to a surprisingly simple description of their late-time behavior. We also introduce a phenomenological model that accurately predicts a universal density profile based entirely on measurable parameters, effectively bridging the gap between theory and observation. This talk establishes the framework necessary to transform voids into pristine cosmological laboratories for the next generation of surveys.

Orateur: Nico SCHUSTER (CPPM)

12:00 The shape of emptiness: improving the signal from cosmic voids using reconstruction

Cosmic voids, the vast, underdense regions of the Universe, are emerging as powerful cosmological laboratories. They carry unique imprints of the growth of structure and the underlying physics driving cosmic acceleration. Among the most promising observables in this context is the void-galaxy cross-correlation function, which encodes both the geometry and dynamics of the cosmic web. In this talk, I will explore how the shape of voids can serve as a sensitive probe of cosmology, offering complementary constraints to traditional large-scale structure measurements. I will present recent developments demonstrating how void shapes respond to different cosmological models, and how systematic effects, particularly those related to redshift-space distortions, can be mitigated through advanced velocity reconstruction techniques.

These results highlight the growing potential of void-based analyses in the era of next-generation spectroscopic surveys, such as Euclid, DESI and Roman, where they will play a crucial role in refining our understanding of gravity, dark energy, and the large-scale structure of the Universe.

Orateur: Giulia DEGNI (CPPM - AMU)

14:00 → 16:00 Tuesday Afternoon: BLOCK 1

14:00 Simons Observatory LAT status

The Simons Observatory is a new CMB experiment located in the Atacama desert, in Chile. It is composed of 6 Small Aperture Telescopes (SATs) and a Large Aperture Telescope (LAT). Some of these telescopes, including the LAT, have already seen first light.

The LAT aims to make arcminute-resolution maps of 50% of the sky in a wide frequency range from 30 to 280 GHz, with an unmatched depth for this sky fraction. Its main scientific goals are primary CMB (temperature and polarization), CMB lensing, and the Sunyaev-Zel'dovich effect, among many others.

In this talk, we will present a status of the LAT, describing the current state of the experiment and planned next steps. We will also show some results of the Initial Science Observations, demonstrating the current performance of the instrument.

Orateur: Merry Duparc (ijclab)

14:30 SPT-3G: latest results and future prospects

SPT-3G is the latest receiver installed on the South Pole Telescope (SPT), a dedicated cosmic microwave background (CMB) telescope at the Amundsen-Scott station in Antarctica. The 16,000 detectors and 10-meter primary mirror allow us to produce low-noise and high-resolution maps of the millimeter-wave sky in temperature and polarisation. These data are rich in cosmological information and power an array of science cases, such as primary CMB anisotropies, secondary CMB anisotropies (including lensing), cluster cosmology, and astrophysical signals. In this talk, I will present the latest cosmology results from the analysis of CMB temperature, polarisation, and lensing power spectra derived from two years of SPT-3G observations of the main survey field covering 4% of the sky. I will give an overview of work in progress, including the analysis of additional observation fields that extend the survey area to 25% of the sky, and discuss the future of the telescope.

Orateur: Lennart Balkenhol

15:00 How to measure cosmology from nothing: void-galaxy cross-correlation with DESI

Cosmic voids provide a novel probe of structure formation and cosmic expansion. They are sensitive to structure growth, dark energy, modified gravity, and sum of neutrino masses. Using the DESI spectroscopic data, we perform void identification and present the preliminary measurements of the void-galaxy cross-correlation function. By measuring the void-galaxy cross-correlation function we can extract constraints on the content of the Universe and the growth rate of structures through the Alcock Paczynski test and by modeling redshift space distortions. The work I will present highlights the potential of cosmic voids for testing cosmological models with DESI.

Orateur: Katayoon GHAEMI (Centre de Physique des Particules de Marseille (CPPM))

15:30 Cosmology from the Rubin Void Size Function

Cosmic voids are a powerful tool to extract cosmological constraints and study galaxy properties’ dependence on the environment. This project builds a pipeline for constraining cosmological parameters using the void size function. We will build the void catalogs from the Roman SkySim5000 galaxy catalogs using the publicly available VIDE void finder. The cosmological constraints’ inference will rely on the state of the art theoretical model for the void size function, which accounts for the impact of tracer bias, redshift-space distortions and the Alcock-Paczynski effect. This project will be the first to confront theoretical models for the void size function to measurements made from voids obtained from photometric redshift tracer data. As such, it will investigate the impact of statistical and systematic photometric redshift uncertainties on void statistics. Extracted constraints will include (but will not be limited to) constraining the dark energy equation of state, sigma 8, the dark matter density, the content of the Universe and the sum of neutrino masses.

Orateur: Pierre BOCCARD (CPPM)

16:00 → 16:30 Coffee Break

16:30 → 18:30 Tuesday Afternoon: BLOCK 2

16:30 Future missions for CMB spectral distortion measurements

The only precise measurement of the cosmic microwave background (CMB) spectrum to date was achieved by COBE-FIRAS in the early 1990s, showing that it is close to a perfect blackbody emission. However, both standard and non-standard physical processes inevitably generate small deviations from this spectrum, known as CMB spectral distortions. They provide a powerful and unexplored probe of the thermal history of the Universe, offering unique insights into early Universe, particle physics and structure formation.

After a brief review of the scientific motivations for CMB spectral distortion measurements, I will present current experimental efforts aimed at their detection. In particular, I will provide the latest updates on the balloon-borne pathfinder BISOU, led by CNES, and on the ESA M8 proposal for spectral distortions, FOSSIL.

Orateur: Xavier Coulon (IAS-CNRS)

17:00 WG TOOLS

Orateur: Guilhem LAVAUX (IAP, CNRS INSU)

17:30 No supernovae? Measuring H₀ from Cepheids, TRGB stars, and masers with BORG peculiar velocities

The Hubble tension demands fresh, independent routes to H₀. I present two-rung and one-rung distance ladders that bypass Type Ia supernovae entirely, connecting Cepheid and TRGB distances, as well as geometric maser distances, directly to cosmological redshifts. The first two-rung ladder uses the SH0ES Cepheid sample with Milky Way, LMC, and NGC 4258 anchors, yielding H₀ = 71.7 ± 1.3 km/s/Mpc at 1.8 per cent precision, though the result depends on assumptions about host-sample selection (arXiv:2509.09665). The second two-rung ladder replaces Cepheids with TRGB distances from the Extragalactic Distance Database (Anand et al. 2021), comprising over 500 HST-based measurements within ~10 Mpc. The one-rung ladder uses six maser-hosting galaxies whose geometric distances connect directly to H₀, free of any stellar physics. All ladders share a common inference backbone: a rigorous Bayesian forward model that self-consistently treats sample selection and peculiar velocities, the latter drawn from the Manticore-Local reconstruction based on the BORG algorithm. Together, these SNe Ia-free ladders provide independent, complementary constraints on the origin of the Hubble tension.

Orateur: Richard Stiskalek (Oxford University)

18:00 Simulation-Based Inference (SBI) for cosmology with type Ia Supernovae (SNe Ia).

Systematic uncertainties associated to calibration, selection functions and astrophysical effects are dominating the error budget of SNe Ia cosmology. Correction methods applied to account for these systematics, and especially for the complex combination of selection functions and astrophysical variability, are questionable, particularly given the current H0 and Λ tensions for which SN Ia data are central.

Recently, the Zwicky Transient Facility (ZTF) survey has produced a volume-limited sample of more than a thousand SNe Ia, allowing to directly probe the distribution of SNe Ia parameters without being affected by selection effects. However, extending the cosmological analysis to higher redshifts, leveraging the full ZTF DR2 dataset, requires a robust treatment of selection effects.

Using datasets realistically produced using skysurvey, we train a Neural Network (NN) to infer the simulation input parameters. This novel inference method, called SBI, is a promising avenue to solve the complex problem of cosmological inference with SNe Ia data, and thus accurately derive H0, w0 and wa.

Orateur: Adam TRIGUI (IP2I)

mer. 3 juin

09:00 → 10:30 Wednesday Morning: BLOCK 1

09:00 Multiplexed Survey Telescope (MUST)

The MUST telescope is being constructed in Lenghu, Qinghai, China. It will be equipped with a 6.5 mirror, a 7deg2 FoV and 20,000 fibre positioners. Observations are expected to start in 2030, and collect 120 millions redshift in 0.1 < z < 5.5 in 8.5 years. This project is led by the Tsinghua University in collaboration with EPFL. An international collaboration policy defines the modalities to join the project. Anybody providing technical or scientific contributions is eligible to data access. In this presentation, I will present the project and the activities being developed in the France.

Orateur: Eric JULLO (Aix-Marseille Université/LAM)

09:30 Bayesian Inference with Differentiable Simulators for the Joint Analysis of Galaxy Clustering and CMB Lensing

We present our work towards a field-level inference (FLI) pipeline for the joint analysis of DESI galaxy clustering and CMB lensing from Planck and ACT. Using synthetic data, we demonstrate an end-to-end Bayesian framework that jointly samples the posterior of the initial density field, galaxy bias, and cosmological parameters (Ωₘ, σ₈).
Our differentiable forward model propagates Gaussian initial conditions through Lagrangian Perturbation Theory (2LPT), applies a Lagrangian bias expansion, and computes CMB lensing convergence along the LOS via the Born approximation. The joint likelihood combines galaxy number density with the convergence map observations, incorporating Planck PR4 reconstruction noise and analytically marginalizing over the unmodeled high-z lensing contribution beyond the simulation volume.
We demonstrate efficient exploration of the high-dimensional posterior using the MCLMC sampler. Preliminary results suggest that the joint analysis tightens parameter constraints. This work is a step toward applying FLI to real data to constrain large-scale physics, such as (local) primordial non-Gaussianity.

Orateur: Jonathan Hawla (CEA Paris-Saclay)

10:00 Multi-probe inferences of late-time cosmic dynamics from Stage-III to Stage-V surveys

Dark matter and dark energy dominate the late-time Universe, but their physical nature remains unknown. I will present results on using the clustering of cosmological tracers (supernovae, galaxies, galaxy clusters) to constrain late-time cosmic dynamics from Stage-III to Stage-V surveys within a Bayesian forward-modelling framework. I will summarise key results from existing data sets, including the most precise determination of the Hubble constant from Cepheids to date, recent investigations for the Euclid mission, and discuss ongoing work for the conceptual planning of the Stage-V Wide Field Spectroscopic Telescope. Finally, I will outline several promising directions that emerge from these results, aimed at pushing the constraining power of these probes to their practical limits with upcoming data sets.

Orateur: Eleni Tsaprazi (Imperial College London)

10:30 → 11:00 Coffee Break

11:00 → 12:30 Wednesday Morning: BLOCK 2

11:00 Overview of DESI and its recent results

The Dark Energy Spectroscopic Instrument (DESI) published its DR2 Baryon Acoustic Oscillation (BAO) measurement in March 2025, based on a spectroscopic sample of over 14 million galaxies and quasars. After an overview of the DESI survey itself, I will present the DR2 BAO measurement and its cosmological implications, with a particular focus on neutrino mass constraints and on the nature of dark energy.
Indeed, the results are well described by the ΛCDM framework, and combined to cosmic microwave background measurements (e.g., from Planck), they yield the most stringent cosmological neutrino mass contraint to date. However, a mild tension arises when combining with CMB measurements, which can be alleviated by the introduction of dynamical dark energy. This may be a hint of departure from our standard ΛCDM cosmological model.

Orateur: Domitille Chebat (CEA Saclay - IRFU - DPhP)

11:30 First measurement of the Hubble constant from gravitational wave-galaxy cross-correlations

We measure for the first time the Hubble constant (H0) from the cross-correlation of galaxies and gravitational waves (GW), by applying the Peak Sirens method. This method consists of finding the peak of the 3D angular cross-spectrum $C_\ell(z,D_L)$ between the galaxy redshifts ($z$) and the GW luminosity distances ($D_L$). Using two GW events from the GWTC-3.0 catalog and the GLADE+ galaxy catalog, we make the first detection of the cross-correlation peak at $5.9\sigma$ confidence. This signal comes mostly from the best localized event in the catalog, GW190814, which alone provides a $3.4\sigma$ significance. Adding also the multimessenger event GW170817, but without using its known redshift, we find $H_0=67^{+18}_{−15}$ km s$^{−1}$Mpc$^{−1}$ and the first observational constraint on the GW bias, $b_{gw}<4.3$ at 95% CI. These measurements set the stage for future novel cosmological constraints with this technique.

Orateur: Charles Dalang (ENS Paris)