Colloque national CMB-France #8

24 nov. 2026, 14:00 → 26 nov. 2026, 12:00 Europe/Paris

Institut Astrophysique de Paris

L'initiative CMB-France vise à mettre en place une animation scientifique au niveau national autour des études CMB (cmb-france.cnrs.fr).

Le CMB-France #8 aura lieu du 24 au 26 Novembre 2026 à l'Institut d'Astrophysique de Paris (Paris).

 

L'inscription est gratuite mais obligatoire: inscription.

 

CMB-France peut prendre en charge les missions (dans la limite des stocks disponibles).
Contactez-nous.

 

L’idée de ces colloques est de:

• présenter les nouveautés scientifiques et les nouveaux résultats dans le domaine
• donner un statut sur l’avancement des expériences ou des projets en cours
• permettre aux jeunes (post-doc, PhD) de présenter leur travail

 

L'agenda sera mis à jour en fonction de vos contributions.

N'hésitez pas à nous contacter pour proposer des choses (discussions, invités, ...).

 

Un diner est prévu mercredi 25 au soir, chez Félicie (174, avenue du Maine 75014 Paris)

 

 

 

 Cette action est soutenue par le PNCG.

mar. 24 novembre

14:00 → 15:30 Sessions 1

14:00 From Cryogenic Detector Readout Chains to a CMB Observatory - Potential Contributions

Cryogenic detection and readout chains for future CMB observatories are far from off-the-shelf products. Their development requires years of coordinated work on detectors, superconducting devices, readout electronics, cryogenic integration and characterisation. These developments must remain closely aligned with the scientific objectives of each experiment, as detector performance, instrument architecture and scientific requirements are tightly coupled.

This contribution will present instrumental developments and expertise at APC and partners that could support future CMB experiments, drawing on activities related to X-IFU, QUBIC, CMB-S4 and CMB-Flex. Potential contributions include TES and KID detector design (including on-chip coupling/filtering), integration/characterization and dedicated low-noise analog readout electronics. These activities provide complementary building blocks that could be adapted to the needs of future instruments, in collaboration with other institutes and industrial partners

The presentation will outline these potential contributions and discuss how they could fit within the broader landscape of French and European capabilities, with the aim of identifying opportunities for collaboration and contributing to the development of future CMB instrumentation/space missions.

Orateur: Damien PRELE (APC)

14:20 Development of flexible superconducting ribbon cables for CMB observation instrumentation

We initiated the development of flexible superconducting ribbon cables, originally intended for the CMB-S4 observatory, to interconnect detectors with the first stage of the electronics. These cables needed to be produced in very large quantities while meeting specific performance requirements. To mitigate risks, we carried out this development in partnership with three different industrial or laboratory entities. The expected performance levels have already been achieved with one of them. This type of cable is of interest for other applications in the field of CMB observation, or more generally low temperature detection.

Orateur: Xavier de la Broïse (CEA/DRF/IRFU)

14:40 CreSZendo: From Clusters to REionization via the SZ Effect with Northern high-resolution Deep Observations

The CreSZendo proposes conducting the largest and deepest high-resolution (10” at 260 GHz) millimetre survey of the sky, focusing on measuring the Sunyaev-Zeldovich (SZ) effect. Thanks to our team's state-of-the-art Kinetic Inductance Detector (KID) technology, we will construct and install a four-band millimetre wavelength camera at the IRAM 30 m telescope. This camera will have an unprecedented mapping speed, being up to twenty times faster than current instruments.
During the epoch of reionisation (EoR), the bulk motion of electrons imprints on the CMB via the kinetic SZ effect. This provides a means of observing patchy structures induced by the formation of bubbles around the first ionising objects. CreSZendo will be the first to map these structures down to sub-arcminute scales, enabling us to directly probe the mean redshift and duration of the EoR epoch.
Later in the history of the universe, clusters of galaxies form at the intersection of filamentary structures. Hot gas in these clusters and filaments interacts with CMB photons, leading to the thermal (tSZ) effect. CreSZendo will detect thousands of clusters and groups of galaxies, exploring the uncharted territory of low mass and very high redshift, where out-of-equilibrium processes are expected to dominate. Additionally, CreSZendo will map filamentary structures in the cosmic web to directly measure their baryon content and reveal missing baryons by cross-correlating with visible and infrared probes from the Euclid satellite.
CreSZendo will generate a wealth of legacy data, including a robust census of extreme dusty starbursts during the epoch of massive galaxy assembly. This will provide insights in dust obscured growth of structures.

Orateur: Juan-Francisco Macias-Perez (LPSC)

15:00 SNO-CMB

Discussion sur la création d'un Service National d'Observation autour des activités CMB.

Orateurs: Guilaine Lagache (LAM), Marian Douspis (IAS)

15:30 → 16:00 Coffee

16:00 → 18:10 Sessions 2

16:00 The Simons Observatory: Commissioning of the Mid-Frequency Small Aperture Telescopes

16:30 The Simons Observatory Large Aperture Telescope: Cosmology with high resolution data of the Cosmic Microwave Background and preliminary Science Verification results

The Simons Observatory (SO) Large Aperture Telescope (LAT) achieved first light in February 2025 and is expected to reach unprecedented depth at high resolutions over a large area of the millimeter sky for the duration of the survey, making it one of the leading Cosmic Microwave Background (CMB) experiments of the next decade. In this talk, I will provide an overview of the LAT science goals, which include understanding the nature of dark energy, constraining primordial non-gaussianity to probe inflation models, and probing dark matter candidates. I will also present preliminary science verification results from the initial period of LAT observations, gathered over the months of September and October 2025. Analysis of these observations aims to validate the instrument’s sensitivity, data quality, and the readiness of the analysis pipelines. I will highlight preliminary CMB lensing measurements, null test results performed with the SO lensing reconstruction pipeline, and cross-correlations between lensing and external tracers, which ensure that the LAT is ready for science observations and poised to achieve its science goals.

Orateur: Karen Perez Sarmiento (Institut d'Astrophysique Spatiale)

16:50 The Simons Observatory: A framework for efficient map-based simulations to mitigate instrumental systematics for CMB surveys

The current and future observations of the CMB are expected to reach unprecedented sensitivity to place stringent constraints on both cosmological parameters and fundamental physics. The ground-based Simons Observatory Large Aperture Telescope started observations last year, and will provide the most sensitive small-scale CMB measurements. A tight control over instrumental systematics is critical, through the assessment of their impact as well as their mitigation.
In this talk, we propose a new generalized framework to efficiently simulate systematics at the map level, along with the Python package smarties optimized to handle high resolution simulations. As a first application, we demonstrate that our results are in agreement with significantly more numerically expensive end-to-end simulations for a set of beam-related systematics. The ability to efficiently produce large ensembles of such simulations will allow for more robust cosmological parameter estimation from the power spectra of the CMB and gravitational lensing potential.

Orateur: Magdy Morshed (INFN Ferrara)

17:10 The Simons Observatory: Initial Science Observations with the Small Aperture Telescopes

In this talk, we tentatively present the first results from the initial science observations, including polarisation maps and power spectra. We describe the analysis pipelines, from data preprocessing to the component separation strategy used to handle Galactic foregrounds, and discuss the main instrumental systematics and their mitigation.

Orateur: Ema Tsang

17:30 Latest news from SPT-3G and SPO

The 10m South Pole Telescope (SPT) maps the CMB sky with arc-minute resolution. With its high sensitivity observations at 95, 150 and 220GHz its third generation camera (SPT-3G) aims at providing unprecedented constraints on the cosmological parameters, hopefully soon to be based on the analyses of 10% of the CMB sky.
The South Pole Observatory combines the SPT observations with those of the BICEP array, focused on the detection of polarisation B modes at large scales, a signature of the gravitational waves generated by an early phase of inflation.
This talk will provide a status report of these projects.

Orateur: Silvia Galli

17:50 Extracting cosmology and astrophysics from the cross-correlation between the thermal Sunyaev-Zeldovich signal and galaxy clustering

The cosmic microwave background (CMB) traces the matter distribution through the imprints left by its interactions with large-scale structure. Together, the CMB and large-scale structure probes provide complementary insights into the matter distribution at different epochs of the Universe. Their distinct theoretical modeling and observational systematics make cross-correlations between them a particularly powerful probe.

I will focus on results about the cross-correlations of the thermal Sunyaev–Zel’dovich (tSZ) effect and the galaxy clustering signal in the context of Euclid. The tSZ signal, originating from the CMB with Compton-y maps from Planck and ACT, is sensitive to the integrated thermal pressure of hot gas in the Universe. The galaxy clustering signal traces the galaxy distribution from collapsed matter.

Orateur: Léa Dumilly (Institut d'Astrophysique Spatiale)

mer. 25 novembre

09:00 → 10:00 Sessions 3

09:00 Atmosphere mitigation in CMB observations

I will present atmocube, a new package for fast and accurate simulation of atmospheric emission and absorption impacting CMB observations from ground-based observatories, and the potential use of co-pointing observations with a multi-channel lidar to characterize and potentially disentangle atmospheric contamination in CMB data streams with dedicated multi-instrument data-analysis techniques.

Orateur: Jacques Delabrouille (IN2P3)

09:20 Time-Domain Component Separation for Atmospheric Emission Removal in CMB Observations

Ground-based observations of the cosmic microwave background (CMB) are severely challenged by atmospheric emission, which outshines the faint astrophysical and CMB signals. In particular, fluctuations in atmospheric water vapour introduce non-stationary, spatially complex correlations in the time-ordered data (TOD), coupling neighbouring detectors and contaminating large angular scale modes in temperature. The presence of instruments temperature-to-polarisation leakage can further pollute polarisation signal. Traditional atmospheric mitigation techniques, such as high-pass filtering in the frequency domain, risk discarding valuable CMB information, especially at low multipoles where the primordial B-mode polarisation signal is expected to peak.

We present a novel time-domain component separation approach that exploits the distinct temporal and spectral signatures of atmospheric emission. By incorporating physical atmospheric parameters such as wind speed and precipitable water vapour (PWV), our method improves atmospheric decontamination while preserving the underlying CMB signal. We validate the approach using simulations that include both a 2D atmospheric model and a full 3D atmospheric cube with realistic temporal evolution, demonstrating accurate reconstruction of sky maps and recovery of large-scale modes. In addition, we show how the atmospheric signal can be leveraged to better characterise the instrumental response and to model the correlated component of the noise covariance matrix, enabling its whitening for subsequent analysis stages.

Orateur: Dr Benjamin Beringue (APC, CNRS)

09:40 Atmosphere mitigation in CMB observations using multi-frequency time-domain component separation

CMB observations from ground-based observatories are limited in sensitivity by the fluctuating emission from the Earth's atmosphere, mostly due to water vapor inhomogeneities. Standard mitigation techniques either remove a significant fraction of the cosmological signal (low-frequency filtering) or leave residuals from instrumental imperfections and polarized atmospheric emission (pair-differencing, half-wave plate modulation). We present a new data analysis framework in which a multi-frequency focal plane includes detectors dedicated to atmosphere monitoring. Time-domain component separation applied to these monitors yields atmospheric templates, which are then used to subtract the atmosphere from the science detectors. We introduce an atmospheric decontamination factor, $A^\mathrm{atm}_\ell$, to quantify the reduction of atmospheric power in the maps. In simulations, this pipeline leaves ~4000 times less atmospheric power than a standard filtering and binning pipeline over $30 \leq \ell \leq 300$.

Orateur: Julien Tang (Université Paris Cité)

10:00 → 10:30 Coffee

10:30 → 12:00 Sessions 4

10:30 DESI: DR2 Results and Perspectives

The Dark Energy Spectroscopic Instrument (DESI) is building the largest 3D map of our universe to measure its expansion history over the past 11 billion years, and thereby, study dark energy. Over a five-year period (Run 1a), DESI has spectroscopically classified nearly 40 million galaxies and quasars over 1/3 of the sky and to redshifts z < 3.5.

The DESI collaboration has completed measurements of the baryon acoustic oscillation (BAO) feature and more generally, of large-scale structure, using data from the first three years of observation (DR2). In this seminar, I will present those measurements and their implications for our understanding of the cosmological model, I will discuss the tensions with LambdaCDM and the question of the nature of dark energy.

I will also present new results from DR2 on the Lyman-alpha forest of quasars. First, a full-shape analysis of the Lyman-alpha forest power spectrum, providing a measurement of the Alcock-Paczyński effect at high redshift (2 < z < 3). Second, a new multi-redshift BAO analysis of the Lyman-alpha forest splitting the data into three redshift bins, probing the matter-dominated expansion at the highest redshifts accessible to DESI.

Finally, I will give a brief outlook on the future of the DESI program. Run 1b, which follows Run 1, extends the survey with an increased tracer density and an enlarged footprint towards the southern sky. Looking further ahead, DESI Run 2 is scheduled to begin in 2029 and will span seven years, targeting the primordial universe at z > 2 and opening a new window on the early-time expansion history and primordial physics.

Orateur: Christophe Yeche (CEA Irfu/SPP)

11:00 Euclid meets CMB lensing: first cosmological constraints from the Euclid Quick Data Release

The Euclid Quick Data Release (Q1) offers the first opportunity to test the cosmological potential of Euclid on real data. In this talk I will present the first cosmological analysis performed on Euclid data, based on the tomographic clustering of photometric galaxies in the three Euclid Deep Fields and their cross-correlation with CMB lensing from Planck and ACT. Despite covering only about 60 square degrees, the data already deliver a combined detection significance above 30σ. I will show how cross-correlating with CMB lensing provides a powerful handle on the systematics that affect early data, from imaging artefacts to photometric redshift errors, and how it helps break the degeneracy between galaxy bias and the growth of structure. The resulting constraints on the matter density, the amplitude of fluctuations and the growth factor are consistent with current state-of-the-art results, and reach comparable precision with a sky area hundreds of times smaller. These results provide a first glimpse of what Euclid, combined with CMB lensing, will achieve with its first major data release.

Orateur: Dr Margherita Lembo (Institut d'Astrophysique de Paris (IAP))

11:20 Generalised map-based component separation method to probe isotropic cosmic birefringence and its application to cross-experiments calibration

In this presentation, I will introduce a generalized map-based component-separation framework that accounts for instrumental polarization angle miscalibrations and propagates calibration prior information into cosmological parameter inference.
This framework is being developed as an extension of the Megatop pipeline, a code designed to probe primordial B modes in the context of the Simons Observatory. It enables the joint inference of the tensor-to-scalar ratio (r), the amplitude of lensing-induced B modes, and the isotropic cosmic birefringence angle.
Finally, I will present forecasts for cross-experiment calibration and constraints on isotropic cosmic birefringence using Simons Observatory alone and in combination with LiteBIRD.

Orateur: Malcolm Landais (Laboratoire Astroparticule & Cosmology)

11:40 Look-elsewhere effect in searches for new physics with the CMB

ΛCDM fits CMB data remarkably well, and new physics is usually searched for by adding one parameter at a time: curvature, N_eff, dark-energy equation of state, varying constants, and others. Several have shown 2 to 3σ hints in Planck data. But when many extensions are tested on the same data, some such deviations are expected from noise alone: this is the look-elsewhere effect. Because the extensions are fitted to the same spectra, their significances are correlated, and the effect cannot be estimated by simply counting tests.

Based on a recent work, I will present an analytical framework that can be used to estimate the probability that a hint arises by chance among a set of extensions, accounting for correlations. For Planck, a 3σ local deviation corresponds to about 2.4σ when correcting for the look-elsewhere effect. Small-scale data, such as from the Simons Observatory LAT, break degeneracies and increase this penalty.

Orateur: Merry DUPARC (IJCLab)

12:00 → 14:00 Lunch

14:00 → 16:00 Sessions 5

14:00 LiteBIRD Status

I will present the current status of the LiteBIRD JAXA mission, after it recently passed the Mission Definition Review at JAXA and it entered into project phase. A new exciting phase has just started, with a new instrumental design and enhanced science goals I will describe in more details.

Orateur: Ludovic Montier (IRAP)

14:30 FOSSIL

15:00 CMB-probe

The 2020 decadal survey in Astronomy and Astrophysics in the US (released in 2021) proposed a program of ground and space-based activities and recommended critical actions to carry-out its identified most compelling science goals. It stated that to address the major science questions identified by the Panel on Cosmology, the cosmic microwave background remains the single most important phenomenon that can be observed. It endorsed a ground-based CMB project, CMB-S4, and mentioned a CMB Probe as a possible candidate for the second Probe solicitation in the 2030s.
Since Astro2020, the CMB landscape has changed significantly. In 2025, CMB-S4 was cancelled, leaving a critical gap after the completion of Stage-3 ground-based experiments. Progress has been made in understanding the impact of Galactic foregrounds on inflationary science constraints. Technologies and launch capabilities relevant for a CMB Probe have progressed. The international landscape vis-a-vis a CMB space mission has evolved. All of these developments compel a critical re-evaluation of the role of a CMB Probe, both for a possible mid-decade review and in advance of upcoming Decadal Survey planning activities.
In this context, a CMB Science Analysis Group has been set-up to review the scientific, technological, and programmatic landscape. In this talk, I will review ongoing activities and future steps in preparation for a potential Probe-Class CMB space mission in the US, to be selected in the 2030s for a launch in the 2040s.

Orateur: Jacques Delabrouille

15:20 COSMOCal project: a calibrator for CMB experiments

Precise measurements of the Cosmic Microwave Background (CMB) polarization anisotropies are essential for probing fundamental physics, particularly through the search for primordial B-modes generated by inflationary gravitational waves and the investigation of cosmic birefringence, a potential signature of physics beyond the Standard Model. Achieving the required accuracy demands unprecedented control of instrumental systematic uncertainties, especially those related to the absolute polarization angle calibration and the characterization of telescope beams. Inaccuracies in these quantities can induce spurious polarization signals, contaminate E- and B-mode measurements, and mimic or obscure genuine cosmological effects.

The COSMOCal project aims to address these challenges by deploying an artificial calibration source in space, providing an independent, stable, and well-characterized reference for the absolute polarization angle calibration and beam characterization of ground-based CMB telescopes. This approach will help disentangle instrumental systematic effects from genuine cosmological signals, improving the robustness of future CMB measurements.

This contribution will present the latest technological developments and programmatic progress of the COSMOCal project, highlighting the key milestones towards its implementation.

Orateur: Alessia RITACCO (CNRS-LPSC)

15:40 Impact of Galactic dust complexity on calibration of future CMB space missions

The search for primordial gravitational waves imprinted in the polarization of the Cosmic Microwave Background (CMB) is a major scientific goal of next-generation CMB experiments. Achieving the sensitivity required to detect primordial B-modes demands precise control of instrumental systematics and a robust separation of the cosmological signal from astrophysical foregrounds. In particular, accurate polarization-angle calibration is essential, as even small miscalibrations can bias the reconstruction of the tensor-to-scalar ratio, r. Moreover, Galactic dust complexity and intrinsic EB correlations can mimic instrumental polarization-angle miscalibration. In this contribution, we investigate how 3D dust complexity affects the recovery of r in space-borne observations. We also discuss how an absolute polarization-angle calibration source can help break the degeneracy between astrophysical and instrumental effects, enabling cross-calibration between CMB experiments and improving the reliability of future B-mode measurements.

Orateur: Silvia Micheli

16:00 → 16:30 Coffee

16:30 → 18:10 Sessions 6

16:30 BISOU: a balloon mission for CMB spectral distortions

BISOU (Balloon Interferometer for Spectral Observations of the primordial Universe) is a CNES balloon-borne spectrometer designed to measure CMB spectral distortions. Its primary goal is the first detection of the CMB y-distortion monopole signal arising from the gravitation heating of gas during large-scale structure formation. BISOU should also measure the CIB and extragalactic CO monopoles, helping us to better understand the galaxie formation at cosmic noon. I will present the BISOU mission, its scientific objectives and instrument concept, together with the current status of the CNES Phase A study. In addition, I will discuss the preparation strategy that is put in place around the development of a laboratory breadboard.

Orateur: Xavier COULON (IAS)

16:50 Systematics in CMB spectral distortions spectrometers: From Instrument Model to Laboratory Breadboard

CMB (Cosmic Microwave Background) spectral distortions can probe the thermal and energetic history of the early Universe. However, the CMB spectrum has not been remeasured since COBE/FIRAS, which constrained deviations from a blackbody to ΔI/I ≲ 10⁻⁵. Detecting these faint signals requires anticipating, from the earliest design stages, how instrumental systematics will limit the measurement.

We present our study of systematic effects for FOSSIL (FTS fOr CMB Spectral diStortIon expLoration) and BISOU (Balloon Interferometer for Spectral Observation of the Universe), a Fourier-transform spectrometer (FTS) designed to probe spectral distortions. First, the instrument was modeled with the TICRA GRASP software, which allows us to model the optical aberrations and the systematics introduced by the moving mirror of the FTS. Then, we present our instrument simulator, which will be able to forecast the error induced on the science measurement by optical systematics. To validate these predictions, a laboratory breadboard of the instrument is being built at the Institut d'Astrophysique Spatiale (IAS), which will allow us to characterise these systematics experimentally and compare them with the simulator, a key step towards the validation of the model ahead of the mission.

Orateur: Morgane LOQUET LE GALL (IAS)

17:10 A reference more perfect than the CMB: the thermal challenge of FOSSIL

The cosmic microwave background (CMB) spectrum measured by COBE-FIRAS is the most perfect blackbody observed so far, with upper limits on spectral distortions of |y| < 1.5×10⁻⁵ and |μ| < 9×10⁻⁵. Smaller distortions, left by energy injection at different epochs of cosmic history, are nevertheless expected below these limits. FOSSIL, a CMB spectral distortion mission proposed to ESA M8, aims to detect them with a differential Fourier Transform Spectrometer that compares the sky to an internal reference blackbody, the BBIR (BlackBody Internal Reference). This measurement principle turns the science goals directly into thermal requirements. The BBIR must be even closer to a perfect Planck spectrum than the CMB itself.
Following a brief description of the measurement concept, I will present the thermo-mechanical architecture of FOSSIL, from passive radiative cooling with V-grooves down to the 4.5 K instrument stage and the sub-kelvin cooling of the detectors. I will then present the BBIR design and the proposed solution to meet the thermal stability and photometric requirements driven by the science objectives. BBIR systematic effects can mimic a distortion and must remain well below the part-per-million level. Finally, I will present how thermal and spectral modelling is used to optimise the BBIR geometry, materials and thermal environment against these requirements.

Orateur: Dr Valentin SAUVAGE (Institut d'Astrophysique Spatiale)

17:30 Map-based asymmetric beam convolution

We present a new method to convolve any real beam represented as a 2D array with a sky map $m$. A joint pointing -- beam operator allows to faithfully and fully recreate the beam convolved signal with a beam of any shape including asymmetric beams. This technique is useful for simulations of observations produced by asymmetric realistic beams, and estimating $T \rightarrow P$ and $E \rightarrow B$ leakage produced by beam asymmetry.

Orateur: Dr Artem Basyrov (APC, CNRS)

17:50 Exact map-space beam convolution via spin-mapmaking

We present a map-based method for exact beam convolution using spin-mapmaking, enabling efficient propagation of non-ideal beam systematics to the map level. Validated against TOD-level 4π convolution for circular, elliptical, and Planck HFI scanning beams, it is exact to numerical precision in the limit where subpixel effects are neglected.

Orateur: Camille Demay (University of Trento, University of Ferrara)

19:30 → 22:00 Diner (chez Félicie)

jeu. 26 novembre

09:00 → 10:20 Sessions 7

09:00 High angular resolution maps of polarized dust emission using scattering transforms on Planck data.

Accurate statistical characterization of Galactic foregrounds is crucial for extracting cosmological information from CMB polarization data. However, the complexity of the interstellar medium leads to non-Gaussian structures that cannot be fully captured by traditional summary statistics, like the power spectrum. On the other hand, simulation and machine-learning approaches, while informative, face limitations: the former rely on simplifying physical assumptions, and the latter require large volumes of realistic training data that are in general not available. Further complicating the problem, polarized foregrounds are never observed in isolation but are always mixed with nuisance signals, including the CMB and instrumental noise.

A promising alternative to traditional methods for the statistical characterization and modeling of the Galactic polarized foregrounds is provided by scattering transforms (ST). These are a mathematically grounded set of summary statistics that efficiently capture multiscale, non-Gaussian features of complex physical fields and can be robustly estimated from limited data.

In this talk, we will discuss recent progress in applying ST to Planck data in order to separate the Galactic polarized foregrounds from the CMB and instrumental noise relying only on the 353 GHz channel in polarization, as well as the ability to use other tracers, such as the intensity at 857 GHz. We will present a map of Galactic polarized foregrounds separated from CMB and instrumental noise. These results highlight the potential of ST in exploiting both spatial and spectral information in the task of Galactic polarized foreground modeling, laying the groundwork for a more complete generative modeling approach, that could subsequently be applied to more recent data, such as ACT, SPT, or SO.

Orateur: Alexandros Tsouros (ENS-PSL)

09:20 Self-supervised framework for component separation in the submillimeter sky

We present a self-supervised framework to simultaneously reconstruct the different components of the submillimeter sky. The framework offers flexibility in the choice of emission models, including nonlinear models with spatially varying spectral parameters, such as the spectral index β in dust-emission models. Instrumental effects and constraints can also be incorporated into the reconstruction forward model, potentially enabling joint component separation and deconvolution.

As a proof of concept, we apply our approach to simulated intensity maps of the cosmic microwave background (CMB), the cosmic infrared background (CIB), and the thermal Sunyaev–Zel’dovich (tSZ) effect from the WebSky simulations, spanning frequencies from 93 to 545 GHz. We assess the reconstructed maps through full-sky statistical analyses at high angular resolution, focusing on residual contamination between components, particularly the impact of CIB–tSZ correlations. We compare our tSZ reconstruction with that obtained using MILCA, a widely used component-separation method, and evaluate the recovered tSZ fluxes at galaxy cluster positions. Our results show that the independently trained networks can reconstruct the different components with reduced contamination compared to MILCA.

Future developments could extend the approach to polarization by combining flexible foreground models with higher-order statistical constraints, such as those based on the scattering transform. This framework could help to address foreground separation challenges in upcoming CMB experiments, including the Simons Observatory and LiteBIRD.

Orateur: Victor Bonjean (LPENS)

09:40 Extragalactic CO as a CMB foreground: an analytic halo model for the CO auto-spectrum and its correlation with the CIB

Rotational lines of carbon monoxide (CO) emitted by star-forming galaxies are redshifted into the frequency bands of CMB experiments. Because these bands are broad, many transitions emitted over a wide redshift range add up to a continuum-like signal that is strongly correlated with the cosmic infrared background (CIB). Current predictions of this foreground rely mostly on simulations. I will present an analytic, full-shape halo model for the CO auto-spectrum and the CO×CIB cross-spectrum in CMB bands. It has separate occupation distributions for CO and infrared emitters, includes central and satellite galaxies and CO transitions up to J = 13, and covers all frequency combinations. CO and infrared luminosities derive from the same star formation rate. At ℓ = 3000, the CO×CIB contribution reaches 0.6 and 1.6 µK² at 150 and 220 GHz, comparable to the kSZ power. The star formation history and CO excitation dominate the modelling uncertainty. Fisher forecasts show that, for an ACT-like experiment, neglecting CO biases the radio and SZ foreground parameters by 1–2.5σ while leaving cosmology unaffected. For a CMB-HD-like experiment, CO is detected at high significance, and ignoring it biases cosmological parameters by up to ~2σ. CO should therefore be part of the foreground model of future high-resolution CMB analyses.

Orateur: Giorgia Zagatti (University of Ferrara)

10:00 Towards cross-correlations between the CMB and the 21cm signal

Because they probe two different, complementary aspects of the same question of cosmic reionisation (where is the neutral gas for one, and where are the free electrons for the other), the high-redshift 21cm signal and CMB secondary anisotropies such as the kSZ and Thomson screening are highly complementary. Their cross-correlation has been shown in many works to be a promising observable for cosmology, with sufficient signal-to-noise ratio (SNR) to be observable either today or with upcoming telescopes. However, none of these forecasts took the reality of observations into account. In this talk, I will show how the predicted SNR evolve when considering the specificities of each telescope used for the cross-correlation, such as precise uv/sky coverage, as well as the impact of foregrounds. Naturally, the SNR take a big hit, so I will present various observing and analysis strategies to gain back the lost SNR and reach a detection of the cross-correlation of the 21cm signal with CMB optical depth fluctuations (also called "patchy tau").

Orateur: Adélie Gorce (IAS, Université Paris-Saclay)

10:20 → 11:00 Coffee

11:00 → 12:00 Sessions 8

11:00 Galactic Science with the LiteBIRD satellite: Spectral characterization of diffuse Galactic polarized emission at the angular power spectrum level

Accurate modeling of polarized Galactic emission has become a major challenge for current and next-generation Cosmic Microwave Background (CMB) B-mode experiments. Ignoring the spectral complexity of thermal dust and Galactic synchrotron emission when integrating along the line of sight and over large sky fractions inevitably leads to biases in CMB polarization analyses.
In this talk, I will review how the future LiteBIRD satellite, which will benefit from an increased number of bands and sensitivity with respect to past CMB experiments such as Planck, will exploit its broader frequency coverage to characterize the spectral properties of interstellar medium emission across the three-dimensional structure of the Milky Way. I will present a study of the polarized spectral energy distributions (SEDs) of diffuse Galactic emission and demonstrate the significant improvements LiteBIRD will achieve over Planck.
Nevertheless, the canonical description of foreground spectral behavior will reach its limits for LiteBIRD. I will show that this challenge can be addressed using moment expansion, in which modified black-body and power law SEDs are Taylor-expanded around the pivot spectral parameters, 𝛽 and T. This enhanced modeling of the Galactic polarized signal enables unbiased measurements of the tensor-to-scalar ratio r, even in the presence of pessimistically complex foregrounds.

Orateur: Samy Vinzl (IRAP)

11:20 Bayesian inference of polarised dust SED variations with scattering transforms: validation on Planck-like simulations

Spatial variations of the polarized dust spectral energy distribution (SED) are a major concern for the search for primordial Cosmic Microwave Background (CMB) $B$-modes, as they could be misinterpreted as a cosmological signal. These variations are however difficult to characterize, since only a few noisy polarized channels trace dust in currently available surveys. For this purpose, we propose a new Bayesian method to characterize these SED variations from noisy Planck data at 217 and 353 GHz. We formulate this task as an inverse problem, where noise and CMB are additive contaminants. We solve it using a recently introduced approach, in which Scattering Transform (ST) statistics provide a low-dimensional representation of the signal. Cross-ST statistics between frequencies allows to capture inter-frequency information. We perform the inference in the ST space, where we recover a posterior distribution over multifrequency models of the polarized dust emission. We validate this approach on a single simulated Planck patch with caracteristics matching the Chamaeleon-Musca region, from which we infer pixel-wise maps of the SED parameters with calibrated uncertainties. With typical SED variations expected from current dust emission models, we detect the overall decorrelation between frequencies, as well as spatial variations of the effective spectral index, but not of the polarization angle. These results pave the way for mapping the polarized dust SED properties in Planck data or in upcoming surveys.

Orateur: Sébastien Pierre (LPENS)

11:40 Non-parametric spectral-matching component separation for measuring CMB B-mode polarization with a satellite mission

The measurement of primordial CMB B-mode polarization is fundamentally limited by our ability to separate this faint signal from bright and complex Galactic foregrounds. Spectral Matching Independent Component Analysis (SMICA) offers a flexible, semi-blind approach to this foreground-cleaning problem. However, its standard implementation relies on a physically unrealistic assumption of non-spatially varying foreground emission, reducing the efficiency of the method. In this work, we develop two complementary strategies to address this limitation at the component-separation level. First, we augment SMICA with effective foreground components beyond the physical dust and synchrotron degrees of freedom. Second, we extend SMICA to operate independently on subsets of the sky, better capturing local variations in the foreground emission. Beyond component separation, we further explore a likelihood-level marginalization procedure using internally reconstructed foreground templates. We demonstrate the performance of the pipeline on simulated LiteBIRD-like observations, using foreground models of increasing complexity based on d1s1 and d10s5. However, as this newly proposed approach is fundamentally limited by a methodological so-called ILC bias, we will also introduce and validate a new global, consistent, maximum-likelihood framework: “Map-based Inference of Spectral Templates with Independent Component Analysis” (MISTICA). This new method follows the core principle of SMICA, retaining the spectral-diversity-based separation within a global likelihood framework, providing a direct estimate of the power spectrum, while allowing for natural incorporation of a spatially varying mixing matrix and handling of partial/cut-sky coverage.

Orateur: Hoang Viet Tran (APC, Université Paris Cité)