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Institut de Physique Th´eorique
CEA
CEAStrong spectral features from asymptotic giant branch stars in distant quiescent galaxies
03 Nov 2024
CNRS
University of CambridgeDating the ages and weighting the stellar populations in galaxies are essential steps when studying galaxy formation through cosmic times. Evolutionary population synthesis models with different input physics are used for this purpose. Moreover, the contribution from the thermally pulsing asymptotic giant branch (TP-AGB) stellar phase, which peaks for intermediate-age 0.6-2 Gyr, has been debated for decades. Here we report the detection of strong cool-star signatures in the rest-frame near-infrared spectra of three young (~1Gyr), massive (~10^10Msun) quiescent galaxies at large look-back time, z=1-2, using JWST/NIRSpec. The coexistence of oxygen- and carbon-type absorption features, spectral edges and features from rare species, such as vanadium and possibly zirconium, reveal a strong contribution from TP-AGB stars. Population synthesis models with a significant TP-AGB contribution reproduce the observations better than those with a weak TP-AGB, which are commonly used. These findings call for revisions of published stellar population fitting results, as they point to populations with lower masses and younger ages and have further implications for cosmic dust production and chemical enrichment. New generations of improved models are needed, informed by these and future observations.
JWST and ALMA discern the assembly of structural and obscured components in a high-redshift starburst galaxy
10 May 2024
California Institute of TechnologyWe present observations and analysis of the starburst, PACS-819, at z=1.45 ( M), using high-resolution (; 0.8 kpc) ALMA and multi-wavelength JWST images from the COSMOS-Web program. Dissimilar to HST/ACS images in the rest-frame UV, the redder NIRCam and MIRI images reveal a smooth central mass concentration and spiral-like features, atypical for such an intense starburst. Through dynamical modeling of the CO J=5--4 emission with ALMA, PACS-819 is rotation-dominated thus has a disk-like nature. However, kinematic anomalies in CO and asymmetric features in the bluer JWST bands (e.g., F150W) support a more disturbed nature likely due to interactions. The JWST imaging further enables us to map the distribution of stellar mass and dust attenuation, thus clarifying the relationships between different structural components, not discernable in the previous HST images. The CO J = 5 -- 4 and FIR dust continuum emission are co-spatial with a heavily-obscured starbursting core (<1 kpc) which is partially surrounded by much less obscured star-forming structures including a prominent arc, possibly a tidally-distorted dwarf galaxy, and a clump, either a sign of an ongoing violent disk instability or a recently accreted low-mass satellite. With spatially-resolved maps, we find a high molecular gas fraction in the central area reaching (/) and short depletion times ( 120 Myrs) across the entire system. These observations provide insights into the complex nature of starbursts in the distant universe and underscore the wealth of complementary information from high-resolution observations with both ALMA and JWST.
DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints
09 Oct 2025
University of Michigan
Yale UniversityThe DESI Collaboration's second data release presents precise Baryon Acoustic Oscillation (BAO) measurements from three years of observations, yielding a 4.2 preference for a dynamical dark energy model over CDM and establishing a stringent upper bound of m < 0.064 eV on neutrino masses in a flat CDM universe. These findings refine cosmological parameters and highlight potential deviations from the standard model.
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AION-1: Omnimodal Foundation Model for Astronomical Sciences
20 Oct 2025
Researchers from the Flatiron Institute, NYU, Princeton, and a large international collaboration developed AION-1, an omnimodal foundation model that integrates 39 distinct astronomical data modalities from five major surveys. This model achieves strong performance in low-data regimes and enables flexible data fusion and cross-modal conditional generation for diverse scientific tasks.
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Quantum Effects for Black Holes with On-Shell Amplitudes
15 Sep 2025
the University of TokyoResearchers developed a framework using on-shell scattering amplitudes to describe quantum black hole effects, including Hawking radiation and dynamics in binary systems. This method unifies classical and quantum black hole phenomena, demonstrating that Hawking radiation can be derived from three-point processes and revealing distinct classical and quantum contributions to binary black hole mass shifts.
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Lost in Latent Space: An Empirical Study of Latent Diffusion Models for Physics Emulation
31 Oct 2025
François Rozet
New York UniversityResearchers at Polymathic AI and the Flatiron Institute conducted an empirical study on latent diffusion models (LDMs) for physics emulation, demonstrating that these models maintain high accuracy and statistical plausibility even at extreme compression rates, while achieving substantial computational speedups compared to traditional methods and pixel-space neural solvers.
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Simulation-Based Inference: A Practical Guide
A comprehensive guide outlines a systematic workflow for applying neural-network-based Simulation-Based Inference (SBI) to perform robust parameter inference for complex scientific models lacking explicit likelihood functions. The guide demonstrates its utility across astrophysics, psychophysics, and neuroscience, emphasizing diagnostic checks for reliable posterior distributions.
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Riemannian Flow Matching for Brain Connectivity Matrices via Pullback Geometry
InriaGenerating realistic brain connectivity matrices is key to analyzing population heterogeneity in brain organization, understanding disease, and augmenting data in challenging classification problems. Functional connectivity matrices lie in constrained spaces, such as the set of symmetric positive definite or correlation matrices, that can be modeled as Riemannian manifolds. However, using Riemannian tools typically requires redefining core operations (geodesics, norms, integration), making generative modeling computationally inefficient. In this work, we propose DiffeoCFM, an approach that enables conditional flow matching (CFM) on matrix manifolds by exploiting pullback metrics induced by global diffeomorphisms on Euclidean spaces. We show that Riemannian CFM with such metrics is equivalent to applying standard CFM after data transformation. This equivalence allows efficient vector field learning, and fast sampling with standard ODE solvers. We instantiate DiffeoCFM with two different settings: the matrix logarithm for covariance matrices and the normalized Cholesky decomposition for correlation matrices. We evaluate DiffeoCFM on three large-scale fMRI datasets with more than 4600 scans from 2800 subjects (ADNI, ABIDE, OASIS-3) and two EEG motor imagery datasets with over 30000 trials from 26 subjects (BNCI2014-002 and BNCI2015-001). It enables fast training and achieves state-of-the-art performance, all while preserving manifold constraints. Code: this https URL
Extended Dark Energy analysis using DESI DR2 BAO measurements
Chinese Academy of Sciences
University of ChicagoWe conduct an extended analysis of dark energy constraints, in support of the
findings of the DESI DR2 cosmology key paper, including DESI data, Planck CMB
observations, and three different supernova compilations. Using a broad range
of parametric and non-parametric methods, we explore the dark energy
phenomenology and find consistent trends across all approaches, in good
agreement with the CDM key paper results. Even with the additional
flexibility introduced by non-parametric approaches, such as binning and
Gaussian Processes, we find that extending CDM to include a
two-parameter is sufficient to capture the trends present in the data.
Finally, we examine three dark energy classes with distinct dynamics, including
quintessence scenarios satisfying , to explore what underlying
physics can explain such deviations. The current data indicate a clear
preference for models that feature a phantom crossing; although alternatives
lacking this feature are disfavored, they cannot yet be ruled out. Our analysis
confirms that the evidence for dynamical dark energy, particularly at low
redshift (), is robust and stable under different modeling
choices.
Where Do Tokens Go? Understanding Pruning Behaviors in STEP at High Resolutions
17 Sep 2025
Université Paris-SaclayVision Transformers (ViTs) achieve state-of-the-art performance in semantic segmentation but are hindered by high computational and memory costs. To address this, we propose STEP (SuperToken and Early-Pruning), a hybrid token-reduction framework that combines dynamic patch merging and token pruning to enhance efficiency without significantly compromising accuracy. At the core of STEP is dCTS, a lightweight CNN-based policy network that enables flexible merging into superpatches. Encoder blocks integrate also early-exits to remove high-confident supertokens, lowering computational load. We evaluate our method on high-resolution semantic segmentation benchmarks, including images up to 1024 x 1024, and show that when dCTS is applied alone, the token count can be reduced by a factor of 2.5 compared to the standard 16 x 16 pixel patching scheme. This yields a 2.6x reduction in computational cost and a 3.4x increase in throughput when using ViT-Large as the backbone. Applying the full STEP framework further improves efficiency, reaching up to a 4x reduction in computational complexity and a 1.7x gain in inference speed, with a maximum accuracy drop of no more than 2.0%. With the proposed STEP configurations, up to 40% of tokens can be confidently predicted and halted before reaching the final encoder layer.
S-JEPA: towards seamless cross-dataset transfer through dynamic spatial attention
07 Oct 2024
Motivated by the challenge of seamless cross-dataset transfer in EEG signal processing, this article presents an exploratory study on the use of Joint Embedding Predictive Architectures (JEPAs). In recent years, self-supervised learning has emerged as a promising approach for transfer learning in various domains. However, its application to EEG signals remains largely unexplored. In this article, we introduce Signal-JEPA for representing EEG recordings which includes a novel domain-specific spatial block masking strategy and three novel architectures for downstream classification. The study is conducted on a 54 subjects dataset and the downstream performance of the models is evaluated on three different BCI paradigms: motor imagery, ERP and SSVEP. Our study provides preliminary evidence for the potential of JEPAs in EEG signal encoding. Notably, our results highlight the importance of spatial filtering for accurate downstream classification and reveal an influence of the length of the pre-training examples but not of the mask size on the downstream performance.
Dynamical Decoupling of Generalization and Overfitting in Large Two-Layer Networks
29 Oct 2025
Stanford UniversityResearch by Andrea Montanari and Pierfrancesco Urbani unveils a dynamical separation of timescales in large two-layer neural network training, where feature learning occurs on faster timescales than overfitting. This dynamic process explains the non-monotonic behavior of test error and the phenomenon of "feature unlearning," offering a mechanistic understanding of generalization in overparameterized models.
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Parallel Application of Slitless Spectroscopy to Analyze Galaxy Evolution (PASSAGE): Survey Overview
University of California, Santa BarbaraDuring the second half of Cycle 1 of the James Webb Space Telescope (JWST), we conducted the Parallel Application of Slitless Spectroscopy to Analyze Galaxy Evolution (PASSAGE) program. PASSAGE received the largest allocation of JWST observing time in Cycle 1, 591 hours of NIRISS observations to obtain direct near-IR imaging and slitless spectroscopy. About two thirds of these were ultimately executed, to observe 63 high-latitude fields in Pure Parallel mode. These have provided more than ten thousand near-infrared grism spectrograms of faint galaxies.
PASSAGE brings unique advantages in studying galaxy evolution: A) Unbiased spectroscopic search, without prior photometric pre-selection. By including the most numerous galaxies, with low masses and strong emission lines, slitless spectroscopy is the indispensable complement to any pre-targeted spectroscopy; B) The combination of several dozen independent fields to overcome cosmic variance; C) Near-infrared spectral coverage, often spanning the full range from 1.0--2.3 m, with minimal wavelength gaps, to measure multiple diagnostic rest-frame optical lines, minimizing sensitivity to dust reddening; D) JWST's unprecedented spatial resolution, in some cases using two orthogonal grism orientations, to overcome contamination due to blending of overlapping spectra; E) Discovery of rare bright objects especially for detailed JWST followup. PASSAGE data are public immediately, and our team plans to deliver fully-processed high-level data products.
In this PASSAGE overview, we describe the survey and data quality, and present examples of these accomplishments in several areas of current interest in the evolution of emission-line galaxy properties, particularly at low masses.
Recent Advances in Optimal Transport for Machine Learning
21 Aug 2024
Researchers from Université Paris-Saclay, CEA, LIST, F-91120, Palaiseau, France conducted a comprehensive survey of Optimal Transport for Machine Learning (OTML) from 2012 to 2023. The work synthesizes advancements in computational efficiency, theoretical extensions, and broad applications across various ML subfields, positioning OT as a robust framework for comparing and transforming probability distributions.
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Constraints on Neutrino Physics from DESI DR2 BAO and DR1 Full Shape
07 Oct 2025
Harvard University
University College LondonThe Dark Energy Spectroscopic Instrument (DESI) Collaboration has obtained robust measurements of baryon acoustic oscillations (BAO) in the redshift range, 0.1 < z < 4.2, based on the Lyman- forest and galaxies from Data Release 2 (DR2). We combine these measurements with external cosmic microwave background (CMB) data from Planck and ACT to place our tightest constraints yet on the sum of neutrino masses. Assuming the cosmological CDM model and three degenerate neutrino states, we find \sum m_\nu<0.0642 eV (95%) with a marginalized error of eV. We also constrain the effective number of neutrino species, finding N_\rm{eff} = 3.23^{+0.35}_{-0.34} (95%), in line with the Standard Model prediction. When accounting for neutrino oscillation constraints, we find a preference for the normal mass ordering and an upper limit on the lightest neutrino mass of m_l < 0.023 eV (95%). However, we determine using frequentist and Bayesian methods that our constraints are in tension with the lower limits derived from neutrino oscillations. Correcting for the physical boundary at zero mass, we report a 95% Feldman-Cousins upper limit of \sum m_\nu<0.053 eV, breaching the lower limit from neutrino oscillations. Considering a more general Bayesian analysis with an effective cosmological neutrino mass parameter, , that allows for negative energy densities and removes unsatisfactory prior weight effects, we derive constraints that are in tension with the same oscillation limit. In the absence of unknown systematics, this finding could be interpreted as a hint of new physics not necessarily related to neutrinos. The preference of DESI and CMB data for an evolving dark energy model offers one possible solution. In the CDM model, we find \sum m_\nu<0.163 eV (95%), relaxing the neutrino tension. [Abridged]
How Foundational are Foundation Models for Time Series Forecasting?
Foundation Models are designed to serve as versatile embedding machines, with strong zero shot capabilities and superior generalization performance when fine-tuned on diverse downstream tasks. While this is largely true for language and vision foundation models, we argue that the inherent diversity of time series data makes them less suited for building effective foundation models. We demonstrate this using forecasting as our downstream task. We show that the zero-shot capabilities of a time series foundation model are significantly influenced and tied to the specific domains it has been pretrained on. Furthermore, when applied to unseen real-world time series data, fine-tuned foundation models do not consistently yield substantially better results, relative to their increased parameter count and memory footprint, than smaller, dedicated models tailored to the specific forecasting task at hand.
Whitening Spherical Gaussian Mixtures in the Large-Dimensional Regime
Whitening is a classical technique in unsupervised learning that can facilitate estimation tasks by standardizing data. An important application is the estimation of latent variable models via the decomposition of tensors built from high-order moments. In particular, whitening orthogonalizes the means of a spherical Gaussian mixture model (GMM), thereby making the corresponding moment tensor orthogonally decomposable, hence easier to decompose. However, in the large-dimensional regime (LDR) where data are high-dimensional and scarce, the standard whitening matrix built from the sample covariance becomes ineffective because the latter is spectrally distorted. Consequently, whitened means of a spherical GMM are no longer orthogonal. Using random matrix theory, we derive exact limits for their dot products, which are generally nonzero in the LDR. As our main contribution, we then construct a corrected whitening matrix that restores asymptotic orthogonality, allowing for performance gains in spherical GMM estimation.
Modelling the effect of stellar metallicity on the XUV evolution of low-mass stars and its impact on exoplanet atmospheres/habitability
25 Sep 2025
University of BristolUnderstanding how exoplanet atmospheres evolve is a key question in the context of habitability. One key process governing this evolution is atmospheric evaporation by stellar X-ray and EUV emission (collectively, XUV). As such, the evolution of exoplanet atmospheres is closely tied to the evolution of the host star's magnetic activity. Many studies have modelled the combined evolution of exoplanet atmospheres and their host stars. However, to date, the impact of the host star's metallicity on stellar activity/exoplanet atmosphere evolution has not been explored. In this work, we investigate how stellar metallicity affects the rotation and activity evolution of solar-like stars as well as the corresponding exoplanet atmospheric evolution. We reconfirm previous results that metal-rich stars spin down more rapidly than metal-poor stars. We also find that the XUV flux that an exoplanet in the habitable zone of its host star receives is larger when the host star is more metal-rich. As such, the atmospheres of exoplanets in the habitable zones of metal-rich stars are evaporated more rapidly than exoplanets in the habitable zones of metal-poor stars. Lastly, we find that the atmospheric evolution is most sensitive to the host star metallicity when the host star has a higher mass. In the highest mass solar-stars, the metallicity can have a larger influence on the atmospheric evolution than the initial rotation period of the star.
Signatures of Topological Symmetries on a Noisy Quantum Simulator
16 Oct 2025
University of Southern CaliforniaTopological symmetries, invertible and otherwise, play a fundamental role in the investigation of quantum field theories. Despite their ubiquitous importance across a multitude of disciplines ranging from string theory to condensed matter physics, controlled realizations of models exhibiting these symmetries in physical systems are rare. Quantum simulators based on engineered solid-state devices provide a novel alternative to conventional condensed matter systems for realizing these models.
In this work, eigenstates of impurity Hamiltonians and loop operators associated with the topological symmetries for the Ising conformal field theory in two space-time dimensions are realized on IBM's Kingston simulator. The relevant states are created on the quantum device using a hybrid quantum-classical algorithm. The latter is based on a variation of the quantum approximate optimization algorithm ansatz combined with the quantum natural gradient optimization method. Signatures of the topological symmetry are captured by measuring correlation functions of different qubit operators with results obtained from the quantum device in reasonable agreement with those obtained from classical computations. The current work demonstrates the viability of noisy quantum simulators as platforms for investigating low-dimensional quantum field theories with direct access to observables that are often difficult to probe in conventional condensed matter experiments.
SPT-3G D1: CMB temperature and polarization power spectra and cosmology from 2019 and 2020 observations of the SPT-3G Main field
25 Jun 2025
University of TorontoWe present measurements of the temperature and E-mode polarization angular power spectra of the cosmic microwave background (CMB) from observations of 4% of the sky with SPT-3G, the current camera on the South Pole Telescope (SPT). The maps used in this analysis are the deepest used in a CMB TT/TE/EE analysis to date. The maps and resulting power spectra have been validated through blind and unblind tests. The measurements of the lensed EE and TE spectra are the most precise to date at l=1800-4000 and l=2200-4000, respectively. Combining our TT/TE/EE spectra with previously published SPT-3G CMB lensing results, we find parameters for the standard LCDM model consistent with Planck and ACT-DR6 with comparable constraining power. We report a Hubble constant of km/s/Mpc from SPT-3G alone, 6.2 sigma away from local measurements from SH0ES. For the first time, combined ground-based (SPT+ACT) CMB primary and lensing data have reached Planck's constraining power on some parameters, a milestone for CMB cosmology. The combination of these three CMB experiments yields the tightest CMB constraints to date, with km/s/Mpc, and the amplitude of clustering . CMB data alone show no evidence for physics beyond LCDM; however, we observe a 2.8 sigma difference in LCDM between CMB and baryon acoustic oscillation (BAO) results from DESI-DR2, which is relaxed in extended models. The combination of CMB and BAO yields 2-3 sigma shifts from LCDM in the curvature of the universe, the amplitude of CMB lensing, or the dark energy equation of state. It also drives mild preferences for models that address the Hubble tension through modified recombination or variations in the electron mass in a non-flat universe. This work highlights the growing power of ground-based CMB experiments and lays a foundation for further cosmological analyses with SPT-3G.
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