arXiv subjects

Raphaël Kou

Publications and source records attributed to Raphaël Kou.

6 recordsLinked to original sources

arXiv2026

cloelib: A Flexible Python Library for Computing Cosmological Observables in the Euclid Era

cloelib is a Python library developed to compute cosmological observables within the Cosmology Likelihood for Observables in Euclid (CLOE) ecosystem (cloe-org). As cosmology enters a precision era driven by galaxy survey missions such as Euclid, there is a growing need for flexible, efficient, and differentiable software capable of supporting next-generation inference pipelines. cloelib addresses these demands through a modular architecture that interfaces seamlessly with established Boltzmann solvers whilst incorporating JAX-based automatic differentiation to enable gradient-based methods. The library defines consistent protocols for background evolution, perturbations, and non-linear structure formation, and supports a wide range of observables, including photometric and spectroscopic large-scale structure probes, as well as cross-correlations with the Cosmic Microwave Background and galaxy clusters. In its finalised form, cloelib is intended to serve as the reference theory computation infrastructure for Euclid's first cosmological release, bridging traditional numerical cosmology with modern optimisation techniques and emerging machine learning approaches to inference.

astro-ph.CO↗

arXiv2025

Unified dark fluid with null sound speed as an alternative to phantom dark energy

Recent BAO measurements from DESI, when combined with CMB and supernovae data, suggest evolving dark energy and in particular point to a possible phantom regime, with an equation of state parameter $w<-1$. We explore an alternative phenomenological way to model dark matter and dark energy based on a unified dark fluid (UDF). By construction, our model reproduces the same background expansion history as DESI's best-fit using the CPL parametrization, but assumes a vanishing rest-frame sound speed and no anisotropic stress. This simple prescription ensures a consistent and physical treatment of perturbations and, in our case, the use of a unified dark sector avoids phantom behaviour. We model CMB, LSS, and redshift-space distortion observables, and find mostly small differences with CPL, suggesting that while stage IV CMB and galaxy surveys will be able to test these models, achieving a decisive distinction between them may prove challenging on linear scales. At the non-linear level, we study spherical collapse in the UDF and show that within this framework, structure formation proceeds very similarly to standard scenarios. Using Planck, DESI BAO DR2, and DES Y5 supernovae data, we demonstrate that this simple UDF model fits current observations nearly as well as CPL, while treating perturbations consistently. Because most cosmological observations are not sensitive to how the dark sector is split, the unified framework can also approximate the phenomenology of interacting dark energy-dark matter scenarios or evolving dark matter, making it a general way to model the data, at least as long as the dark components have a vanishing sound speed, which is the most distinctive feature of our analysis. Our results highlight that a unified dark fluid with evolving equation of state and null sound speed is sufficient to pass current constraints without invoking a phantom component.

astro-ph.CO↗

arXiv2025

Improving CMB constraints on early Universe physics with LSS: A multi-probe forecast including cross-covariance

Extensions to the $Λ\textrm{CDM}$ model prior to recombination can modify the growth of perturbations around radiation-matter equality, leaving a distinct signature in the matter power spectrum. Upcoming large-scale structure surveys will be sensitive to these features, allowing tests of early physics that are complementary to the CMB observations. In this paper, we forecast how well the combination of galaxy clustering, weak lensing and CMB lensing two point statistics, also known as $6\times2$pt analysis, will tighten constraints on extensions to the $Λ\textrm{CDM}$ model in the early Universe. We find significant improvements, in particular in the case of early dark energy, where the uncertainty on its density parameter could be divided by a factor of $3$ to $4$ when combining Euclid observables with Simons Observatory or CMB-S4, compared to using CMB observations alone. Testing for different scale cuts, we find that much of the constraining power comes from the largest scales which are less prone to systematic uncertainties. We take into account the most significant terms in the cross-covariance between large-scale structure tracers and CMB power spectra, which arises from gravitational lensing. Assessing the impact of this additional cross-covariance on the constraints, we find small corrections for most parameters, except for $A_s$ and $τ$ where the lensing induced covariance leads to a more significant degradation of constraints. This forecast analysis highlights the potential of combining CMB and galaxy survey data to test the cosmological model. In particular, early Universe physics, relevant before recombination, stands out as a promising area that benefits substantially from this approach.

astro-ph.CO↗

arXiv2025

A flexible parameterization to test early physics solutions to the Hubble tension with future CMB data

One approach to reconciling local measurements of a high expansion rate with observations of acoustic oscillations in the CMB and galaxy clustering (the "Hubble tension") is to introduce additional contributions to the $Λ$CDM model that are relevant before recombination. While numerous possibilities exist, none are currently well-motivated or preferred by data. However, future CMB experiments, which will measure acoustic peaks to much smaller scales and resolve polarization signals with higher signal-to-noise over large sky areas, should detect almost any such modification at high significance. We propose a model-agnostic method to capture most relevant possible deviations from $Λ$CDM due to additional non-interacting components, while remaining sufficiently constraining to enable detection across various scenarios. The phenomenological model uses a fluid model with four parameters governing additional density contributions that peak at different redshifts, and two sound speed parameters. We forecast possible constraints with Simons Observatory, explore parameter degeneracies that arise in $Λ$CDM, and demonstrate that this method could detect a range of specific models. Which of the new parameters gets excited can indicate the nature of any new physics, while the generality of the model allows for testing with future data in a way that should not be plagued by a posteriori choices or publication bias. When testing our model with Planck data, we find good consistency with the $Λ$CDM model, but the data also allows for large Hubble parameter, especially if the sound speed of an additional component is not too different to that of radiation.

astro-ph.CO↗

arXiv2023

Constraining f(R) gravity with cross-correlation of galaxies and cosmic microwave background lensing

We look for signatures of the Hu-Sawicki f(R) modified gravity theory, proposed to explain the observed accelerated expansion of the universe; in observations of the galaxy distribution, the cosmic microwave background (CMB), and gravitational lensing of the CMB. We study constraints obtained by using observations of only the CMB primary anisotropies, before adding the galaxy power spectrum and its cross-correlation with CMB lensing. We show that cross-correlation of the galaxy distribution with lensing measurements is crucial to breaking parameter degeneracies, placing tighter constraints on the model. In particular, we set a strong upper limit on $\log{\lvert f_{R_0}\lvert }<-4.61$ at 95% confidence level. This means that while the model may explain the accelerated expansion, its impact on large-scale structure closely resembles General Relativity. Studies of this kind with future data sets will probe smaller potential deviations from General Relativity.

astro-ph.CO↗

arXiv2023

Cosmic Census: Relative Distributions of Dark Matter, Galaxies and Diffuse Gas

Galaxies, diffuse gas, and dark matter make up the cosmic web that defines the large-scale structure of the Universe. We constrained the joint distribution of these constituents by cross-correlating galaxy samples binned by stellar mass from the Sloan Digital Sky Survey CMASS catalog with maps of lensing convergence and the thermal Sunyaev-Zeldovich (tSZ) effect from the Planck mission. Fitting a halo-based model to our measured angular power spectra (galaxy-galaxy, galaxy-lensing convergence, and galaxy-tSZ) at a median redshift of $z=0.53$, we detected variation with stellar mass of the galaxy satellite fraction and galaxy spatial distribution within host halos. We find a tSZ-halo hydrostatic mass bias, $b_h$, such that $(1-b_h)=0.6\pm0.05$, with a hint of a larger bias, $b_h$, at the high stellar mass end. The normalization of the galaxy-cosmic microwave background lensing convergence cross-power spectrum shows that galaxies trace the matter distribution without an indication of stochasticity ($A=0.98\pm 0.09$). We forecast that next-generation cosmic microwave background experiments will improve the constraints on the hydrostatic bias by a factor of two and will be able to constrain the small-scale distribution of dark matter, hence informing the theory of feedback processes.

astro-ph.CO↗