arXiv subjects

Ziad Sakr

Publications and source records attributed to Ziad Sakr.

At least 19 recordsLinked to original sources

arXiv2026

Cosmology from Nx2pt Analyses of SKAO Wide-Area Surveys

SKAO surveys will provide an unprecedented window into the large-scale structure of the universe through HI 21cm galaxy and intensity mapping surveys, and radio continuum surveys. We present forecasts for the cosmological constraining power of ``$N\times2\mathrm{pt}$'' analyses -- which combine galaxy clustering, galaxy weak lensing, galaxy-galaxy lensing signals and 21cm Intensity Maps. By assuming cosmology surveys from an SKA-Mid AA4, we show that such an $N\times2\mathrm{pt}$ analysis will be able to deliver measurements of $\sim1\%$ precision on $Λ$CDM cosmological parameters. We also explore dynamical dark energy in the $w_0, w_a$ model, the sum of neutrino masses $\rm M_ν$, and the background curvature $Ω_{\rm k}$.

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arXiv2026

Beyond ΛCDM with the SKA Observatory -- II: Unveiling the Secrets of the Early Universe

The origins of the universe remain one of the biggest mysteries in modern cosmology. While the Planck satellite has provided a wealth of information about the early universe, there is still much to be discovered. The Square Kilometre Array Observatory (SKAO) offers a unique opportunity to probe the universe's infancy, going beyond the current limitations of our knowledge. By measuring the power spectrum of biased tracers of the dark matter distribution on the largest cosmological scales and exploring beyond 2-point statistics, SKAO will enable us to refine our understanding of the primordial universe, including the shape of the inflationary power spectrum and the presence of primordial non-Gaussianity. In this chapter we will review recent works looking at the potential of SKAO's surveys, and how synergies with other surveys can revolutionize our understanding of the origins of the cosmos.

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arXiv2026

Cosmology from HI galaxy surveys with the SKA

The 21cm line from neutral hydrogen is expected to be a ubiquitous (albeit faint) tracer of galaxies in the late Universe. With SKAO-MID, large wide-field surveys of several million HI-containing galaxies will become feasible, resulting in catalogues of sufficient size to measure large-scale structure observables such as baryon acoustic oscillations and redshift-space distortions. While optical galaxy surveys over comparable areas are generally deeper, radio surveys of this kind have a number of other advantages, such as broader sampling of the halo mass function and the possibility of measuring luminosity distances via the Tully-Fisher relation. In this chapter, we provide predictions for the galaxy number counts versus redshift that will be achievable with a wide-field HI galaxy survey on SKAO-MID, along with corresponding forecasts for cosmological observables. Given the substantial uncertainty in the HI mass function with redshift, we bracket our predictions using a handful of different modelling methods.

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arXiv2026

Cosmology with Multi-Wavelength Line Intensity Mapping Synergies in the SKAO Era

Line intensity mapping (LIM) has emerged as a powerful tool for surveying the large-scale structure of the Universe across cosmic time by measuring spatial fluctuations in the cumulative emission of spectral lines from unresolved sources or the intergalactic medium. Besides the most abundant 21-cm hyperfine line of neutral hydrogen, there are bright far-infrared fine-structure lines like [CII] 158 $μ$m, [OIII] 88 $μ$m, [NII] 122/205 $μ$m, and [OI] 63 $μ$m, as well as mid-/high-$J$ CO rotational transitions, hydrogen Ly$α$ and H$α$ as potential LIM probes. A key opportunity lies in combining and cross-correlating 21-cm intensity maps from SKAO with other line intensity maps, targeted by a range of ongoing and forthcoming LIM experiments that probe overlapping cosmic volumes. Cross-correlation between 21-cm maps and other line tracers mitigates uncorrelated systematics and enhances sensitivity to the underlying matter distribution, while multi-line analyses help disentangle cosmological and astrophysical parameters. Beyond cross-power spectra, higher-order and morphological statistics -- such as cross-bispectra, marked correlations, and morphological measures -- capture non-Gaussian features and the environmental dependence of structure formation. This chapter explores the synergies that can be achieved by combining SKAO observations with other line-intensity mapping experiments, demonstrating how such joint analyses can unlock new insights into galaxy evolution and cosmology.

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arXiv2026

Weak Lensing with SKAO: Cosmic Shear Cosmology

We discuss the power of weak gravitational lensing surveys with the SKAO in constraining cosmological parameters and the properties of radio star-forming galaxy samples. As well as reviewing progress to date on cosmic shear in radio experiments, we show forecasts for parameter constraints using the Mid telescope both alone and in cross-correlation with contemporaneous optical surveys. By selecting a sample of resolved, high-redshift star-forming galaxies in Band 2, surveys with the AA4 configuration will be capable of measuring the growth of structure on large scales in the Universe through the effect of weak gravitational lensing on their shapes. Assuming the high fidelity reconstruction of such galaxy shapes to be possible, we find that SKAO will measure the $S_8$ structure formation parameter to a level of $5\%$ alone and $3\%$ in full combination with either LSST or the \emph{Euclid} satellite. These measurements will be highly important due to their radically different sensitivities to key weak lensing systematics, both instrumental and astrophysical, and as such provide a vital robustness test to a pillar of modern cosmological measurements. Radio surveys also provide unique and potentially game-changing information in the form of polarisation and galaxy kinematics, which allow the cleaner separation of lensing from intrinsic galaxy shapes and can increase statistical power by factors $\sim5$-$10$.

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arXiv2026

Cosmology from Clustering of Continuum Galaxies

The distribution of radio continuum galaxies is a useful, fast, and accessible probe of the matter distribution in the Universe, enlightening us about the Universe's initial conditions, the physics of dark matter, and the nature of the mysterious dark energy. However, radio continuum galaxies alone cannot easily be localised in the radial direction, and cross-identification of host sources from optical catalogues is challenging across wide area surveys. Moreover, there are several redshift-dependent properties of radio galaxy populations that all need accurate modelling to make reliable inferences about fundamental physics. These include accurate measurements of the redshift distribution of radio sources ($dN/dz$), the coupling between radio galaxies and the underlying matter distribution (quantified by the galaxy bias, $b(z)$), and the true flux distribution $N(S,z)$ of the radio sources (magnification bias). The amount of encoded cosmological information depends on the survey properties and the level of homogeneity across its footprint. In this chapter, we demonstrate the cosmological potential of a 20,000 sq. deg survey with the SKAO in AA4 configuration, using 10,000 hours of observations. Such a survey will reach $\mathcal{O}(μ\mathrm{Jy/beam)}$ sensitivities and detect $\mathcal{O}$(300-400 million) radio sources, the largest sample of radio continuum galaxies to date. This surpasses the number of sources assumed for the previous SKA cosmology Red Book. We predict the angular clustering of such a survey, using mocks accounting for potential telescope systematics, and discuss which data corrections may be needed when these systematics cannot be accurately modelled.}

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arXiv2026

On the robustness of the angular homogeneity scale $θ_H$: a comparative analysis of computational approaches

The assumption of large-scale homogeneity is a cornerstone of modern cosmology and underlies the validity of the FLRW framework. Testing the scale at which the Universe transitions to homogeneity remains a key observational challenge, particularly with the increasing precision of galaxy surveys. We aim to assess the robustness of the angular homogeneity scale, $θ_H$, by systematically comparing different computational approaches used in its estimation and by quantifying the impact of methodological choices and physical clustering scales. We analyse mock galaxy catalogues from the MICE Grand Challenge simulation. The angular fractal dimension $D_2(θ_H)$ is computed using the Landy-Szalay estimator and direct pair-counting methods. We implement different approaches, including symbolic regression, to model $D_2(θ_H)$ and determine $θ_H$. Uncertainties are estimated using resampling techniques and alternative parametric error propagation methods. We find that the estimation of $θ_H$ is sensitive to methodological choices in the analysis, such as survey area, redshift bin, numerical implementation and fitting strategy. While its redshift evolution is robust, its absolute value is sensitive to both modelling choices and the presence of local clustering features. Our results highlight the importance of methodological systematics in homogeneity studies, showing that the determination of $θ_H$ depends not only on the data, but also on the adopted analysis strategy. Flexible approaches such as symbolic regression provide a useful framework to model these effects, but also emphasize the need for careful modelling and survey design. This has important implications for future large-scale structure analyses aiming to test the Cosmological Principle with high precision.

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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.

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arXiv2026

A more inclusive effective dark fluid equation of state parameter: constraints from SKA and Euclid like surveys

We forecast constraints on an effective dark fluid equation of state parameter $w_{\rm eff}$ that encapsulates modified gravity theories that modifies both the Universe background expansion as well as its large scale structures growth. This is achieved through relating Friedmann equations' dark fluid pressure and density content, thus $w_{\rm eff}$, to modified gravity parameterized models by mean of the Newtonian potential equation parameter $μ_0$, the gravitational slip parameter $η_0$ and a redshift dependent Hubble parameter $H_{0,{\rm bck}}$. We adopt next stage SKA survey specifications, alone or in combination with concurrently expected DR3 Euclid survey release, paying attention to the modeling and recipe of the implementation of the galaxy clustering and lensing probes obtained from the two surveys. We consider two data mock models: one with deviation of the intermediate parameters at the level of 10 \% (yielding however $w_{\rm eff}=-1.03$) and another sub-percently close to $Λ$CDM. We found that the three parameters deviation from $Λ$CDM could only be detected at 1 $σ$ from SKA alone, while this improves to $\sim$ 2 $σ$ when we combine with Euclid. An improvement of the order of 30\% on the bounds is reached after projecting the three parameters into a single $w_{\rm eff}$ parameter. However, this affects both cases and thus it does not change much, though it improves the level of detection with respect to $Λ$CDM values. We conclude that synergy from both surveys benefits to tighten our constraints, but also that our highly generalized parameterization, although impacting at both the background and the perturbation level, will be hard to disentangle from $Λ$CDM at the level at which our forecast is performed and it still needs, to the least, data from more advanced stages of the adopted surveys to hope reach this target.

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arXiv2026

The shape function of the observed growth index

The growth index $γ$ is a powerful trigger for detecting deviations from $Λ$CDM. However, its value is often determined by considering an asymptotic constant value that works for all redshift, or else following a chosen parameterisation. Here we formulate the growth index as function of three quantities that could be directly related to observables in redshift bins, $fσ_8(z_i)$, $f(z_i)$ and $H(z_i)$. We determine its value and its derivative at observed nodal center of redshift bins and use the shape function method, after showing insightful connection with its underlying governing virtual-work conservation principle, to construct a redshift dependence of the $γ$ without assuming a specific parameterization. We then use the resulting shape function to test if we can disentangle between different scenarios where there are discrepancies between its three constituent measured components. We also tested whether it can be used to rule out models of modified gravity, or extended parametric models of the growth index that capture more general behaviors with an additional parameter as function of the scale factor or dark energy. Adopting forecasted measurements from next generation surveys on the three quantities used to construct $γ$, we find that reported discrepancies between them could be detected with our method, but at the bins where the errors and lost of precision from our addition of degrees of freedom is small with respect to the deviation of $γ$. The same could be concluded for first order extensions to $γ$ or common modified gravity models, and to a lesser degree for dynamical dark energy models after supposing the latest DESI values. We conclude that this method is a strong tool to investigate cosmology in a model-independent way especially with forthcoming data delivered by further stage-IV surveys with more stringent uncertainties.(Abridged)

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arXiv2026

Forecast on the generalised dark matter properties from a Euclid-like survey

The Stage~IV \textit{Euclid} mission will deliver spectroscopic galaxy redshifts together with photometric positions and shapes, enabling cosmological analyses through spectroscopic galaxy clustering (GCsp), photometric galaxy clustering (GCph), weak-lensing cosmic shear (WL), and their cross-correlation (XC). In this work we forecast the constraining power of a Euclid-like survey on the Generalised Dark Matter (GDM) parameters $w_{\rm gdm}$ and $c^{2}_{s,{\rm gdm}}$. Our analysis extends previous forecasting pipeline used for standard cold dark matter. For GCsp, we adopt a semi-analytic nonlinear RSD model, with free terms for each bin. For the photometric probes, we compute the nonlinear GDM matter power spectrum using dedicated simulations, and we modify the lensing and clustering window and the intrinsic-alignment prescription. We consider several survey configurations and explore three fiducial values of $σ_8$ motivated by current CMB and low-redshift measurements. In an optimistic setting, for fiducial values $σ_8 \simeq 0.81$ and $σ_8 \simeq 0.77$, we find relative errors of $4.01\%$ (GCsp), $5.01\%$ (GCph+WL+XC), and $1.96\%$ (all probes) on $c^{2}_{s,{\rm gdm}}$, and $3.26\%$ (GCph+WL+XC) and $1.85\%$ (all probes) on $w_{\rm gdm}$. For a lower fiducial value $σ_8 \simeq 0.67$, that could strongly disfavor $Λ$GDM, we find constraints of $5\%$ (GCsp), $5\%$ (GCph+WL+XC), and $2.45\%$ (all probes) on $c^{2}_{s,{\rm gdm}}$, and $3.43\%$ (GCph+WL+XC) and $2.04\%$ (all probes) on $w_{\rm gdm}$. We also found that, combining all probes, whether in the pessimistic or optimistic settings, a Euclid-like survey will be able to disentangle between the three scenarios. These results show that the survey will be able to constrain the GDM parameters and distinguish between normalisations of the matter fluctuations.(Abridged)

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arXiv2025

Model independent approach towards measuring expansion and growth factor from next generation galaxy clustering and lensing angular power spectrum

In this work we perform Fisher forecasts on the expansion and the growth factors following model independent approaches from 3x2pt joint analysis of the galaxy lensing, clustering, and their cross-correlated spectra at the linear, and extending as well to non linear scales. For that, instead of choosing a specific model for the matter power spectrum, the main ingredient of these probes, we express it by parametrizing its components, such as the expansion and the growth factor, and those of the standard halo model and excursion set theory in several z bins, besides to the different bias and non-linear bias modelling functions. We apply the technique to Euclid, Rubin and SKA public specifications in the range 0.2 < z < 1.8 and show that one can then obtain model-independent constraints of the expansion E(z i ) and the growth factor G(z i ). We also show the change in gain in precision at each z- shell when going from pessimistic cut at linear scales to more optimistic non-linear settings, or the difference between using each survey alone or a combination of all of them, or the impact from fixing or adding more degrees of freedom in the non-linear modeling. We found that, in the most agnostic case, one can still reach high precision on E(z i ) in the order of the percent level when combining the three surveys at once while the growth factor G(z i ) has for the same settings one order of magnitude weaker constraints. We also found for both factors, an improvement that can reach one order of magnitude in precision when passing from linear to non-linear scales. We conclude that we will be able to constrain the two important factors of the background evolution and structure formation of the Universe when using non linear scales and the combined power of future surveys even in the most agnostic approaches.

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arXiv2025

Applying the BF method on the DESI evidence for dynamical dark energy models

Recent baryon acoustic oscillation measurements from the DESI, when combined with CMB data and Type Ia supernovae observations, indicate a preference for dynamical dark energy when considering the Chevallier-Polarski-Linder (CPL) model, over the standard ΛCDM or the wCDM model. However, the Bayes factor, a key metric for model comparison, remains inconclusive on which model is preferred. This paper applies the BF method, that integrates both Bayesian and frequentist approaches to DESI data to address the limitations of purely frequentist or Bayesian methods. It consists in considering the Bayes factor as a random variable and calculates its distribution, that results from values computed in a frequentist approach after perturbing the data following the model considered. We apply this hybrid method to DESI data, comparing the CPL and w models under various prior conditions, including weak and strong priors, and theory-informed priors. We find that, when the traditional bayes factor is considered, that weak priors favor the w model over CPL, while strong priors favor CPL. Additionally, theory-informed priors further enhance the preference for the w model. While when we apply the BF method, the preference for CPL over w is seen in all cases albeit with similar but reduced impact on the p-value from the different prior considerations. We also tried to generalize further, by perturbing as well the covariance matrix following the model considered, and found that, in general, the current data in that case is not stringent enough to disentangle between the two models. Our results demonstrate that varying the Bayes factor as a random variable, providing that the covariance matrix is kept as model independent, provides a robust model comparison, reducing the impact of prior dependence as well as offering quantitative assessment of the preferences of the competing models.(abridged)

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arXiv2025

On the validity of the continuity equation in a modified gravity framework with CMB, DES 3x2pt and tomographic ISW data

In this work we propose a phenomenological modification to the continuity equation at the linear perturbation level and test it using combinations of the CMB temperature, polarization and lensing potential angular spectrum, the ISW-galaxy cross power spectrum and the 3$\times$2pt lensing and galaxy clustering from DES survey. We investigate two parametrisations of this modification, both proportional to a new parameter $A_c$, which is assumed to be either constant in time, or proportional to the scale factor $a$, in order to be more relevant at late times. We find DES and ISW data to be consistent with the standard continuity equation when $A_c$ is constant, but 2--3$σ$ hints of a non-zero modification appear when Planck data is included. The model $A_c \propto a$ results in stronger tensions. We also test the effects of including the common extra parameters $μ$ and $η$ that modify the Poisson equation and Weyl potential. Although $A_c$, $μ$ and $η$ are correlated, we still find non-zero $A_c$ when Planck data is included or without Planck if $A_c \propto a$ and only either $η$ or $μ$ are allowed to vary. We conclude that violations of the continuity equation should be considered with care when testing additional deviations from general relativity.

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arXiv2025

Uncovering the bias in the evidence for dynamical dark energy through minimal and generalized modeling approaches

In this letter we argue that the CPL parameterisation for the dark energy equation of state is biased towards preferring such model over the constant $w$ while the latter bounds are still compatible with LCDM. For that we compare constraints on the EoS parameters $w_0$ and early time type $w_a$ (CPL) against those with a late time parameterisation on $w_a$ (LZ) and the constant $w$ model, using CMB, Supernovae and BAO from DESI datasets. We found, the same as was the case with CPL model, preference for dynamical dark energy within the LZ model, but for values almost symmetrically distributed with respect to their LCDM limits. This is due to the fact that the presence of $w_0$ allows to recast each parametrisation into making it compensate the preference for $w\sim -1$ in the opposite direction. To further test our hypothesis, we fixed $w_0$ to -1 and followed a minimal approach by considering models that deviates by one free parameter, or we extend to more general models that either group both late and early effects, or allow the presence of two dark energy fluid alike and constant alike component. We found that all the variants, except the original CPL are still compatible with LCDM, with likelihoods peaking close to $w_0 = -1$, $w_a = 0$, or 0.68 for $Ω_{\rm CC}$, with the constant $w$ and the late time $w_a$ having the smallest constraints. Although we found that the evidence from CPL is stronger than those for the more minimal cases, however the preference increases further for the more generalized parameterizations, while still staying compatible with LCDM in terms of the significance levels. We conclude that considering CPL model is not sufficient to test deviation from the standard model and that it is necessary to conduct further minimal or more general approaches to better understand the outcomes from model testing and inference methods.(abridged)

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arXiv2025

The case for a low dark matter density in dynamical dark energy model from local probes

In this work we investigate, through a Bayesian study, the ability of a local low matter density $Ω_{\rm M}$, in discrepancy with the value usually inferred from the CMB angular power spectrum, to accommodate observations from local probes without being in tension with the local values of the Hubble constant $H_0$ or the matter fluctuation $σ_8$ parameters. For that, we combine multiple local probes, with the criteria that they either can constrain the matter density parameter independently from the CMB constraints, or can help in doing so after making their relevant observations more model independent by relaxing their relevant calibration parameters. We assume however, either a dynamical dark energy model, or the standard $Λ$CDM model, when computing the corresponding theoretical observables. We also add, in almost all of our Monte Carlo runs, the latest Baryonic acoustic oscillations (BAO) measurements from the DESI year one release to our core group. We found that, within $Λ$CDM model, for different combinations of our probes, we can accommodate a low matter density along with the $H_0$ and $σ_8$ values usually obtained from local probes, providing we promote the sound drag $r_s$ component in BAO calculations to a free parameter, and that even if we combine with the Pantheon+ Supernova sample. Assuming $w_0w_a$CDM, we also found that relaxing $r_s$ allow us to accommodate $Ω_{\rm M}$, $H_0$ and $σ_8$ within their local values, with still however a preference for $w_0w_a$ values far from $Λ$CDM. However, when including Pantheon+ Supernova sample, we found that the latter preference for high matter density pushes $σ_8$ to much smaller values, mitigating by then a low matter density solution to the two common tensions. We conclude that a low matter density value, helps in preserving the concordance within $Λ$CDM model. (abridged)

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arXiv2025

Model-independent forecasts for the cosmological anisotropic stress

The effective anisotropic stress $η$ is a key variable in the characterization of many classes of modified gravity theories, as it allows the testing for a long-range force additional to gravity. In this paper we forecast the precision with which future large surveys can determine $η$ in a way that only relies on directly observable quantities obtained from the spectroscopic measurements of the clustering of galaxies and the photometric based observation of the projected lensing and galaxy clustering correlations and their cross signal. Our method does not require further assumptions about the initial power spectrum, the modified gravity model, the expansion rate, or the bias. We consider various cases: $η$ free to vary in space and time, or with only redshift dependence, or constant. We take as a reference specifications that approximate a Euclid-like photometric or a combined one with a DESI-like spectroscopic survey. Among our results, we find that a future large-scale lensing and clustering survey can constrain $η$ to at least 30\% if $z$, $k$ independent, and to less than 10\% on average for the $z$ dependence only, to finally reach 5\% values in the constant case.

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arXiv2024

The distribution of Bayes' ratio

The ratio of Bayesian evidences is a popular tool in cosmology to compare different models. There are however several issues with this method: Bayes' ratio depends on the prior even in the limit of non-informative priors, and Jeffrey's scale, used to assess the test, is arbitrary. Moreover, the standard use of Bayes' ratio is often criticized for being unable to reject models. In this paper, we address these shortcoming by promoting evidences and evidence ratios to frequentist statistics and deriving their sampling distributions. By comparing the evidence ratios to their sampling distributions, poor fitting models can now be rejected. Our method additionally does not depend on the prior in the limit of very weak priors, thereby safeguarding the experimenter against premature rejection of a theory with a uninformative prior, and replaces the arbitrary Jeffrey's scale by probability thresholds for rejection. We provide analytical solutions for some simplified cases (Gaussian data, linear parameters, and nested models), and we apply the method to cosmological supernovae Ia data. We dub our method the FB method, for Frequentist-Bayesian.

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