arXiv subjects

Rahima Mokeddem

Publications and source records attributed to Rahima Mokeddem.

4 recordsLinked to original sources

arXiv2025

Accessing the homogeneity scale with 21 cm intensity mapping surveys

The homogeneity scale, $R_{\rm H}$, offers a fundamental test of the Cosmological Principle, yet it has not yet been measured with 21cm intensity mapping surveys. A key limitation for such a measurement is the telescope beam, which artificially smooths the observed signal. We quantify this effect using the two-point correlation function and the correlation dimension, $\mathcal{D}_2(r)$, to model how beam convolution suppresses intrinsic clustering. For any given redshift $z$, we identify a maximum beam width, $\sigma_{\rm max}(z)$, beyond which the homogeneity scale cannot be recovered. This limit defines an inaccessible region in the $\sigma \times z$ parameter space, where $R_{\rm H}$ is erased by beam smoothing. Applying this framework to several current and upcoming radio telescopes, we assess their ability to probe $R_{\rm H}$. Our results provide the first quantitative forecast of the instrumental requirements for measuring the cosmic homogeneity scale with 21cm IM, and establish a theoretical basis for future observational applications.

astro-ph.CO↗

arXiv2025

Cosmological Remapping for Efficient Generation of 21 cm Intensity Mapping Mocks

We present a novel application of cosmological rescaling, or "remapping," to generate 21 cm intensity mapping mocks for different cosmologies. The remapping method allows for computationally efficient generation of N-body catalogs by rescaling existing simulations. In this work, we employ the remapping method to construct dark matter halo catalogs, starting from the Horizon Run 4 simulation with WMAP5 cosmology, and apply it to different target cosmologies, including WMAP7, Planck18 and Chevallier-Polarski-Linder (CPL) models. These catalogs are then used to simulate 21 cm intensity maps. We use the halo occupation distribution (HOD) method to populate halos with neutral hydrogen (HI) and derive 21 cm brightness temperature maps. Our results demonstrate the effectiveness of the remapping approach in generating cosmological simulations for large-scale structure studies, offering an alternative for testing observational data pipelines and performing cosmological parameter forecasts without the need for computationally expensive full N-body simulations. We also analyze the precision and limitations of the remapping, in light of the rescaling parameters $s$ and $s_m$, as well as the effects of the halo mass and box size thresholds.

astro-ph.CO↗

arXiv2025

Probing cosmic isotropy: Hubble constant and matter density large-angle variations with the Pantheon+SH0ES data

In this study we investigate potential large-angle anisotropies in the angular distribution of the cosmological parameters $H_0$ (the Hubble constant) and $\Omega_m$ (the matter density) in the flat-$\Lambda$CDM framework, using the Pantheon+SH0ES supernovae catalog. For this we perform a directional analysis by dividing the celestial sphere into a set of directions, and estimate the best-fit cosmological parameters across the sky using a MCMC approach. Our results show a dominant dipolar pattern for both parameters in study, suggesting a preferred axis in the universe expansion and in the distribution of matter. However, we also found that for $z \gtrsim 0.015$, this dipolar behavior is not statistically significant, confirming the expectation -- in the $\Lambda$CDM scenario -- of an isotropic expansion and a uniform angular distribution of matter (both results at $1\,\sigma$ confidence level). Nevertheless, for nearby supernovae, at distances $\lesssim 60$ Mpc or $z \lesssim 0.015$, the peculiar velocities introduce a highly significant dipole in the angular distribution of $H_0$. Furthermore, we perform various robustness tests that support our findings, and consistency tests of our methodology.

astro-ph.CO↗

arXiv2023

Excess of lensing amplitude in the Planck CMB power spectrum

Precise measurements of the Planck cosmic microwave background (CMB) angular power spectrum (APS) at small angles have stimulated accurate statistical analyses of the lensing amplitude parameter $A_{L}$. To confirm if it satisfies the value expected by the flat-$Λ$CDM concordance model, i.e. $A_{L} = 1$, we investigate the spectrum difference obtained as: the difference of the measured Planck CMB APS and the Planck best-fit $Λ$CDM APS model. To know if this residual spectrum corresponds to statistical noise or if it has a hiden signature that can be accounted for with a larger lensing amplitude $A_{L} > 1$, we apply the Ljung-Box statistical test and find, with high statistical significance, that the spectrum difference is not statistical noise. This spectrum difference is then analysed in detail using simulated APS, based on the Planck $Λ$CDM best-fit model, where the lensing amplitude is a free parameter. We explore different binnations of the multipole order \,$\ell$\, and look for the best-fit lensing amplitude parameter that accounts for the spectrum difference in a $χ^2$ procedure. We find that there is an excess of signal that is well explained by a $Λ$CDM APS with a non-null lensing amplitude parameter $A_{lens}$, with values in the interval $[0.10,0.29]$ at 68\% confidence level. Furthermore, the lensing parameter in the Planck APS should be $1 + A_{lens} > 1$ at $\sim 3 σ$ of statistical confidence. Additionally, we perform statistical tests that confirm the robustness of this result. Important to say that this excess of lensing amplitude, not accounted in the Planck's flat-$Λ$CDM model, could have an impact on the theoretical expectation of large-scale structures formation once the scales where it was detected correspond to these matter clustering processes.

astro-ph.CO↗