Get started
Work

Cosmology-dependent covariance in galaxy cluster number counts: consequences for parameter inference

Year: 2026
Type: article
Abstract: Galaxy clusters provide powerful constraints on cosmology through their abundance as a function of mass and redshift. Parameter inference from cluster counts requires modelling the data covariance entering the likelihood, including contributions from Poisson shot noise and super-sample covariance (SSC) induced by long-wavelength density fluctuations. While both the mean counts and their covariance depend on cosmology, evaluating the full covariance during parameter inference can be computationally expensive, particularly for SSC terms. As a result, many analyses adopt approximations in which the covariance is computed at a single fiducial cosmology, either from simulations or theoretical predictions, and kept fixed during inference. In this work, we investigate the impact of constructing the covariance matrix at an incorrect fiducial cosmology and quantify how this assumption propagates into the estimation of Ω c , σ 8 , and w . We perform a systematic analysis in which the covariance is either varied consistently with the sampled cosmology or fixed at displaced cosmological models, and we also explore intermediate strategies in which selected components, such as the computationally expensive SSC term, are kept fixed while others vary. Our analysis includes observational effects relevant for optical surveys, including mass-proxy scatter and photometric redshift uncertainties, and focuses on configurations representative of LSST-like surveys. We find that the estimators of Ω c , σ 8 , and w remain unbiased even when the covariance is constructed at an incorrect cosmology; however, fixing the covariance can lead to significant over- or underestimation of credible regions. The magnitude and sign of this effect are primarily driven by parameters controlling the amplitude of matter fluctuations, such as ln(10 10 A s ) and S 8 , and for LSST-like surveys, an inconsistent covariance specification can artificially modify the apparent S 8 tension inferred from cluster counts. We further show that a single covariance update evaluated at the recovered best-fit cosmology is sufficient to restore the correct uncertainties. These results indicate that while fixed-covariance approximations may remain adequate for some single-probe analyses, a fully cosmology-dependent treatment is required for consistent multi-probe studies.
DOI: 10.1088/1475-7516/2026/10/050
Open Access status: green
OpenAlex ID: W7166700736
FWCI:
Cites:
Cited by:
Related to: