The SRG/eROSITA All-Sky Survey

The evolution of the cluster mass function traces the growth of the linear density perturbations and can be utilized to constrain the parameters of cosmological and alternative gravity models. In this context, we present new constraints on potential deviations from general relativity by investigating the Hu-Sawicki parametrization of the f (R) gravity with the first Spectrum Roentgen Gamma (SRG)/eROSITA All-Sky Survey (eRASS1) cluster catalog in the western Galactic hemisphere in combination with the overlapping Dark Energy Survey Year-3, KiloDegree Survey, and Hyper Suprime-Cam data for weak lensing mass calibration. For the first time, we present constraints obtained from cluster abundances only. When we consider massless neutrinos, we find a strict upper limit of log |fR0| < −4.31 at a 95% confidence level. Massive neutrinos suppress structure growth at small scales, and thus have the opposite effect of f (R) gravity. We consequently investigate the joint fit of the mass of the neutrinos with the modified gravity parameter. We obtain log |fR0| < −4.08 jointly with ∑ mν < 0.49 eV at a 95% confidence level, which is tighter than the limits in the literature utilizing cluster counts only. At log |fR0| = −6, the number of clusters is not significantly changed by the theory. Consequently, we do not find any statistical deviation from general relativity in the study of eRASS1 cluster abundance. Deeper surveys with eROSITA, increasing the number of detected clusters, will further improve constraints on log |fR0| and investigate alternative gravity theories.