Probing cosmic anisotropy from galaxy clusters via the dipole fitting method

Abstract

The cosmological principle, as the cornerstone of the standard cosmological model, requires that the universe be homogeneous and isotropic on large scales. As a fundamental assumption, it is constantly subjected to testing via various datasets and methods. In this work, we used the dipole fitting (DF) method to correct the logarithmic luminosity ($\log{L_{X}}$) of galaxy clusters, search for cosmic anisotropic signals, and establish a statistical isotropic analysis scheme. Compared to the type Ia supernovae (SNe Ia), the galaxy clusters offer a superior spatial distribution, which enhances the reliability of the identified anisotropic signals. Using a sample of 313 galaxy clusters (observed by Chandra and XMM-Newton), we identified the preferred direction (l, b) = (${257.82^{\circ}}_{-52.88}^{+58.01}$, $-31.30{^{\circ}}_{-39.46}^{+35.92}$) of the cosmic anisotropy. The corresponding magnitude of anisotropy is $A$ = $-5.4 \times 10^{-4}$. Subsample reanalyses categorized by instrumentation (Chandra and XMM-Newton) and redshift (low-redshift, $z \leq 0.10$; high-redshift, $z > 0.10$) revealed more significant anisotropic signals. The XMM-Newton dataset yields a statistical significance of $1.8\sigma$ (Bootstrap) and $1.9\sigma$ (Randomized), which are considerably higher than those from the Chandra or total datasets. Meanwhile, the reanalyses also reveal that the choice of equipment and the sample redshift influence the preferred direction, anisotropic magnitude, and statistical significance obtained from galaxy clusters. Overall, the DF method can be well integrated with galaxy clusters and applied to the detection of cosmic anisotropy.