New constraints on cosmic anisotropy from galaxy clusters using an improved 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 applied the dipole fitting (DF) method to galaxy clusters to search for cosmic anisotropic signals and establish a statistical isotropy analysis scheme. Compared to Type Ia supernovae (SNe Ia), 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 two preferred directions (l, b) = (${257.82^{\circ}}_{-52.88}^{+58.01}$, $-31.30{^{\circ}}_{-39.46}^{+35.92}$) and ($80.89{^{\circ}}_{-52.46}^{+60.97}$, $31.75{^{\circ}}_{-40.16}^{+35.19}$). The former aligns with the direction of faster cosmic expansion, while the latter points toward slower expansion. The corresponding magnitude of anisotropy is $|A| \approx 5.2 \sim 5.4 \times 10^{-4}$, with statistical isotropy analyses yielding a confidence level of $\sim 1.0\sigma$. Subsample reanalyses categorized by instrumentation (Chandra and XMM-Newton) and redshift (low-redshift, $z \leq 0.10$; high-redshift, $z > 0.10$) reveal that the choice of equipment and the sample redshift influence the preferred direction, anisotropic magnitude, and statistical significance. Notably, the XMM-Newton dataset yields a statistical significance of $2.26\sigma$ (Bootstrap) and $2.86\sigma$ (Randomized), which are considerably higher than those from the Chandra or total datasets.