Περίληψη :
The isotropy of the Universe is a fundamental pillar of the standard cosmological model. Surprisingly enough, however, this hypothesis still remains ambiguous based on several local probes. We use a novel method to test the local isotropy of the Universe using galaxy cluster scaling relations. Firstly, we utilize the X-ray luminosity-temperature (Lx-T) scaling relation of the eeHIFLUGCS, ACC, and XCS-DR1 cluster samples. While the temperature measurement does not depend on cosmology, the luminosity does, through the luminosity distance. Scanning the full extragalactic sky, we detect a region that seems to share a significantly different Lx-T relation than the rest of the sky across all three samples (>4σ). Many X-ray and cluster related reasons that could potentially explain this anisotropy were tested, but none was able to alleviate the tension. As more and more possible systematics are excluded, one can explain this anisotropy as an unexpectedly strong bulk flow motion, or even as local anisotropies in the expansion rate of the Universe. Similarly, 9 additional cluster scaling relations with independent cluster measurements are utilized to discover the anisotropy origin. Interestingly, our results seem to support the hypothesis that the anisotropies are of cosmological origin or large bulk flows. No matter what the anisotropy origin is, past X-ray (cluster) studies might need to consider how this effect would bias their results.
