Speaker: Konstantinos Migkas (chair: Laure Bouscasse)
Title: Is the Universe isotropic? Using galaxy clusters to find out…
The Cosmological Principle is a fundamental pillar of the standard cosmological models. The way we understand the Universe is heavily based on the latter being isotropic. Surprisingly enough however, this hypothesis still remains ambiguous. In order to introduce a new test for the isotropy of the Universe, we use the eeHIFLUGCS galaxy cluster sample to investigate the directional behavior of the X-ray luminosity-temperature (L_X-T) relation of galaxy clusters. A tight correlation is known to exist between the two quantities of the X-ray-emitting intracluster medium of galaxy clusters, and while the luminosity measurement depends on the chosen cosmology through the luminosity distance, the temperature, flux and redshift determinations are cosmology-independent. Thus, the isotropy of the luminosity distance over the full extragalactic sky can be mirrored in the behavior of the L_X-T scaling relation. The eeHIFLUGCS sample is ideal for such a study due to its large number of clusters (contains the 400 brightest X-ray clusters in the sky), and the uniform full-sky coverage it provides. Consistently with the results of our previous work, the behavior of the L_X-T relation of eeHIFLUGCS heavily depends on the direction of the sky. The presence of a dipole-like anisotropy is detected with a sim 3.5 sigma (99.95\%) significance, centered at the Galactic coordinates of (l,b)~(275 deg, -15 deg), 66 deg away from the CMB dipole, but roughly consistent with the results of other cosmological probes. Several reasons that could potentially explain this were tested, such as the existence of unknown hydrogen “dark” clouds, the effect of bulk flows, the bias introduced by double clusters etc., but none was able to explain the obtained results in a satisfactory degree. Interestingly enough, other cluster samples appear to have a similar behavior throughout the sky, while being completely independent to each other and to eeHIFLUGCS. Combining all 3 samples, results to 844 different galaxy clusters with indepentent luminosity and temperature measurements. Performing a joint analysis, the final anisotropy of the obtained Hubble constant is further intensified (>6 sigma), with a dipole appearing towards (l,b)~(258 deg, -9 deg), separated by 56 deg from the CMB dipole. This result strongly demonstrates that X-ray studies that assume perfect isotropy in the properties of galaxy clusters, can produce heavily biased results (e.g. X-ray mass determination of galaxy clusters for cosmological studies). The identification of the factor that causes such behavior is of the utmost importance.