Solar system peculiar motion from the Hubble diagram of quasars and testing the Cosmological Principle

Abstract: We determine here peculiar motion of the Solar system, first time from the $m-z$ Hubble diagram of quasars. Observer's peculiar motion causes a systematic shift in the $m-z$ plane between sources lying along the velocity vector and those in the opposite direction, providing a measure of the peculiar velocity. Accordingly, from a sample of $\sim 1.2 \times 10^5$ mid-infrared quasars with measured spectroscopic redshifts, we arrive at a peculiar velocity $\sim 22$ times larger than that from the CMBR dipole, but direction matching within $\sim 2\sigma$. Previous findings from number count, sky brightness or redshift dipoles observed in samples of distant AGNs or SNe Ia too had yielded values two to ten times larger than the CMBR value, %but this by far is the largest value arrived at for the peculiar motion, though the direction in all cases agreed with the CMBR dipole. Since a genuine solar peculiar velocity cannot vary from one dataset to the other, an order of magnitude, statistically significant, discordant dipoles, might imply that we may instead have to look for some other cause for the genesis of these dipole, including that of the CMBR. At the same time, a common direction for all these dipoles, determined from completely independent surveys by different groups employing different techniques, might indicate that these dipoles are not resulting from some systematics in the observations or in the data analysis, but could instead suggest a preferred direction in the Universe due to an inherent anisotropy, which, in turn, would be against the Cosmological Principle (CP), the most basic tenet of the modern cosmology.