Tomographic redshift dipole: Testing the cosmological principle

The cosmological principle posits that the universe is statistically homogeneous and isotropic on large scales, implying all matter shares the same rest frame. This principle suggests that velocity estimates of our motion from various sources should agree with the cosmic microwave background (CMB) dipole's inferred velocity of 370 km/s. Yet, for over two decades, analyses of radio galaxy and quasar catalogs have found velocities at odds with the CMB dipole, with tensions up to 5$\sigma$. In a blind analysis of BOSS and eBOSS spectroscopic data from galaxies and quasars across $0.2<z<2.2$, we applied a novel dipole estimator for a tomographic approach, robustly correcting biases and quantifying uncertainties with realistic mock catalogs. Our findings with eBOSS data ($0.6<z<2.2$), indicating a velocity of $196^{+92}_{-79}$ km/s, demonstrate a $2\sigma$ agreement with the CMB dipole when considering the full 3D vector distribution and a 3-to-6$\sigma$ tension with previous number count studies. This result supports the cosmological principle, emphasizing the consistency of our motion with the CMB across vast cosmic distances. On the other hand, the BOSS data revealed potential unmodeled systematics; the estimator could not be minimized using the LOWZ set ($0.2<z<0.4$), and the CMASS set ($0.4<z<0.6$) presented results that pointed towards the southern hemisphere, conflicting with the CMB dipole. Addressing the disparities with earlier number count analyses and understanding possible systematics in spectroscopic measurements will be essential to further validate the cosmological principle.