The CatWISE2020 Quasar dipole: A Reassessment of the Cosmic Dipole Anomaly

Abstract: The cosmological principle, which asserts a statistically homogeneous and isotropic universe on large scales, is a foundational assumption of the standard cosmological model. A critical test of this principle involves the kinematic interpretation of the Cosmic Microwave Background temperature dipole, conventionally attributed to our peculiar motion relative to the cosmic rest frame. The Ellis-Baldwin test provides a probe of this kinematic interpretation by searching for a matching Doppler-driven dipole in the number counts of extragalactic radio sources. Recent measurements from the CatWISE2020 quasar catalog have reported a dipole amplitude significantly exceeding the kinematic expectation, with a claimed significance of $4.9\sigma$. We present a comprehensive reassessment of this test using the same dataset, incorporating major sources of uncertainty in the statistical inference. We use a simulation framework based on the FLASK package, incorporating lognormal realizations of the large-scale structure, the quasar clustering bias, the survey's radial selection function, and its exact sky coverage. Our simulations account for the kinematic dipole, the intrinsic clustering dipole, shot noise, and survey geometry effects. The analysis yields a revised significance of the kinematic dipole excess of $3.63\sigma$ in the absence of a clustering dipole, and $3.44\sigma$ in the presence of a randomly oriented clustering dipole. When the clustering dipole is aligned with the kinematic dipole direction, the significance decreases further to $3.27\sigma$. Our analysis demonstrates that although the anomaly is reduced in significance, it cannot be explained solely as a result of the clustering dipole or mode coupling arising from the survey mask.