Testing the Cosmological Principle with CatWISE Quasars: A Bayesian Analysis of the Number-Count Dipole

Abstract: The Cosmological Principle, that the Universe is homogeneous and isotropic on sufficiently large scales, underpins the standard model of cosmology. However, a recent analysis of 1.36 million infrared-selected quasars has identified a significant tension in the amplitude of the number-count dipole compared to that derived from the CMB, thus challenging the Cosmological Principle. Here we present a Bayesian analysis of the same quasar sample, testing various hypotheses using the Bayesian evidence. We find unambiguous evidence for the presence of a dipole in the distribution of quasars with a direction that is consistent with the dipole identified in the CMB. However, the amplitude of the dipole is found to be 2.7 times larger than that expected from the conventional kinematic explanation of the CMB dipole, with a statistical significance of $5.7\sigma$. To compare these results with theoretical expectations, we sharpen the $\Lambda$CDM predictions for the probability distribution of the amplitude, taking into account a number of observational and theoretical systematics. In particular, we show that the presence of the Galactic plane mask causes a considerable loss of dipole signal due to a leakage of power into higher multipoles, exacerbating the discrepancy in the amplitude. By contrast, we show using probabilistic arguments that the source evolution of quasars improves the discrepancy, but only mildly so. These results support the original findings of an anomalously large quasar dipole, independent of the statistical methodology used.

• The paper finds that the quasar number-count dipole is 2.7 times greater than predicted by the kinematic CMB dipole with a 5.7σ significance.
• It employs a rigorous Bayesian framework to account for masking biases, clustering effects, and sample selection in the CatWISE quasar data.
• The results challenge the standard cosmic isotropy assumption, suggesting the need for revisions to existing ΛCDM cosmological models.

Bayesian Analysis of the Number-Count Dipole: Examining the Cosmological Principle with CatWISE Quasars

The paper "Testing the Cosmological Principle with CatWISE Quasars: A Bayesian Analysis of the Number-Count Dipole" presents a rigorous investigation into the alignment of the cosmological large-scale structure with the underlying assumptions of isotropy and homogeneity, often referred to as the Cosmological Principle. Using a vast sample of quasars selected via the CatWISE2020 catalog, the authors apply a sophisticated Bayesian statistical framework to explore the validity of the kinematic interpretation of the cosmic microwave background (CMB) dipole.

The crux of the analysis lies in the intriguing discrepancy between the amplitude of the number-count dipole derived from quasars and that predicted by the conventional kinematic explanation from the CMB dipole. The research identifies an anomalously large dipole amplitude, approximately 2.7 times the expected value, with a significant statistical deviation of $5.7\sigma$. Such a result starkly challenges the expected explanation anchored in the standard cosmic model.

A central component of the analysis involves considering the geometric configuration of the mask applied to the quasar sample due to galactic and ecliptic influences on the quasar density. The paper meticulously accounts for this through parametric modeling that accommodates varying assumptions about these influences, expressed in the exploration of hypotheses with a Bayesian evidence comparison. The introduction of corrections for the ecliptic bias particularly stands out, supporting the presence of a non-isotropic distribution in the universe.

The theoretical derivation of the expected probability distribution for the dipole amplitude accounts for full-sky versus partial-sky observations, employing cosmological predictions from the $\Lambda$CDM model to inform the expected power distributions of the dipole. This includes rigorous calculations of cross-correlations between kinematic and clustering dipoles and the inclusion of clustering biases, shot noise, and source evolution throughout the analysis. Nonetheless, it is evident in this study that neither the suppression of power due to the masking effects nor potential systematic biases fully account for the observed dipole anomaly.

The implications of these findings carry profound theoretical weight, necessitating a reevaluation of how cosmic isotropy is quantitatively understood and modeled within current cosmological frameworks. Should these results hold upon further scrutiny and comparison with other surveys, it would suggest an incomplete understanding and characterization of cosmic anisotropy on large scales, potentially leading to adjustments in the fundamental assumptions behind cosmological modeling.

In terms of future directions, extending this Bayesian analysis to other independent datasets, such as additional quasar catalogs and different celestial structures like galaxies and radio sources could yield more comprehensive insights into the nature of cosmic anisotropy. Additionally, refining theoretical models that incorporate redshift dependencies, evolution biases, and other systematic effects could enhance the predictive power of cosmological interpretations, paving the way for potentially new physics that accounts for such discordance with current models.

In conclusion, this Bayesian analysis enriches our understanding of cosmological distribution with empirical evidence pointing to substantial deviations from isotropy, prompting a reevaluation of existing cosmic structure models and the assumptions that underpin them.