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 $4.9σ$. 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 $3.63σ$ in the absence of a clustering dipole, and $3.44σ$ 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 $3.27σ$. 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.

Figures (6)

• Figure 1: Top: Sky mask for the CatWISE2020 quasar sample which excludes the Galactic plane ($|b| < 30^\circ$), regions around bright stars, and other areas with poor photometry, removing 52.6% of the sky. Bottom: Sky distribution of the CatWISE2020 quasar catalog used for dipole anisotropy studies.
• Figure 2: Top and middle panels: The mode-coupling matrix $M_{\ell\ell'}$ for the CatWISE2020 survey mask shown as a 2D heatmap (top) and as a 3D representation (middle). Prominent off-diagonal elements demonstrate significant power leakage from higher multipoles ($\ell'$) into lower multipoles ($\ell$). Bottom panel: $M_{1\ell'}$ plotted versus $\ell'$ illustrating the leakage of power from multipole $\ell'$ into the measured dipole ($\ell=1$). The plot clearly shows the decay of the coupling with increasing $\ell'$ and the enhanced leakage from odd multipoles compared to even ones (see the inset). For comparison, the coupling for a simple symmetrical galactic cut is also shown.
• Figure 3: The fractional deviation of the measured dipole amplitude, $(D_f - D_i)/D_i$, as a function of injected multipole power. The solid lines represent the mean across 1000 realizations, while the shaded regions indicate the $\pm1\sigma$ uncertainty intervals. Results are shown for the CatWISE2020 mask (orange) and symmetrical $10^\circ$ (green) and $30^\circ$ (blue) galactic cuts. Panels (a) and (c) show the results for even multipoles ($C_2$, $C_4$), where no systematic trend is observed. Panels (b) and (d) show the results for odd multipoles ($C_3$, $C_5$), demonstrating a clear positive correlation between the injected power and the fractional dipole deviation. For comparison, the solid and dotted black vertical lines represent the median $C_{\ell}$, and their corresponding 95% confidence limits, from FLASK simulations. Note that the $C_{\ell}$ values indicated here are not normalized by the monopole.
• Figure 4: Shot noise-subtracted angular power spectrum of the CatWISE2020 quasar distribution overdensity field ($\Delta n$) compared with FLASK simulations. The red points show the measured power spectrum from the real data, computed using NaMaster and averaged over 5 multipole bins. The solid black line represents the mean from 1000 FLASK mock realisations, while the shaded blue region indicates the $\pm 2 \sigma$ uncertainty derived from the simulations. Faint colored symbols display individual power spectra from the first 5 FLASK realisations. The CatWISE2020 measurements lie within the $2\sigma$ confidence interval for multipoles $\ell > 5$, indicating consistency with the $\Lambda$CDM-based FLASK simulations on intermediate and small angular scales. This figure contains the same information as figure 6 of Tiwari:2023.
• Figure 5: Probability density functions (PDFs) of the dipole amplitude ($\times 1000$) from six simulation scenarios. Across all subplots, the CatWISE2020 catalog dipole amplitude ($D^{obs}\times 10^3 = 15.54$) is indicated by the vertical red dashed line. The blue dashed and black dotted vertical lines represent the median and $3\sigma$ confidence level, respectively, specific to each simulation set.