Planck CMB Anomalies: Astrophysical and Cosmological Secondary Effects and the Curse of Masking

Abstract: Large-scale anomalies have been reported in CMB data with both WMAP and Planck data. These could be due to foreground residuals and or systematic effects, though their confirmation with Planck data suggests they are not due to a problem in the WMAP or Planck pipelines. If these anomalies are in fact primordial, then understanding their origin is fundamental to either validate the standard model of cosmology or to explore new physics. We investigate three other possible issues: 1) the trade-off between minimising systematics due to foreground contamination (with a conservative mask) and minimising systematics due to masking, 2) astrophysical secondary effects (the kinetic Doppler quadrupole and kinetic Sunyaev-Zel'dovich effect), and 3) secondary cosmological signals (the integrated Sachs-Wolfe effect). We address the masking issue by considering new procedures that use both WMAP and Planck to produce higher quality full-sky maps using the sparsity methodology (LGMCA maps). We show the impact of masking is dominant over that of residual foregrounds, and the LGMCA full-sky maps can be used without further processing to study anomalies. We consider four official Planck PR1 and two LGMCA CMB maps. Analysis of the observed CMB maps shows that only the low quadrupole and quadrupole-octopole alignment seem significant, but that the planar octopole, Axis of Evil, mirror parity and cold spot are not significant in nearly all maps considered. After subtraction of astrophysical and cosmological secondary effects, only the low quadrupole may still be considered anomalous, meaning the significance of only one anomaly is affected by secondary effect subtraction out of six anomalies considered. In the spirit of reproducible research all reconstructed maps and codes will be made available for download here http://www.cosmostat.org/anomaliesCMB.html.

• The paper demonstrates that advanced component separation techniques, like LGMCA, reduce the need for aggressive masking in CMB anomaly analysis.
• It finds that while astrophysical effects such as the kinetic Doppler quadrupole and Sunyaev-Zel'dovich signals introduce alignments, their low amplitudes limit overall impact.
• Subtracting cosmological secondary signals, including the ISW effect, diminishes several anomalies, yet the persistent low quadrupole remains challenging.

Analysis of Planck CMB Anomalies: A Critical Examination of Secondary Effects and Masking Techniques

The study of cosmic microwave background (CMB) anomalies has gained significant attention, particularly in assessing the statistical isotropy and Gaussianity of early Universe fluctuations. The paper "Planck CMB Anomalies: Astrophysical and Cosmological Secondary Effects and the Curse of Masking" by Rassat et al. investigates large-scale anomalies identified in CMB data from WMAP and Planck. These anomalies, if confirmed as primordial, could suggest deviations from the standard cosmological model or point towards new physics.

Summary of Key Findings

The authors explore three critical concerns potentially linked to these anomalies:

1. Masking Issues: The traditional approach involves using masks to mitigate foreground residuals, particularly emissions from our galaxy. The authors develop and utilize the LGMCA method to produce high-quality, full-sky maps for CMB analysis. Their analysis indicates that masking can exert a stronger effect on observed anomalies than foreground residuals, challenging the necessity of aggressive masking techniques when sophisticated foreground subtraction methods like LGMCA are employed.
2. Astrophysical Secondary Effects: The kinetic Doppler quadrupole (kDq) and kinetic Sunyaev-Zel'dovich (kSZ) effects are examined as potential contributors to observed anomalies. While the intrinsic quadrupole and octopole alignments of the kSZ signal are noteworthy, its low amplitude precludes significant influence on anomalies. Subtraction of these effects after characterizing them provides a clearer view of primordial anomalies.
3. Cosmological Secondary Effects: The integrated Sachs-Wolfe (ISW) effect is studied using galaxy data from 2MASS and NVSS. While the removal of known secondary cosmological signals is shown to diminish the significance of certain anomalies, the fundamental question remains as to why such alignments occur, warranting further examination.

Analysis of Large-Scale Anomalies

The investigation scrutinizes several large-scale anomaly statistics, including the low quadrupole, quadrupole-octopole alignment, planar octopole, Axis of Evil, parity asymmetry, and the cold spot. It concludes that:

• Quadrupole-Octopole Alignment:

Initially appearing significant in various Planck map renditions, this alignment loses statistical significance once secondary effects are duly subtracted.

• Low Quadrupole:

Remains consistently anomalous across maps, even after various secondary effects are factored out. This persistence highlights the need to consider it as a potentially fundamental feature warranting further cosmological insight.

• Other Anomalies:

Such as the Axis of Evil and mirror parity, largely lose their significance upon refined analysis, suggesting they may originate from secondary effects or systematic errors introduced by masking.

Practical and Theoretical Implications

The revelations from this paper hold substantial implications in both practical methodologies and theoretical models of cosmology. Practically, the findings question the continued heavy reliance on aggressive masking when robust component separation techniques can minimize biases and reduce the potential distortion of large-scale CMB statistics. Theoretically, the results stress the importance of sophisticated model inclusion of astrophysical and cosmological secondary effects in data interpretation for accurate validation of cosmological models.

Future Directions

Looking forward, there is ample scope for further research into advanced component separation techniques that could offer even cleaner CMB maps, allowing for refined insights into the authenticity and source of these anomalies. Advances in astronomical data integration will further enhance our understanding of secondary cosmological effects. Continued efforts in these domains could prove instrumental in addressing existing CMB anomalies, and by extension, informing new physics beyond the standard cosmological models.

Ultimately, Rassat et al.'s research lays a foundation for re-evaluating established methodologies in CMB analyses and encourages a more nuanced approach to discerning the aggregate implications of secondary effects and masking choices on the perceived anomalies in the cosmic landscape.