Planck 2018 results. VII. Isotropy and Statistics of the CMB (1906.02552v2)

Abstract: Analysis of the Planck 2018 data set indicates that the statistical properties of the cosmic microwave background (CMB) temperature anisotropies are in excellent agreement with previous studies using the 2013 and 2015 data releases. In particular, they are consistent with the Gaussian predictions of the $\Lambda$CDM cosmological model, yet also confirm the presence of several so-called "anomalies" on large angular scales. The novelty of the current study, however, lies in being a first attempt at a comprehensive analysis of the statistics of the polarization signal over all angular scales, using either maps of the Stokes parameters, $Q$ and $U$, or the $E$-mode signal derived from these using a new methodology (which we describe in an appendix). Although remarkable progress has been made in reducing the systematic effects that contaminated the 2015 polarization maps on large angular scales, it is still the case that residual systematics (and our ability to simulate them) can limit some tests of non-Gaussianity and isotropy. However, a detailed set of null tests applied to the maps indicates that these issues do not dominate the analysis on intermediate and large angular scales (i.e., $\ell \lesssim 400$). In this regime, no unambiguous detections of cosmological non-Gaussianity, or of anomalies corresponding to those seen in temperature, are claimed. Notably, the stacking of CMB polarization signals centred on the positions of temperature hot and cold spots exhibits excellent agreement with the $\Lambda$CDM cosmological model, and also gives a clear indication of how Planck provides state-of-the-art measurements of CMB temperature and polarization on degree scales.

• The paper confirms that CMB temperature anisotropies follow Gaussian ΛCDM predictions while identifying large-scale anomalies.
• The study introduces a refined polarization analysis using purified inpainting to effectively reduce E/B mixing and systematic residuals.
• Null hypothesis tests across intermediate scales find no non-Gaussian signatures in polarization, reinforcing standard cosmological models.

Analysis of Planck 2018 Results on Isotropy and Statistics of the CMB

The publication under review encapsulates a thorough investigation of the cosmic microwave background (CMB) data, recorded by the Planck satellite during its 2018 mission. The focal point of this paper is to assess the isotropy and Gaussian nature of the CMB, particularly investigating large-scale anomalies and the polarization anisotropies.

Key Findings

• Temperature Anisotropies: The temperature fluctuations of the CMB align with the Gaussian predictions of the $\Lambda$ Cold Dark Matter (ΛCDM) model. The analysis corroborates previous datasets from the 2013 and 2015 Planck releases.
• Large-scale Anomalies: Despite general consistency with Gaussian statistics, the data confirm the presence of anomalies at large angular scales, noted in prior studies as well. These anomalies include hemispherical power asymmetry and deviations at higher multipoles that align with the direction of the Planck scanning strategy.
• Polarization Signal: A significant effort is focused on improving the analysis of the polarization data. The paper presents a refined methodology that mitigates systematic residuals, previously problematic in the 2015 polarization maps.
• Null Hypothesis Tests: On intermediate and large angular scales (ℓ ≲ 400), the paper finds no conclusive evidence of cosmological non-Gaussianity or anomalies akin to those observed in temperature anisotropies using the polarization data.
• Stacking Analyses: Examining CMB polarization around temperature extrema shows good agreement with the ΛCDM model, highlighting the accuracy of Planck polarization measurements on degree scales.

Methodological Advances

The paper introduces a comprehensive approach for analyzing the polarization data, employing "E- and B-mode map reconstruction" through a method termed as "purified inpainting". This technique is conducive to reducing E/B mixing—an issue that arises due to mask application in creating $E$ and $B$ mode maps from Stokes parameters, $Q$ and $U$.

Implications and Future Directions

1. Anomalies and Cosmic Inflation: The results uphold the standard cosmological model but continue to recognize large-scale anomalies. These anomalies present opportunities for constraining theoretical models of cosmic inflation and any associated deviations from isotropy.
2. Data Consistency and Systematics: The paper underscores the necessity of refining instrument models and simulations to improve the characterization of systematic effects, particularly essential for future CMB polarization studies.
3. Polarization as a Probe: Although limited by current noise levels and systematic uncertainties, polarization remains a critical probe for cross-verifying and extending our understanding of observed anomalies.

Future work will likely focus on gaining higher sensitivities in polarization measurements to detect subtle effects that could inform us about physics beyond the standard model. Enhanced observational techniques and upcoming missions could provide a clearer picture and more robust tests of isotropy and Gaussianity in our Universe's structure.