Planck 2018 results. V. CMB power spectra and likelihoods (1907.12875v2)

Abstract: This paper describes the 2018 Planck CMB likelihoods, following a hybrid approach similar to the 2015 one, with different approximations at low and high multipoles, and implementing several methodological and analysis refinements. With more realistic simulations, and better correction and modelling of systematics, we can now make full use of the High Frequency Instrument polarization data. The low-multipole 100x143 GHz EE cross-spectrum constrains the reionization optical-depth parameter $\tau$ to better than 15% (in combination with with the other low- and high-$\ell$ likelihoods). We also update the 2015 baseline low-$\ell$ joint TEB likelihood based on the Low Frequency Instrument data, which provides a weaker $\tau$ constraint. At high multipoles, a better model of the temperature-to-polarization leakage and corrections for the effective calibrations of the polarization channels (polarization efficiency or PE) allow us to fully use the polarization spectra, improving the constraints on the $\Lambda$CDM parameters by 20 to 30% compared to TT-only constraints. Tests on the modelling of the polarization demonstrate good consistency, with some residual modelling uncertainties, the accuracy of the PE modelling being the main limitation. Using our various tests, simulations, and comparison between different high-$\ell$ implementations, we estimate the consistency of the results to be better than the 0.5$\sigma$ level. Minor curiosities already present before (differences between $\ell$<800 and $\ell$>800 parameters or the preference for more smoothing of the $C_\ell$ peaks) are shown to be driven by the TT power spectrum and are not significantly modified by the inclusion of polarization. Overall, the legacy Planck CMB likelihoods provide a robust tool for constraining the cosmological model and represent a reference for future CMB observations. (Abridged)

• The paper introduces a hybrid approach combining low- and high-multipole analyses to enhance CMB likelihood evaluations.
• It achieves over 15% tightening on reionization optical-depth and 20-30% improved precision in high-multipole cosmological parameters.
• The results establish a robust legacy framework for future CMB observations and advanced testing of the ΛCDM model.

Overview of the Planck 2018 CMB Power Spectra and Likelihoods

The presented Planck 2018 results focus on the legacy Cosmic Microwave Background (CMB) likelihoods derived from the final data release, building upon the methodologies adopted in previous Planck releases of 2013 and 2015. The paper outlines refined approaches to CMB spectrum analysis, highlighting methodological improvements, data-processing refinements, and simulation-driven validations essential for the final legacy analysis. The structure involves a hybrid method using different approximations at low multipoles (ℓ < 30) and high multipoles (ℓ ≥ 30).

Methodological and Data Processing Advancements

• Hybrid Approach and Likelihood Construction: The 2018 analysis maintains a two-pronged approach—with low multipole analysis emphasizing integrative techniques supported by realistic simulations and substantial methodological refinements. Particular focus is placed on making full use of the CMB polarization data from the High Frequency Instrument (HFI) channels, previously limited by systematics corrections and modeling accuracies.
• High-Multipole Spectra: For high multipoles, advancements in temperature-to-polarization leakage and calibration processes have facilitated more accurate constraint placement on cosmological parameters like the cold dark matter density parameter (Ω_CDM), baryonic matter density (Ω_b), and the Hubble constant (H_0), with noted improvements surpassing 20%-30% in precision over TT-only constraints. Notably, these leaps in precision are partly attributed to the better modeling of these leakages and inefficiencies.

Results and Validation

The key numerical results demonstrate substantial tightening of constraints on the reionization optical-depth parameter (τ), improved by more than 15% in combination with other CMB spectra data components. Similarly, the paper presents thorough validations using extensive simulations to test the consistency and robustness of their methodological improvements, especially focusing on the low-multipole EE cross-spectra.

Implications and Theoretical Significance

• Parameter Constraints and Impacts: A landmark implication of this paper is the narrowed error bars and improved confidence intervals around significant cosmological parameters, notably τ, reflecting advancements in noise and systematic control, along with refined correction modeling. This significant reduction in parameter uncertainties influences cosmological models' accuracy and precision, driving advancements in theoretical predictions and practical cosmological explorations, including inflation and structure formation models.
• Groundwork for Future Observations: These results provide an important legacy reference framework for future CMB observations, offering a meticulously validated standard for comparing forthcoming observational data. Continued improvements in systematic control and data processing will be critical as cosmology explores low-signal regions, enhancing the overall depth of astronomical inquiries into the universe's evolution.

Future Prospects

Moving forward, theoretical and observational cosmology will leverage these constrained models to explore extensions beyond the current ΛCDM framework. This includes exploring potential alterations or additions to the cosmic model, such as varying dark energy equations of state or investigating neutrino mass hierarchies. The understanding gleaned from Planck's legacy data will be pivotal in these pursuits, setting a benchmark for upcoming missions and ground telescopes, which will aim to extend the CMB's astrophysical and cosmological insights.

In conclusion, Planck's 2018 final CMB results set a new standard in precision cosmology. Beyond the numerical advancements, the refined methods and corrections signify critical steps towards further unraveling the cosmic history. This paper not only cements Planck's role as a keystone in cosmic exploration but also illuminates paths for future work in the quest to map the universe’s most ancient light.