Planck 2018 results. I. Overview and the cosmological legacy of Planck (1807.06205v2)

Abstract: The European Space Agency's Planck satellite, which was dedicated to studying the early Universe and its subsequent evolution, was launched on 14 May 2009. It scanned the microwave and submillimetre sky continuously between 12 August 2009 and 23 October 2013, producing deep, high-resolution, all-sky maps in nine frequency bands from 30 to 857GHz. This paper presents the cosmological legacy of Planck, which currently provides our strongest constraints on the parameters of the standard cosmological model and some of the tightest limits available on deviations from that model. The 6-parameter LCDM model continues to provide an excellent fit to the cosmic microwave background data at high and low redshift, describing the cosmological information in over a billion map pixels with just six parameters. With 18 peaks in the temperature and polarization angular power spectra constrained well, Planck measures five of the six parameters to better than 1% (simultaneously), with the best-determined parameter (theta_*) now known to 0.03%. We describe the multi-component sky as seen by Planck, the success of the LCDM model, and the connection to lower-redshift probes of structure formation. We also give a comprehensive summary of the major changes introduced in this 2018 release. The Planck data, alone and in combination with other probes, provide stringent constraints on our models of the early Universe and the large-scale structure within which all astrophysical objects form and evolve. We discuss some lessons learned from the Planck mission, and highlight areas ripe for further experimental advances.

• The paper establishes precise limits on the ΛCDM model using Planck’s high-resolution full-sky CMB maps.
• The paper exploits advanced polarization and lensing analyses to validate cosmic structure formation and the role of dark energy.
• The paper refines reionization history and neutrino mass constraints through improved data processing and comprehensive simulations.

Overview of the Cosmological Legacy of the Planck 2018 Release

The paper "Planck 2018 results. I. Overview and the cosmological legacy of Planck" presents an exhaustive account of the Planck satellite mission, primarily aimed at delineating its contributions to cosmology through its observations of the cosmic microwave background (CMB). Serving as the Planck mission’s final data release, it integrates significant improvements and methodological refinements over its previous iterations, presenting a comprehensive dataset for cosmology.

The Planck mission, launched by the European Space Agency, achieved remarkable advances in the measurement of CMB anisotropies. Describing data from the microwave to the submillimeter range, the mission produced high-resolution, full-sky maps across nine frequency bands. It quantifies the sky with unprecedented precision by mapping the temperature fluctuations and polarization of the CMB, providing stringent tests on cosmological parameters and gravitational models.

Key Scientific Advances

1. Constraints on Cosmological Models:
   • The Planck 2018 data sets the most precise limits on the parameters of the standard cosmological model, $\Lambda$CDM. The six parameters of this model, which delineate the Universe's composition and geometry with remarkable accuracy—apart from the $\tau$ parameter which characterizes optical depth.
   • Planck confirms the Universe's isotropy and homogeneity, underpinning the inflationary model's predictions and reinforcing the foundational assumptions of modern cosmology.
2. Lensing and Structure Formation:
   • Planck's full-sky lensing map delivers the highest signal-to-noise ratio obtained to date from cosmic lensing, providing insights into the distribution of mass in the Universe. These measurements affirm gravitational instability as the driver of cosmic structure formation, validating the presence of cold dark matter and elucidating the role of dark energy in the Universe's accelerated expansion.
3. Neutrino Physics:
   • The dataset constrains the sum of neutrino masses to less than 0.12 eV, tightly bound the count of neutrino species, suggesting no significant deviation from the Standard Model's prediction, except for light thermal relics allowed by earlier models.
4. Reionization History:
   • The release features significant improvements in polarization data, allowing for an updated assessment of cosmic reionization, suggesting it was late and fast, providing pinpoint constraints on the ionization fraction history and reducing the need for non-standard early Universe models.

Methodological Enhancement

The 2018 Planck release utilized improved data processing techniques resulting in more accurate map-making and calibration strategies. HFI and LFI processing advancements reduced systematic uncertainties, notably in polarization measurements. The inclusion of a sophisticated end-to-end simulation package bolstered the characterization of the mission data's noise properties, allowing robust foreground subtraction and component separation.

Implications and Future Directions

Planck’s comprehensive dataset strengthens the case for the $\Lambda$CDM model while establishing constraints on possible extensions, including primordial non-Gaussianity limits and dark energy behavior. Although the Planck mission has concluded, it will serve as a benchmark for upcoming CMB studies that aim to probe secondary phenomena like the influence of gravitational waves, precision lensing studies, and the deep polarization measurements.

Comparative assessments highlight areas of minor tension, such as the debated $H_0$ value contrasting local measurements, offering a rich field for future exploration. The dataset's legacy encompasses both a foundation for testing fundamental physics and a continued impetus for probing cosmological mysteries with future missions complementing Planck's observations. The paper concludes by acknowledging that while the 2018 release is comprehensive, ongoing analyses and forthcoming missions will continue to refine our understanding of cosmic dynamics.