Planck 2018 results. I. Overview and the cosmological legacy of Planck

TL;DR

Planck 2018 consolidates a comprehensive cosmological legacy from the CMB, confirming the remarkable success of the $\Lambda$CDM framework with high-precision constraints on its six base parameters and on crucial extensions. The paper details improved data processing, polarization-focused foreground separation, and a rich multi-frequency sky model, including an all-sky CMB map and a robust lensing reconstruction. It reports stringent cosmological bounds: $\Omega_K$ consistent with flatness, $\sum m_\nu < 0.12\,\mathrm{eV}$ (95% CL), $N_{\rm eff} = 3.01 \pm 0.35$, and a reionization optical depth $\tau = 0.056 \pm 0.007$, among others, with $n_s < 1$ at high significance. While Planck finds overall concordance with other probes, mild tensions remain (e.g., Ly$\alpha$ distances and low-$z$ $S_8$ hints, and the $H_0$ discrepancy with local measurements), highlighting areas for future observational and theoretical work. The Legacy also emphasizes that Planck priors underpin contemporary cosmology and that future experiments will build on its groundwork to probe primordial gravitational waves, reionization details, and possible new physics.

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.

Figures (2)

• Figure 1: Fluctuations of sky emission in each of nine Planck frequency bands, after removal of a common dipole component. The fluctuations are expressed as equivalent temperature variations at each of the seven lowest frequencies, so that fluctuations with a thermal spectrum will appear the same in each map (except for the effects of the varying resolution of the maps). The highest frequencies, which monitor the dust emission, are expressed in more conventional units.
• Figure 2: Sky polarization in seven polarized frequency bands of Planck. The first two columns show the $Q$ and $U$ Stokes parameters; the last column indicates the polarized intensity, $P=\sqrt{Q^2+U^2}$ (although this emphasizes the strength of polarization in noisy regions). In addition to the rich science that they enable on their own, these maps set the baseline for all future CMB polarization experiments, for example by defining the most cosmologically challenged areas.