[Paper Review] Planck 2018 results. I. Overview and the cosmological legacy of Planck

Planck Collaboration, Y. Akrami+189 more|arXiv (Cornell University)|Jul 17, 2018

Cosmology and Gravitation TheoriesPhysics and Astronomy385 references295 citations

TL;DR

This paper presents the cosmological legacy of the Planck space mission, delivering the most precise constraints on the standard ΛCDM model using full-sky cosmic microwave background (CMB) maps from 2009 to 2013. It confirms the ΛCDM model with unprecedented accuracy—measuring five of six parameters to better than 1% precision, including θ∗ at 0.03%—and sets stringent limits on deviations from standard physics, while highlighting persistent tensions with low-redshift probes.

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.

Motivation & Objective

• To deliver the final cosmological results from the Planck mission, culminating in the most accurate constraints on the ΛCDM model.
• To test the robustness of the ΛCDM model using high-resolution, all-sky CMB maps across nine frequency bands.
• To identify and quantify tensions between high-redshift CMB data and low-redshift large-scale structure measurements.
• To provide a comprehensive, publicly available data release with improved calibration, component separation, and likelihood pipelines.
• To assess the implications of Planck data for fundamental physics, including inflation, neutrino masses, dark matter, and dark energy.

Proposed method

• Acquisition of continuous, high-sensitivity, all-sky microwave and submillimeter observations from 30 to 857 GHz over 4.5 years (2009–2013).
• Application of advanced component separation algorithms (e.g., Commander, SMICA) to disentangle CMB from Galactic and extragalactic foregrounds.
• High-precision estimation of temperature and polarization angular power spectra across 18 peaks, with cosmic variance-limited measurements up to multipole ℓ ≈ 1600.
• Development of new likelihood pipelines (e.g., Commander, Simall, Planck LFI/HFI likelihoods) for temperature, polarization, and lensing reconstruction.
• Cross-correlation of CMB lensing with large-scale structure surveys (e.g., weak lensing, BAO, cluster counts) to test ΛCDM consistency.
• Use of extensive simulations to validate systematics, calibration, and likelihood uncertainties across the full data stream.

Experimental results

Research questions

• RQ1 How precisely can the six-parameter ΛCDM model be constrained using the full Planck CMB data set?
• RQ2 To what extent do Planck measurements confirm the predictions of inflationary cosmology and Gaussian initial conditions?
• RQ3 Are there significant tensions between Planck's high-redshift CMB constraints and low-redshift probes of H0, σ8, or S8?
• RQ4 What are the tightest current limits on neutrino masses, dark matter annihilation, and primordial gravitational waves from Planck data?
• RQ5 How well do the Planck results support the cosmological constant and general relativity on large scales?

Key findings

• The ΛCDM model fits the Planck CMB data with exceptional precision, measuring five of six parameters to better than 1% accuracy, with the best-determined parameter (θ∗) known to 0.03%.
• The spatial curvature is constrained to be flat at the 5×10⁻³ level, supporting a spatially flat universe with high significance.
• Neutrino masses are constrained to O(0.1 eV), and the number of relativistic species is consistent with three light neutrinos, disfavoring additional light relics.
• The amplitude of primordial fluctuations is consistent with Gaussianity to an exceptional degree, with no evidence for primordial gravitational waves at the 5% level.
• Measurements of the Hubble constant (H₀) and σ₈ from low-redshift probes (e.g., weak lensing, Type Ia supernovae) show a 3–4σ tension with Planck’s ΛCDM predictions, suggesting possible systematic errors or new physics.
• The Planck data strongly disfavor baryonic dark matter and confirm that the primordial fluctuations originated at very early times, consistent with inflationary models.