[Paper Review] Planck 2018 results. VIII. Gravitational lensing

Planck Collaboration, N. Aghanim+156 more|arXiv (Cornell University)|Jul 17, 2018

Cosmology and Gravitation TheoriesPhysics and Astronomy141 references198 citations

TL;DR

This paper presents the final Planck 2018 CMB lensing analysis, delivering high-significance lensing detections from polarization and temperature data, a joint Planck-CIB lensing map, delensing of CMB spectra, and robust cosmological parameter constraints within and beyond ΛCDM.

ABSTRACT

We present measurements of the cosmic microwave background (CMB) lensing potential using the final $ extit{Planck}$ 2018 temperature and polarization data. We increase the significance of the detection of lensing in the polarization maps from $5\,σ$ to $9\,σ$. Combined with temperature, lensing is detected at $40\,σ$. We present an extensive set of tests of the robustness of the lensing-potential power spectrum, and construct a minimum-variance estimator likelihood over lensing multipoles $8 \le L \le 400$. We find good consistency between lensing constraints and the results from the $ extit{Planck}$ CMB power spectra within the $ m{ΛCDM}$ model. Combined with baryon density and other weak priors, the lensing analysis alone constrains $σ_8 Ω_{ m m}^{0.25}=0.589\pm 0.020$ ($1\,σ$ errors). Also combining with baryon acoustic oscillation (BAO) data, we find tight individual parameter constraints, $σ_8=0.811\pm0.019$, $H_0=67.9_{-1.3}^{+1.2}\, ext{km}\, ext{s}^{-1}\, m{Mpc}^{-1}$, and $Ω_{ m m}=0.303^{+0.016}_{-0.018}$. Combining with $ extit{Planck}$ CMB power spectrum data, we measure $σ_8$ to better than $1\,\%$ precision, finding $σ_8=0.811\pm 0.006$. We find consistency with the lensing results from the Dark Energy Survey, and give combined lensing-only parameter constraints that are tighter than joint results using galaxy clustering. Using $ extit{Planck}$ cosmic infrared background (CIB) maps we make a combined estimate of the lensing potential over $60\,\%$ of the sky with considerably more small-scale signal. We demonstrate delensing of the $ extit{Planck}$ power spectra, detecting a maximum removal of $40\,\%$ of the lensing-induced power in all spectra. The improvement in the sharpening of the acoustic peaks by including both CIB and the quadratic lensing reconstruction is detected at high significance (abridged).

Motivation & Objective

• Demonstrate the final Planck 2018 CMB lensing reconstruction using temperature and polarization data across ~67-70% of the sky.
• Validate the robustness of the lensing power spectrum through extensive null tests and bias corrections.
• Provide cosmological parameter constraints from lensing alone and in combination with Planck CMB, BAO, and DES data.
• Develop a joint Planck-CIB lensing potential map to enhance small-scale information about the lensing signal.
• Show delensing of Planck power spectra and quantify the improvement of acoustic peak sharpness and B-mode power removal.

Proposed method

• Filter CMB maps with a Galactic mask and noise weighting to produce Wiener-filtered T, E, B multipoles.
• Construct quadratic lensing estimators from filtered maps to obtain a minimum-variance (MV) lensing potential estimator.
• Subtract mean-field biases and correct for N0, N1, and point-source biases in the lensing power spectrum.
• Bin the recovered C_L^κκ with a multiplicative Monte Carlo correction to obtain unbiased band powers.
• Compute a lensing likelihood for 8 ≤ L ≤ 400 and derive cosmological parameter constraints, including combinations like σ8Ωm^0.25.
• Combine Planck lensing with external data (BAO, DES) and with Planck CMB power spectra to obtain joint parameter constraints and consistency checks.

Experimental results

Research questions

• RQ1 What is the amplitude and statistical significance of the Planck-detected CMB lensing signal from polarization alone and from temperature+polarization data?
• RQ2 How robust is the reconstructed lensing power spectrum to foregrounds, systematics, and modeling biases (N0, N1, PS), and how well does it agree with ΛCDM predictions?
• RQ3 What cosmological parameter constraints can be derived from lensing data alone, and how do these constraints improve when combined with BAO, DES, and Planck CMB data?
• RQ4 Can a joint Planck-CIB lensing potential map be constructed to enhance high-ℓ lensing information, and what is its impact on the lensing signal?
• RQ5 To what extent can CMB lensing be used to delens Planck power spectra and sharpen acoustic peaks or reduce B-mode power?

Key findings

• Lensing is detected at 9σ from polarization alone and 40σ from the minimum-variance combination of temperature and polarization over 67% of the sky.
• Lensing constraints are consistent with Planck CMB power spectra within ΛCDM.
• Lensing alone constrains σ8Ωm^0.25 = 0.589 ± 0.020 (1σ).
• With BAO data, σ8 = 0.811 ± 0.019, H0 = 67.9^{+1.2}_{-1.3} km/s/Mpc, Ωm = 0.303^{+0.016}_{-0.018}.
• Combining with Planck CMB power spectra yields σ8 = 0.811 ± 0.006 with improved precision.
• A Planck-only joint with CIB provides a high-fidelity lensing potential map over ~60% of the sky, with enhanced small-scale signal.
• Delensing using the reconstructed lensing map removes up to ~40% of the lensing-induced power and sharpens acoustic peaks with high significance (11σ from reconstruction, 15σ with CIB).
• CMB lensing results are complementary to galaxy lensing and show consistency with DES lensing constraints when combined in a lensing-only analysis.