The Atacama Cosmology Telescope: DR6 Gravitational Lensing Map and Cosmological Parameters

TL;DR

ACT DR6 delivers a wide-area, high-fidelity CMB lensing mass map and uses its power spectrum, combined with BAO and Planck lensing, to measure the amplitude of matter fluctuations with unprecedented precision. The results affirm ΛCDM predictions, yielding σ8 ≈ 0.812–0.819 and H0 ≈ 68 km s^-1 Mpc^-1, while constraining the sum of neutrino masses to ∑mν < 0.13 eV (95% c.l.). Cross-checks with galaxy weak lensing show mild tension in S8, underscoring the value of cross-probe analyses across redshift ranges and scales. The work also provides a robust data release, including mass maps, masks, simulations, and a public likelihood, paving the way for future neutrino physics and dark-energy studies with upcoming CMB surveys.

Abstract

We present cosmological constraints from a gravitational lensing mass map covering 9400 sq. deg. reconstructed from CMB measurements made by the Atacama Cosmology Telescope (ACT) from 2017 to 2021. In combination with BAO measurements (from SDSS and 6dF), we obtain the amplitude of matter fluctuations $σ_8 = 0.819 \pm 0.015$ at 1.8% precision, $S_8\equivσ_8({Ω_{\rm m}}/0.3)^{0.5}=0.840\pm0.028$ and the Hubble constant $H_0= (68.3 \pm 1.1)\, \text{km}\,\text{s}^{-1}\,\text{Mpc}^{-1}$ at 1.6% precision. A joint constraint with CMB lensing measured by the Planck satellite yields even more precise values: $σ_8 = 0.812 \pm 0.013$, $S_8\equivσ_8({Ω_{\rm m}}/0.3)^{0.5}=0.831\pm0.023$ and $H_0= (68.1 \pm 1.0)\, \text{km}\,\text{s}^{-1}\,\text{Mpc}^{-1}$. These measurements agree well with $Λ$CDM-model extrapolations from the CMB anisotropies measured by Planck. To compare these constraints to those from the KiDS, DES, and HSC galaxy surveys, we revisit those data sets with a uniform set of assumptions, and find $S_8$ from all three surveys are lower than that from ACT+Planck lensing by varying levels ranging from 1.7-2.1$σ$. These results motivate further measurements and comparison, not just between the CMB anisotropies and galaxy lensing, but also between CMB lensing probing $z\sim 0.5-5$ on mostly-linear scales and galaxy lensing at $z\sim 0.5$ on smaller scales. We combine our CMB lensing measurements with CMB anisotropies to constrain extensions of $Λ$CDM, limiting the sum of the neutrino masses to $\sum m_ν < 0.13$ eV (95% c.l.), for example. Our results provide independent confirmation that the universe is spatially flat, conforms with general relativity, and is described remarkably well by the $Λ$CDM model, while paving a promising path for neutrino physics with gravitational lensing from upcoming ground-based CMB surveys.

Figures (17)

• Figure 1: Mass-map weights for CMB and galaxy weak lensing, normalized to the maximum value. The blue solid curve shows the relative weights different redshifts receive in a mass map reconstructed from CMB lensing (as in this work) and the orange solid curve shows the same for a sample of galaxies at $z=1$ (typical of current galaxy lensing surveys). The dashed curves show the corresponding source distribution, with that for the CMB centered at the redshift of last-scattering around $z=1100$. The comoving distances to the peak redshifts are roughly 1 Gpc (galaxy lensing) and 5 Gpc (CMB lensing). An angular scale of $\sim 1 \deg$ or a lens multipole of $L=200$ then corresponds to comoving wave-numbers at those distances of roughly 0.2 Mpc$^{-1}$ (galaxy lensing) and 0.04 Mpc$^{-1}$ (CMB lensing).
• Figure 2: Overlap of the ACT mass map (red) with various ongoing galaxy surveys. The green contours show a rough union of the footprint of SDSS, the DECam Legacy Survey and the Mayall z-band Legacy Survey, with Dark Energy Spectroscopic Instrument (DESI) data expected to be available in part of this region 1807.0928710.3847/1538-3881/ab089d. The grayscale background is a Galactic dust map from Planck1502.01588.
• Figure 3: ACT DR6 CMB lensing mass map presented in this work. The map covers $9400\,{\rm{deg}}^2$ or sky fraction $f_{\rm sky}=0.23$ with a signal-to-noise significantly greater than unity over a wide range of scales. We show the Wiener-filtered CMB lensing convergence in an orthographic projection with bright orange corresponding to peaks of the dark-matter dominated mass distribution and dark purple regions corresponding to voids. Dark-matter dominated structures on few-degree scales corresponding to the peak of the lensing power spectrum can be seen by eye (see also Figure \ref{['fig:Nell']}). The grayscale background is a Galactic dust map from Planck1502.01588; our analysis mask is designed to avoid dusty regions of the sky. The region in the gray box is shown in Figure \ref{['fig:zoom']}.
• Figure 4: A zoom-in of a $900\,{\rm{deg}}^2$ region of the ACT DR6 mass map shown as the Wiener-filtered gravitational potential (related to the convergence through $\nabla^2\phi=-2\kappa$). The distribution of dusty galaxies constituting the CIB measured by Planck is overlaid as contours. The overdensities in red correspond well with the bright/white mass-dominated regions of the mass map and the underdensities in blue correspond well with the darker mass-devoid regions.
• Figure 5: The ACT DR6 CMB lensing power spectrum measurement, from Qu23. The bandpowers of the two-point statistics of the DR6 mass map are shown as red data points. The black solid curve shows the prediction for this signal in the $\Lambda$CDM model based on the measurement of the primary CMB anisotropies by the Planck satellite; i.e., this prediction is not a fit to the ACT data. The prediction and our measurement (presented in detail in the companion paper, Qu23) are in excellent agreement, showing the success of the $\Lambda\rm{CDM}$ model in propagating a measurement of the radiation anisotropies at age of the universe $t\simeq 375,000$ years ($z\simeq 1100$) to the matter fluctuations at $t\simeq 1-9$ billion years ($z\simeq 0.5-5$). We also show samples (orange) from $\Lambda$CDM chains of the Planck primary CMB anisotropy measurements to highlight the uncertainty in the early universe prediction. The dotted, dashed, and dot-dashed curves show the noise power spectra (i.e., the variance of the reconstruction noise per mode) in the mass maps produced by Planck PR3 1807.06210, PlanckNPIPE2206.07773, and this work, respectively. The ACT mass map is signal-dominated out to $L\simeq 150$, providing a high-fidelity view of the dark-matter-dominated mass distribution. The dark gray regions are not included in our analysis and the light gray region is included in our "extended" analyses. The top axis shows the comoving wave-number $k=L/\chi(z_p)$ at the peak redshift of the CMB lensing kernel $z_p=2$.