The Atacama Cosmology Telescope: A Measurement of the DR6 CMB Lensing Power Spectrum and its Implications for Structure Growth

We present new measurements of cosmic microwave background (CMB) lensing over 9400 sq. deg. of the sky. These lensing measurements are derived from the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) CMB dataset, which consists of five seasons of ACT CMB temperature and polarization observations. We determine the amplitude of the CMB lensing power spectrum at 2.3% precision (43σ significance) using a novel pipeline that minimizes sensitivity to foregrounds and to noise properties. To ensure our results are robust, we analyze an extensive set of null tests, consistency tests, and systematic error estimates and employ a blinded analysis framework. The baseline spectrum is well fit by a lensing amplitude of A_lens=1.013±0.023 relative to the Planck 2018 CMB power spectra best-fit ΛCDM model and A_lens=1.005±0.023 relative to the ACT DR4 + WMAP best-fit model. From our lensing power spectrum measurement, we derive constraints on the parameter combination S^CMBL_8 ≡σ_8 (Ω_m/0.3)^0.25 of S^CMBL_8= 0.818±0.022 from ACT DR6 CMB lensing alone and S^CMBL_8= 0.813±0.018 when combining ACT DR6 and Planck NPIPE CMB lensing power spectra. These results are in excellent agreement with ΛCDM model constraints from Planck or ACT DR4 + WMAP CMB power spectrum measurements. Our lensing measurements from redshifts z∼0.5–5 are thus fully consistent with ΛCDM structure growth predictions based on CMB anisotropies probing primarily z∼1100. We find no evidence for a suppression of the amplitude of cosmic structure at low redshifts