The Atacama Cosmology Telescope: A Measurement of the DR6 CMB Lensing Power Spectrum and its Implications for Structure Growth (2304.05202v2)

Abstract: 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 ($43\sigma$ 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 $A_{\mathrm{lens}}=1.013\pm0.023$ relative to the Planck 2018 CMB power spectra best-fit $\Lambda$CDM model and $A_{\mathrm{lens}}=1.005\pm0.023$ relative to the $\text{ACT DR4} + \text{WMAP}$ best-fit model. From our lensing power spectrum measurement, we derive constraints on the parameter combination $S^{\mathrm{CMBL}}_8 \equiv \sigma_8 \left({\Omega_m}/{0.3}\right)^{0.25}$ of $S^{\mathrm{CMBL}}_8= 0.818\pm0.022$ from ACT DR6 CMB lensing alone and $S^{\mathrm{CMBL}}_8= 0.813\pm0.018$ when combining ACT DR6 and Planck NPIPE CMB lensing power spectra. These results are in excellent agreement with $\Lambda$CDM model constraints from Planck or $\text{ACT DR4} + \text{WMAP}$ CMB power spectrum measurements. Our lensing measurements from redshifts $z\sim0.5$--$5$ are thus fully consistent with $\Lambda$CDM structure growth predictions based on CMB anisotropies probing primarily $z\sim1100$. We find no evidence for a suppression of the amplitude of cosmic structure at low redshifts

Overview of the ACT DR6 CMB Lensing Power Spectrum Measurement

"The Atacama Cosmology Telescope: A Measurement of the DR6 CMB Lensing Power Spectrum and its Implications for Structure Growth" presents a comprehensive analysis of the CMB lensing power spectrum using data from the Atacama Cosmology Telescope's (ACT) sixth Data Release (DR6). This measurement is pivotal in enhancing our understanding of cosmic structure formation and in addressing current tensions in cosmological measurements.

The paper details the results derived from ACT's five-season CMB temperature and polarization observations, resulting in a remarkably precise measurement of the CMB lensing power spectrum with a significance of $43\sigma$. This precision is achieved through a novel analysis pipeline specifically optimized to minimize noise and foreground contamination, ensuring a robust measurement of the lensing amplitude.

Key Numerical Results and Claims

1. Lensing Amplitude and Power Spectrum Constraints: The analysis finds that the lensing power spectrum amplitude relative to the Planck 2018 CMB power spectra best-fit $\Lambda$CDM model is $A_{\mathrm{lens}}=1.013\pm0.023$. A similar consistency is found with an $A_{\mathrm{lens}}=1.005\pm0.023$ when compared against the $\text{ACT DR4} + \text{WMAP}$ best-fit model. These results underline a significant consistency with the $\Lambda$CDM structure growth predictions, confirming the validity of these fundamental cosmological parameters over large cosmic scales.
2. Parameter Constraint: The analysis provides constraints for the $S^{\mathrm{CMBL}_8 \equiv \sigma_8 \left({\Omega_m}/{0.3}\right)^{0.25}$ parameter at $0.818\pm0.022$ from ACT DR6 CMB lensing alone and $0.813\pm0.018$ upon combining ACT DR6 data with Planck NPIPE CMB lensing power spectra.

Implications

• Agreement Across Models: The measured lensing power spectrum aligns well with predictions from the standard $\Lambda$CDM model, reinforcing the model's robustness in characterizing cosmic structure growth. This agreement persists even when utilizing data from independent experiments (Planck and ACT+WMAP), thus fortifying confidence in these complementary datasets.
• Resolution to $S_8$ Tension: The ACT DR6 results do not indicate any suppression of cosmic structure amplitude at low redshifts, a factor that was contributing to the so-called $S_8$ tension where some recent surveys reported a lower than expected amplitude. The implications suggest that discrepancies or systematic issues in other studies may need reevaluation since ACT DR6 measurements independently verify the expected amplitude.
• Future Prospects: The results set a precedent for future developments in improving CMB lensing measurements' precision, particularly from ground-based facilities like ACT. As a precursor to surveys from the upcoming Simons Observatory, these findings indicate potential for further breakthroughs in understanding cosmic structure.

Conclusion

This paper is significant in consolidating the fixed lensing potential as a robust tool for testing extensions to the $\Lambda$CDM model and probing fundamental physics such as neutrino masses. The congruence with Planck and ACT+WMAP results underscores the mutual reinforcement and reliability of modern cosmological observations across different platforms. As future CMB surveys refine their techniques, the insights from this work could be instrumental in resolving outstanding questions in cosmology.