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1. Atacama Cosmology Telescope: B-mode delensing with DR6 data and external tracers of large-scale structure

   https://doi.org/10.1103/ztry-pnlx  

   Hertig, Emilie; Baleato_Lizancos, Antón; Qu, Frank J; Bond, J Richard; Calabrese, Erminia; Challinor, Anthony; Devlin, Mark J; Dunkley, Jo; Louis, Thibaut; Madhavacheril, Mathew S; et al (June 2026, Physical Review D)

   Large-scale $B$ -mode polarization of the cosmic microwave background (CMB) is a prime target for current and future experiments in search of primordial gravitational waves. With increasingly sensitive instruments being deployed, secondary $B$ -modes induced by the weak gravitational lensing of CMB photons are becoming an important limitation and need to be removed, a process known as delensing. In this work, we combine internally reconstructed CMB lensing maps from the Atacama Cosmology Telescope (ACT) data release 6 (DR6) with galaxy samples from unWISE and a map of the cosmic infrared background (CIB) fluctuations from Planck to produce a well-correlated tracer of the CMB lensing field. Our coadded tracer, shown to be 55%–85% correlated with the true lensing convergence at multipoles $L\le 2000$ , is then convolved with ACT DR6 $E$ -mode polarization to yield a template of the lensing $B$ -modes. We assess its performance on a wide range of scales by using it to delens ACT DR6 and Planck $B$ -modes over 23% of the sky, removing around 39% of the lensing power at $100\le l\le 1500$ and 47% at $30\le l\le 300$ , respectively. Our template achieves the highest delensing efficiency to date and will be useful for the analysis of early polarization maps from the Simons Observatory. We finally outline prospects for further improvements by including additional large-scale structure tracers from upcoming cosmological surveys. 

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

   https://doi.org/10.3847/1538-4357/acfe06  

   Qu, Frank J; Sherwin, Blake D; Madhavacheril, Mathew S; Han, Dongwon; Crowley, Kevin T; Abril-Cabezas, Irene; Ade, Peter_A R; Aiola, Simone; Alford, Tommy; Amiri, Mandana; et al (February 2024, The Astrophysical Journal)

   Abstract We present new measurements of cosmic microwave background (CMB) lensing over 9400 deg²of the sky. These lensing measurements are derived from the Atacama Cosmology Telescope (ACT) Data Release 6 (DR6) CMB data set, 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 (43σsignificance) using a novel pipeline that minimizes sensitivity to foregrounds and to noise properties. To ensure that our results are robust, we analyze an extensive set of null tests, consistency tests, and systematic error estimates and employ a blinded analysis framework. Our CMB lensing power spectrum measurement provides constraints on the amplitude of cosmic structure that do not depend on Planck or galaxy survey data, thus giving independent information about large-scale structure growth and potential tensions in structure measurements. The baseline spectrum is well fit by a lensing amplitude ofA_lens= 1.013 ± 0.023 relative to the Planck 2018 CMB power spectra best-fit ΛCDM model andA_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 $S_8^{\mathrm{CMBL}}\equiv {\sigma }_8{\left({\mathrm{\Omega }}_m/0.3\right)}^{0.25}$ of $S_8^{\mathrm{CMBL}}=0.818\pm 0.022$ from ACT DR6 CMB lensing alone and $S_8^{\mathrm{CMBL}}=0.813\pm 0.018$ when combining ACT DR6 and PlanckNPIPECMB 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 redshiftsz∼ 0.5–5 are thus fully consistent with ΛCDM structure growth predictions based on CMB anisotropies probing primarilyz∼ 1100. We find no evidence for a suppression of the amplitude of cosmic structure at low redshifts. 

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

   https://doi.org/10.3847/1538-4357/acff5f  

   Madhavacheril, Mathew S; Qu, Frank J; Sherwin, Blake D; MacCrann, Niall; Li, Yaqiong; Abril-Cabezas, Irene; Ade, Peter_A R; Aiola, Simone; Alford, Tommy; Amiri, Mandana; et al (February 2024, The Astrophysical Journal)

   Abstract We present cosmological constraints from a gravitational lensing mass map covering 9400 deg²reconstructed from measurements of the cosmic microwave background (CMB) made by the Atacama Cosmology Telescope (ACT) from 2017 to 2021. In combination with measurements of baryon acoustic oscillations and big bang nucleosynthesis, we obtain the clustering amplitudeσ₈= 0.819 ± 0.015 at 1.8% precision, $S_8\equiv {\sigma }_8{\left({\mathrm{\Omega }}_{\mathrm{m}}/0.3\right)}^{0.5}=0.840\pm 0.028$ , and the Hubble constantH₀= (68.3 ± 1.1) km s⁻¹Mpc⁻¹at 1.6% precision. A joint constraint with Planck CMB lensing yieldsσ₈= 0.812 ± 0.013, $S_8\equiv {\sigma }_8{\left({\mathrm{\Omega }}_{\mathrm{m}}/0.3\right)}^{0.5}=0.831\pm 0.023$ , andH₀= (68.1 ± 1.0) km s⁻¹Mpc⁻¹. These measurements agree with ΛCDM extrapolations from the CMB anisotropies measured by Planck. We revisit constraints from the KiDS, DES, and HSC galaxy surveys with a uniform set of assumptions and find thatS₈from all three are lower than that from ACT+Planck lensing by levels ranging from 1.7σto 2.1σ. This motivates further measurements and comparison, not just between the CMB anisotropies and galaxy lensing but also between CMB lensing probingz∼ 0.5–5 on mostly linear scales and galaxy lensing atz∼ 0.5 on smaller scales. We combine with CMB anisotropies to constrain extensions of ΛCDM, limiting neutrino masses to ∑m_ν< 0.13 eV (95% c.l.), for example. We describe the mass map and related data products that will enable a wide array of cross-correlation science. Our results provide independent confirmation that the universe is spatially flat, conforms with general relativity, and is described remarkably well by the ΛCDM model, while paving a promising path for neutrino physics with lensing from upcoming ground-based CMB surveys. 

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4. Galactic Foreground Constraints on Primordial B-mode Detection for Ground-based Experiments

   https://doi.org/10.3847/1538-4357/ac3092  

   Hervías-Caimapo, Carlos; Bonaldi, Anna; Brown, Michael L.; Huffenberger, Kevin M. (January 2022, The Astrophysical Journal)

   Abstract Contamination by polarized foregrounds is one of the biggest challenges for future polarized cosmic microwave background (CMB) surveys and the potential detection of primordial B -modes. Future experiments, such as Simons Observatory (SO) and CMB-S4, will aim at very deep observations in relatively small ( f sky ∼ 0.1) areas of the sky. In this work, we investigate the forecasted performance, as a function of the survey field location on the sky, for regions over the full sky, balancing between polarized foreground avoidance and foreground component separation modeling needs. To do this, we simulate observations by an SO-like experiment and measure the error bar on the detection of the tensor-to-scalar ratio, σ ( r ), with a pipeline that includes a parametric component separation method, the Correlated Component Analysis, and the use of the Fisher information matrix. We forecast the performance over 192 survey areas covering the full sky and also for optimized low-foreground regions. We find that modeling the spectral energy distribution of foregrounds is the most important factor, and any mismatch will result in residuals and bias in the primordial B -modes. At these noise levels, σ ( r ) is not especially sensitive to the level of foreground contamination, provided the survey targets the least-contaminated regions of the sky close to the Galactic poles. 

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5. Atacama Cosmology Telescope: Multiprobe cosmology with unWISE galaxies and ACT DR6 CMB lensing

   https://doi.org/10.1103/PhysRevD.111.083516  

   Farren, Gerrit S; Krolewski, Alex; Qu, Frank J; Ferraro, Simone; Calabrese, Erminia; Dunkley, Jo; Embil_Villagra, Carmen; Hill, J Colin; Kim, Joshua; Madhavacheril, Mathew S; et al (April 2025, Physical Review D)

   We present a joint analysis of the cosmic microwave background (CMB) lensing power spectra measured from the Data Release 6 of the Atacama Cosmology Telescope (ACT) and PR4, cross-correlations between the ACT and lensing reconstruction and galaxy clustering from unWISE, and the unWISE clustering auto-spectrum. We obtain 1.5% constraints on the matter density fluctuations at late times parametrized by the best constrained parameter combination $S_8^{3\mathrm{x}2\mathrm{pt}}\equiv {\sigma }_8({\mathrm{\Omega }}_m/0.3{)}^{0.4}=0.815\pm 0.012$ . The commonly used $S_8\equiv {\sigma }_8({\mathrm{\Omega }}_m/0.3{)}^{0.5}$ parameter is constrained to $S_8=0.816\pm 0.015$ . In combination with baryon acoustic oscillation (BAO) measurements we find ${\sigma }_8=0.815\pm 0.012$ . We also present sound-horizon-independent estimates of the present day Hubble rate of $H_0={66.4}_{-3.7}^{+3.2}\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$ from our large scale structure data alone and $H_0={64.3}_{-2.4}^{+2.1}\mathrm{km}{\mathrm{s}}^{-1}{\mathrm{Mpc}}^{-1}$ in combination with uncalibrated supernovae from . Using parametric estimates of the evolution of matter density fluctuations, we place constraints on cosmic structure in a range of high redshifts typically inaccessible with cross-correlation analyses. Combining lensing cross- and autocorrelations, we derive a 3.3% constraint on the integrated matter density fluctuations above $z=2.4$ , one of the tightest constraints in this redshift range and fully consistent with a $\mathrm{\Lambda }$ cold dark matter ( $\mathrm{\Lambda }\mathrm{CDM}$ ) model fit to the primary CMB from . Finally, combining with primary CMB observations and using the extended low redshift coverage of these combined datasets we derive constraints on a variety of extensions to the $\mathrm{\Lambda }\mathrm{CDM}$ model including massive neutrinos, spatial curvature, and dark energy. We find in flat $\mathrm{\Lambda }\mathrm{CDM}\sum m_{\nu }<0.12\mathrm{eV}$ at 95% confidence using the large scale structure data, BAO measurements from Sloan Digital Sky Survey, and primary CMB observations. 

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