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The Atacama Cosmology Telescope: A demonstration of CMB lensing measurement from daytime data

Irene Abril-Cabezas ^1,2,*, Frank J. Qu^3,4, Joshua Kim ⁵, Mathew S. Madhavacheril ⁵, Karen Perez-Sarmiento ⁵, Zachary Atkins ^5,6, Erminia Calabrese ⁷, Anthony Challinor ^8,2,1, Mark J. Devlin ⁵ et al.

Adriaan J. Duivenvoorden⁹, Jo Dunkley ¹⁰, Alexander van Engelen¹¹, Simone Ferraro ^12,13, Emily Finson ¹⁴, Carlos Hervías-Caimapo^3,4,15, Matt Hilton ^16,17, Arthur Kosowsky ¹⁸, Aleksandra Kusiak ^8,2, Thibaut Louis ¹⁹, Niall MacCrann^1,2, Kavilan Moodley ¹⁷, Toshiya Namikawa ^20,1,2, Sigurd Næss²¹, Lyman A. Page ⁶, Adrien La Posta ²², Emmanuel Schaan ^3,4, Neelima Sehgal ¹⁴, Blake D. Sherwin^1,2, Carlos E. Sierra^3,4, Cristóbal Sifón ²³, Suzanne T. Staggs⁶, Emilie Storer^24,6, and Edward J. Wollack ²⁵

¹DAMTP, Centre for Mathematical Sciences, University of Cambridge, Wilberforce Road, Cambridge CB3 OWA, United Kingdom
²Kavli Institute for Cosmology Cambridge, Madingley Road, Cambridge, CB3 0HA, United Kingdom
³Kavli Institute for Particle Astrophysics and Cosmology, 382 Via Pueblo Mall, Stanford, California 94305-4060, USA
⁴SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
⁵Department of Physics and Astronomy, University of Pennsylvania, 209 South 33rd Street, Philadelphia, Pennsylvania 19104, USA
⁶Joseph Henry Laboratories of Physics, Jadwin Hall, Princeton University, Princeton, New Jersey 08544, USA
⁷School of Physics and Astronomy, Cardiff University, The Parade, Cardiff, Wales CF24 3AA, United Kingdom
⁸Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, United Kingdom
⁹Max-Planck-Institut fur Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany
¹⁰Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, New Jersey 08544, USA
¹¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA
¹²Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720, USA
¹³Berkeley Center for Cosmological Physics, Department of Physics, University of California, Berkeley, California 94720, USA
¹⁴Physics and Astronomy Department, Stony Brook University, Stony Brook, New York 11794, USA
¹⁵Instituto de Astrofísica and Centro de Astro-Ingeniería, Facultad de Física, Pontificia Universidad Católica de Chile, Avenida Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹⁶Wits Centre for Astrophysics, School of Physics, University of the Witwatersrand, Private Bag 3, 2050, Johannesburg, South Africa
¹⁷Astrophysics Research Centre, School of Mathematics, Statistics, and Computer Science, University of KwaZulu-Natal, Westville Campus, Durban 4041, South Africa
¹⁸Department of Physics and Astronomy, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, USA
¹⁹Université Paris-Saclay, CNRS/IN2P3, IJCLab, 91405 Orsay, France
²⁰Center for Data-Driven Discovery, Kavli IPMU (WPI), UTIAS, The University of Tokyo, Kashiwa, 277-8583, Japan
²¹Institute of Theoretical Astrophysics, University of Oslo, Norway
²²Department of Physics, University of Oxford, Keble Road, Oxford, OX1 3RH, United Kingdom
²³Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
²⁴Physics Department, McGill University, Montreal, QC H3A 0G4, Canada
²⁵NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, Maryland 20771, USA

^*Contact author: ia404@cam.ac.uk

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Phys. Rev. D 114, 063524 – Published 14 September, 2026

DOI: https://doi.org/10.1103/3ksm-wlxr

Abstract

We present a cosmic microwave background (CMB) lensing power spectrum analysis using daytime data (11am–11pm UTC) gathered by the Atacama Cosmology Telescope (ACT) over the period 2017–2022 (ACT Data Release 6). This dataset is challenging to analyze because the Sun heats and deforms the telescope mirror, complicating the characterization of the telescope. We perform more than one hundred null and consistency checks to ensure the robustness of our measurement and its compatibility with nighttime observations. We detect the CMB lensing power spectrum at $18 σ$ significance, with an amplitude $A_{lens} = 1.071 \pm 0.060$ with respect to the prediction from the best-fit Planck-ACT CMB power spectrum $Λ CDM$ cosmology. In combination with the Dark Energy Spectroscopic Instrument (DESI) Baryon Acoustic Oscillation (BAO) data, this corresponds to a constraint on the amplitude of matter fluctuations $σ_{8} = 0.842 \pm 0.026$ . The analysis presented here is especially relevant for ground-based millimeter-wave CMB experiments at the Atacama site, paving the way for future analyses making use of both nighttime and daytime data to place tight constraints on cosmological parameters.

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Phys. Rev. D 114, 063524– Published 14 September, 2026

Vol. 114, Iss. 6 — 15 September 2026

Received 28 November 2025
Accepted 3 August 2026

DOI: https://doi.org/10.1103/3ksm-wlxr

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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The Atacama Cosmology Telescope: A demonstration of CMB lensing measurement from daytime data

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