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    SPT-3G D1: Constraints on inflationary gravitational waves with two years of SPT-3G data

    J. A. Zebrowski1,2,3,*,†, C. L. Reichardt4, A. J. Anderson3,1,2, B. Ansarinejad4, M. Archipley2,1, L. Balkenhol5, P. S. Barry6, K. Benabed5, A. N. Bender7,1,2 et al. (SPT-3G Collaboration)

    A. N. Bender7,1,2, B. A. Benson3,1,2, F. Bianchini8,9,10, L. E. Bleem7,1,2, F. R. Bouchet5, L. Bryant11, E. Camphuis5, J. E. Carlstrom1,11,12,7,2, C. L. Chang7,1,2, P. Chaubal4, P. M. Chichura12,1, A. Chokshi13, T.-L. Chou2,1, A. Coerver14, T. M. Crawford2,1, C. Daley15,16, T. de Haan17, K. R. Dibert2,1, M. A. Dobbs18,19, M. Doohan4, A. Doussot5, D. Dutcher20, W. Everett21, C. Feng22, K. R. Ferguson23,24, K. Fichman12,1, A. Foster20, S. Galli5, A. E. Gambrel1, R. W. Gardner11, F. Ge8,9,25, N. Goeckner-Wald9,8, R. Gualtieri7,26, F. Guidi5, S. Guns14, N. W. Halverson27,28, E. Hivon5, G. P. Holder22, W. L. Holzapfel14, J. C. Hood1, A. Hryciuk12,1, N. Huang14, F. Kéruzoré7, A. R. Khalife5, L. Knox25, M. Korman29, K. Kornoelje2,1, C.-L. Kuo8,9,10, Y. Li1, K. Levy4, A. E. Lowitz1, C. Lu22, G. P. Lynch25, A. Maniyar8,9,10, E. S. Martsen2,1, F. Menanteau16,30, M. Millea14, J. Montgomery18, Y. Nakato9, T. Natoli1, G. I. Noble31,32, Y. Omori2,1, A. Ouellette22, Z. Pan7,1,12, P. Paschos11, K. A. Phadke16,30, A. W. Pollak13, K. Prabhu25, W. Quan7,12,1, S. Raghunathan30, M. Rahimi33, A. Rahlin2,1, M. Rouble18, J. E. Ruhl34, E. Schiappucci33, A. Simpson2,1, J. A. Sobrin3,1, A. A. Stark35, J. Stephen11, C. Tandoi16, B. Thorne25, C. Trendafilova30, C. Umilta22, J. D. Vieira16,22,30, A. G. Vieregg1,11,12,2, A. Vitrier5, Y. Wan16,30, N. Whitehorn24, W. L. K. Wu8,10, and M. R. Young3,1 (SPT-3G Collaboration)

    • 1Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA
    • 2Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA
    • 3Fermi National Accelerator Laboratory, MS209, P.O. Box 500, Batavia, Illinois 60510, USA
    • 4School of Physics, University of Melbourne, Parkville, Victoria 3010, Australia
    • 5Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis bd Arago, 75014 Paris, France
    • 6School of Physics and Astronomy, Cardiff University, Cardiff, CF24 3AA, United Kingdom
    • 7High-Energy Physics Division, Argonne National Laboratory, 9700 South Cass Avenue, Lemont, Illinois 60439, USA
    • 8Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, California 94305, USA
    • 9Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, California 94305, USA
    • 10SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA
    • 11Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA
    • 12Department of Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA
    • 13University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637, USA
    • 14Department of Physics, University of California, Berkeley, California 94720, USA
    • 15Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191, Gif-sur-Yvette, France
    • 16Department of Astronomy, University of Illinois Urbana-Champaign, 1002 West Green Street, Urbana, Illinois 61801, USA
    • 17High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
    • 18Department of Physics and McGill Space Institute, McGill University, 3600 Rue University, Montreal, Quebec H3A 2T8, Canada
    • 19Canadian Institute for Advanced Research, CIFAR Program in Gravity and the Extreme Universe, Toronto, Ontario M5G 1Z8, Canada
    • 20Joseph Henry Laboratories of Physics, Jadwin Hall, Princeton University, Princeton, New Jersey 08544, USA
    • 21Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, Colorado 80309, USA
    • 22Department of Physics, University of Illinois Urbana-Champaign, 1110 West Green Street, Urbana, Illinois 61801, USA
    • 23Department of Physics and Astronomy, University of California, Los Angeles, California 90095, USA
    • 24Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824, USA
    • 25Department of Physics and Astronomy, University of California, One Shields Avenue, Davis, California 95616, USA
    • 26Department of Physics and Astronomy, Northwestern University, 633 Clark Street, Evanston, Illinois 60208, USA
    • 27CASA, Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, Colorado 80309, USA
    • 28Department of Physics, University of Colorado, Boulder, Colorado 80309, USA
    • 29Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106, USA
    • 30Center for AstroPhysical Surveys, National Center for Supercomputing Applications, Urbana, Illinois 61801, USA
    • 31Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
    • 32David A. Dunlap Department of Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, Ontario M5S 3H4, Canada
    • 33School of Physics, University of Melbourne, Parkville, VIC 3010, Australia
    • 34Department of Physics, Case Western Reserve University, Cleveland, Ohio, 44106, USA
    • 35Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, Massachusetts 02138, USA

    • *Contact author: j.z@uchicago.edu
    • †NASA Einstein Fellow.
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    Phys. Rev. D 112, 123520 – Published 8 December, 2025

    DOI: https://doi.org/10.1103/33wm-c6pt

    Abstract

    We present a measurement of the B-mode polarization power spectrum of the cosmic microwave background anisotropies at 32≤ℓ<502 for three bands centered at 95, 150, and 220 GHz using data from the SPT-3G receiver on the South Pole Telescope. This work uses SPT-3G observations from the 2019 and 2020 winter observing seasons of a ∼1500  deg2 patch of sky that directly overlaps with fields observed with the BICEP/Keck family of telescopes and covers part of the proposed Simons Observatory and CMB-S4 deep fields. Employing new techniques for mitigating polarized atmospheric noise, the SPT-3G data demonstrates a white noise level of 9.3 (6.7)  μK−arcmin at ℓ∼500 for the 95 GHz (150 GHz) data, with a 1/ℓ noise knee at ℓ=128 (182). We fit the observed six auto- and cross-frequency B-mode power spectra to a model including lensed ΛCDM B-modes and a combination of Galactic and extragalactic foregrounds. This work characterizes foregrounds in the vicinity of the BICEP/Keck survey area, finding foreground power consistent with that reported by the BICEP/Keck collaboration within the same region and a factor of ∼3 higher power over the full SPT-3G survey area. Using SPT-3G data over the BICEP/Keck survey area, we place a 95% upper limit on the tensor-to-scalar ratio of r<0.25 and find the statistical uncertainty on r to be σ(r)=0.067.

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    Phys. Rev. D 112, 123520– Published 8 December, 2025
    • Received 7 May 2025
    • Accepted 17 September 2025
    Article part of CHORUS
    Accepted manuscript will be available starting 8 December, 2026.
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