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BICEP/Keck XXI: Constraints on early-Universe parity violation from multipole-dependent birefringence

P. A. R. Ade¹, Z. Ahmed ^2,3, M. Amiri ⁴, D. Barkats ⁵, R. Basu Thakur ⁶, C. A. Bischoff ⁷, D. Beck ^8,*, J. J. Bock^6,9, H. Boenish⁵ et al. (BICEP/Keck Collaboration)

H. Boenish⁵, V. Buza¹⁰, B. Cantrall ^8,2, J. R. Cheshire, IV ⁶, J. Connors¹¹, J. Cornelison ¹², M. Crumrine¹³, A. J. Cukierman⁶, E. Denison¹¹, L. Duband¹⁴, M. Echter⁵, M. Eiben ¹⁵, B. D. Elwood ^5,16, S. Fatigoni ⁶, J. P. Filippini ¹⁷, A. Fortes⁸, M. Gao⁶, C. Giannakopoulos⁷, N. Goeckner-Wald⁸, D. C. Goldfinger ⁸, S. Gratton^18,19, J. A. Grayson⁸, A. Greathouse ⁶, P. K. Grimes ⁵, G. Hall^20,8, G. Halal ⁸, M. Halpern⁴, E. Hand⁷, S. A. Harrison⁵, S. Henderson^2,3, T. D. Hoang ¹³, J. Hubmayr¹¹, H. Hui ⁶, K. D. Irwin⁸, J. H. Kang ⁶, K. S. Karkare ²¹, S. Kefeli⁶, J. M. Kovac ^5,16, C. Kuo⁸, K. Lasko ^13,20, K. Lau ⁶, M. Lautzenhiser⁷, A. Lennox¹⁷, T. Liu ^8,†, S. C. Mackey ^10,22, N. Maher¹³, K. G. Megerian⁹, L. Minutolo⁶, L. Moncelsi ⁶, Y. Nakato⁸, H. T. Nguyen^6,9, R. O’Brient^6,9, S. N. Paine⁵, A. Patel⁶, M. A. Petroff ⁵, A. R. Polish ^5,16, T. Prouve¹⁴, C. Pryke ¹³, C. D. Reintsema¹¹, S. Richter⁵, T. Romand⁶, M. Salatino⁸, A. Schillaci⁶, B. Schmitt⁵, R. Schwartz¹³, C. D. Sheehy¹³, B. Singari ^13,20, A. Soliman^6,9, T. St. Germaine⁵, A. Steiger ⁶, B. Steinbach⁶, R. Sudiwala¹, G. Teply⁶, K. L. Thompson^2,8, C. Tucker ¹, A. D. Turner⁹, C. Vergès ²³, A. G. Vieregg^10,22, A. Wandui ⁶, A. C. Weber⁹, J. Willmert ¹³, C. L. Wong^5,16, W. L. K. Wu ^2,3, H. Yang⁸, C. Yu ^10,12, L. Zheng ⁵, C. Zhang ², and S. Zhang⁶ (BICEP/Keck Collaboration)

¹School of Physics and Astronomy, Cardiff University, Cardiff, CF24 3AA, United Kingdom
²Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, California 94305, USA
³SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
⁴Department of Physics and Astronomy, University of British Columbia, Vancouver, British Columbia, V6T 1Z1, Canada
⁵Center for Astrophysics, Harvard and Smithsonian, Cambridge, Massachusetts 01238, USA
⁶Department of Physics, California Institute of Technology, Pasadena, California 91125, USA
⁷Department of Physics, University of Cincinnati, Cincinnati, Ohio 45221, USA
⁸Department of Physics, Stanford University, Stanford, California 94305, USA
⁹Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA
¹⁰Kavli Institute for Cosmological Physics, University of Chicago, Chicago, Illinois 60637, USA
¹¹National Institute of Standards and Technology, Boulder, Colorado 80305, USA
¹²High-Energy Physics Division, Argonne National Laboratory, Lemont, Illinois 60439, USA
¹³School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA
¹⁴Service des Basses Températures, Commissariat à l’Énergie Atomique, 38054 Grenoble, France
¹⁵Faculty of Physical Sciences, University of Iceland, 102 Reykjavík, Iceland
¹⁶Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
¹⁷Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
¹⁸Centre for Theoretical Cosmology, DAMTP, University of Cambridge, Cambridge CB3 0WA, United Kingdom
¹⁹Kavli Institute for Cosmology Cambridge, Cambridge CB3 0HA, United Kingdom
²⁰Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA
²¹Department of Physics, Boston University, Boston, Massachusetts 02215, USA
²²Department of Physics, University of Chicago, Chicago, Illinois 60637, USA
²³Physics Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

^*Contact author: dobeck@stanford.edu
^†Contact author: tongtianliu@stanford.edu

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Phys. Rev. D 113, 123536 – Published 24 June, 2026

DOI: https://doi.org/10.1103/v8y3-m75b

Abstract

We present the first constraints on multipole-dependent cosmic birefringence using CMB polarization data from the BK18 dataset, which combines observations from BICEP2, Keck Array, and BICEP3 at frequencies of 95, 150, and 220 GHz. Photon coupling to an axionlike field leads to the rotation of CMB polarization, inducing nonzero $E B$ cross-correlations. We show that a multipole-dependent rotation $β (ℓ)$ imprints a distinct signature in the polarization spectra that can be constrained. Specifically, we consider an early dark energy (EDE) scenario in which a pseudoscalar field couples to photons through a Chern-Simons interaction, generating a polarization rotation with multipole dependence. We introduce a phenomenological $β (ℓ)$ as a step function, obtaining constraints on the step function size consistent with zero, with uncertainties less than $\sim 0.15 °$ (68% CL). In addition, using multi-frequency $E E$ , $B B$ , and $E B$ cross spectra, along with robust BICEP/Keck foreground treatment and likelihood framework, we derive constraints on the axion-photon coupling amplitude $g$ for several choices of EDE parameters. For the baseline best-fit value $f_{EDE} = 0.087$ from the Planck 2018 analysis, we obtain $g = 0.11 \pm 0.37$ (68% CL), consistent with previous limits.

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Phys. Rev. D 113, 123536– Published 24 June, 2026

Vol. 113, Iss. 12 — 15 June 2026

Received 20 March 2026
Accepted 12 May 2026

DOI: https://doi.org/10.1103/v8y3-m75b

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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BICEP/Keck XXI: Constraints on early-Universe parity violation from multipole-dependent birefringence

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