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Cosmic birefringence from a joint analysis of ACT and Planck

Johannes R. Eskilt ^*

Institute of Theoretical Astrophysics, University of Oslo, P. O. Box 1029 Blindern, N-0315 Oslo, Norway

^*Contact author: j.r.eskilt@astro.uio.no

Phys. Rev. D 114, 043553 – Published 26 August, 2026

DOI: https://doi.org/10.1103/hcsm-39qw

Abstract

Dark matter and dark energy could be ultralight pseudoscalar fields that couple to electromagnetism through a Chern-Simons term. This would cause a parity-breaking rotation of the plane of linearly polarized light from the cosmic microwave background, known as cosmic birefringence, which is not predicted by $Λ$ cold dark matter. Recent hints of a nonzero cosmic birefringence angle have been found in both the Planck Data Release 4 and Atacama Cosmology Telescope Data Release 6 datasets independently. This work jointly analyzes the polarized maps of both, leveraging the cross-power spectra between the two telescopes. Taking the reported instrumental priors at face value, the joint analysis yields $β = 0.27 7 ° \pm 0.05 7 °$ , which is nonzero with a statistical significance of $4.8 σ$ . A robustness test mitigating contamination from dust emission excludes $β = 0 °$ at $3.5 σ$ , and retrieving $β = 0 °$ would require both the dust $E B$ modeling and instrumental priors to fail simultaneously. However, unresolved systematics in the data must be understood before we can draw strong cosmological conclusions.

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