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  • Letter
  • Open Access

Cosmic birefringence from the Atacama Cosmology Telescope Data Release 6

Patricia Diego-Palazuelos1,* and Eiichiro Komatsu1,2,3

  • *Contact author: diegop@MPA-Garching.MPG.DE
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Phys. Rev. D 113, L101302 – Published 8 May, 2026

DOI: https://doi.org/10.1103/pbc3-t52s

Abstract

The polarized light of the cosmic microwave background is sensitive to new physics that violates parity symmetry. For example, the interaction of photons with the fields of elusive dark matter and dark energy could cause a uniform rotation of the plane of linear polarization across the sky, an effect known as cosmic birefringence. We extract the cosmological rotation angle, β, using Bayesian analysis of parity-violating correlations, EB and TB, of polarization data from the Atacama Cosmology Telescope (ACT) Data Release 6. We use prior probabilities for instrumental miscalibration angles derived from the optics model for the ACT telescope and instruments, and marginalize over a residual intensity-to-polarization leakage. We measure β=0.215°±0.074° (68% confidence level), which excludes β=0 with a statistical significance of 2.9σ. Although there remain systematics in the ACT data that are not understood and do not allow us to draw strong cosmological conclusions, this result is consistent with previous independent results from the WMAP and Planck missions. It is suggestive that independent data sets and analyses using different methodologies have yielded the same sign and comparable magnitudes for β.

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Phys. Rev. D 113, L101302– Published 8 May, 2026
  • Received 25 July 2025
  • Revised 16 September 2025
  • Accepted 18 March 2026
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