Joint ACT–Planck analysis strengthens hint of cosmic birefringence but systematics remain

A new arXiv preprint reports the strongest statistical hint yet that the universe may rotate the polarization of the cosmic microwave background, but its author also says unresolved measurement problems still rule out any firm cosmological claim.

In the paper, “Cosmic birefringence from a joint analysis of ACT and Planck,” Johannes R. Eskilt of the Institute of Theoretical Astrophysics at the University of Oslo combined public polarization data from the European Space Agency’s Planck mission and the Atacama Cosmology Telescope, or ACT. The study reports a cosmic birefringence angle of 0.277 degrees, plus or minus 0.057 degrees.

“Taking the reported instrumental priors at face value, the joint analysis yields β = 0.277° ± 0.057°, which is non-zero with a statistical significance of 4.8σ,” Eskilt wrote in the abstract. The arXiv record shows version 1 was posted Aug. 6, and the PDF is dated Aug. 10.

Cosmic birefringence is the hypothesized rotation of the plane of linearly polarized light from the cosmic microwave background, the relic radiation left over from the early universe. If that signal is real and cosmological rather than caused by instruments or foreground contamination, it would point to parity-violating physics beyond the standard ΛCDM model of cosmology.

Eskilt’s analysis is based on public polarized maps from Planck Data Release 4, also called NPIPE or PR4, and ACT Data Release 6. The paper combines cross-power spectra between the two telescopes with ACT-by-ACT and Planck-by-Planck spectra, a joint approach meant to strengthen the measurement and check consistency across data sets.

The paper also reports a more conservative test aimed at reducing contamination from Galactic dust, one of the main known trouble spots in this line of work. In that robustness test, the paper says a zero birefringence result is still excluded at 3.5 sigma.

“A robustness test mitigating contamination from dust emission excludes β = 0° at 3.5σ, and retrieving β = 0° would require both the dust EB modeling and instrumental priors to fail simultaneously. However, unresolved systematics in the data must be understood before we can draw strong cosmological conclusions,” Eskilt wrote.

That caveat is central to the result. The two main vulnerabilities discussed in this literature are instrument polarization-angle miscalibration and polarized Galactic foregrounds, especially dust, which can create the kind of EB correlations researchers are trying to measure. The new paper does not present cosmic birefringence as part of standard cosmology. Instead, it notes that such a signal is not predicted by ΛCDM and could arise in theories where dark matter or dark energy involve ultra-light pseudoscalar fields coupled to electromagnetism through a Chern-Simons term.

The result builds on a string of earlier hints rather than arriving as a standalone surprise. In 2020, Yuto Minami and Eiichiro Komatsu reported about 0.35 degrees, plus or minus 0.14 degrees, using Planck 2018 HFI maps. In 2022, Eskilt and Komatsu reported 0.342 degrees, with an uncertainty of roughly 0.09 degrees, from a combined Planck and WMAP analysis, a result at about 3.6 sigma. And in 2025, an ACT Data Release 6 analysis reported 0.215 degrees, plus or minus 0.074 degrees, or about 2.9 sigma, while also warning about unresolved ACT systematics.

Eskilt says the new joint analysis reproduces prior single-dataset results, including an ACT-only value of 0.207 degrees, plus or minus 0.073 degrees, consistent with earlier ACT work.

Taken together, the new paper strengthens an existing pattern: Planck, WMAP and ACT data have all pointed in roughly the same direction, and the combined ACT-Planck analysis raises the statistical significance further. But the field’s bottom line has not changed. This is the strongest hint so far, not a settled detection, and any claim that the cosmos itself is twisting the microwave background will depend on independent checks that can rule out calibration errors and dust-related foreground effects.