Cosmic birefringence from a joint analysis of ACT and Planck

Dark matter and dark energy could be ultra-light 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 CDM. Recent hints of a non-zero cosmic birefringence angle have been found in both the Planck Data Release 4 and ACT 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 beta = 0.277 plus or minus 0.057, which is non-zero with a statistical significance of 4.8 sigma. A robustness test mitigating contamination from dust emission excludes beta = 0 at 3.5 sigma, and retrieving beta=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.