Is CMB polarization rotated by a parity-violating field?
In plain words
A hypothetical very light field filling space could rotate the polarization of light on its journey from the early universe. A small rotation angle has been reported, but it is hard to separate from instrument calibration errors.
Precise statement
Measure the isotropic polarization rotation angle $\beta$, which converts $E$-modes into $B$-modes and produces nonzero EB correlations; analyses of Planck and WMAP using Galactic foreground EB as calibration reported $\beta \sim 0.34 \pm 0.09\ \mathrm{degrees}$ (about $3.6\sigma$, 2022). Answer whether $\beta$ is nonzero with detector polarization angles calibrated independently to below $0.05\ \mathrm{degrees}$.
What would settle it
Measurement of $\beta$ with ground-based or space CMB polarization experiments using artificial calibration sources or lunar and planetary references.
Status in the literature
Unverified note
ACT DR6 (2025, arXiv:2509.13654) measured $\beta = 0.215 \pm 0.074\,\mathrm{degrees}\ (2.9\,\sigma)$, consistent with Planck and WMAP but still limited by polarization-angle calibration.