Standard electromagnetism and ΛCDM are parity-even: mirror-reflecting the universe should leave CMB statistics unchanged, which forces the TB and EB cross-power spectra, correlations between temperature or E-mode patterns and the parity-odd B-modes, to vanish identically. They appear not to vanish. Minami and Komatsu (2020) extracted a uniform polarization rotation angle of beta = 0.35 degrees +/- 0.14 from the Planck 2018 EB spectra, and Eskilt and Komatsu (2022, arXiv:2205.13962) sharpened the measurement with Planck PR4 and WMAP data to roughly 0.30 to 0.34 degrees at 3.6 sigma significance. Read at face value, every CMB photon's polarization plane rotated by a third of a degree somewhere along its 13.8-billion-year journey: cosmic birefringence, direct evidence of parity-violating physics.
The measurement's historic obstacle, degeneracy with instrumental polarization-angle miscalibration, was broken by the Minami-Komatsu method of using the Galactic foreground emission, which shares the instrument's miscalibration but not the cosmological rotation, as an internal calibrator. The signal has survived PR4 reprocessing, frequency-dependence tests, and independent reanalyses (Sullivan et al. 2025), though it remains below the conventional discovery threshold and its foreground-modeling assumptions are still debated. Within ΛCDM there is no source for it: the model contains no parity-violating sector, so explanations require new physics by construction, with the default candidate an axion-like pseudoscalar field coupled to electromagnetism through a Chern-Simons term, tuned to rotate polarization by the observed amount.
The standing is tantalizing: a 3.6 sigma parity violation that, if confirmed, falsifies the symmetry assumptions of the standard model of cosmology outright. LiteBIRD and CMB-S4 polarization, with far better angle calibration and sensitivity, will either push the signal past 5 sigma or kill it, and cross-checks against the other large-angle anomaly axes will test whether the parity violation knows about the rest of the anomalous sky.