Physical Review Letters

We search for evidence of parity-violating physics in the Planck 2018 polarization data, and report on a new measurement of the cosmic birefringence angle, $\beta$. The previous measurements are limited by the systematic uncertainty in the absolute polarization angles of the Planck detectors. We mitigate this systematic uncertainty completely by simultaneously determining $\beta$ and the angle miscalibration using the observed cross-correlation of the $E$- and $B$-mode polarization of the cosmic microwave background and the Galactic foreground emission. We show that the systematic errors are effectively mitigated and achieve a factor-of-$2$ smaller uncertainty than the previous measurement, finding $\beta=0.35 \pm 0.14\,\deg$ (68% C.L.), which excludes $\beta = 0$ at $99.2$% C.L. This corresponds to the statistical significance of $2.4\sigma$.

New Extraction of the Cosmic Birefringence from the Planck 2018 Polarization Data