Cosmic Birefringence: searching for parity-violating physics with the CMB polarization
The cross-correlation between the E- and B-mode polarization of the Cosmic Microwave Background (CMB) can be used as a probe for parity-violating physics in the Universe. Parity-violating processes such as a Chern-Simons coupling to axion-like particles or the Faraday rotation induced by primordial magnetic fields are expected to rotate the plane of linear polarization and produce a non-null EB correlation. We commonly refer to that rotation as “cosmic birefringence” because it is as if space itself acted like a birefringent crystal.
Several experiments have already tried to measure birefringence from the CMB EB correlation. However, past measurements tended to be dominated by systematic uncertainties and the limited precision of polarization angle calibration. In this talk, I will present the methodology proposed by Minami et al. (2019) for the simultaneous determination of both birefringence and miscalibrated polarization angles. This novel approach overcomes the limitation imposed by the insufficient knowledge of instrument calibration by using Galactic foreground emission as our calibrator, allowing for a systematic-free measurement of cosmic birefringence.
We have recently applied this methodology to polarization data from the high-frequency instrument of the Planck mission public release 4 (NPIPE reprocessing). For nearly full-sky data, we initially find a birefringence angle of β = 0.30° ± 0.11° (68% C.L.). We also find that the values of β decrease as the Galactic mask is enlarged, which can be interpreted as the effect of polarized foreground emission. We have used two independent models to account for this component and mitigate its impact on β. Although results are promising, we do not assign cosmological significance to the measured value of β until we improve our knowledge of the foreground polarization. If confirmed, this signal could be attributed to an ultra-light axion-like field and would be evidence of parity-violating physics outside the weak interaction.