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Cosmic microwave background birefringence is a tiny rotation of the sky’s polarized light. Measuring it is hard because the same rotated pattern can also come from a common misalignment of the detectors’ polarization angles. The Minami–Komatsu (MK) method separates a true cosmic rotation from an instrumental offset by using polarized emission from our Galaxy. The new paper introduces a different test that does not rely on that Galactic model and so provides a complementary check on the MK-based result.
The authors build a “birefringence-blind” differential estimator. It uses combinations of antisymmetric and symmetric cross-spectra between maps. For any pair of maps, a common rotation (what would include a cosmic birefringence signal) cancels exactly, and the estimator returns the angle difference Δαij = αi − αj between the two detector sets. By combining many map pairs and fixing a gauge (a reference choice for the overall angle), they reconstruct the pattern of detector-dependent calibration offsets across the instrument.
They applied this procedure to eight Planck NPIPE detector-set maps. The differential reconstruction of detector angles agrees with the MK reconstruction that used Galactic foreground polarization. Quantitatively, the two patterns give χ2 = 8.18 for seven degrees of freedom, corresponding to a probability-to-exceed (PTE) of 0.32. All shared-data residuals are below 1.8σ. These numbers indicate good internal consistency between the two analysis routes on the same maps.
Why this matters: EB-based searches for cosmic birefringence (EB means the cross-correlation between two polarization patterns called E and B) are sensitive to tiny angle shifts. Having an independent estimator that is blind to a common rotation gives confidence that the MK separation is not driven by a particular foreground model or by a single analysis choice. The new differential test can be added as a validation layer for future analyses that aim to detect or constrain cosmic birefringence.
Important caveats remain. Both the differential reconstruction and the MK reconstruction use the same Planck maps, so this agreement is an independent method and modeling route but not an independent-data confirmation. That means it strengthens trust in the calibration foundation of the MK-inferred birefringence angle without providing a separate absolute measurement of it. When the authors perform a conditional reconstruction that fixes the common mode, they obtain β = 0.37 ± 0.12°, but that number is tied to the shared data and the conditional choices in the analysis.