The cosmic microwave background (CMB) is ancient light that holds information about the early universe. If its polarization is rotated by a tiny angle, it could be a trace of new particles, such as axions. However, the rotation from a cosmic effect is indistinguishable from an error in the tilt of telescope detectors. The existing Minami–Komatsu method uses polarized dust in our Galaxy to separate these two contributions, but the result depends on the model of that dust. This work adds an independent check that does not use that model, and applies it for the first time to real Planck data.
The authors use eight polarization maps from different sets of Planck detectors at frequencies 100, 143, 217, and 353 GHz. For each pair of maps, they construct two combinations of spectra: symmetric and antisymmetric. The ratio of these combinations gives the difference in angles between detectors, while the common cosmic rotation cancels completely. Then all pairwise differences are combined into a network where maps are nodes and angle differences are links. The network is solved by least squares, accounting for correlations between pairs that share common maps. The resulting pattern of angles is compared with the result of the Minami–Komatsu method.
Comparison of 28 detector pairs showed that the angles from the differential method and those from the Minami–Komatsu method agree: the chi-square statistic is 8.18 with seven degrees of freedom (probability 0.32). All deviations are less than 1.8 sigma. This means that the detector-dependent calibration pattern given by the foreground model is also reproduced by a method that does not use that model. However, this is not an independent measurement of the absolute value of the cosmic rotation, since both methods use the same sky maps.
The work shows that the calibration of Planck detector angles obtained from the galactic dust model is not an artifact of that model. The pattern of angles itself is confirmed by an independent method that relies only on the polarization properties of the CMB itself. This strengthens confidence in the measured value of cosmic birefringence of about 0.37 degrees. The method can be used as an additional check for future experiments such as Simons Observatory and LiteBIRD.
The differential test can be used in future CMB polarization experiments. It requires no new observations and can accompany any analysis based on a foreground model. To obtain the absolute value of the cosmic rotation, an external calibrator with accuracy better than 0.1 degrees is needed, which is not yet available for Planck. In the future, such a calibrator could come from a dedicated source or an overlapping telescope.
The method will affect data analysis for current and future CMB polarization experiments such as BICEP/Keck, SPT-3G, Simons Observatory, and LiteBIRD. It may also be useful for calibrating angles in other astronomical instruments where precise polarization orientation matters.
The next step is to validate the analytic covariance matrix and the separable foreground approximation with full simulations that include spatially varying dust spectra and frequency decorrelation. Also, external calibrators with accuracy better than 0.1 degrees need to be developed for an absolute measurement of cosmic birefringence.
This work is connected to a fundamental problem in observational cosmology: how to distinguish a cosmic signal from systematic instrumental errors. It offers a new way to check polarization calibration that does not depend on a model of the galactic foreground. This is important for a reliable measurement of cosmic birefringence and, more broadly, for the search for new physics beyond the Standard Model.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors developed a differential polarization calibration method that compares angles between pairs of Planck detectors and completely removes the common cosmic rotation. Applying the method to Planck data, they showed that the angle pattern is consistent with the result of the Minami–Komatsu method (chi-square 8.18 with 7 degrees of freedom, probability 0.32), which strengthens confidence in the measured value of cosmic birefringence.
Where its strength is
The strength of the work is that it offers a calibration check that does not depend on the model of galactic dust, and it successfully applies it to real data, confirming the consistency of two independent approaches.
Where it could go next
Worth trying
Why this may matter
If the method proves effective on future data, it could become a standard tool for controlling systematic errors in measurements of cosmic birefringence, which is important for the search for new physics.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.59, against a usual 0.64. It looks like few have drilled nearby.
The angle of cosmic birefringence that scientists are looking for is only about 0.3 degrees, roughly the angle subtended by a pinky nail at arm's length. Yet the calibration accuracy of detectors must be better than 0.1 degrees to measure it reliably.