Is the universe twisted? A new check on a possible twist in the universe's oldest light

The article discusses a new method to test the polarization calibration of CMB detectors, aiming to distinguish between cosmic birefringence and instrumental errors. Researchers at UC San Diego developed a technique that compares maps from different detector sets, using a new estimator to identify calibration differences. The method is designed to be insensitive to common rotations across all maps, allowing it to assess how well individual detectors are calibrated relative to each other. The study found that the new approach is consistent with existing analyses, even though it relies on different assumptions.

The team applied the method to eight Planck polarization maps and compared the resulting calibration patterns with the established Minami–Komatsu analysis. The two approaches were consistent, but the new method is not able to determine the overall polarization angle or the absolute cosmic birefringence signal. This means that the method can only check for calibration errors, not for the actual physical rotation of the universe. The study suggests that independent calibration checks are essential before interpreting small signals as evidence for new physics.

The new technique could help in searching for primordial B modes, which are associated with gravitational waves from the early universe. The method could identify relative calibration errors that convert E modes into spurious B modes. This could support future experiments, such as those at the Simons Observatory, in their efforts to detect primordial B modes. The study emphasizes the importance of rigorous calibration checks to ensure that small signals are not misinterpreted as new physics. The research is published in The Astrophysical Journal Letters.

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