The cosmic microwave background (CMB) is the universe’s oldest light, leftover thermal radiation from about 380,000 years after the big bang. Maps of the CMB offer a picture of the “baby universe” as it was 13.8 billion years ago.
About 380,000 years after the big bang, the scattering of radiation by free electrons generated a small amount of linear polarization as the universe became transparent. The process is somewhat analogous to the polarization produced when sunlight scatters in Earth’s atmosphere.
Scientists have found hints that this polarization may have rotated slightly during its journey across the cosmos. If confirmed, this phenomenon, known as cosmic birefringence, could provide evidence for physics beyond the Standard Model and potentially offer clues about the nature of dark matter and dark energy (see sidebar).
However, even a minuscule error in the orientation of a telescope’s polarization detectors can produce almost exactly the same effect. Researchers at UC San Diego developed a method to test the relative polarization-angle calibration of different detector sets. The team developed a new estimator and applied it to existing observations from the European Space Agency’s Planck satellite.
The study, led by postdoctoral fellow Anto I. Lonappan, Chancellor’s Distinguished Professor of Physics Brian Keating and associate professor of physics Kam Arnold, appears in The Astrophysical Journal Letters.
Calibrating the calibrators
Recent analyses have suggested that CMB polarization may have rotated only a fraction of a degree during the nearly 14 billion years the light has traveled. The difficulty is that the same rotation can be produced if the polarization-sensitive detectors in a telescope are miscalibrated by a similarly small angle. From the CMB signal alone, scientists cannot distinguish between a uniform cosmic rotation and a common error in detector orientation because both produce exactly the same observed effect.
The UC San Diego team approached the problem by comparing maps made from different groups of detectors. A genuine cosmic rotation would be common to all the maps. In the new method, that common rotation cancels when the maps are compared, leaving only differences in their polarization calibration.
“The signal we are looking for is incredibly small, so we have to be certain that we are seeing the universe and not our instrument,” said Lonappan. “Our method gives us a complementary way to check that distinction. Before interpreting a tiny rotation as new physics, we want to know that the calibration itself can be trusted.”
The team applied the method to eight Planck polarization maps and compared the resulting calibration pattern with that obtained from the established analysis currently used to separate instrumental rotation from cosmic birefringence, known as the Minami–Komatsu analysis. The two approaches were consistent, despite relying on different assumptions.
The new technique is intentionally insensitive to any rotation that is common to all maps. As a result, it can determine how well different detector sets are calibrated relative to one another, but it cannot by itself establish the overall polarization angle or the absolute cosmic birefringence signal. That still requires an independent absolute calibration reference.
As a conditional demonstration, the researchers anchored their differential reconstruction to the common calibration mode inferred by the existing analysis. This reproduced a cosmic-birefringence angle of 0.37 ± 0.12 degrees, consistent with Minami–Komatsu.
Searching for a new standard
There are two types of CMB polarization: E modes and B modes. E modes, first detected in 2002, are created by density shifts in the universe and have symmetric patterns. B modes, on the other hand, have curl-like patterns. Gravitational lensing—the distortion of CMB light by matter between us and the early universe—produces B modes that have already been observed, while primordial gravitational waves could produce an additional B-mode signal that has not yet been detected.
Discovery of primordial B modes would provide powerful evidence about gravitational waves in the early universe and could strongly support particular models of cosmic inflation. The calibration method developed at UC San Diego could support primordial B-mode searches by identifying relative angle-calibration errors that convert E modes into spurious B modes.
As experiments such as those at the Simons Observatory, where Keating is the principal investigator, push polarization measurements to increasingly high precision, independent calibration checks will become an important safeguard before very small signals are interpreted as evidence for new physics.
“The detection of primordial B modes would transform our understanding of the early universe, so the measurement must survive rigorous calibration checks,” said Keating. “Anto has developed a useful new framework for testing relative polarization-angle calibration. This is ultimately about knowing when we can trust a measurement.”
Publication details
Anto I. Lonappan et al, Differential Polarization Calibration: A Consistency Test for Cosmic Birefringence, The Astrophysical Journal Letters (2026). DOI: 10.3847/2041-8213/aea401
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University of California – San Diego
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Is the universe twisted? A new check on a possible twist in the universe’s oldest light (2026, September 24)
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