A genuine cosmic signal would appear in every map; an instrumental error would vary between detector groups. By subtracting one map from another, the method cancels any rotation common to all maps and leaves only the residual differences.

What the data show

The researchers applied their method to eight polarization maps from the European Space Agency's Planck satellite. They then compared their calibration pattern with an established technique called the Minami–Komatsu analysis, the current standard for separating instrumental rotation from cosmic birefringence.

The two approaches agreed, despite relying on different assumptions. As a conditional test, the team 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 prior Minami–Komatsu findings.

What the method cannot do

The new technique is deliberately insensitive to any rotation common to all maps. It can therefore measure how well different detector sets are calibrated relative to each other, but it cannot by itself determine the absolute polarization angle.

Establishing the absolute cosmic-birefringence signal still requires an independent absolute calibration reference, and none yet exists for CMB data. In short, the framework distinguishes between detector sets but does not pin down the universe's twist on its own.

Why this matters for primordial B modes

The CMB carries two main types of polarization patterns, known as E modes and B modes. E modes, with symmetric patterns from density shifts in the early universe, were first detected in 2002. B modes have curl-like patterns.

Gravitational lensing — the bending of CMB light by intervening matter — already produces some B modes. But primordial gravitational waves from the inflationary epoch would generate an additional B-mode signal that no experiment has yet detected.

Finding those primordial B modes would deliver strong evidence for cosmic inflation, the rapid expansion thought to have occurred a fraction of a second after the Big Bang. It would also sharpen models of how the early universe evolved.

A small angle-calibration error can convert E modes into spurious B modes, mimicking the signal experiments hunt for. The new method could help future experiments rule out that contamination before claiming a discovery.

What comes next

Upcoming projects, including the Simons Observatory in Chile, where Keating is principal investigator, aim to push CMB polarization measurements well beyond Planck's sensitivity. As precision climbs, calibration checks will increasingly decide whether a faint signal counts as new physics or an instrument artifact.

"The detection of primordial B modes would transform our understanding of the early universe, so the measurement must survive rigorous calibration checks," Keating said. "Anto has developed a useful new framework for testing relative polarization-angle calibration. This is ultimately about knowing when we can trust a measurement."

Paper: 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.