This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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.
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.
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