Gaining confidence on general relativity with cosmic polarization rotation

TL;DR

This paper reviews astrophysical searches for cosmic polarization rotation over 26 years, which tests the Einstein equivalence principle and supports general relativity by showing no detectable polarization rotation.

Contribution

It provides a comprehensive review of observational efforts and results related to cosmic polarization rotation, highlighting the constraints on new physics beyond general relativity.

Findings

No significant cosmic polarization rotation detected within 1 degree

Current measurements support the validity of the Einstein equivalence principle

Future prospects may improve sensitivity to potential violations

Abstract

The theory of general relativity, for which we celebrate the centennial at this Symposium, is based on the Einstein equivalence principle. This principle could be violated through a pseudoscalar-photon interaction, which would also produce a rotation of the polarization angle for radiation traveling over very long distances. Therefore, if we could show that this cosmic polarization rotation does not exist, our confindence in general relativity would be greatly increased. We review here the astrophysical searches for cosmic polarization rotation, which have been made in the past 26 years using the polarization of radio galaxies and of the cosmic microwave background. So far no rotation has been detected within about 1 degree. We discuss current problems and future prospects for cosmic polarization rotation measurements.