Indication of anisotropy in electromagnetic propagation over cosmological distances

We report a systematic rotation of the plane of polarization of electromagnetic radiation propagating over cosmological distances. The effect is extracted independently from Faraday rotation, and found to be correlated with the angular positions and distances to the sources. Monte Carlo analysis yields probabilistic P values of order 10(-3) for this to occur as a fluctuation. A fit yields a birefringence scale of order 10(25)h(0)/h m. Dependence on redshift z rules out a local effect. Barring hidden systematic bias in the data, the correlation indicates a new cosmological effect.