  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) meters.
Dependence on redshift z rules out a local effect. Barring hidden systematic
bias in the data, the correlation indicates a new cosmological effect.
