  Based on a sample of 355 quasars with significant optical polarization, we
found that quasar polarization vectors are not randomly oriented over the sky
as naturally expected. The probability that the observed distribution of
polarization angles is due to chance is lower than 0.1%. The polarization
vectors of the light from quasars are aligned although the sources span huge
regions of the sky (~ 1 Gpc). Groups of quasars located along similar lines of
sight but at different redshifts (typically z ~ 0.5 and z ~ 1.5) are
characterized by different preferred directions of polarization. These
characteristics make the observed alignment effect difficult to explain in
terms of a local contamination by interstellar polarization in our Galaxy.
Interpreted in terms of a cosmological-size effect, we show that the dichroism
and birefringence predicted by a mixing between photons and very light
pseudoscalar particles within a magnetic field can qualitatively reproduce the
observations. We find that circular polarization measurements could help
constrain this mechanism.
