Cosmic anisotropies from quasars: from polarization to structural-axis
  alignments
  The comparison of the orientations of the optical-polarization vectors of
quasars that are separated by billions of light-years has led to the discovery
that they are aligned instead of pointing in random directions as expected.
This discovery has been confirmed and the significance of the correlations
enhanced. We devoted this doctoral thesis to an in-depth analysis of these
striking observations that imply Gpc-scale correlations. We developed a new and
independent statistical method which is dedicated to the study and the
characterization of the distribution of the orientations of vectorial
quantities that are perpendicular to the lines of sight of a set of sources
spread on the celestial sphere. This allowed us to confirm independently the
large-scale polarization-vector alignments and to refine the limits of the
alignment regions through an unbiased characterization of the signal. We also
provided a detailed analysis of a large sample of polarization measurements
made at radio wavelengths in which similar polarization-vector alignments are
found. The regions of alignments of the quasar-radio-polarization vectors are
found to be close to these of optical alignments. This might suggest that
quasar axes themselves could be aligned. Based on new observations, we further
analyzed the optical-polarization vectors of quasars that belong to two large
groups. Taking into account the link between the optical-polarization vectors
and the morphologies of the quasars, we found that the spin axes of the quasars
align with the axis of the large-quasar group to which they belong. We
reinforced our findings using radio-polarization data and a large sample of
large-quasar groups. We additionally found that the preferred orientations of
the quasar spin axes depend on the richness of their host groups.
