Large-scale alignments of quasar polarisations: a detailed study of the
  spinless-particle scenario
  The main motivation for our work has been a puzzling observation concerning
quasars. No one expected the existence of correlations in the polarisation of
visible light coming from objects separated by gigaparsecs, until they were
first reported in the form of a redshift-dependent effect that has become more
and more significant with the growth of the data sample.
  In close connection with the observational group, we have studied in detail
the most widely considered scenario, involving axion-photon mixing in
extragalactic magnetic fields.
  After a systematic investigation, we conclude that it is very unlikely that
these observations can be accounted for by axion-like particles, given current
data and constraints. We also derive new limits on the parameters describing
these particles.
  This thesis gives in particular a detailed account of the consequences of
axion-photon mixing on polarisation, studies the influence of averages over the
frequency (including a wave-packet treatment of the mixing), and discusses the
consequences of different magnetic-field morphologies.
