Searching for Axion-like Particles with Active Galactic Nuclei
  Strong mixing between photons and axion-like particles in the magnetic fields
of clusters of galaxies induces a scatter in the observed luminosities of
compact sources in the cluster. This is used to construct a new test for
axion-like particles; applied to observations of active galactic nuclei it is
strongly suggestive of the existence of a light axion-like particle.
