Axion-like particles: possible hints and constraints from the
  high-energy Universe
  The high-energy Universe is potentially a great laboratory for searching new
light bosons such as axion-like particles (ALPs). Cosmic sources are indeed the
scene of violent phenomena that involve strong magnetic field and/or very long
baselines, where the effects of the mixing of photons with ALPs could lead to
observable effects. Two examples are archetypal of this fact, that are the
Universe opacity to gamma-rays and the imprints of astrophysical magnetic
turbulence in the energy spectra of high-energy sources. In the first case,
hints for the existence of ALPs can be proposed whereas the second one is used
to put constraints on the ALP mass and coupling to photons.
