  Many extensions of the Standard Model predict the existence of ALPs, which
are very light spin-zero bosons with a two-photon coupling. Photon-ALP
oscillations occur in the presence of an external magnetic field, and ALPs can
lead to observable effects on the measured photon spectrum of astrophysical
sources. An intriguing situation arises when blazars are observed with the
Cherenkov Telescopes H.E.S.S., MAGIC, CANGAROO III and VERITAS. The
extragalactic background light (EBL) produced by galaxies during cosmic
evolution gives rise to a source dimming which becomes important in the VHE
band. This dimming can be considerably reduced by photon-ALP oscillations in
the large-scale magnetic fields, and the resulting blazar spectra become harder
than expected. We find that for ALPs lighter than 5 x 10^{-10} eV the photon
survival probability is larger than predicted by conventional physics above a
few hundred GeV. This is a clear-cut prediction which can be tested with the
planned Cherenkov Telescope Array and HAWC. Moreover, we offer a new
interpretation of the VHE blazars detected so far, according to which the large
spread in the values of the observed spectral index is mainly due to the wide
spread in the source distances rather than to large variations of their
internal physical properties.

High Energy Astrophysical Phenomena;High Energy Physics - Phenomenology