Evidence for a New Light Boson from Cosmological Gamma-Ray Propagation?
  An anomalously large transparency of the Universe to gamma rays has recently
been discovered by the Imaging Atmospheric Cherenkov Telescopes (IACTs)
H.E.S.S. and MAGIC. We show that observations can be reconciled with standard
blazar emission models provided photon oscillations into a very light
Axion-Like Particle occur in extragalactic magnetic fields. A quantitative
estimate of this effect is successfully applied to the blazar 3C279. Our
prediction can be tested with the satellite-borne Fermi/LAT detector as well as
with the ground-based IACTs H.E.S.S., MAGIC, CANGAROOIII, VERITAS and the
Extensive Air Shower arrays ARGO-YBJ andMILAGRO. Our result also offers an
important observational test for models of dark energy wherein quintessence is
coupled to the photon through an effective dimension-five operator.
