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 to a very light Axion‐Like Particle occur in extragalactic magnetic fields. A quantitative estimate of this effect is successfully applied to blazar 3C279. Our predictions can be tested in the near future by new observations performed both with IACTs and with the ARGO‐YBJ air shower detector. 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.