Photon propagation and the very high energy γ-ray spectra of blazars: how transparent is the Universe?

Recent findings by gamma-ray Cherenkov telescopes suggest a higher transparency of the Universe to very high energy (VHE) photons than expected from current models of the extragalactic background light. It has been shown that such transparency can be naturally explained by the DARMA scenario, in which the photon mixes with a new, very light, axion-like particle predicted by many extensions of the Standard Model of elementary particles. We discuss the implications of DARMA for observations of blazar VHE gamma-ray spectra, and show that it successfully accounts for the observed correlation between spectral slope and redshift when the same intrinsic emission spectrum is adopted for faraway sources and for nearby ones. DARMA also predicts the observed blazar spectral index to become asymptotically independent of redshift for faraway sources. Our prediction can be tested with the satellite-borne Fermi/LAT (Large Area Telescope) detector as well as with the ground-based Cherenkov telescopes HESS, MAGIC, CANGAROO III, VERITAS and the Extensive Air Shower arrays ARGO-YBJ and Milagro.