The Extragalactic Background Light and the Detection of the Cosmic Gamma-Ray Horizon

The first statistically significant detection of the cosmic gamma-ray horizon (CGRH) that is independent of any extragalactic background light (EBL) model is presented in this talk. The CGRH is a fundamental quantity in cosmology. It gives an estimate of the opacity of the Universe to very-high energy (VHE) gamma-ray photons due to photon-photon pair production with the EBL. The only estimations of the CGRH to date are predictions from EBL models and lower limits from gamma-ray observations of cosmological blazars and gamma-ray bursts. Here, we present synchrotron/synchrotron self-Compton models (SSC) of the spectral energy distribution of 15 blazars based on (almost) simultaneous observations from radio up to the highest energy gamma-rays taken with the Fermi satellite. These synchrotron/SSC models predict the unattenuated VHE fluxes, which are compared with the observations by imaging atmospheric Cherenkov telescopes. This comparison provides an estimation of the optical depth of the EBL, which allows a derivation of the CGRH through a Monte Carlo analysis that is EBL-model independent. We find that the observed CGRH is compatible with the current knowledge of the EBL. We conclude showing that the detection of the CGRH allows us to estimate the expansion rate of the Universe from gamma-ray attenuation.