Detection of the cosmic \gamma-ray horizon from multiwavelength
  observations of blazars
  The first statistically significant detection of the cosmic \gamma-ray
horizon (CGRH) that is independent of any extragalactic background light (EBL)
model is presented. 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 homogeneous synchrotron/synchrotron self-Compton (SSC) models of the
spectral energy distributions 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 estimate of the optical depth
of the EBL, which allows a derivation of the CGRH through a maximum likelihood
analysis that is EBL-model independent. We find that the observed CGRH is
compatible with the current knowledge of the EBL.
