Transparency of the Universe to gamma-rays

Using the most recent observational data concerning the extragalactic background light and the radio background for a source at an arbitrary redshift in the range z s ≤ 3, we compute the energy E 0 of an observed γ-ray photon in the range 10 ≤ E 0 ≤ 1013 GeV such that the resulting optical depth τγ(E 0, z s) takes the values 1, 2, 3 and 4.6 corresponding to an observed flux dimming of e −1 0.37, e −2 0.14, e −3 0.05 and e −4.6 0.01, respectively. Below a distance D 8 kpc, we find that τγ(E 0, DH 0/c) < 1 for any value of E 0. In the limiting case of a local Universe (z s 0), we compare our result with the one derived in 1997 by Coppi and Aharonian. The present achievement is of paramount relevance for the planned ground-based detectors like Cherenkov Telescope Array, High Altitude Water Cherenkov Experiment and Hundred Square-km Cosmic ORigin Explorer.