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 gamma-ray photon in the range 10 < E-0 < 10(13) GeV such that the resulting optical depth tau(gamma)(E-0, z(s)) takes the values 1, 2, 3 and 4.6 corresponding to an observed flux dimming of e(-1) similar or equal to 0.37, e(-2) similar or equal to 0.14, e(-3) similar or equal to 0.05 and e(-4.6) similar or equal to 0.01, respectively. Below a distance D similar or equal to 8 kpc, we find that tau(gamma)(E-0, DH0/c) < 1 for any value of E-0. In the limiting case of a local Universe (z(s) similar or equal to 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.