Transparency of the Universe to gamma rays

Using the most recent observational data concerning the Extragalactic Back-ground Light and the Radio Background, for a source at a redshift zs ≤ 3 we compute the energy E0 of an observed γ-ray photon in the range 10 GeV ≤ E0 ≤ 1013 GeV such that the resulting optical depth τγ(E0, zs) takes the values 1, 2, 3 and 4.6, cor-responding 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 source distance D ' 8 kpc we find that τγ(E0, DH0/c) < 1 for any value of E0. In the limiting case of a local Universe (zs ' 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 CTA, HAWC and HiSCORE. Key words: diffuse radiation – gamma-rays: observations