Probing of extragalactic background light with TeV gamma‐ray observations

Extragalactic Background Light (EBL) radiation contains an information about the cosmological processes including the history of stars and galaxies formation. The detection of γ ‐ray sources in the energy range 0.8–40 TeV at red‐shifts from z = 0.018 to z = 1.375 with SHALON telescopes led to the constraint of the EBL density, as the TeV γ ‐rays can be absorbed due to interaction of low‐energy photons of EBL. It is compared with data from measurements and models. Also, the observations of sources with z > 2 from first and second Fermi LAT AGN catalogue have been started in the year 2014, and the first results on B2 242 + 43 (z = 2.243) and B2 0743 + 25 (z = 2.979) quasars are presented. The detection of TeV γ ‐ray sources at high red‐shifts is the evidence of less average spectral density of EBL and thus the less star formation rate at early evolution stage than it is previously believed.