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Alessandro De Angelis coauthored calculations of the transparency of intergalactic space to gamma rays. The work connects an assumed background photon population with energy-dependent optical depth and the attenuation of distant sources.^[ref]

High-energy photon propagation is affected by interactions with background radiation, including pair production. Interpreting a source's observed spectrum therefore requires both an emission model and a propagation model; a missing high-energy component need not originate entirely at the source.