The Gamma-Ray Horizon: Attenuation on the Extragalactic Background Light as a Function of Energy and Redshift

Gamma rays above tens of GeV are absorbed by pair production on the extragalactic background light, so the observable universe shrinks with increasing photon energy. Using a compact representation of the published background-light models as constrained by Fermi Large Area Telescope blazar spectra, this paper computes the optical depth, the attenuation factor and the horizon redshift at which the optical depth reaches unity, for energies from 10 GeV to 1 TeV. It finds horizon 100GeV = 0.5; horizon 1TeV = 0.103; attenuation 100GeV z1 = 0.0482. The horizon at 1 TeV lies at very low redshift, which is why the TeV sky is a local survey. The study is simulated and computed, not measured.