  Photon-photon scattering of gamma-rays on the cosmic microwave background has
been studied using the low energy approximation of the total cross section by
Zdziarski & Svensson (1989); Svensson & Zdziarski (1990). Here, the cosmic
horizon due to photon-photon scattering is accurately determined using the
exact cross section and we find that photon-photon scattering dominates over
the pair production at energies smaller than 1.68 GeV and at redshifts larger
than 180.
