Cosmology with 100-TeVγ-ray telescopes

We show that the secondary photons which are by-products of the energy loss of extragalactic cosmic rays interacting with the cosmic background have a characteristic energy of 10-100 TeV. For a model where the extragalactic cosmic rays are uniformly distributed in space and in time over the past ${10}^9$ yr the flux of 100-TeV photons is ${10}^{-5\mathrm{}}$ of the cosmic-ray flux. Such fluxes are attractively close to the resolution of the new generation of $\gamma$-ray telescopes and their detection can provide important cosmological information.