We calculate the intergalactic photon density as a function of both energy and redshift for 0 < z < 6 for photon energies from .003 eV to the Lyman limit cutoff at 13.6 eV in a \Lambda CDM universe with \Omega _ { \Lambda } = 0.7 and \Omega _ { m } = 0.3 .
The basic features of our backwards evolution model for galaxies were developed in Malkan and Stecker ( 1998 and 2001 ) .
With a few improvements , we find that this evolutionary model gives predictions of new deep number counts from Spitzer as well as a calculation spectral energy distribution of the diffuse infrared background which are in good agreement with the data .
We then use our calculated intergalactic photon densities to extend previous work on the absorption of high energy \gamma -rays in intergalactic space owing to interactions with low energy photons and the 2.7 K cosmic microwave background radiation .
We calculate the optical depth of the universe , \tau , for \gamma -rays having energies from 4 GeV to 100 TeV emitted by sources at redshifts from 0 to 5 .
We also give an analytic fit with numerical coefficients for approximating \tau ( E _ { \gamma } ,z ) .
As an example of the application of our results , we calculate the absorbed spectrum of the blazar PKS 2155-304 at z = 0.117 and compare it with the spectrum observed by the H.E.S.S .
air Cherenkov \gamma -ray telescope array .