Reconstructing the \gamma-ray Photon Optical Depth of the Universe to
  z~4 from Multiwavelength Galaxy Survey Data
  We reconstruct \gamma-ray opacity of the Universe out to z<3-4 using an
extensive library of 342 observed galaxy luminosity function surveys extending
to high redshifts. We cover the whole range from UV to mid-IR (0.15-25mic)
providing for the first time a robust empirical calculation of the
\gamma\gamma-optical depth out to several TeV. Here, we use the same database
as Helgason et al. 2012 where the EBL was reconstructed from luminosity
functions out to 4.5mic and was shown to recover observed galaxy counts to high
accuracy. We extend our earlier library of LFs to 25mic such that it covers the
energy range of pair production with \gamma-rays 1) in the entire Fermi/LAT
energy range, and 2) at higher TeV energies probed by ground-based Cherenkov
telescopes. In the absence of significant contributions to the cosmic diffuse
background from unknown populations, such as the putative Population III era
sources, the Universe appears to be largely transparent to \gamma-rays at all
Fermi/LAT energies out to z~2 whereas becoming opaque to TeV photons already at
z<0.2 and reaching \tau~10 at z=1. Comparing with the currently available
Fermi/LAT GRB and blazar data shows that there is room for significant
emissions originating in the first stars era.
