Gamma-ray spectra and the extragalactic background light

Attenuation of very high-energy gamma rays by pair-production with UV, optical and IR extragalactic background light (EBL) photons provides a link between the history of galaxy formation and gamma-ray astronomy. We present results from our latest semi-analytic models (SAMs), which employ the main ingredients thought to be important to galaxy formation and evolution, as well as an improved method for reprocessing of starlight by dust to mid- and far-IR wavelengths. These SAMs are based upon a hierarchical structural formation scenario, and are successful in reproducing a large variety of observational constraints such as number counts, redshift-dependent luminosity and mass functions, and color bimodality. Our fiducial model is based upon a WMAP5 cosmology and estimates the dust emission spectral energy distribution using templates based on Spitzer data. This model predicts a background flux considerably lower than found in optical and near-IR measurements that rely on subtraction of zodiacal and galactic foregrounds, and near the lower bounds set by number counts of resolvable sources at many wavelengths. We show predictions for the effect on extragalactic gamma-ray observations, and conclude with a discussion of the implications of our work and how the science of gamma-ray astronomy will continue to help constrain cosmology.