Evolving Stellar Background Radiation and Gamma-Ray Optical Depth

We present a semi-empirical model for the evolving far-infrared to ultraviolet diffuse background produced by stars in galaxies. The model is designed to reproduce the results of deep galaxy surveys, and therefore may be considered as a cosmology-independent lower limit to the extragalactic background light. Using this model and recent HEGRA data, we infer the intrinsic spectrum at multi-TeV gammaray energies for Mkn 501 and find that it is consistent with a power law of spectral index 2.49± 0.04. In turn, this finding renders it rather unlikely that the present-day infrared background has an intensity as high as claimed by Finkbeiner et al. [1]. Future 10 GeV to TeV observations could be used to either constrain the ultraviolet-to-infrared background model at high redshifts or cosmological parameters.