ar X iv : 0 70 4 . 28 88 v 2 [ as tr o - ph ] 1 0 M ay 2 00 7 The Extragalactic γ Ray Background

One way to understand the nonthermal history of the universe is by establishing the origins of the unresolved and truly diffuse extragalactic γ rays. Dim blazars and radio/γ galaxies certainly make an important contribution to the galactic γ-ray background given the EGRET discoveries, and previous treatments are reviewed and compared with a new analysis. Studies of the γ-ray intensity from cosmic rays in star-forming galaxies and from structure formation shocks, as well as from dim GRBs, are briefly reviewed. A new hard γ-ray source class seems required from the predicted aggregate intensity compared with the measured intensity.