Voids as Alternatives to Dark Energy and the Propagation of Gamma Rays through the Universe

Abstract

We test the opacity of a void Universe to TeV energy gamma rays having obtained the extra-galactic background light in that Universe using a simple model and the observed constraints on the star formation rate history. We find that the void Universe has significantly more opacity than a Lambda-CDM Universe, putting it at odds with observations of BL-Lac objects. We argue that while this method of distinguishing between the two cosmologies contains uncertainties, it circumvents any debates over fine-tuning.