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

TL;DR

This paper investigates whether a void universe model can explain gamma-ray transparency and finds it predicts higher opacity than observed, challenging its viability as an alternative to dark energy.

Contribution

The study introduces a method to test void universe models against gamma-ray observations, providing a new way to evaluate their consistency with astrophysical data.

Findings

Void universe has higher gamma-ray opacity than Lambda-CDM.

Void universe predictions conflict with observations of BL-Lac objects.

Method offers a way to distinguish cosmological models without fine-tuning.

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.