EBL Inhomogeneity and Hard-Spectrum Gamma-Ray Sources

TL;DR

This paper investigates how cosmic voids along the line of sight affect gamma-ray opacity and finds that their impact is minimal, less than 1%, challenging previous assumptions about EBL inhomogeneity effects.

Contribution

The study provides a detailed analysis showing that cosmic voids have a negligible effect on gamma-ray opacity, refining our understanding of EBL inhomogeneity impacts.

Findings

Void effects on gamma-ray opacity are less than 1%.

Inhomogeneities have minimal impact compared to simple linear models.

Results challenge previous assumptions about EBL inhomogeneity influence.

Abstract

The unexpectedly hard very-high-energy (VHE; $E > 100$ GeV) $\gamma$-ray spectra of a few distant blazars have been interpreted as evidence for a reduction of the $\gamma\gamma$ opacity of the Universe due to the interaction of VHE $\gamma$-rays with the extragalactic background light (EBL) compared to the expectation from our current knowledge of the density and cosmological evolution of the EBL. One of the suggested solutions to this problem consisted of the inhomogeneity of the EBL. In this paper, we study the effects of such inhomogeneities on the energy density of the EBL (which then also becomes anisotropic) and the resulting $\gamma\gamma$ opacity. Specifically, we investigate the effects of cosmic voids along the line of sight to a distant blazar. We find that the effect of such voids on the $\gamma\gamma$ opacity, for any realistic void size, is only of the order of $\lesssim 1$ % and much smaller than expected from a simple linear scaling of the $\gamma\gamma$ opacity with the line-of-sight galaxy under-density due to a cosmic void.