Speaker
Description
The flux of high-energy astrophysical γ rays is attenuated by the production of electron-positron pairs from scattering off of extragalactic background light (EBL). We use the most up-to-date information on galaxy populations to compute their contributions to the pair-production optical depth. We find that the optical depth inferred from γ-ray measurements exceeds that expected from galaxies at the ∼2σ level. If the excess is modeled as a frequency-independent re-scaling of the standard contribution to the EBL from galaxies, then it is detected at the 2.7σ level (an overall 14−30% increase of the EBL). If the frequency dependence of the excess is instead modeled as a two-photon decay of a dark-matter axion, then the excess is favored over the null hypothesis of no excess at the 2.1σ confidence level. While we find no evidence for a dark-matter signal, the analysis sets the strongest current bounds on the photon-axion coupling over the 8−25 eV mass range.
