Speaker
Description
Multi-TeV gamma rays interact with optical and infrared background photons through electron-positron pair production. Their observed spectra therefore probe the extragalactic background light, particularly photons in the (0.01)–(0.1\,\mathrm{eV}) range. I will explain how gamma-ray observations reconstruct this background and why their agreement with integrated galaxy counts leaves little room for additional diffuse emission. This absence of excess can constrain new physics that produces soft photons, including axion decays and radiative neutrino decays induced by electromagnetic transition moments. Recent observations, from large archival TeV samples to GRB 221009A, illustrate both the sensitivity and the main astrophysical uncertainties. In this way, the transparency of the Universe becomes a probe of feebly interacting particles.