The Cosmological Optical Convergence: Extragalactic Background Light from TeV Gamma Rays

The intensity of the extragalactic background (EBL), the accumulated optical and infrared emissions since the first stars, is the subject of a decades-long tension in the optical band. These photons form a target field that attenuates the γ-ray flux from extragalactic sources. This Letter reports the first γ-ray measurement of the EBL spectrum at z = 0 that is purely parametric and independent of EBL evolution with redshift over a wavelength range from 0.18 to 120 μm. Our method extracts the EBL absorption imprint on more than 260 archival TeV spectra from the STeVECat catalog by marginalizing nuisance parameters describing the intrinsic emission and instrumental uncertainties. We report an intensity at 600 nm of 6.9 ± 1.9 nW m−2 sr−1 × h70, which is indistinguishable from the intensity derived from integrated galaxy light (IGL) and compatible with direct measurements taken beyond Pluto’s orbit. We exclude with 95% confidence diffuse contributions to the EBL with an intensity relative to the IGL, fdiff, greater than 20% and provide a measurement of the expansion rate of the Universe at z = 0, km s−1 Mpc −1 × (1 + fdiff), which is EBL-model independent. IGL, direct, and γ-ray measurements agree on the EBL intensity in the optical band, finally reaching a cosmological optical convergence.