A GeV–TeV Measurement of the Extragalactic Background Light

The extragalactic background light (EBL) can be probed via the absorption imprint it leaves in the spectra of gamma-ray sources ($\gamma \gamma \to {e}^{-}{e}^{+}$). We recently developed a dedicated technique to reconstruct the EBL, and its evolution with redshift, from γ-ray optical depth data using a large sample of blazars detected by the Fermi Large Area Telescope. Here, we extend this data set to the TeV regime using ground-based Cherenkov observations of 38 blazars and report the first homogeneous measurement of the EBL spectral intensity covering the ultraviolet to infrared wavelengths (∼0.1–100 μm). A minimal EBL throughout the wavelength range with respect to integrated galaxy light is found, allowing little additional unresolved emission from faint or truly diffuse populations setting an upper limit of ≲4 nW m−2 sr−1 at 1.4 μm. In particular, the cosmic optical background at z = 0 is found to be ${27.8}_{-2.0}^{+2.1}\,\mathrm{nW}\,{{\rm{m}}}^{-2}\ {\mathrm{sr}}^{-1}$. This work lays the foundation for accurate gamma-ray measurements of the EBL across its whole spectral range using a combination of GeV and TeV data.