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

TL;DR

This paper presents a novel gamma-ray based measurement of the extragalactic background light (EBL) spectrum at redshift zero, providing an independent, model-free estimate that aligns with galaxy light observations and constrains cosmic expansion.

Contribution

It introduces a purely parametric gamma-ray method to measure the EBL spectrum independently of redshift evolution, confirming optical convergence and constraining diffuse contributions.

Findings

Measured EBL intensity at 600 nm: 6.9 ± 1.9 nW/m²/sr

Excluded diffuse EBL contributions greater than 20% of galaxy light

Estimated H₀ = 67^{+7}_{-6} km/s/Mpc, independent of EBL models

Abstract

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 $\gamma$-ray flux from extragalactic sources. This paper reports the first $\gamma$-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 $120\,\mu$m. 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 $6.9 \pm 1.9$ nW m$^{-2}$ sr$^{-1}\,\times\, h_{70}$, 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, $f_\mathrm{diff}$, greater than $20\,\%$ and provide a measurement of the expansion rate of the universe at $z=0$, $H_0 = 67^{+7}_{-6}$ km s$^{-1}$ Mpc$^{-1}\,\times\, (1+f_\mathrm{diff})$, which is EBL-model independent. IGL, direct and $\gamma$-ray measurements agree on the EBL intensity in the optical band, finally reaching a cosmological optical convergence.