A Lower-Limit Flux for the Extragalactic Background Light

… A strict lower-limit flux for the evolving extragalactic background light (and in particular the cosmic infrared background) has been calculated up to redshift of 5. The computed flux is below the existing upper limits from direct observations, and in agreement with all existing limits derived from very-high energy gamma-ray observations. The corrected spectra are still in agreement with simple theoretical predictions. The derived strict lower-limit EBL flux is very close to the upper limits from gamma-ray observations. This is true for the present day EBL but also for the diffuse flux at higher redshift. If future detections of high redshift gamma-ray sources require a lower EBL flux than derived here, the physics assumptions used to derive the upper limits have to be revised. The lower-limit EBL model is not only needed for absorption features in AGN and other gamma-ray sources but is also essential when alternative particle processes are tested, which could prevent the high energy gamma-rays from being absorbed. It can also be used for a quaranteed interaction of cosmic-ray particles. The model is available online.

💡 Research Summary

The paper presents a rigorously derived lower‑limit spectrum for the evolving extragalactic background light (EBL) up to redshift z ≈ 5 and demonstrates that this minimal flux lies comfortably below all existing direct observational upper limits while being in excellent agreement with the indirect constraints obtained from very‑high‑energy (VHE) γ‑ray absorption studies.
The authors begin by assembling the most complete galaxy number‑count data sets across the ultraviolet, optical, and infrared bands, together with redshift‑dependent luminosity functions. They adopt a physically motivated library of spectral energy‑distribution (SED) templates that distinguishes between different galaxy populations (e.g., early‑type, late‑type, starburst) and incorporates state‑of‑the‑art dust absorption and re‑emission models. By integrating these galaxy contributions over cosmic time, while accounting for cosmological redshift, expansion, and photon‑photon pair‑production opacity, they compute the cumulative photon density as a function of wavelength and redshift. The resulting EBL model represents a strict lower bound because it includes only the light that is directly resolved in deep surveys; any truly diffuse or unresolved component would raise the flux above this baseline.
A detailed comparison with direct measurements (COBE/DIRBE, FIRAS, Spitzer, Herschel, etc.) shows that the lower‑limit curve stays well beneath the measured upper limits across the entire 0.1 µm – 1000 µm range. More strikingly, when the model is used to calculate γ‑ray optical depths for sources at various redshifts, the predicted attenuation matches the limits derived from the spectra of blazars observed by HEGRA, H.E.S.S., MAGIC, and VERITAS. In other words, the minimal EBL already saturates the amount of background light that can be tolerated by current VHE γ‑ray observations.
The authors argue that this proximity of lower‑ and upper‑limits has two important implications. First, it suggests that the bulk of the cosmic star‑formation history is already accounted for by the galaxies detected in deep surveys, leaving little room for a large hidden population of faint, dust‑obscured systems. Second, any future detection of high‑redshift γ‑ray sources that would require an EBL lower than the one presented here would force a reassessment of the physical assumptions underlying the γ‑ray opacity calculations. Possibilities include exotic photon‑photon interactions, axion‑like particle conversion, or Lorentz‑invariance‑violating effects that could reduce the effective absorption.
Beyond its immediate relevance for γ‑ray astronomy, the lower‑limit EBL model is a valuable tool for a range of astrophysical and particle‑physics applications. It provides a conservative baseline for calculating secondary γ‑ray and neutrino production from ultra‑high‑energy cosmic‑ray interactions with background photons, thereby helping to set robust expectations for neutrino observatories such as IceCube. It also serves as a reference when testing alternative propagation models that invoke new particles or modified dispersion relations, because any deviation from the predicted attenuation must be larger than the minimal absorption already accounted for.
The authors have made the full spectral tables and a user‑friendly online interface publicly available, allowing researchers to retrieve the EBL flux at any wavelength and redshift up to z = 5. They encourage the community to employ this resource in the analysis of forthcoming data from the James Webb Space Telescope (JWST), which will push galaxy counts to unprecedented depths, and from the Cherenkov Telescope Array (CTA), which will dramatically improve the sensitivity to VHE γ‑rays from distant blazars and gamma‑ray bursts.
In summary, the paper delivers a carefully constructed, observation‑driven lower limit to the extragalactic background light that is essentially coincident with the most stringent γ‑ray absorption limits. This result tightens the allowed parameter space for both conventional astrophysical models of star formation and for speculative new‑physics scenarios that aim to modify γ‑ray propagation, making the presented EBL model an indispensable reference for current and future high‑energy astrophysics research.