# The extragalactic background light revisited and the cosmic photon-photon opacity

## TL;DR

This paper revises the extragalactic background light (EBL) estimates using recent infrared survey data, leading to lower photon densities and reduced cosmic opacity, which impacts high-energy astrophysics interpretations.

## Contribution

It provides an updated EBL model based on new survey data, reducing photon densities at longer wavelengths and refining cosmic opacity estimates.

## Key findings

- Lower photon densities in mid- and far-infrared compared to previous models.
- Reduced cosmic photon-photon opacity, affecting high-energy photon propagation.
- Alleviates tension in TeV blazar observations, reducing need for new physics.

## Abstract

In addition to its relevant astrophysical and cosmological significance, the Extragalactic Background Light (EBL) is a fundamental source of opacity for cosmic high energy photons, as well as a limitation for the propagation of high-energy particles in the Universe. We review our previously published determinations of the EBL photon density in the Universe and its evolution with cosmic time, in the light of recent surveys of IR sources at long wavelengths. We exploit deep survey observations by the Herschel Space Observatory and the Spitzer telescope, matched to optical and near-IR photometric and spectroscopic data, to re-estimate number counts and luminosity functions longwards of a few microns, and the contribution of resolved sources to the EBL. These new data indicate slightly lower photon densities in the mid- and far-infrared and sub-millimeter compared to previous determinations. This implies slightly lower cosmic opacity for photon-photon interactions. The new data do not modify previously published EBL modeling in the UV-optical and near-IR up to several microns, while reducing the photon density at longer wavelengths. This improved model of the EBL alleviates some tension that had emerged in the interpretation of the highest-energy TeV observations of local \textit{blazar}s, reducing the case for new physics beyond the standard model (like violations of the Lorenz Invariance, LIV, at the highest particle energies), or for exotic astrophysics, that had sometimes been called for to explain it. Applications of this improved EBL model on current data are considered, as well as perspectives for future instrumentation, the Cherenkov Telescope Array (CTA) in particular.