# A $\gamma$-ray determination of the Universe's star-formation history

## TL;DR

This paper uses gamma-ray observations of distant sources to measure the extragalactic background light, enabling reconstruction of the Universe's star-formation history over most of cosmic time, consistent with galaxy survey data.

## Contribution

It introduces a novel gamma-ray based method to trace the Universe's star-formation history, extending measurements to earlier epochs.

## Key findings

- Star-formation rate peaks at redshift z~2.
- EBL measurements are consistent with galaxy surveys.
- Hints of a decline in faint galaxy abundance at z~6.

## Abstract

The light emitted by all galaxies over the history of the Universe produces the extragalactic background light (EBL) at ultraviolet, optical, and infrared wavelengths. The EBL is a source of opacity for $\gamma$ rays via photon-photon interactions, leaving an imprint in the spectra of distant $\gamma$-ray sources. We measure this attenuation using {739} active galaxies and one gamma-ray burst detected by the {\it Fermi} Large Area Telescope. This allows us to reconstruct the evolution of the EBL and determine the star-formation history of the Universe over 90\% of cosmic time. Our star-formation history is consistent with independent measurements from galaxy surveys, peaking at redshift $z\sim2$. Upper limits of the EBL at the epoch of re-ionization suggest a turnover in the abundance of faint galaxies at $z\sim 6$.