Cosmic Gamma-Ray Background from Star-Forming Galaxies

Understand

The origin of the extragalactic gamma-ray background is a pressing cosmological mystery.

• The Fermi Gamma-Ray Space Telescope has recently measured the intensity and spectrum of this background; both are substantially different from previous measurements.
• We present a novel calculation of the gamma-ray background from normal star-forming galaxies.
• Contrary to long-standing expectations, we find that numerous but individually faint normal galaxies may comprise the bulk of the Fermi signal, rather than rare but intrinsically bright active galaxies.