Cosmic Gamma-Ray Background from Star-Forming Galaxies
  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. This result has wide-ranging implications, including:
the possibility to probe the cosmic star formation history with gamma rays; the
ability to infer the cosmological evolution of cosmic rays and galactic
magnetic fields; and an increased likelihood of identifying subdominant
components from rare sources (e.g., dark matter clumps) through their large
anisotropy.
