  The measured cosmic gamma ray background (CGB) spectrum at MeV energies is in
reasonable agreement with the predicted contribution from type Ia supernovae
(SNIa). But the characteristic features in the SNIa gamma ray spectrum,
weakened by integration over source redshifts, are hard to measure, and
additionally the contributions from other sources in the MeV range are
uncertain, so that the SNIa origin of the MeV CGB remains unproven. Since
different CGB sources have different clustering properties and redshift
distributions, by combining the CGB spectrum and angular correlation
measurements, the contributions to the CGB could be identified and separated.
The SNIa CGB large-scale structure follows that of galaxies. Its rms
fluctuation at degree scales has a characteristic energy dependence, ranging
from $\sim 1%$ to order of unity and can be measured to several percent
precision by proposed future satellites such as the Advanced Compton Telescope.
With the identification of the SNIa contribution, the SNIa rate could be
measured unambiguously as a function of redshift up to $z \sim 1$, by combining
both the spectrum and angular correlation measurements, yielding new
constraints on the star formation rate to even higher redshifts. Finally, we
show that the gamma ray and neutrino backgrounds from supernovae should be
closely connected, allowing an important consistency test from the measured
data. Identification of the astrophysical contributions to the CGB would allow
much greater sensitivity to an isotropic high-redshift CGB contribution arising
in extra dimension or dark matter models.
