  Why is non-Gaussianity interesting? One of generic predictions from
inflationary scenarios is that primordial fluctuations are exactly Gaussian in
linear order; however, the non-linearity in the inflation will produce weak
non-Gaussianity. Thus, measuring the non-Gaussianity in the cosmic microwave
background radiation anisotropy is a probe of the non-linear physics in the
very early universe. Since the angular three point function is zero for the
Gaussian field, it is sensitive to the non-Gaussianity. We predict its harmonic
transform counterpart, the angular {\it Bispectrum}, down to arcminutes angular
scales, including the full effect of the radiation transfer function. We find
that even the Planck experiment cannot detect the primary bispectrum from the
inflation, as long as the single field slow-roll inflation is right.
Non-linearities in the low redshift universe also produce the non-Gaussianity.
We find that secondary bispectra are detectable by both MAP and Planck
experiments. The secondary bispectra probe the non-linear physics of the
low-redshift universe. Although this could be a contaminant to the primary
signal, MAP and Planck experiments are found to be able to separate the primary
from secondary effects well. We present a tentative comparison of the primary
bispectrum to the published COBE 4 year bispectrum. The data put a weak
constraint on the parameter, and the constraint would become much tighter when
we use all modes available in the COBE data, and certainly forthcoming
satellite experiments. As a conclusion, the bispectrum is a key measure to
confirm or destroy the simple inflationary scenario in non-linear order that
seems quite successful in linear order.
