The angular bispectrum of the cosmic microwave background

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Abstract
COBE has provided us with the whole skymap of the CBR anisotropies. However, even if the noise level is negligible when the four year COBE data are available, the cosmic variance will limit us from obtaining information about the Gaussian nature of the primordial fluctuations from the COBE dataset. This important issue is addressed in this paper by studying the angular bispectrum of the cosmic microwave background. A general form of the angular bispectrum is given and the cosmic variance of the angular bispectrum for Gaussian fluctuations is calculated. The encouraging result from the study is that the cosmic variance of skewness and three point function is dramatically reduced by removing the quadrupole term from the sky map, which leads us to suggest that the quadrupole term be removed in searching for non-Gaussian signals in COBE data. The discouraging result is that the non-Gaussian signals in most physically motivated non-Gaussian models are too small compared with cosmic variance. Unless the amplitudes are unreasonably large, the non-Gaussian signals are detectable through COBE only in the models where the angular bispectrum is flat or increases with increasing multipole moment

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