Evidence for Non-Gaussianity in the [ITAL]COBE[/ITAL] DMR 4 Year Sky Maps

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Abstract
We introduce and study the distribution of an estimator for the normalized bispectrum of the cosmic microwave background (CMB) anisotropy. We use it to construct a goodness-of-fit statistic to test the co-added 53 and 90 GHz COBE Diffuse Microwave Radiometer 4 year maps for non-Gaussianity. Our results indicate that Gaussianity is ruled out at the confidence level in excess of 98%. This value is a lower bound, given all the investigated systematics. The dominant non-Gaussian contribution is found near the multipole of order ℓ=16. Our attempts to explain this effect as caused by the diffuse foreground emission from the Galaxy have failed. We conclude that unless there exists a microwave foreground emission that spatially correlates with neither the DIRBE nor Haslam maps, the cosmological CMB anisotropy is genuinely non-Gaussian.