Bounds on non-Gaussianity in the variance from small-scale cosmic microwave background observations

We compare the latest results from CMB experiments at scales around l_e~150 over different parts of the sky to test the hypothesis that they are drawn from a Gaussian distribution, as is usually assumed. Using both the diagonal and the full covariance chi^2 tests, we compare the data with different sets of strategies and find in all cases incompatibility with the Gaussian hypothesis above the 1sigma level. We show how to include a generic non-Gaussian signal in the data analysis. Results from CMB observations can be made compatible with each other by assuming a non-Gaussian distribution for the signal, with a kurtosis at a level B_4=<delta^4_T>_c/<delta^2_T>^2~=90. A possible interpretation of this result is that the initial fluctuations at the surface of last scattering are strongly non-Gaussian. Another interpretation is that the systematic errors have been underestimated in all experiments by a factor of 2. Other explanations include foreground contamination, non-linear effects or a combination of these.