Statistics of bipolar representation of CMB maps

Gaussianity of temperature fluctuations in the Cosmic Microwave Background (CMB) implies that the statistical properties of the temperature field can be completely characterized by its two-point correlation function. The two-point correlation function can be expanded in full generality in the Bipolar Spherical Harmonic (BipoSH) basis. Looking for significant deviations from zero for BipoSH coefficients derived from observed CMB maps forms the basis of the strategy used to detect isotropy violation. In order to quantify “significant deviation” we need to understand the distributions of these coefficients. We analytically evaluate the moments and the distribution of the coefficients of expansion (ALMl1l2), using the characteristic function approach. We show that for BipoSH coefficients with M=0 an analytical form for the moments up to any arbitrary order can be derived. For the remaining BipoSH coefficients with M≠0, the moments derived using the characteristic function approach need to be supplemented with a correction term. The correction term is found to be important particularly at low multipoles. We provide a general prescription for calculating these corrections, however we restrict the explicit calculations only up to kurtosis. We confirm our results with measurements of BipoSH coefficients on numerically simulated statistically isotropic CMB maps