Probability density of the multipole vectors for a Gaussian cosmic microwave background

We review Maxwell's multipole vectors, and elucidate some of their mathematical properties, with emphasis on the application of this tool to the cosmic microwave background (CMB). In particular, for a completely random function on the sphere (corresponding to the statistically isotropic Gaussian model of the CMB), we derive the full probability density function of the multipole vectors. This function is used to analyse the internal configurations of the third-year Wilkinson Microwave Anisotropy Probe quadrupole and octopole, and we show that the observations are consistent with the Gaussian prediction. A particular aspect is the planarity of the octopole, which we find not to be anomalous.