Review Article

Large-Angle Anomalies in the CMB

Figure 4

Quadrupole and octopole ( and ) temperature anisotropy of the WMAP sky map in galactic coordinates, shown with the ecliptic plane and the cosmological dipole. Included are the multipole vectors (solid diamonds): two for the quadrupole (red diamonds) and three for the octopole (green diamonds). We also show the four normals (solid squares) to the planes defined by vectors that describe the quadrupole and octopole temperature anisotropy; one normal is defined by the quadrupole (red square) and three by the octopole (green squares). Note that three out of four normals lie very close to the dipole direction. The probability of this alignment being accidental is about one part in a thousand. Moreover, the ecliptic plane traces out a locus of zero of the combined quadrupole and octopole over a broad swath of the sky—neatly separating a hot spot in the northern sky from a cold spot in the south. These apparent correlations with the solar system geometry are puzzling and currently unexplained.