2.5.2. Quadrupole anisotropy
The discovery of a quadrupole moment in the cosmic microwave background radiation would provide perhaps the strongest evidence for large-scale inhomogeneity or anisotropy in the Universe. At present, there is no definitive measurement of an intrinsic quadrupole-like anisotropy. Balloon-borne experiments by Fixsen et al. (1983) and Lubin et al. (1983) at 1.2 and 0.3 cm wavelength have imposed an upper limit of 6.2 × 10-5 on the r.m.s. amplitude of the quadrupole anisotropy. The only contradictory experiment that still maintains a detection is by Fabbri et al. (1980), who utilized a balloon-borne far infrared bolometer to search for large scale anisotropy in the background radiation in the 0.05-3 mm wavelength region. In addition to confirming the dipole anisotropy, a second harmonic term was detected. The peak amplitude, with very limited sky coverage, was 3( ± 1) × 10-4, and consistent with alignment along the dipole axis. It seems likely that this signal is due to contamination by galactic dust emission, in view of the null result by the other groups. In particular, the Princeton experiment not only confirmed the dipole anisotropy, but detected the motion of the earth relative to the sun and the microwave background.
Detection of an intrinsic quadrupole anisotropy could be readily explained with a minimum of assumptions in the context of the gravitational instability theory for the origin of large scale structure in the Universe. Indeed, one would like to measure this effect in order to forge a definitive link between theory and observation. One difficulty with past observations has been that of separating out the diffuse galactic radio emission, which possesses a large-scale spatial distribution that contains a quadrupole-like component. It seems clear that a definitive detection will require complete sky coverage as well as improved sensitivity at the 0.1 mK level.