A characteristic feature of the CBR, noted at the time of its discovery by Penzias and Wilson (1965), is its approximate isotropy (see Appendix A). Approximately equal intensity in all directions is expected if the radiation is a relic of the Hot Big Bang. On the other hand, there are a variety of mechanisms that can induce small amplitude variations in intensity, or anisotropies, into an initially uniform CBR; some of these were outlined in Chapter 2 and will be discussed in detail in Chapter 8.

Careful measurements of the angular distribution of the CBR have therefore been pursued both to confirm the cosmic, Hot Big Bang, origin of the CBR and to search for small amplitude anisotropies imprinted in it. In this chapter we deal with observations of the angular distribution of the CBR on the largest angular scales, θ ≳ 10°, and in particular with the dipole and quadrupole moments of the CBR. The value of about 10° for the boundary between ‘large’ scale anisotropies (discussed in this chapter) and smaller scale anisotropies (Chapter 7) is obviously rather artificial. When we turn in Chapter 8 to the implications of the measurements of and upper limits on CBR anisotropies, we will be drawing on the results of both Chapters 6 and 7.