Soon after the discovery of the CBR it was recognized that measurements of, or upper limits on, its anisotropy on scales of degrees or less would provide unique information about the origin and development of structure within the Universe. Such observations are of particular value because they probe cosmic times well before the appearance of any luminous objects in the Universe such as galaxies or stars. The surface of last scattering from which the CBR photons reach us is more distant than any QSO or galaxy yet detected (these lie at z < 5), and the CBR thus carries information about the state of the Universe at earlier times. It is quite likely, in fact, that the CBR photons we study reach us from the first few hundred thousand years of the history of the Universe; and, as we shall see in Chapter 8, the CBR may encode information from the epoch of inflation nearly 50 orders of magnitude earlier still.

With so much to learn, astronomers have worked hard to detect fluctuations in the temperature of the CBR. Indeed, there have been as many searches for structure in the background on scales ≤ 10° as measurements of the dipole and of the spectrum combined.