An Introduction to Radio Astronomy

The ‘alabaster curtain’ of the cosmic microwave background, the CMB, defines the limit of the directly observable Universe. Beyond that limit, the mean free path of photons is so short that the blackbody radiation field and the primitive plasma of ions and electrons are locked together in tight coupling. As expansion of the plasma proceeds, its density and temperature drop, and, when the Universe reaches an age that appears to be about 370 000 years, the era of decoupling is reached, the ions and electrons combine into atoms, and the photons escape through the nearly transparent neutral medium, to be observed as the CMB. Indirect paths of study of the Universe beyond the era of decoupling are few; one of these is the first era of nucleosynthesis, discussed in Chapter 14. There is the hope that neutrinos or more exotic particles might give another observation channel, reaching beyond the era of first nucleosythesis.

The Universe that we observe around us is clearly inhomogeneous, and that inhomogeneity had to grow from far smaller, unstable density fluctuations, urged on by the relentless force of gravity. After the discovery of the CMB, the search for spatial structure began almost immediately; it was generally recognized, by experimenters and theorists alike, that the testimony of those first perturbations must be present at some level in the apparently featureless facade of the CMB, motivating an intensive and difficult search.