# The Post-Recombination Universe — the Dark Ages

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## Abstract

In the final part of this volume, we consider the post-recombination Universe and the physical processes responsible for the formation of galaxies and clusters of galaxies as we know them at the present day. The post-recombination era spans the redshift range from about 1000 to zero and it is convenient to divide it into two parts. The more recent of these, spanning the redshift range 0 < z < 5, may be termed the observable Universe of galaxies, in the sense that this is the range of redshifts over which galaxies and quasars have now been observed. Although there have been remarkable developments in detector and telescope technology over recent years, it is still the case that only the most luminous objects can be readily detected at large redshifts, z ≥ 1. As we will show in Chap. 17, there is plentiful evidence that the populations of galaxies and quasars have evolved dramatically over the redshift range 0 < z < 5. There is now good evidence that much of the star formation activity in galaxies and the synthesis of the heavy elements took place during these late phases of the post-recombination era, as we discuss in Chap. 18. Many of the most important physical processes which led to the formation of galaxies as we know them took place during this era, which, fortunately, is now accessible to astronomical observation. Many of these phenomena will be discussed in Chap. 20.

The earlier phase of the post-recombination era, the redshift interval 1000 > z > 5, is often referred to as the Dark Ages. At the beginning of this redshift interval, we can learn a considerable amount about the early development of the perturbations from which galaxies and larger scale structures formed from observations of the Cosmic Background Radiation. Until galaxies became visible as bound, star-forming systems at redshifts z ≤ 5, however, there are few observational tools which can be used to study precisely what took place during the immediate post-recombination era. The perturbations were still in the linear regime at z ~ 1000 but, as they collapsed to form bound systems, their evolution became non-linear. Many of the processes which led to the variety of structures we observe today must have taken place during these epochs. In this chapter we investigate some aspects of the physical processes which are likely to be important during the post-recombination epochs, before the galaxies became visible at redshifts z < 5.
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