For ``Physics Reports'' special issue in memory of D.N. Schramm; (astro-ph/9912345)
Abstract. The universe would have been completely dark between the epoch of recombination and the development of the first non-linear structure. But at redshifts beyond 5 - perhaps even beyond 20 - stars formed within `subgalaxies' and created the first heavy elements; these same systems (together perhaps with `miniquasars') generated the UV radiation that ionized the IGM, and maybe also the first significant magnetic fields. Although we can already probe back to z ≅ 5, these very first objects may be so faint that their detection must await next-generation optical and infrared telescopes. Observations in other wavebands may offer indirect clues to when reionization occurred. Despite the rapid improvements in numerical simulations, the processes of star formation and feedback are likely to remain a challenge for the next decade.
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