'First light' in the universe: what ended the 'dark age'?

The universe would have been completely dark between the epoch of recombina tion and the development of the first non-linear structure. But at redshift s beyond 5 - perhaps even beyond 20 - stars formed within 'subgalaxies' and created the first heavy elements; these same systems (together perhaps wit h 'miniquasars') generated the UV radiation that ionised the IGM, and may b e also the first significant magnetic fields. Although we can already probe back to z similar or equal to 5, these very first objects may be so faint that their detection must await next-generation optical and infrared telesc opes. Observations in other wavebands may offer indirect clues to when reio nisation occurred. Despite the rapid improvements in numerical simulations, the processes of star formation and feedback are likely to remain a challe nge for the next decade. (C) 2000 Elsevier Science B.V. All rights reserved .