The formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state. In popular cosmological models, the first sources of light began to form at a redshift z=30 and reionized most of the hydrogen in the universe by z=7. Current observations are at the threshold of probing the hydrogen reionization epoch. The study of high-redshift sources is likely to attract major attention in observational and theoretical cosmology over the next decade. (C) 2001 Elsevier Science B.V. All rights reserved. [References: 5]
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