The formation of the first stars and quasars marks the transformation of the universe from its smooth initial state to its clumpy current state.
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. Also available on astro-ph, here . Note that in the astro-ph version, several figures were compressed, resulting in a large reduction in size with just a slight reduction in quality.