Black Holes From the Dark Ages: Exploring the Reionization Era and Early   Structure Formation With Quasars and Gamma-Ray Bursts
  The cosmic reionization era, which includes formation of the first stars, galaxies, and AGN, is now one of the most active frontiers of cosmological research. We review briefly our current understanding of the early structure formation, and use the ideas about a joint formation of massive black holes (which power the early QSOs) and their host galaxies to employ high-redshift QSOs as probes of the early galaxy formation and primordial large-scale structure. There is a growing evidence for a strong biasing in the formation of the first luminous sources, which would lead to a clumpy reionization. Absorption spectroscopy of QSOs at z > 6 indicates the end of the reionization era at z ~ 6; yet measurements from the WMAP satellite suggest and early reionization at z ~ 10 - 20. The first generation of massive stars, perhaps aided by the early mini-quasars, may have reionized the universe at such high redshifts, but their feedback may have disrupted the subsequent star and galaxy formation, leading to an extended and perhaps multimodal reionization history ending by z ~ 6. Observations of gamma-ray bursts from the death events of these putative Population III stars may provide essential insight into the primordial structure formation, reionization, early chemical enrichment, and formation of seed black holes which may grow to become central engines of luminous quasars. 
S. G. Djorgovski