First Light and the Reionization of the Universe
  The development of primordial inhomogeneities into the non-linear regime and the formation of the first bound objects mark the transition from a simple cooling universe -- described by just a few parameters -- to a very messy hot one -- the realm of radiative, hydrodynamic and star formation processes. In popular cold dark matter cosmological scenarios, stars and quasars may have first appeared in significant numbers around a redshift of 10 or so, as the gas within subgalactic halos cooled rapidly due to atomic processes and fragmented. This early generation of sources generated the ultraviolet radiation and kinetic energy that ended the cosmic ``dark ages'' and reheated and reionized most of the hydrogen in the cosmos by a redshift of 6. The detailed thermal history of the intergalactic medium -- the main repository of baryons at high redshift -- during and soon after these crucial formative stages depends on the power-spectrum of density fluctuations on small scales and on a complex network of poorly understood `feedback' mechanisms, and is one of the missing link in galaxy formation and evolution studies. 
Piero Madau (University of California Santa Cruz)