  Observations made using large ground-based and space-borne telescopes have
probed cosmic history all the way from the present-day to a time when the
Universe was less than a tenth of its present age. Earlier on lies the
remaining frontier, where the first stars, galaxies, and massive black holes
formed. They fundamentally transformed the early Universe by endowing it with
the first sources of light and chemical elements beyond the primordial hydrogen
and helium produced in the Big Bang. The interplay of theory and upcoming
observations promises to answer the key open questions in this emerging field.
