Understanding the formation and evolution of the first stars and
galaxies represents one of the most exciting frontiers in astronomy.
Since the universe was filled with hydrogen atoms at early times,
the most promising method for observing the epoch of the first stars
is to use the prominent 21-cm spectral line of hydrogen. Current
observational efforts are focused on the cosmic reionization era,
but observations of the pre-reionization cosmic dawn are also
beginning and promise exciting discoveries. While observationally
unexplored, theoretical studies predict a rich variety of
observational signatures from the astrophysics of the early galaxies
that formed during cosmic dawn. As the first stars formed, their
radiation (plus that from stellar remnants) produced feedback that
radically affected both the intergalactic medium and the character
of newly-forming stars. Lyman-alpha radiation from stars
generated a strong 21-cm absorption signal, observation of which is
currently the only feasible method of detecting the dominant
population of galaxies at redshifts as early as z~25. Another
major player is cosmic heating; if due to soft X-rays, then it
occurred fairly early (z~15) and produced the strongest
pre-reionization signal, while if it is due to hard X-rays, as now
seems more likely, then it occurred later and may have dramatically
affected the 21-cm sky even during reionization. In terms of
analysis, much focus has gone to studying the angle-averaged power
spectrum of 21-cm fluctuations, a rich dataset that can be used to
reconstruct the astrophysical information of greatest interest. This
does not, however, diminish the importance of finding additional
probes that are complementary or amenable to a more
model-independent analysis. Examples include the global
(sky-averaged) 21-cm spectrum, and the line-of-sight anisotropy of
the 21-cm power spectrum. Another striking feature may result from a
recently recognized effect of a supersonic relative velocity between
the dark matter and gas. This effect enhanced large-scale
clustering and, if early 21-cm fluctuations were dominated by small
galactic halos, it produced a prominent pattern on 100 Mpc scales.
Work in this field, focused on understanding the whole era of
reionization and cosmic dawn with analytical models and numerical
simulations, is likely to grow in intensity and importance, as the
theoretical predictions are finally expected to confront 21-cm
observations in the coming years.
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