Physical Cosmology From the 21-cm Line
  We describe how the high-redshift 21-cm background can be used to improve
both our understanding of the fundamental cosmological parameters of our
Universe and exotic processes originating in the dark sector. The 21-cm
background emerging during the cosmological Dark Ages, the era between hydrogen
recombination and the formation of the first luminous sources (likely at z ~
30), is difficult to measure but provides several powerful advantages for these
purposes: in addition to the lack of astrophysical contamination, it will allow
probes of very small scale structure over a very large volume. Additionally,
the 21-cm background is sensitive to the thermal state of the intergalactic
hydrogen and therefore probes any exotic processes (including, e.g., dark
matter scattering or decay and primordial black holes) during that era. After
astrophysical sources have formed, cosmological information can be separated
from astrophysical effects on the 21-cm background through methods such as
redshift space distortions, joint modeling, and by searching for indirect
effects on the astrophysical sources themselves.
