Astro 2020 Science White Paper: Fundamental Cosmology in the Dark Ages
  with 21-cm Line Fluctuations
  The Dark Ages are the period between the last scattering of the cosmic
microwave background and the appearance of the first luminous sources, spanning
approximately 1100 < z < 30. The only known way to measure fluctuations in this
era is through the 21-cm line of neutral hydrogen. Such observations have
enormous potential for cosmology, because they span a large volume while the
fluctuations remain linear even on small scales. Observations of 21-cm
fluctuations during this era can therefore constrain fundamental aspects of our
Universe, including inflation and any exotic physics of dark matter. While the
observational challenges to these low-frequency 21-cm observations are
enormous, especially from the terrestrial environment, they represent an
important goal for cosmology.
