Astro2020 Science White Paper: Insights Into the Epoch of Reionization
  with the Highly-Redshifted 21-cm Line
  The epoch of reionization, when photons from early galaxies ionized the
intergalactic medium about a billion years after the Big Bang, is the last
major phase transition in the Universe's history. Measuring the characteristics
of the transition is important for understanding early galaxies and the cosmic
web and for modeling dwarf galaxies in the later Universe. But such
measurements require probes of the intergalactic medium itself. Here we
describe how the 21-cm line of neutral hydrogen provides a powerful probe of
the reionization process and therefore important constraints on both the
galaxies and intergalactic absorbers at that time. While existing experiments
will make precise statistical measurements over the next decade, we argue that
improved 21-cm analysis techniques - allowing imaging of the neutral gas itself
- as well as improved theoretical models, are crucial for testing our
understanding of this important era.
