Observing Neutral Hydrogen Above Redshift 6: The "Global" Perspective
  Above redshift 6, the dominant source of neutral hydrogen in the Universe
shifts from localized clumps in and around galaxies and filaments to a
pervasive, diffuse component of the intergalactic medium (IGM). This transition
tracks the global neutral fraction of hydrogen in the IGM and can be studied,
in principle, through the redshifted 21 cm hyperfine transition line. During
the last half of the reionization epoch, the mean (global) brightness
temperature of the redshifted 21 cm emission is proportional to the neutral
fraction, but at earlier times (10 < z < 25), the mean brightness temperature
should probe the spin temperature of neutral hydrogen in the IGM. Measuring the
(of order 10 mK) mean brightness temperature of the redshifted 21 cm line as a
function of frequency (and hence redshift) would chart the early evolution of
galaxies through the heating and ionizing of the IGM by their stellar
populations. Experiments are already underway to accomplish this task or, at
least, provide basic constraints on the evolution of the mean brightness
temperature. We provide a brief overview of one of these projects, the
Experiment to the Detect the Global EOR Signature (EDGES), and discuss
prospects for future results.
