Observing neutral hydrogen above redshift 6: The "Global" perspective

Above redshift 6, the dominant Source of neutral hydrogen in the Universe shifts front localized clumps in and around galaxies and filaments to a pervasive, diffuse component of the intergalactic medium (IGM). This transition tracks die global mitral 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 die reioinzation epoch, the mean (global) brightness temperature of the redshifted 21 cm emission is proportional to die neutral fraction, but at earlier times (110 < z < 25), the mean brightness temperature should probe die spin temperature of neutral hydrogen in the IGM. Measuring the (of order 10 mK) mean brightness temperature of the redshifted 2 1 cm line as a function of frequency (and hence redshift) would chart die 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 Oil the evolution of die 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.