A wide range of cosmological observations have been proposed that make use of the 21 cm emission of neutral Hydrogen. These include: Measurement of the expansion history h(z), via the mapping of large scale structure and baryon acoustic oscillations at redshifts from 0 to 1; Detection of the redshift of the Epoch of Reionization, believed to lie someplace in the redshift range 7 to 12; and Measurement of the linear growth of large scale structure in the neutral dark age at redshifts as high as 50. Because 21 cm observations provide three-dimensional information many theorists are intrigued by the huge information content such observations promise to provide.
The physics examined by such observations may someday include a test of the Dark Energy equation of state, and study of linear growth of dark matter structures via ordinary attractive gravity, as well as information from very early epochs on star formation and radiative transport. But, even though the potential for such observation is exciting, the actual observations remain very challenging to carry out. First, the telescopes are big. These observations will be made using telescopes of collecting area at least 10,000 m2, and with hundreds or even thousands of wide-band receiver channels. Second, both Galactic and extragalactic foreground emissions exceed the sought after 21 cm signals by many orders of magnitude. Third, the frequency range of these observation is heavily contaminated by man-made interference. Removal of these foregrounds, and the man made interference, will require understanding and calibration of the telescope response with unprecedented precision.
I will report on our use of the Green Bank Telescope, the Giant Metrewave Radio Telescope in attempts to detect 21 cm emission at redshifts one and nine. I will also describe initial results of tests of 21 cm Cylinder Radio Telescope Prototype we have built in Pittsburgh.