There are three distinct regimes in which radio observations of the redshifted 21 cm line of HI can contribute directly to cosmology in unique ways. The regimes are naturally divided by redshift, from high to low, into: inflationary physics, the Dark Ages and reionization, and galaxy evolution and Dark Energy. Each measurement presents its own set of technical, theoretical, and observational challenges, making “what we need to know” not so much an astrophysical question at this early stage as a comprehensive experimental question. A wave of new pathfinder projects are exploring the fundamental aspects of what we need to know (and what we should expect to learn in the coming years) in order to achieve the goals of the Square Kilometer Array (SKA) and beyond.

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