Hyperfine Level–changing Collisions of Hydrogen Atoms and Tomography of the Dark Age Universe

We present accurate rates for spin exchange processes in collisions of hydrogen atoms for temperatures in the range 1 K < T < 300 K. A fully quantal, multichannel theory is employed in the calculations. We discuss the respective roles of spin exchange and long-range magnetic interactions in collisional redistribution of sublevel populations. We use the rates to calculate the spin temperature of hydrogen in the universe at 20 < z < 200 and explore its sensitivity to the collision data. We stress the important role of atomic collisions in enabling 21 cm tomography, recently proposed by Loeb & Zaldarriaga, of the "dark age" universe.