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.