We review some of the fundamental physics necessary for computing the
highly-redshifted spin-flip background. We first discuss the radiative transfer
of the 21-cm line and define the crucial quantities of interest. We then review
the processes that set the spin temperature of the transition, with a
particular focus on Wouthuysen-Field coupling, which is likely to be the most
important process during and after the Cosmic Dawn. Finally, we discuss
processes that heat the intergalactic medium during the Cosmic Dawn, including
the scattering of Lyman-alpha, cosmic microwave background, and X-ray photons.Comment: To appear as a book chapter in "The Cosmic 21-cm Revolution: Charting
the first billion years of our Universe," Ed Andrei Mesinger (Bristol: IOP
Publishing Ltd) AAS-IOP ebooks http://www.iopscience.org/books/aas. arXiv
admin note: text overlap with arXiv:astro-ph/060803
