The Fundamentals of the 21-cm Line

TL;DR

This paper reviews the fundamental physics behind the 21-cm line, focusing on radiative transfer, spin temperature processes like Wouthuysen-Field coupling, and heating mechanisms during the Cosmic Dawn.

Contribution

It provides a comprehensive overview of the key physical processes influencing the 21-cm signal during early cosmic epochs.

Findings

Clarifies the role of Wouthuysen-Field coupling in setting spin temperature.

Summarizes heating mechanisms affecting the intergalactic medium.

Highlights the importance of Lyman-alpha, CMB, and X-ray photons in cosmic heating.

Abstract

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.