"Version: 20191201"--Title page verso.Includes bibliographical references.1. Theoretical framework : the fundamentals of the 21 cm line -- 1.1. Radiative transfer of the 21 cm line -- 1.2. The spin temperature -- 1.3. Heating of the intergalactic medium2. Astrophysics from the 21 cm background -- 2.1. Properties of the high-z intergalactic medium -- 2.2. Sources of the UV and x-ray background -…