Exploring the cosmic dawn and epoch of reionization with the 21 cm line

The dark age of the Universe, when no luminous object existed, ended with the birth of the first stars, galaxies, and black holes. This epoch is called cosmic dawn. Cosmic reionization is the major transition of the intergalactic medium in the Universe driven by ionizing photons emitted from luminous objects. Although the epoch through the dark ages to reionization is a milestone in the Universe, our knowledge of it is not yet sufficient. The cosmic 21 cm signal, which is emitted from neutral hydrogen, is expected to open a new window into this epoch. In this review paper, we first introduce the basic physics of the 21 cm line and how the first stars impact on the 21 cm line signal. Next, we briefly summarize how we extract astrophysical information from the 21 cm line signal by means of statistical and machine-learning approaches. We also discuss the synergy between the 21 cm line signal and other emission lines. Finally, we summarize the current status of 21 cm experiments.