It is widely agreed that studying the 21 cm emission line from neutral hydrogen may be our best hope for understanding the creation of the first structures during the dark ages and the Epoch of Reionization (EoR). This hyperfine transition occurs as a result of the spin-spin interaction between the electron and the proton in hydrogen: The energy of the parallel spins (triplet) state is greater than that of the anti-parallel spins (singlet). This transition is strongly banned, with a transition probability of $2.6 \times 10^{-15} \mathrm{s}^{-1}$ being exceedingly low. Despite its low chance, the 21 cm hyperfine transition is one of the most important tools in observational astronomy due to the abundance of hydrogen in the Universe. The relative occupation number of the ground state and the occupation number of the excited state determine the strength of the 21 cm emission or absorption. Collisions or the so-called Wouthuysen-Field effect can both excite the 21 cm line. In this article, we give a brief review of the basic physics of 21 cm emission and highlight the current main worldwide 21 cm signal experiments. We also discuss several related concepts in astronomy and cosmology with these observations.
Absorption in the 21 cm Hydrogen Line at $z>10$ as a Sensitive Tool for the Construction of a Cosmological Model on Small Scales
Institute for Nuclear Research, Russian Academy of Sciences（俄罗斯科学院核物理研究所）；Astro Space Center, Lebedev Physical Institute, Russian Academy of Sciences（俄罗斯科学院列别杰夫物理研究所天文航天中心）
A quasi-molecular mechanism of formation of hydrogen in the early Universe -- a scheme of calculation
在我们最近的论文（Kereslidze et all 2019 a，2021）中，提出了一种非标准的准分子机制，并应用于处理宇宙学重组。人们假设，在宇宙演化的预复合阶段，一个电子与两个相邻的质子结合，产生了氢分子离子$H_2^+$，处于高度激发态，然后下降到较低的激发态或解离。在这项工作中，我们阐述了计划的计算自由束缚辐射跃迁到吸引态的$H_2^+$的红移$z$的功能。连同早期开发的$H_2^+$中的束缚-束缚辐射跃迁的处理，详细的计算方案可用于设计快速和完整的宇宙学复合代码。
