Searching for the hydrogen 21 cm line in cosmos

TL;DR

This paper reviews the physics of the 21 cm hydrogen line and discusses current experiments aiming to detect this signal from the early universe, which is crucial for understanding cosmic dawn and reionization.

Contribution

It provides a comprehensive overview of the physics behind the 21 cm line and summarizes the main ongoing observational efforts in this field.

Findings

Summarizes key physics of 21 cm emission and absorption.

Highlights major worldwide 21 cm signal experiments.

Discusses implications for cosmology and early universe studies.

Abstract

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.