The 21-cm Line as a Probe of Reionization

TL;DR

This paper reviews how the 21-cm hydrogen line can be used to study the early universe's structure formation and reionization, discussing physics, key parameters, and observational challenges.

Contribution

It provides a comprehensive overview of the physics, astrophysical parameters, and observational strategies related to the 21-cm line as a probe of reionization.

Findings

Describes the physics of the 21-cm transition.

Analyzes the evolution of the 21-cm background.

Discusses observational challenges and future prospects.

Abstract

One of the most exciting probes of the early phases of structure formation and reionization is the spin-flip line of neutral hydrogen, with a rest wavelength of 21 cm. This chapter introduces the physics of this transition and the astrophysical parameters upon which it depends, including discussions of the radiation fields that permeate the intergalactic medium that fix the brightness of this transition. We describe the critical points in the evolution of the 21-cm background and focus on the sky-averaged brightness and the power spectrum as representative measurements. Finally, we include a discussion of observations and the challenges they face in the near future.