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The First Galaxies in the Universe$
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Abraham Loeb and Steven R. Furlanetto

Print publication date: 2013

Print ISBN-13: 9780691144917

Published to Princeton Scholarship Online: October 2017

DOI: 10.23943/princeton/9780691144917.001.0001

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The 21-cm Line

The 21-cm Line

(p.408) Chapter Twelve The 21-cm Line
The First Galaxies in the Universe

Abraham Loeb

Steven R. Furlanetto

Princeton University Press

This chapter describes how the 21-cm line is used to study the high-z Universe. It introduces the spin-flip or the hyperfine line—a transition driven by the interaction of the spins of the proton and electron, whose relative directions affect the energy of the electron's orbit. An atom in the upper state eventually undergoes a spin-flip transition, emitting a photon with a wavelength of 21 cm. As the chapter shows, this transition is extremely weak, so the effective intergalactic medium (IGM) optical depth is only of the order of 1 percent: this makes the entire neutral IGM accessible during the cosmic dawn. Moreover, the transition energy is so low that it provides a sensitive thermometer of the low-temperature IGM, and as a low-frequency radio transition, it can be seen across the entirety of the IGM against the cosmic microwave background.

Keywords:   21-cm line, high-z Universe, intergalactic medium, hyperfine line, spin-flip transition, spin-flip background, statistical fluctuations, spin-flip fluctuations

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