Furlanetto 2006 21cm氢线作为高红移宇宙的探针

主要内容:
中性氢原子基态电子可以处在于质子磁矩平行或者反平行两种状态，前者比后者能量低对应21cm光子的能量。这种跃迁几率非常小，但宇宙中中性氢非常多，还是能观测到这种禁戒线，并成为独一无二的观测中性氢丰度的探针。

文章将21cm线作为工具，综述研究宇宙中中性氢的分布，包括IGM，大尺度结构，及更早期的宇宙。

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文章信息:
Cosmology at low frequencies: The 21 cm transition and the high-redshift Universe
Pages 181-301
Steven R. Furlanetto, S. Peng Oh and Frank H. Briggs

Observations of the high-redshift Universe with the 21 cm hyperfine line of neutral hydrogen promise to open an entirely new windowonto the early phases of cosmic structure formation. Here we reviewthe physics of the 21 cm transition, focusing on processes relevant at high redshifts, and describe the insights to be gained from such observations. These include measuring the matter power spectrum at z∼50, observing the formation of the cosmic web and the first luminous sources, and mapping the reionization of the intergalactic medium. The epoch of reionization is of particular interest, because large HII regions will seed substantial fluctuations
in the 21 cm background. We also discuss the experimental challenges involved in detecting this signal, with an emphasis on the Galactic and extragalactic foregrounds. These increase rapidly toward low frequencies and are especially severe for the highest redshift applications. Assuming that these difficulties can be overcome, the redshifted 21 cm line will offer unique insight into the high-redshift Universe, complementing other probes but providing the only direct, three-dimensional view of structure formation from z∼200 to 6.