Editing Reionization (section)

===21-cm line===
Even with the quasar data roughly in agreement with the CMB anisotropy data, there are still a number of questions, especially concerning the energy sources of reionization and the effects on, and role of, [[structure formation]] during reionization. The [[hydrogen line|21-cm line]] in hydrogen is potentially a means of studying this period, as well as the "dark ages" that preceded reionization. The 21-cm line occurs in neutral hydrogen, due to differences in energy between the spin triplet and spin singlet states of the electron and proton. This transition is [[forbidden line|forbidden]], meaning it occurs extremely rarely. The transition is also highly [[temperature]] dependent, meaning that as objects form in the "dark ages" and emit Lyman-alpha [[photon]]s that are absorbed and re-emitted by surrounding neutral hydrogen, it will produce a 21-cm line signal in that hydrogen through [[Wouthuysen-Field coupling]].<ref>{{cite journal |author=Barkana |first1=Rennan |last2=Loeb |first2=Abraham |name-list-style=amp |year=2005 |title=Detecting the Earliest Galaxies through Two New Sources of 21 Centimeter Fluctuations |journal=The Astrophysical Journal |volume=626 |issue=1 |pages=1–11 |arxiv=astro-ph/0410129 |bibcode=2005ApJ...626....1B |doi=10.1086/429954 |s2cid=7343629}}</ref><ref name="Alvarez_enhanced">{{cite journal |author=Alvarez |first1=M. A. |last2=Pen |first2=Ue-Li |last3=Chang |first3=Tzu-Ching |date=2010 |title=Enhanced Detectability of Pre-reionization 21 cm Structure |journal=The Astrophysical Journal Letters |volume=723 |issue=1 |pages=L17–L21 |arxiv=1007.0001 |bibcode=2010ApJ...723L..17A |doi=10.1088/2041-8205/723/1/L17 |s2cid=118436837}}</ref> By studying 21-cm line emission, it will be possible to learn more about the early structures that formed. Observations from the [[Experiment to Detect the Global Epoch of Reionization Signature]] (EDGES) points to a signal from this era, although follow-up observations will be needed to confirm it.<ref>{{cite news|url=https://www.nature.com/articles/d41586-018-02616-8|title=Astronomers detect light from the Universe's first stars|date= 28 February 2018|access-date=1 March 2018}}</ref> Several other projects hope to make headway in this area in the near future, such as the [[Precision Array for Probing the Epoch of Reionization]] (PAPER), [[Low-Frequency Array (LOFAR)|Low Frequency Array]] (LOFAR), [[Murchison Widefield Array]] (MWA), [[Giant Metrewave Radio Telescope]] (GMRT), Mapper of the IGM Spin Temperature (MIST), the [[Dark Ages Radio Explorer]] (DARE) mission, and the [[Large Aperture Experiment to Detect the Dark Ages|Large-Aperture Experiment to Detect the Dark Ages]] (LEDA).