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Hydrogen line
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===In cosmology=== The line is of great interest in [[Big Bang]] cosmology because it is the only known way to probe the cosmological "[[Dark Ages (cosmology)|dark ages]]" from [[Recombination (cosmology)|recombination]] (when stable hydrogen atoms first formed) to the [[reionization]] epoch. After including the [[redshift]] range for this period, this line will be observed at frequencies from 200 MHz to about 15 MHz on Earth.<ref>{{cite journal | title=Radio Recombination Lines at Decameter Wavelengths: Prospects for the Future | last1=Peters | first1=Wendy M. | last2=Clarke | first2=T. | last3=Lazio | first3=J. | last4=Kassim | first4=N. | display-authors=1 | journal=Astronomy & Astrophysics | volume=525 | id=A128 | date=January 2011 | doi=10.1051/0004-6361/201014707 | arxiv=1010.0292 | bibcode=2011A&A...525A.128P | s2cid=53582482 }}</ref> It potentially has two applications. First, by [[Intensity mapping|mapping the intensity]] of redshifted 21 centimeter radiation it can, in principle, provide a very precise picture of the [[matter power spectrum]] in the period after recombination.<ref name=Fialkov_Loeb_2013>{{cite journal | title=The 21-cm Signal from the cosmological epoch of recombination | last1=Fialkov | first1=A. | last2=Loeb | first2=A. | journal=Journal of Cosmology and Astroparticle Physics | issue=11 | id=066 | date=November 2013 | volume=2013 | page=066 | doi=10.1088/1475-7516/2013/11/066 | arxiv=1311.4574 | bibcode=2013JCAP...11..066F | s2cid=250754168 }}</ref> Second, it can provide a picture of how the universe was re‑ionized,<ref name=Mellema_et_al_2006>{{cite journal | title=Simulating cosmic reionization at large scales - II. The 21-cm emission features and statistical signals | last1=Mellema | first1=Garrelt | last2=Iliev | first2=Ilian T. | last3=Pen | first3=Ue-Li | last4=Shapiro | first4=Paul R. | journal=Monthly Notices of the Royal Astronomical Society | volume=372 | issue=2 | pages=679–692 | display-authors=1 | date=October 2006 | doi=10.1111/j.1365-2966.2006.10919.x | doi-access=free | arxiv=astro-ph/0603518 | bibcode=2006MNRAS.372..679M | s2cid=16389221 }}</ref> as neutral hydrogen which has been ionized by radiation from stars or quasars will appear as holes in the 21 cm background.<ref>{{cite journal | title=Redshifted 21 cm Emission from the Pre-Reionization Era. II. H II Regions around Individual Quasars | last1=Kohler | first1=Katharina | last2=Gnedin | first2=Nickolay Y. | last3=Miralda-Escudé | first3=Jordi | last4=Shaver | first4=Peter A. | display-authors=1 | journal=The Astrophysical Journal | volume=633 | issue=2 | pages=552–559 | date=November 2005 | doi=10.1086/444370 | arxiv=astro-ph/0501086 | bibcode=2005ApJ...633..552K | s2cid=15210736 }}</ref><ref name=Pritchard_Loeb_2012/> However, 21 cm observations are very difficult to make. Ground-based experiments to observe the faint signal are plagued by interference from television transmitters and the [[ionosphere]],<ref name=Mellema_et_al_2006/> so they must be made from very secluded sites with care taken to eliminate interference. Space based experiments, including on the far side of the Moon (where they would be sheltered from interference from terrestrial radio signals), have been proposed to compensate for this.<ref>{{cite journal | title=Transformative science from the lunar farside: observations of the dark ages and exoplanetary systems at low radio frequencies | last=Burns | first=Jack O. | journal=Philosophical Transactions of the Royal Society A | volume=379 | issue=2188 | id=20190564 | date=January 2021 | doi=10.1098/rsta.2019.0564 | pmid=33222645 | pmc=7739898 | arxiv=2003.06881 | bibcode=2021RSPTA.37990564B }}</ref> Little is known about other foreground effects, such as [[synchrotron radiation|synchrotron emission]] and [[bremsstrahlung|free–free emission]] on the galaxy.<ref>{{cite journal | title=21 cm Tomography with Foregrounds | last1=Wang | first1=Xiaomin | last2=Tegmark | first2=Max | last3=Santos | first3=Mário G. | last4=Knox | first4=Lloyd | display-authors=1 | journal=The Astrophysical Journal | volume=650 | issue=2 | pages=529–537 | date=October 2006 | doi=10.1086/506597 | arxiv=astro-ph/0501081 | bibcode=2006ApJ...650..529W | s2cid=119595472 }}</ref> Despite these problems, 21 cm observations, along with space-based gravitational wave observations, are generally viewed as the next great frontier in observational cosmology, after the [[Cosmic microwave background radiation#Polarization|cosmic microwave background polarization]].<ref>{{cite journal | title=Peering into the dark (ages) with low-frequency space interferometers | last1=Koopmans | first1=Léon V. E. | last2=Barkana | first2=Rennan | last3=Bentum | first3=Mark | last4=Bernardi | first4=Gianni | last5=Boonstra | first5=Albert-Jan | last6=Bowman | first6=Judd | last7=Burns | first7=Jack | last8=Chen | first8=Xuelei | last9=Datta | first9=Abhirup | last10=Falcke | first10=Heino | last11=Fialkov | first11=Anastasia | last12=Gehlot | first12=Bharat | last13=Gurvits | first13=Leonid | last14=Jelić | first14=Vibor | last15=Klein-Wolt | first15=Marc | last16=Lazio | first16=Joseph | last17=Meerburg | first17=Daan | last18=Mellema | first18=Garrelt | last19=Mertens | first19=Florent | last20=Mesinger | first20=Andrei | last21=Offringa | first21=André | last22=Pritchard | first22=Jonathan | last23=Semelin | first23=Benoit | last24=Subrahmanyan | first24=Ravi | last25=Silk | first25=Joseph | last26=Trott | first26=Cathryn | last27=Vedantham | first27=Harish | last28=Verde | first28=Licia | last29=Zaroubi | first29=Saleem | last30=Zarka | first30=Philippe | display-authors=1 | journal=Experimental Astronomy | volume=51 | issue=3 | pages=1641–1676 | date=June 2021 | doi=10.1007/s10686-021-09743-7 | pmid=34511720 | pmc=8416573 | arxiv=1908.04296 | bibcode=2021ExA....51.1641K }}</ref>
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