Editing Hydrogen line (section)

==Uses==
===In radio astronomy===
The 21&nbsp;cm spectral line appears within the [[radio spectrum]] (in the [[L band|L&nbsp;band]] of the [[Ultra high frequency|UHF band]] of the [[microwave window]]). Electromagnetic energy in this range can easily pass through the Earth's atmosphere and be observed from the Earth with little interference.<ref>{{cite book
 | title=The American Practical Navigator: An Epitome of Navigation. 2002 Bicentennial Edition
 | first=Nathaniel | last=Bowditch | author-link=Nathaniel Bowditch
 | publisher=National Imagery and Mapping Agency
 | chapter=10. Radio Waves | page=158 | year=2002
 | chapter-url=https://thenauticalalmanac.com/Bowditch-%20American%20Practical%20Navigator/Chapt-10%20RADIO%20WAVES.pdf
 | access-date=2023-04-28
 | quote="Skywaves are not used in the UHF band because the ionosphere is not sufficiently dense to reflect the waves, which pass through it into space. ... Reception of UHF signals is virtually free from fading and interference from atmospheric noise."
}}</ref> The hydrogen line can readily penetrate clouds of interstellar [[cosmic dust]] that are [[opacity (optics)|opaque]] to [[visible light]].<ref>{{cite book
 | title=The Fullness of Space
 | first=Gareth | last=Wynn-Williams | year=1992
 | page=36 | isbn=9780521426381
 | publisher=Cambridge University Press
 | url=https://books.google.com/books?id=wjxrloC2gyMC&pg=PA36
}}</ref> Assuming that the hydrogen atoms are uniformly distributed throughout the galaxy, each line of sight through the galaxy will reveal a hydrogen line. The only difference between each of these lines is the Doppler shift that each of these lines has. Hence, by assuming [[circular motion]], one can calculate the relative speed of each arm of our galaxy. The [[rotation curve]] of our galaxy has been calculated using the {{Val|21|u=cm}} hydrogen line. It is then possible to use the plot of the rotation curve and the velocity to determine the distance to a certain point within the galaxy. However, a limitation of this method is that departures from circular motion are observed at various scales.<ref>{{cite journal
 | title=The Large-Scale Distribution of Hydrogen in the Galaxy
 | last=Kerr | first=Frank J. | author-link=Frank John Kerr
 | journal=Annual Review of Astronomy and Astrophysics
 | volume=7 | page=39 | year=1969
 | doi=10.1146/annurev.aa.07.090169.000351
 | bibcode=1969ARA&A...7...39K }}</ref>

Hydrogen line observations have been used indirectly to calculate the mass of galaxies,<ref>{{cite journal
 | title=Integral Properties of Spiral and Irregular Galaxies
 | last=Roberts | first=Morton S.
 | journal=Astronomical Journal
 | volume=74 | pages=859–876 | date=September 1969
 | doi=10.1086/110874 | bibcode=1969AJ.....74..859R | doi-access=free }}</ref> to put limits on any changes over time of the [[fine-structure constant]],<ref>{{cite journal
 | title=New limits on the possible variation of physical constants.
 | last1=Drinkwater | first1=M. J. | last2=Webb | first2=J. K–
 | last3=Barrow | first3=J. D. | last4=Flambaum | first4=V. V.
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=295 | pages=457–462
 | date=April 1998 | issue=2 | doi=10.1046/j.1365-8711.1998.2952457.x 
 | doi-access=free | arxiv=astro-ph/9711290 | bibcode=1998MNRAS.295..457D
| s2cid=5938714 }}</ref> and to study the dynamics of individual galaxies. The [[magnetic field]] strength of [[interstellar space]] can be measured by observing the [[Zeeman effect]] on the 21-cm line; a task that was first accomplished by [[Gerrit L. Verschuur|G. L. Verschuur]] in 1968.<ref>{{cite journal
 | title=Positive Determination of an Interstellar Magnetic Field by Measurement of the Zeeman Splitting of the 21-cm Hydrogen Line
 | last=Verschuur | first=G. L. | date=September 1968
 | journal=Physical Review Letters
 | volume=21 | issue=11 | pages=775–778
 | doi=10.1103/PhysRevLett.21.775 | bibcode=1968PhRvL..21..775V
}}</ref> In theory, it may be possible to search for [[antihydrogen]] atoms by measuring the [[Polarization (physics)|polarization]] of the 21-cm line in an external magnetic field.<ref>{{cite journal
 | title=The 21 cm absorption line profile as a tool for the search for antimatter in the universe
 | last1=Solovyev | first1=Dmitry | last2=Labzowsky | first2=Leonti
 | journal=Progress of Theoretical and Experimental Physics
 | volume=2014 | issue=11 | id=111E016 | date=November 2014
 | pages=111E01 | doi=10.1093/ptep/ptu142 | bibcode=2014PTEP.2014k1E01S 
| doi-access=free }}</ref>

Deuterium has a similar hyperfine spectral line at 91.6 cm (327 MHz), and the relative strength of the 21 cm line to the 91.6 cm line can be used to measure the deuterium-to-hydrogen (D/H) ratio. One group in 2007 reported D/H ratio in the [[galactic anticenter]] to be 21 ± 7 parts per million.<ref>{{Cite journal |last1=Rogers |first1=A. E. E. |last2=Dudevoir |first2=K. A. |last3=Bania |first3=T. M. |date=2007-03-09 |title=Observations of the 327 MHz Deuterium Hyperfine Transition |url=https://iopscience.iop.org/article/10.1086/511978/meta |journal=The Astronomical Journal |language=en |volume=133 |issue=4 |pages=1625–1632 |doi=10.1086/511978 |bibcode=2007AJ....133.1625R |s2cid=15541399 |issn=1538-3881|url-access=subscription }}</ref>

===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&nbsp;MHz to about 15&nbsp;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&nbsp;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&nbsp;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&nbsp;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&nbsp;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>

===Relevance to the search for non-human intelligent life===
[[Image:Pioneer plaque hydrogen.svg|thumb|The hyperfine transition of hydrogen, as depicted on the Pioneer and Voyager spacecraft.]]

The [[Pioneer plaque]], attached to the [[Pioneer 10]] and [[Pioneer 11]] spacecraft, portrays the hyperfine transition of neutral hydrogen and used the wavelength as a standard scale of measurement. For example, the height of the woman in the image is displayed as eight times 21&nbsp;cm, or 168&nbsp;cm. Similarly the frequency of the hydrogen spin-flip transition was used for a unit of time in a map to Earth included on the Pioneer plaques and also the [[Voyager 1]] and [[Voyager 2]] probes. On this map, the position of the Sun is portrayed relative to 14&nbsp;[[pulsar]]s whose rotation period circa 1977 is given as a multiple of the frequency of the hydrogen spin-flip transition. It is theorized by the plaque's creators that an advanced civilization would then be able to use the locations of these pulsars to locate the [[Solar System]] at the time the spacecraft were launched.<ref>{{cite web
 | title=The Pioneer Plaque: Science as a Universal Language
 | first=Jake | last=Rosenthal | date=January 20, 2016
 | publisher=The Planetary Society
 | url=https://www.planetary.org/articles/0120-the-pioneer-plaque-science-as-a-universal-language
 | access-date=2023-04-26
}}</ref><ref>{{cite journal
 | title=Introducing Humans to the Extraterrestrials: the Pioneering Missions of the Pioneer and Voyager Probes
 | first=Klara Anna | last=Capova | date=October 18, 2021
 | journal=Frontiers in Human Dynamics
 | volume=3 | publisher=Frontiers Media S.A.
 | doi=10.3389/fhumd.2021.714616 | doi-access=free }}</ref>

The 21&nbsp;cm hydrogen line is considered a favorable frequency by the [[SETI]] program in their search for signals from potential extraterrestrial civilizations. In 1959, Italian physicist [[Giuseppe Cocconi]] and American physicist [[Philip Morrison]] published "Searching for interstellar communications", a paper proposing the 21&nbsp;cm hydrogen line and the potential of microwaves in the search for interstellar communications. According to George Basalla, the paper by Cocconi and Morrison "provided a reasonable theoretical basis" for the then-nascent SETI program.<ref>{{cite book |last=Basalla |first=George |date=2006 |title=Civilized Life in the Universe |publisher=[[Oxford University Press]] |isbn=978-0-19-517181-5 |pages=[https://archive.org/details/civilizedlifeinu0000basa/page/133 133–135] |url=https://archive.org/details/civilizedlifeinu0000basa/page/133 }}</ref> Similarly, [[:ru:Маковецкий, Пётр Васильевич|Pyotr Makovetsky]] proposed SETI use a frequency which is equal to either
:{{0}}{{pi}} × {{val|1420.40575177|u=MHz}} ≈ {{val|4.46233627|u=GHz}}

or
:[[Turn (geometry)|2{{pi}}]] × {{val|1420.40575177|u=MHz}} ≈ {{val|8.92467255|u=GHz}}

Since [[pi|{{pi}}]] is an [[irrational number]], such a frequency could not possibly be produced in a natural way as a [[harmonic]], and would clearly signify its artificial origin. Such a signal would not be overwhelmed by the H&nbsp;I line itself, or by any of its harmonics.<ref>{{cite web |last=Makovetsky |first=P. |title=Смотри в корень |trans-title=Look at the root |language=ru |url=http://n-t.ru/ri/mk/sk109-4.htm}}</ref>