Editing Reionization (section)

== Detection methods ==
Looking back so far in the history of the universe presents some observational challenges. There are, however, a few observational methods for studying reionization.

===Quasars and the Gunn-Peterson trough===
One means of studying reionization uses the [[spectrum|spectra]] of distant [[quasar]]s. Quasars release an extraordinary amount of energy, being among the brightest objects in the universe. As a result, some quasars are detectable from as long ago as the epoch of reionization. Quasars also happen to have relatively uniform spectral features, regardless of their position in the sky or distance from the [[Earth]]. Thus it can be inferred that any major differences between quasar spectra will be caused by the interaction of their emission with [[atom]]s along the line of sight. For [[wavelength]]s of light at the energies of one of the [[Lyman series|Lyman transitions]] of hydrogen, the [[scattering cross-section]] is large, meaning that even for low levels of neutral hydrogen in the [[intergalactic medium]] (IGM), [[Absorption (electromagnetic radiation)|absorption]] at those wavelengths is highly likely.

For nearby objects in the universe, spectral absorption lines are very sharp, as only photons with energies just right to cause an atomic transition can cause that transition. However, the large distances between the quasars and the telescopes which detect them mean that the [[Metric expansion of space|expansion of the universe]] causes light to undergo noticeable redshifting. This means that as light from the quasar travels through the IGM and is redshifted, wavelengths which had been below the Lyman alpha wavelength are stretched, and will at some point be just equal to the wavelength needed for the Lyman Alpha transition. This means that instead of showing sharp spectral absorption lines, a quasar's light which has traveled through a large, spread out region of neutral hydrogen will show a [[Gunn-Peterson trough]].<ref>{{cite journal |author=Gunn |first1=J. E. |last2=Peterson |first2=B. A. |name-list-style=amp |date=1965 |title=On the Density of Neutral Hydrogen in Intergalactic Space |journal=The Astrophysical Journal |volume=142 |pages=1633–1641 |bibcode=1965ApJ...142.1633G |doi=10.1086/148444|doi-access=free }}</ref>

The redshifting for a particular quasar provides temporal information about reionization. Since an object's redshift corresponds to the time at which it emitted the light, it is possible to determine when reionization ended. Quasars below a certain redshift (closer in space and time) do not show the Gunn-Peterson trough (though they may show the [[Lyman-alpha forest]]), while quasars emitting light prior to reionization will feature a Gunn-Peterson trough. In 2001, four quasars were detected by the [[Sloan Digital Sky Survey]] with redshifts ranging from ''z''&nbsp;=&nbsp;5.82 to ''z''&nbsp;=&nbsp;6.28. While the quasars above ''z''&nbsp;=&nbsp;6 showed a Gunn-Peterson trough, indicating that the IGM was still at least partly neutral, the ones below did not, meaning the hydrogen was ionized. As reionization is expected to occur over relatively short timescales, the results suggest that the universe was approaching the end of reionization at ''z''&nbsp;=&nbsp;6.<ref>{{cite journal |author=Becker |first=R. H. |display-authors=etal |date=2001 |title=Evidence For Reionization at z ~ 6: Detection of a Gunn-Peterson Trough In A z=6.28 Quasar |journal=Astronomical Journal |volume=122 |issue=6 |pages=2850–2857 |arxiv=astro-ph/0108097 |bibcode=2001AJ....122.2850B |doi=10.1086/324231 |s2cid=14117521}}</ref> This, in turn, suggests that the universe must still have been almost entirely neutral at ''z''&nbsp;>&nbsp;10. On the other hand, long absorption troughs persisting down to z&nbsp;<&nbsp;5.5 in the Lyman-alpha and Lyman-beta forests suggest that reionization potentially extends later than ''z''&nbsp;=&nbsp;6.<ref>{{Cite journal |last1=Becker |first1=George D. |last2=Bolton |first2=James S. |last3=Madau |first3=Piero |last4=Pettini |first4=Max |last5=Ryan-Weber |first5=Emma V.|author5-link=Emma Ryan-Weber |last6=Venemans |first6=Bram P. |date=2015-03-11 |title=Evidence of patchy hydrogen reionization from an extreme Lyα trough below redshift six |url=http://academic.oup.com/mnras/article/447/4/3402/1748740/Evidence-of-patchy-hydrogen-reionization-from-an |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=447 |issue=4 |pages=3402–3419 |doi=10.1093/mnras/stu2646 |doi-access=free |issn=1365-2966|arxiv=1407.4850 }}</ref><ref>{{Cite journal |last1=Zhu |first1=Yongda |last2=Becker |first2=George D. |last3=Bosman |first3=Sarah E. I. |last4=Keating |first4=Laura C. |last5=D’Odorico |first5=Valentina |last6=Davies |first6=Rebecca L. |last7=Christenson |first7=Holly M. |last8=Bañados |first8=Eduardo |last9=Bian |first9=Fuyan |last10=Bischetti |first10=Manuela |last11=Chen |first11=Huanqing |last12=Davies |first12=Frederick B. |last13=Eilers |first13=Anna-Christina |last14=Fan |first14=Xiaohui |last15=Gaikwad |first15=Prakash |date=2022-06-01 |title=Long Dark Gaps in the Lyβ Forest at z < 6: Evidence of Ultra-late Reionization from XQR-30 Spectra |journal=The Astrophysical Journal |volume=932 |issue=2 |pages=76 |doi=10.3847/1538-4357/ac6e60 |doi-access=free |issn=0004-637X|arxiv=2205.04569 |bibcode=2022ApJ...932...76Z }}</ref>

===CMB anisotropy and polarization===
The anisotropy of the [[cosmic microwave background]] on different angular scales can also be used to study reionization. Photons undergo scattering when there are free electrons present, in a process known as [[Thomson scattering]]. However, as the universe expands, the density of free electrons will decrease, and scattering will occur less frequently. In the period during and after reionization, but before significant expansion had occurred to sufficiently lower the electron density, the light that composes the CMB will experience observable Thomson scattering. This scattering will leave its mark on the CMB [[anisotropy]] map, introducing secondary anisotropies (anisotropies introduced after recombination).<ref>{{cite journal | first=Manoj | last=Kaplinghat | display-authors=etal | date=2003 | title=Probing the Reionization History of the universe using the Cosmic Microwave Background Polarization | journal=The Astrophysical Journal | volume=583 | issue=1 | pages=24–32 | bibcode=2003ApJ...583...24K | doi=10.1086/344927|arxiv = astro-ph/0207591 | s2cid=11253251 }}</ref> The overall effect is to erase anisotropies that occur on smaller scales. While anisotropies on small scales are erased, [[polarization (waves)|polarization]] anisotropies are actually introduced because of reionization.<ref name="Dore_Patchy">{{cite journal |author=Dore |first=O. |display-authors=etal |year=2007 |title=Signature of patchy reionization in the polarization anisotropy of the CMB |journal=Physical Review D |volume=76 |issue=4 |pages=043002 |arxiv=astro-ph/0701784 |bibcode=2007PhRvD..76d3002D |doi=10.1103/PhysRevD.76.043002 |s2cid=119360903}}</ref> By looking at the CMB anisotropies observed, and comparing with what they would look like had reionization not taken place, the electron column density at the time of reionization can be determined. With this, the age of the universe when reionization occurred can then be calculated.

The [[Wilkinson Microwave Anisotropy Probe]] allowed that comparison to be made. The initial observations, released in 2003, suggested that reionization took place from 30 >&nbsp;''z''&nbsp;>&nbsp;11.<ref>{{cite journal |author=Kogut |first=A. |display-authors=etal |year=2003 |title=First Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Temperature-Polarization Correlation |journal=The Astrophysical Journal Supplement Series |volume=148 |issue=1 |pages=161–173 |arxiv=astro-ph/0302213 |bibcode=2003ApJS..148..161K |doi=10.1086/377219 |s2cid=15253442}}</ref> This redshift range was in clear disagreement with the results from studying quasar spectra. However, the three year WMAP data returned a different result, with reionization beginning at ''z''&nbsp;=&nbsp;11 and the universe ionized by ''z''&nbsp;=&nbsp;7.<ref>{{cite journal |author=Spergel |first=D. N. |display-authors=etal |year=2007 |title=Three-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Implications for Cosmology |journal=The Astrophysical Journal Supplement Series |volume=170 |issue=2 |pages=377–408 |arxiv=astro-ph/0603449 |bibcode=2007ApJS..170..377S |doi=10.1086/513700 |s2cid=1386346}}</ref> This is in much better agreement with the quasar data.

Results in 2018 from [[Planck (spacecraft)|Planck]] mission, yield an instantaneous reionization redshift of z = 7.68 ± 0.79.<ref name=Planck_Results_2018>{{cite journal| author=Planck Collaboration| title=Planck 2018 results. VI. Cosmological parameters| journal=Astronomy & Astrophysics| year=2020| volume=641| pages=A6| doi=10.1051/0004-6361/201833910|arxiv=1807.06209| bibcode=2020A&A...641A...6P| s2cid=119335614}}</ref>

The parameter usually quoted here is τ, the "optical depth to reionization," or alternatively, z<sub>re</sub>, the redshift of reionization, assuming it was an instantaneous event. While this is unlikely to be physical, since reionization was very likely not instantaneous, z<sub>re</sub> provides an estimate of the mean redshift of reionization.

=== Lyman alpha emission ===
[[Lyman-alpha|Lyman alpha]] light from galaxies offers a complementary tool set to study reionization. The Lyman alpha line is the n=2 to n=1 transition of neutral hydrogen and can be produced copiously by galaxies with young stars.<ref>{{Cite journal |last1=Partridge |first1=R. B. |last2=Peebles |first2=P. J. E. |date=March 1967 |title=Are Young Galaxies Visible? |url=http://adsabs.harvard.edu/doi/10.1086/149079 |journal=The Astrophysical Journal |language=en |volume=147 |pages=868 |doi=10.1086/149079 |bibcode=1967ApJ...147..868P |issn=0004-637X}}</ref> Moreover, Lyman alpha photons interact strongly with neutral hydrogen in intergalactic gas through resonant scattering, wherein neutral atoms in the ground (n=1) state absorb Lyman alpha photons and almost immediately re-emit them in a random direction.  This obscures Lyman alpha emission from galaxies that are embedded in neutral gas.<ref>{{Cite journal |last1=Miralda-Escude |first1=Jordi |last2=Rees |first2=Martin J. |date=1998-04-10 |title=Searching for the Earliest Galaxies Using the Gunn-Peterson Trough and the Lyα Emission Line |url=https://iopscience.iop.org/article/10.1086/305458 |journal=The Astrophysical Journal |language=en |volume=497 |issue=1 |pages=21–27 |doi=10.1086/305458 |arxiv=astro-ph/9707193 |bibcode=1998ApJ...497...21M |issn=0004-637X}}</ref> Thus, experiments to find galaxies by their Lyman alpha light can indicate the ionization state of the surrounding gas. An average density of galaxies with detectable Lyman alpha emission means the surrounding gas must be ionized, while an absence of detectable Lyman alpha sources may indicate neutral regions. A closely related class of experiments measures the Lyman alpha line strength in samples of galaxies identified by other methods (primarily [[Lyman-break galaxy|Lyman break galaxy]] searches).<ref>{{Cite journal |last1=Stark |first1=Daniel P. |last2=Ellis |first2=Richard S. |last3=Chiu |first3=Kuenley |last4=Ouchi |first4=Masami |last5=Bunker |first5=Andrew |date=2010-11-01 |title=Keck spectroscopy of faint 3 < z < 7 Lyman break galaxies - I. New constraints on cosmic reionization from the luminosity and redshift-dependent fraction of Lyman α emission: The Lyα emitting fraction at high redshift |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=408 |issue=3 |pages=1628–1648 |doi=10.1111/j.1365-2966.2010.17227.x|doi-access=free |arxiv=1003.5244 }}</ref><ref>{{Cite journal |last1=Pentericci |first1=L. |last2=Fontana |first2=A. |last3=Vanzella |first3=E. |last4=Castellano |first4=M. |last5=Grazian |first5=A. |last6=Dijkstra |first6=M. |last7=Boutsia |first7=K. |last8=Cristiani |first8=S. |last9=Dickinson |first9=M. |last10=Giallongo |first10=E. |last11=Giavalisco |first11=M. |last12=Maiolino |first12=R. |last13=Moorwood |first13=A. |last14=Paris |first14=D. |last15=Santini |first15=P. |date=2011-12-20 |title=Spectroscopic confirmation of z ∼ 7 Lyman break galaxies: Probing the earliest galaxies and the epoch of reionization |url=https://iopscience.iop.org/article/10.1088/0004-637X/743/2/132 |journal=The Astrophysical Journal |volume=743 |issue=2 |pages=132 |doi=10.1088/0004-637X/743/2/132 |arxiv=1107.1376 |bibcode=2011ApJ...743..132P |issn=0004-637X}}</ref><ref>{{Cite journal |last1=Tilvi |first1=V. |last2=Papovich |first2=C. |last3=Finkelstein |first3=S. L. |last4=Long |first4=J. |last5=Song |first5=M. |last6=Dickinson |first6=M. |last7=Ferguson |first7=H. C. |last8=Koekemoer |first8=A. M. |last9=Giavalisco |first9=M. |last10=Mobasher |first10=B. |date=2014-09-17 |title=Rapid decline of Lyα emission toward the reionization era |url=https://iopscience.iop.org/article/10.1088/0004-637X/794/1/5 |journal=The Astrophysical Journal |volume=794 |issue=1 |pages=5 |doi=10.1088/0004-637X/794/1/5 |arxiv=1405.4869 |bibcode=2014ApJ...794....5T |issn=1538-4357}}</ref>

The earliest application of this method was in 2004, when the tension between late neutral gas indicated by quasar spectra and early reionization suggested by CMB results was strong. The detection of Lyman alpha galaxies at redshift z=6.5 demonstrated that the intergalactic gas was already predominantly ionized<ref>{{Cite journal |last1=Malhotra |first1=Sangeeta |last2=Rhoads |first2=James E. |date=2004-12-10 |title=Luminosity Functions of Lyα Emitters at Redshifts z = 6.5 and z = 5.7: Evidence against Reionization at z ≤ 6.5 |url=https://iopscience.iop.org/article/10.1086/427182 |journal=The Astrophysical Journal |language=en |volume=617 |issue=1 |pages=L5–L8 |doi=10.1086/427182 |arxiv=astro-ph/0407408 |bibcode=2004ApJ...617L...5M |issn=0004-637X}}</ref> at an earlier time than the quasar spectra suggested. Subsequent applications of the method suggested some residual neutral gas as recently as z=6.5,<ref>{{Cite journal |last1=Hu |first1=E. M. |last2=Cowie |first2=L. L. |last3=Barger |first3=A. J. |last4=Capak |first4=P. |last5=Kakazu |first5=Y. |last6=Trouille |first6=L. |date=2010-12-10 |title=An atlas of z = 5.7 and z = 6.5 Lyα emitters |url=https://iopscience.iop.org/article/10.1088/0004-637X/725/1/394 |journal=The Astrophysical Journal |volume=725 |issue=1 |pages=394–423 |doi=10.1088/0004-637X/725/1/394 |arxiv=1009.1144 |bibcode=2010ApJ...725..394H |issn=0004-637X}}</ref><ref>{{Cite journal |last1=Kashikawa |first1=Nobunari |last2=Shimasaku |first2=Kazuhiro |last3=Matsuda |first3=Yuichi |last4=Egami |first4=Eiichi |last5=Jiang |first5=Linhua |last6=Nagao |first6=Tohru |last7=Ouchi |first7=Masami |last8=Malkan |first8=Matthew A. |last9=Hattori |first9=Takashi |last10=Ota |first10=Kazuaki |last11=Taniguchi |first11=Yoshiaki |last12=Okamura |first12=Sadanori |last13=Ly |first13=Chun |last14=Iye |first14=Masanori |last15=Furusawa |first15=Hisanori |date=2011-06-20 |title=Completing the census of Lyα emitters at the reionization epoch $^,$ |url=https://iopscience.iop.org/article/10.1088/0004-637X/734/2/119 |journal=The Astrophysical Journal |volume=734 |issue=2 |pages=119 |doi=10.1088/0004-637X/734/2/119 |arxiv=1104.2330 |bibcode=2011ApJ...734..119K |issn=0004-637X}}</ref><ref>{{Cite journal |last1=Ouchi |first1=Masami |last2=Shimasaku |first2=Kazuhiro |last3=Furusawa |first3=Hisanori |last4=Saito |first4=Tomoki |last5=Yoshida |first5=Makiko |last6=Akiyama |first6=Masayuki |last7=Ono |first7=Yoshiaki |last8=Yamada |first8=Toru |last9=Ota |first9=Kazuaki |last10=Kashikawa |first10=Nobunari |last11=Iye |first11=Masanori |last12=Kodama |first12=Tadayuki |last13=Okamura |first13=Sadanori |last14=Simpson |first14=Chris |last15=Yoshida |first15=Michitoshi |date=2010-11-01 |title=Statistics of 207 Lyα emitters at a redshift near 7: constraints on reionization and galaxy formation models |url=https://iopscience.iop.org/article/10.1088/0004-637X/723/1/869 |journal=The Astrophysical Journal |volume=723 |issue=1 |pages=869–894 |doi=10.1088/0004-637X/723/1/869 |arxiv=1007.2961 |bibcode=2010ApJ...723..869O |issn=0004-637X}}</ref> but still indicate that a majority of intergalactic gas was ionized prior to z=7.<ref>{{Cite journal |last1=Wold |first1=Isak G. B. |last2=Malhotra |first2=Sangeeta |last3=Rhoads |first3=James |last4=Wang |first4=Junxian |last5=Hu |first5=Weida |last6=Perez |first6=Lucia A. |last7=Zheng |first7=Zhen-Ya |last8=Khostovan |first8=Ali Ahmad |last9=Walker |first9=Alistair R. |last10=Barrientos |first10=L. Felipe |last11=González-López |first11=Jorge |last12=Harish |first12=Santosh |last13=Infante |first13=Leopoldo |last14=Jiang |first14=Chunyan |last15=Pharo |first15=John |date=2022-03-01 |title=LAGER Lyα Luminosity Function at z ∼ 7: Implications for Reionization |journal=The Astrophysical Journal |volume=927 |issue=1 |pages=36 |doi=10.3847/1538-4357/ac4997 |doi-access=free |arxiv=2105.12191 |bibcode=2022ApJ...927...36W |issn=0004-637X}}</ref>

Lyman alpha emission can be used in other ways to probe reionization further. Theory suggests that reionization was patchy, meaning that the clustering of Lyman alpha selected samples should be strongly enhanced during the middle phases of reionization.<ref>{{Cite journal |last1=McQuinn |first1=Matthew |last2=Hernquist |first2=Lars |last3=Zaldarriaga |first3=Matias |last4=Dutta |first4=Suvendra |date=October 2007 |title=Studying reionization with Lyα emitters |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=381 |issue=1 |pages=75–96 |doi=10.1111/j.1365-2966.2007.12085.x |doi-access=free |arxiv=0704.2239 |bibcode=2007MNRAS.381...75M |issn=0035-8711}}</ref> Moreover, specific ionized regions can be pinpointed by identifying groups of Lyman alpha emitters.<ref>{{Cite journal |last1=Tilvi |first1=V. |last2=Malhotra |first2=S. |last3=Rhoads |first3=J. E. |last4=Coughlin |first4=A. |last5=Zheng |first5=Z. |last6=Finkelstein |first6=S. L. |last7=Veilleux |first7=S. |last8=Mobasher |first8=B. |last9=Wang |first9=J. |last10=Probst |first10=R. |last11=Swaters |first11=R. |last12=Hibon |first12=P. |last13=Joshi |first13=B. |last14=Zabl |first14=J. |last15=Jiang |first15=T. |date=2020-03-01 |title=Onset of Cosmic Reionization: Evidence of an Ionized Bubble Merely 680 Myr after the Big Bang |journal=The Astrophysical Journal Letters |volume=891 |issue=1 |pages=L10 |doi=10.3847/2041-8213/ab75ec |doi-access=free |arxiv=2001.00873 |bibcode=2020ApJ...891L..10T |issn=2041-8205}}</ref><ref>{{Cite journal |last1=Hu |first1=Weida |last2=Wang |first2=Junxian |last3=Infante |first3=Leopoldo |last4=Rhoads |first4=James E. |last5=Zheng |first5=Zhen-Ya |last6=Yang |first6=Huan |last7=Malhotra |first7=Sangeeta |last8=Barrientos |first8=L. Felipe |last9=Jiang |first9=Chunyan |last10=González-López |first10=Jorge |last11=Prieto |first11=Gonzalo |last12=Perez |first12=Lucia A. |last13=Hibon |first13=Pascale |last14=Galaz |first14=Gaspar |last15=Coughlin |first15=Alicia |date=2021-01-25 |title=A Lyman-α protocluster at redshift 6.9 |url=https://www.nature.com/articles/s41550-020-01291-y |journal=Nature Astronomy |language=en |volume=5 |issue=5 |pages=485–490 |doi=10.1038/s41550-020-01291-y |arxiv=2101.10204 |bibcode=2021NatAs...5..485H |issn=2397-3366}}</ref>

===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).