Editing Reionization (section)

===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>