Editing Reionization (section)

== Concept ==
[[Image:reion diagram.jpg|thumb|upright=1.4|Schematic timeline of the universe, depicting reionization's place in cosmic history.]]

Reionization refers to a change in the [[intergalactic medium]] from neutral hydrogen to ions. The neutral hydrogen had been ions at an earlier stage in the history of the universe, thus the conversion back into ions is termed a ''re''ionization. The reionization was driven by energetic photons emitted by the first stars and galaxies.<ref name="FanReview2006">{{Cite journal |last=Fan |first=Xiaohui |last2=Carilli |first2=C.L. |last3=Keating |first3=B. |date=2006-09-01 |title=Observational Constraints on Cosmic Reionization |url=https://www.annualreviews.org/doi/10.1146/annurev.astro.44.051905.092514 |journal=Annual Review of Astronomy and Astrophysics |language=en |volume=44 |issue=1 |pages=415–462 |doi=10.1146/annurev.astro.44.051905.092514 |issn=0066-4146|arxiv=astro-ph/0602375 }}</ref> 

In the timeline of the universe, neutral hydrogen gas was originally formed when  primordial hydrogen nuclei (protons) combined with electrons. Light with sufficient energy will ionize neutral hydrogen gas. At early times, light was so dense and energetic that hydrogen atoms would be immediately re-ionized. As the universe expanded and cooled, the rate of recombination of [[electron]]s and [[proton]]s to form neutral hydrogen was higher than the [[ionize|ionization]] rate.  At around 379,000 years after the Big Bang ([[redshift]] ''z''&nbsp;=&nbsp;1089), this [[recombination (cosmology)|recombination]] left most normal matter in the form of neutral hydrogen.<ref name=tong>{{cite web |author=David Tong |website=Lectures on Cosmology |title="Lecture 2: The Hot Universe" |publisher=University of Cambridge |url=http://www.damtp.cam.ac.uk/user/tong/cosmo.html}}</ref> 

The universe was opaque before the recombination, due to the [[Thomson scattering|scatter]]ing of photons of all wavelengths off free electrons (and free protons, to a significantly lesser extent), but it became increasingly transparent as more electrons and protons combined to form neutral hydrogen atoms. While the electrons of neutral hydrogen can absorb photons of some wavelengths by rising to an [[Lyman series|excited state]], a universe full of neutral hydrogen will be relatively opaque only at those few wavelengths. The remaining light could travel freely and become the [[cosmic microwave background radiation]]. The only other light at this point would be provided by those excited hydrogen atoms, marking the beginning of an era called the [[ Dark Ages (cosmology) | Dark Ages]] of the universe.<ref name=Wise-2019>{{Cite journal |last=Wise |first=John H. |date=2019 |title=Cosmic reionisation |url=https://www.tandfonline.com/doi/full/10.1080/00107514.2019.1631548 |journal=Contemporary Physics |volume=60 |issue=2 |pages=145–163 |doi=10.1080/00107514.2019.1631548 |arxiv=1907.06653 |issn=0010-7514}}</ref> 

The second phase change occurred once objects started to form in the early [[universe]] emitting radiation energetic enough to re-ionize neutral hydrogen. As these objects formed and [[Electromagnetic radiation|radiated]] energy, the universe reverted from being composed of neutral atoms, to once again being an ionized [[plasma (physics)|plasma]]. This occurred between 150 million and one billion years after the Big Bang (at a redshift 20&nbsp;>&nbsp;''z''&nbsp;>&nbsp;6)<ref name=Wise-2019/>{{rp|150}} At that time, however, matter had been diffused by the expansion of the universe, and the scattering interactions of photons and electrons were much less frequent than before electron-proton recombination. Thus, the universe was full of low density ionized hydrogen and remained transparent, as is the case today.

It is believed that the [[Big Bang nucleosynthesis|primordial helium]] also experienced a similar reionization phase change, but at a later epoch in the history of the universe.<ref>{{Cite journal |last1=Furlanetto |first1=Steven R. |last2=Oh |first2=S. Peng |date=July 2008 |title=The History and Morphology of Helium Reionization |url=https://iopscience.iop.org/article/10.1086/588546 |journal=The Astrophysical Journal |language=en |volume=681 |issue=1 |pages=1–17 |doi=10.1086/588546 |arxiv=0711.1542 |bibcode=2008ApJ...681....1F |issn=0004-637X}}</ref>