Editing Reionization (section)

===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.