Reionization

In the fields of Big Bang theory and cosmology, reionization is the process that caused electrically neutral atoms in the primordial universe to reionize after the lapse of the "dark ages".
Detecting and studying the reionization process is challenging but multiple avenues have been pursued.
This reionization was driven by the formation of the first stars and galaxies.

Concept

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 reionization. The reionization was driven by energetic photons emitted by the first stars and galaxies.
In the timeline of the universe, neutral hydrogen gas was originally formed when primordial hydrogen nuclei 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 electrons and protons to form neutral hydrogen was higher than the ionization rate. At around 379,000 years after the Big Bang, this recombination left most normal matter in the form of neutral hydrogen.
The universe was opaque before the recombination, due to the scattering of photons of all wavelengths off free electrons, 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 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 of the universe.
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 radiated energy, the universe reverted from being composed of neutral atoms, to once again being an ionized plasma. This occurred between 150 million and one billion years after the Big Bang 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 primordial helium also experienced a similar reionization phase change, but at a later epoch in the history of the universe.

Stages

Theoretical models give a timeline of the reionization process.
In the first stage of reionization, each new star is surrounded by neutral hydrogen. Light emitted by the star ionizes gas immediately around the star. Then light can reach further out to ionize gas. The ions can recombine, competing with the ionization process. The ionized gas will be hot and it will expand, clearing out the region around the star. The sphere of ionized gas expands until the amount of light from the star that can cause ionizations balances the recombination, a process that takes hundreds of millions of years. At some point the shell of ionization from each star in a galaxy begin to overlap and the ionization frontier pushes out into the intergalactic medium.

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 spectra of distant quasars. 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 atoms along the line of sight. For wavelengths of light at the energies of one of the Lyman transitions of hydrogen, the scattering cross-section is large, meaning that even for low levels of neutral hydrogen in the intergalactic medium, 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 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.
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 do not show the Gunn-Peterson trough, 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 = 5.82 to z = 6.28. While the quasars above z = 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 = 6. This, in turn, suggests that the universe must still have been almost entirely neutral at z > 10. On the other hand, long absorption troughs persisting down to z < 5.5 in the Lyman-alpha and Lyman-beta forests suggest that reionization potentially extends later than z = 6.

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. The overall effect is to erase anisotropies that occur on smaller scales. While anisotropies on small scales are erased, polarization anisotropies are actually introduced because of reionization. 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 > z > 11. 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 = 11 and the universe ionized by z = 7. This is in much better agreement with the quasar data.
Results in 2018 from Planck mission, yield an instantaneous reionization redshift of z = 7.68 ± 0.79.
The parameter usually quoted here is τ, the "optical depth to reionization," or alternatively, z_re, 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_re provides an estimate of the mean redshift of reionization.

Lyman alpha emission

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. Moreover, Lyman alpha photons interact strongly with neutral hydrogen in intergalactic gas through resonant scattering, wherein neutral atoms in the ground 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. 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.
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 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, but still indicate that a majority of intergalactic gas was ionized prior to z=7.
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. Moreover, specific ionized regions can be pinpointed by identifying groups of Lyman alpha emitters.