Reionization
In the fields of Big Bang theory and cosmology, reionization is the process that caused electrically neutral atoms in the universe to reionize after the lapse of the "dark ages".
At around 379,000 years after the Big Bang (redshift z = 1089), this recombination left most normal matter in the form of neutral hydrogen.
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.
[3] The second phase change occurred once gas clouds started to condense in the early universe that were 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 a redshift 20 > z > 6)[3]: 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.
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.
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.
However, the distances between quasars and the telescopes which detect them are large, which means that the expansion of the universe causes light to undergo noticeable redshifting.
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.
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.
[16][17][18] 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[19] at an earlier time than the quasar spectra suggested.
While observations have come in which narrow the window during which the epoch of reionization could have taken place, it is still uncertain which objects provided the photons that reionized the IGM.
To ionize neutral hydrogen, an energy larger than 13.6 eV is required, which corresponds to photons with a wavelength of 91.2 nm or shorter.
Altogether, the critical parameter for any source considered can be summarized as its "emission rate of hydrogen-ionizing photons per unit cosmological volume.
[37][38] Compact dwarf star-forming galaxies like the GPs are considered excellent low-redshift analogs of high-redshift Lyman-alpha and LyC emitters (LAEs and LCEs, respectively).
[37][38] Subsequently, motivated, a series of surveys have been conducted using Hubble Space Telescope's Cosmic Origins Spectrograph (HST/COS) to measure the LyC directly.
[41][42][43][44][45][46] These efforts culminated in the Low-redshift Lyman Continuum Survey,[47] a large HST/COS program which nearly tripled the number of direct measurements of the LyC from dwarf galaxies.
The results from the Low-redshift Lyman Continuum Survey have provided the empirical foundation necessary to identify and understand LCEs at the Epoch of Reionization.
[48][49][50] With new observations from JWST, populations of LCEs are now being studied at cosmological redshifts greater than 6, allowing for the first time a detailed and direct assessment of the origins of cosmic Reionization.
[52] Quasars, a class of active galactic nuclei (AGN), were considered a good candidate source because they are highly efficient at converting mass to energy, and emit a great deal of light above the threshold for ionizing hydrogen.
They are more efficient and effective ionizers than Population II stars, as they emit more ionizing photons,[58] and are capable of reionizing hydrogen on their own in some reionization models with reasonable initial mass functions.
Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it.
