Quasars and The Gunn-Peterson Trough
One means of studying reionization uses the spectra of distant quasars. Quasars release an extraordinary amount of energy, meaning they are among the brightest objects in the universe. Some quasars are even detectable as far back as the epoch of reionization. Quasars also happen to have relatively uniform spectral features, regardless of position in sky or distance from Earth. Thus it can be inferred that any major differences between quasar spectra will be caused by interaction with atoms along the line of sight. For wavelengths of light at the energies of one of the Lyman transitions in hydrogen, the scattering cross-section is large, meaning that even for low levels of neutral hydrogen in the intergalactic medium (IGM), 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 sufficient to cause an atomic transition can cause the transition. 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. This means that as light from the quasar travels through the IGM and is redshifted, wavelengths which had been above the Lyman Alpha limit 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 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 that occurs allows for temporal information about reionization to be learned. Since an object's redshift corresponds to the time at which it emitted the light we see, it is possible to determine when reionization ended. Quasars below a certain redshift will 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 = 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. 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, indicates that the universe must still have been almost entirely neutral at z > 10.
Read more about this topic: Reionization, Detection Methods