Gunn-Peterson trough
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In Astronomical spectroscopy, a feature of the spectra of quasars with redshift greater than 6 that is explained as due to absorption of quasar emission due to neutral hydrogen (hydrogen atoms). The effect exists because intergalactic hydrogen should absorb photons with energies redshifted into the Lyman series at different points along the line of sight.
Nearby quasars and galaxies show no such absorption indicating that the material between the galaxies at recent times is ionized. Quasars with redshift greater than 6 show a depression between the hydrogen lines of the quasar to a region with about a redshift of 6. This suggests that the material in the very early universe was not completely ionized.
Following the results of WMAP I (2003), the time of reionization as seen by Gunn-Peterson appeared to conflict with the estimates of the electron column density as measured by WMAP. However following the release of WMAP III data (2006), the two methods appeared to give a result that is somewhat less conflicting.
This effect was proposed by Gunn and Peterson in 1965 and was first observed in quasars discovered by the Sloan Digital Sky Survey in 2001.
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