Gunn–Peterson trough

In astronomical spectroscopy, the Gunn–Peterson trough is a feature of the spectra of bright high redshift sources, particularly quasars and gamma-ray burst afterglows, due to the presence of neutral hydrogen in the Intergalactic medium (IGM) before the epoch of reionization. The trough is characterized by suppression of electromagnetic emission from the source at wavelengths just below that of a redshifted Lyman series line. As the radiation came towards us and was progressively redshifted, photons at wavelengths higher than that line were never absorbed by the hydrogen because they did not have enough energy to cause the ground-state hydrogen to jump to the higher state corresponding to the line, whereas photons of somewhat higher energy were absorbed when they got redshifted to the wavelength of the line (for instance 91 nanometres for the Lyman alpha line). Photons of even higher energy did not get redshifted to the appropriate wavelength until after the hydrogen of the intergaalactic medium was reionized, and so were not absorbed. The trough is thus the span of wavelengths where absorption did take place. The low-wavelength end of the trough is at a redshift of about 6 from the true Lyman wavelength, since reioonization occurred around the time corresponding to that redshift, whereas the high-wavelength end of the trough is at the redshift of the source, greater than 6.

This effect was originally predicted in 1965 by James E. Gunn and Bruce Peterson, and independently by Peter Scheuer.