In astronomical spectroscopy, the Gunn–Peterson trough is a feature of the spectra of quasars due to the presence of neutral hydrogen in the Intergalactic medium (IGM). The trough is characterized by suppression of electromagnetic emission from the quasar at wavelengths less than that of the Lyman-alpha line at the redshift of the emitted light. This effect was originally predicted in 1965 by James E. Gunn and Bruce Peterson.