In astronomical spectroscopy, the Lyman-alpha forest is a series of absorption lines in the spectra of distant galaxies and quasars arising from the Lyman-alpha electron transition of the neutral hydrogen atom. As the light encounters a neutral hydrogen atom, it is absorbed at a fixed frequency in the rest frame of the neutral hydrogen. The expansion of the Universe then stretches or redshifts the light, moving the absorption feature to a lower frequency. Multiple sets of absorption lines are formed when the light from the quasar or galaxy encounters multiple neutral hydrogen atoms at different redshifts. Seen from a distance, the multiple absorption lines resemble a 'forest'.