Lyman-alpha forest

Astronomical spectroscopic term

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.

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