Editing Lyman-alpha forest (section)

==Physical background==
[[File:Lyman-alpha forest.gif|thumb|upright=2|A quasar spectrum with Lyman absorbers being continuously redshifted due to cosmic expansion forming a "forest" of lines.]]

For a neutral [[hydrogen]] atom, spectral lines are formed when an electron transitions between energy levels. The Lyman series of spectral lines are produced by electrons transitioning between the ground state and higher energy levels (excited states). The Lyman-alpha transition corresponds to an electron transitioning between the ground state (''n''&nbsp;=&nbsp;1) and the first excited state (''n''&nbsp;=&nbsp;2). The Lyman-alpha spectral line has a laboratory wavelength (or rest wavelength) of 1216&nbsp;[[Angstrom|Å]], which is in the [[ultraviolet]] portion of the [[electromagnetic spectrum]].<ref name='modern_astrophysics'>{{cite book | last1 = Carroll | first1 = Bradley W. | last2 = Ostlie | first2 = Dale A. | title = An Introduction to Modern Astrophysics | chapter = The Interaction of Light and Matter | publisher = Addison-Wesley Publishing Company, Inc. | date = 1996 | location = New York, New York | pages = 134–142 | isbn = 978-0-201-54730-6}}</ref>

The Lyman-alpha absorption lines in the quasar spectra result from intergalactic gas through which the [[galaxy]] or quasar's light has traveled. Since [[H I region|neutral hydrogen clouds]] in the intergalactic medium are at different degrees of [[redshift]] (due to their varying distance from Earth), their absorption lines are observed at a range of [[wavelength]]s. Each individual cloud leaves its [[fingerprint]] as an [[absorption line]] at a different position in the observed [[absorption spectrum|spectrum]].