The Lyman-alpha forest reference article from the English Wikipedia on 24-Apr-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Lyman-alpha forest

The Lyman alpha Forest is the sum of absorption lines seen in spectra of distant galaxies / quasars, beginning from the Lyman alpha line at 121.6 nm to lower wavelength (higher photon energies).

These absorption lines result from intergalactic gas through which the quasars light has travelled. Since the absorption and emission of light follow the laws of quantum mechanics, only photons with specific energies can be absorbed. This causes each individual absorption line.

The forest is then created by the fact that that we live in an expanding universe. While the light travels, photons are streched along with space, thus increasing their wavelength and lowering their energy.

This means the intergalactic neutral hydrogen clouds at different positions between earth and the light source see the photons at different wavelength (see also redshift). Therefore each individual cloud leaves its fingerprint as an absorption line at a different position in the spectrum we see from earth.

See also: http://astron.berkeley.edu/~jcohn/lya.html

-- 12:41, 26 Mar 2004 (UTC)

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