The Lyman-Alpha Forest is a series of Absorption Lines in Electromagnetic Radiation from distant galaxies and Quasars consisting of Lyman Alpha, the first Spectral Line of the Lyman Series (121.6 nm), Redshifted to various degrees. As light passes through intervening Intergalactic Clouds (i.e., in the Intergalactic Medium) neutral Hydrogen (such as in an HI Region), radiation is absorbed, and by recognizing and measuring the shifted Lyman-alpha lines, characteristics of the intervening clouds can be determined including location and size. This is useful for studying the Large Scale Structure of the universe. The Baryon Oscillation Spectroscopic Survey uses this technique to find evidence of Baryon Acoustic Oscillations.
Large Scale Structure (LSS)