Lyman-alpha forest

The Lyman-alpha forest is a dense pattern of absorption lines in distant quasars’ spectra, produced by intervening clouds of neutral hydrogen at different cosmic distances.

Lyman-alpha transition: The hydrogen transition between its ground state and first excited state, with a wavelength of about 121.6 nanometers. This atomic transition creates the absorption features that make up the forest.
Lyman-alpha forest tomography: A method that reconstructs three-dimensional intergalactic structure from absorption measured along multiple nearby quasar sightlines. It extends one-dimensional forest spectra into maps of large-scale matter.
Hydrogen spectral series: Groups of hydrogen spectral lines produced by transitions between an electron’s bound energy levels. The Lyman series supplies the ultraviolet transitions behind the forest.
Warm dark matter: A dark matter model whose particles’ thermal motion suppresses the formation of small-scale cosmic structure. Its predicted suppression can be tested against small-scale structure inferred from forest absorption.
Cosmological redshift: The increase in a photon’s wavelength as the universe expands during its journey to an observer. Different redshifts place intervening clouds’ Lyman-alpha absorption at different observed wavelengths.
Intergalactic medium: The diffuse gas occupying the space between galaxies in the universe. The forest is a principal observational probe of this widespread gas.
Lyman series: The hydrogen spectral series formed by transitions that end in the atom’s ground state. Its Lyman-alpha line is the forest’s defining absorption feature.
Ultraviolet background: The diffuse ultraviolet radiation field that ionizes and heats intergalactic gas. Uncertainty in its intensity changes the neutral hydrogen inferred from the forest.
Quasar: An extremely luminous active galactic nucleus powered by matter accreting onto a supermassive black hole. Its bright, distant continuum provides the background light absorbed by foreground hydrogen.
Cosmic web: The large-scale network of interconnected filaments, sheets, and voids formed by matter in the universe. Forest absorption traces the hydrogen distribution within the web’s diffuse filaments.

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