Cosmology
Lyman Alpha Forest
Many absorption lines in quasar spectra — probing intergalactic gas
The Lyman alpha forest is a series of hundreds of narrow absorption lines in quasar spectra. Each line corresponds to a hydrogen Lyα absorption (1216 Å rest frame) caused by an intervening neutral hydrogen cloud. Different redshifts → different observed wavelengths → "forest" of lines blueward of the Lyα emission peak. Provides direct probe of intergalactic medium evolution from z ≈ 6 to today. Reveals: cosmic web structure, hydrogen reionization era, baryon distribution, dark matter clumping.
- SourceHydrogen Lyα at 1216 Å
- CauseIntervening neutral H clouds in IGM
- Spectral locationBlueward of Lyα emission peak in quasar
- Wavelength1216 (1+z) Å for cloud at redshift z
- ProbesIGM from z ~ 6 to today
- ResolutionHundreds of clouds along single line of sight
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Why Lyα forest matters
- IGM probe. Direct measurement of intergalactic gas.
- Reionization era. When universe became transparent.
- Cosmological constraints. Independent of CMB.
- Galaxy formation. Link gas to galaxies.
- Dark matter. Probe small-scale clustering.
- Cosmic structure. 3D mapping of gas distribution.
- Tomography. Tomographic structure of universe.
Common misconceptions
- Forest is just one line. Hundreds along single quasar.
- Lyα forest is theoretical. Standard cosmological observation.
- Forest only at high z. Relevant from z ~ 6 to today.
- Quasar spectrum is featureless. Multiple emission/absorption features.
- Forest tells us nothing new. Independent constraints on cosmology.
- Lyα is unique to forest. Many other lines too.
Frequently asked questions
What's the Lyman alpha forest?
When light from distant quasar passes through intergalactic medium, neutral hydrogen along the way absorbs Lyα photons (at 1216 Å rest). Different redshifts → different observed wavelengths. Spectrum shows forest of absorption lines blueward of the quasar's Lyα emission peak. Each line is a separate cloud at different redshift along line of sight.
How is it observed?
Optical/UV spectroscopy of high-z quasars. High-resolution spectrographs (HIRES, ESPRESSO, etc.). Hundreds of absorption lines per quasar. Different quasars provide different lines of sight — statistical analysis reveals 3D structure of IGM.
What does it tell us?
(1) Hydrogen distribution along line of sight. (2) Cosmic web structure traced by gas. (3) Reionization era — when IGM became transparent (ionization changes absorption). (4) Baryon density evolution. (5) Dark matter clumping (gas traces DM). (6) Galaxy formation feedback. (7) Cosmological parameters.
What about reionization?
Universe's first stars/quasars ionized neutral hydrogen, ending dark ages. Reionization era: ~150 Myr to ~1 Gyr after BB (z = 6-15). Lyman alpha forest sensitive to this — fewer absorption lines at higher z indicates more ionized gas. Helps date reionization.
How is it useful for cosmology?
Power spectrum of Lyα forest fluctuations. Sensitive to small-scale matter density. Probes lower mass scales than CMB (which traces only large-scale fluctuations). Constrains: dark matter properties (warm vs cold), neutrino masses, inflation. Combined with CMB and BAO: complete picture of cosmology.
Are there higher-z analogs?
Yes. Lyman beta, gamma, ... — even higher transitions. Some studies use HeII Lyα — different physics. Combined: multi-wavelength tomography of IGM. Future: high-z LSST observations, other surveys may reveal more.
How is forest used in modern surveys?
BOSS and eBOSS quasar samples have ~10⁵ quasars with measured Lyα forests. DESI extending to ~10⁶. Analysis: power spectrum, cross-correlations with galaxies/CMB. Major source of cosmological constraints. Particle physics constraints: warm dark matter, neutrinos.