Lyman-α

Spectrum of a quasar at redshift 3.20 from DESI DR2. Flux is plotted against observed wavelength from 3600 to 8000 ångström, with rest-frame wavelength on the top axis. Two shaded regions mark where the Lyman-α forest is measured: region B in purple from 920 to 1020 ångström rest frame, and region A in indigo from 1040 to 1205 ångström. The C IV and C III regions are shaded in teal and green, and a strong Lyman-α emission peak rises near 5100 ångström. — A DESI DR2 quasar at z = 3.20 (TargetID 39627696665266273). The shaded bands mark the two regions where the Lyman-α forest is measured — region A (1040–1205 Å rest frame) and region B (920–1020 Å) — with the C IV and C III regions used to check spectro-photometric calibration.
Figure 2 from DESI DR2 Results I: Baryon Acoustic Oscillations from the Lyman-α Forest, M. Abdul Karim et al. (DESI Collaboration), *Physical Review D* 112, 083514 (2025).

Absorption by neutral hydrogen along the line of sight to distant quasars traces the intergalactic medium at redshifts beyond 2, and gives DESI its longest lever arm on the expansion history — the only probe reaching deep into the matter-dominated era.

This page covers the forest program itself: how a measurement is built, and the one-dimensional power spectrum. The forest's distance and growth results are grouped with the other tracers by method.

Lyman-α BAO →

The standard ruler at z ≈ 2.3, from DR1 and DR2, with the validation and catalog papers behind each.

Lyman-α Full Shape →

The Alcock-Paczyński measurement from the broadband — a 1% constraint, twice as tight as BAO from the same data.

Pipeline references →

How a forest measurement is built: imaging, target selection, classification, visual inspection, templates and absorber catalogs. Nine papers, in order.

1D Power Spectrum

A separate observable from the three-dimensional correlation functions: the power spectrum measured along individual lines of sight. P_1D reaches much smaller scales, and constrains warm dark matter, the sum of the neutrino masses and the thermal state of the intergalactic medium.

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