= Solution
The <Lyman-alpha forest> is the collection of <absorption lines> blueward of a distant <quasar>'s own <Lyman-alpha line>. Foreground neutral hydrogen removes photons at rest wavelength $\lambda_\alpha\simeq1215.67$ angstroms, observed at $\lambda=(1+z)\lambda_\alpha$ because of <cosmological redshift>. Gas at many different foreground redshifts therefore produces many lines. The weak forest traces diffuse, mostly ionized <intergalactic medium>, rather than a population of fully neutral clouds; much stronger, self-shielded absorbers are a separate regime. https://ned.ipac.caltech.edu/level5/Madau6/Madau2.html[Madau's account of quasar absorption systems] distinguishes these column-density regimes.
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