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What is the Lyman-alpha forest?

When the spectrum of a QSO contains a very large number of absorption lines, the majority of those absorption lines are usually caused by Lyman-alpha clouds. These sub-galaxy-sized clumps of gas populate the distant universe at different redshifts. Each cloud produces a single absorption line caused by hydrogen atoms called the Lyman-alpha line (hence the name “Lyman-alpha cloud”). If there are enough Lyman-alpha clouds between us and the QSO, a large swath of the QSO spectrum can literally look “chopped up” by all the Lyman-alpha lines produced by these clouds, appearing at all different redshifts. The effect is that of a forest of trees sprouting up and down in the spectrum—hence the name “Lyman-alpha forest.”


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