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Gunn-Peterson effect and the Lyman-alpha forest

  1. Nov 19, 2003 #1


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    After the recombination epoch neutral hydrogen was formed in the universe. Since neutral hydrogen absorbs radiation in the Lyman-alpha transition, a dark epoch begun, which was finished with the reionization of hydrogen due to radiation sources.

    The electromagnetic spectrum from quasars, which are at enough spatio-temporal distance (z > 6 ?) should show specific Lyman-alpha absobtion lines (inside the Ly-a forest) due to neutral hydrogen. This is called the Gunn-Peterson effect.


    1. is my understanding correct?
    2. is the Gunn Peterson effect only a part of the the Ly-a forest, what is the differencie or how can this effect be registered…?

  2. jcsd
  3. Nov 19, 2003 #2


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  4. Aug 14, 2008 #3

    In principle, neutral hydrogen pockets left over towards the end of reionization
    could absorb all the hydrogen spectrum wave length.

    In the discovery of the Gunn-Peterson effect, the SDSS group observed absorption
    both in Ly-alpha and Ly-beta.
  5. Aug 24, 2008 #4

    Your understanding is correct, except regarding the Gunn-Peterson effect (otherwise known as the Gunn-Peterson trough). The Ly-a forest is so-called because the spectrum of the quasar looks like a forest, a bunch of absorption lines caused by the quasar light being absorbed by intervening clouds of neutral hydrogen at different distances (and therefore redshifts) from us.

    If the quasar is far enough away, the light we’re seeing was emitted from the quasar before the universe was reioinized. This means that the “forest” of absorption lines becomes so dense, it looks like a trough (this is the Gunn-Peterson trough). That is, no light gets through to us for a certain span of redshifted Ly-a wavelengths, and that span defines the “edges” of the trough. The edge of the trough towards the red side of the spectrum is the Ly-a wavelength with a redshift corresponding to the redshift (distance) of the quasar. The blue side of the trough corresponds to the redshift or distance or time when the universe became reionized.
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