The Lyman-alpha forest is an interesting feature found in the spectra of sufficiently distant objects. This series of absorption lines extends over a range of frequencies, and are a result of the Lyman-alpha electron transition of neutral hydrogen in between the source and the observer.

Lyman-alpha forest

My question is: What information can be gleaned about the universe from this spectral feature? In other words, what areas of research use the Lyman-alpha forest as a tool, instead of seeing it as noise?

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    $\begingroup$ The Lyman alpha forest encodes the neutral fraction of the density of hydrogen along the line of sight convolved by its temperature and Doppler shifted by its peculiar velocity. Hence it allows you to study all three fields, which is extremely useful, e.g. to understand the physical state of the intergalactic medium, its kinematics, it geometry. $\endgroup$
    – chris
    Commented Mar 15, 2014 at 20:29
  • $\begingroup$ @chris Great comment. I'd encourage you to make this into an answer. $\endgroup$
    – astromax
    Commented Mar 16, 2014 at 12:47

1 Answer 1


According to UCLA's web-tutorial Lyman Alpha Forest (Wright, 2004), there are many 'clouds' of gas between us and a distant quasar (as an example) that absorb

ultraviolet light at the wavelength of the Lyman alpha line of hydrogen at a wavelength of 122 nm.

However, as the clouds of gas have less of a redshift than that of the distant quasar, then their absorption lines are less redshifted than the distant object - a cartoon example is shown below (from the UCLA website):

enter image description here

Now, the significance is of the forest is that it represents clouds that are smaller than the smallest galaxies, thus

We can only see these very low mass clouds by the absorption they produce in the strongest line of the most abundant element: Lyman alpha. Thus by studying the Lyman alpha forest we can learn about the density fluctuations in the Universe on the smallest observable scales.

A similar explanation of the significance of the forest, is given by Quasars, the Lyman Alpha Forest, and the Reionization of the Universe (Mortlock et al. 2011) is

quasars like ULAS J1120+0641 are bright and at high redshift, lower redshift intervening material can absorb some of their light, leaving fingerprints on the final spectrum that we observe on Earth. Because hydrogen is the most abundant element in the universe, it should come as no surprise that it leaves the most prominent spectral signature, in the form of a forest of absorption lines.

Specifically, of major significance is that Mortlock et al. (2011) state that

The Lyman alpha forest can be used to trace this re-ionization of the universe.

  • $\begingroup$ Thanks for the response. Any specifics about how to use it as a distance measurement? $\endgroup$
    – astromax
    Commented Oct 16, 2013 at 12:44
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    $\begingroup$ @astromax: Late comment, and probably you know by now, but may be useful for future readers: Since the rest wavelength of Lyα is known (λ_r = 1216 Å), the observed wavelength λ_o tells you the redshift z of the cloud responsible for the absorption (and the quasar emitting the light); z = λ_r/λ_o – 1. This can be converted into a distance using a formula that is too complicated to write in a comment, but can be found here. $\endgroup$
    – pela
    Commented Feb 27, 2015 at 12:05
  • $\begingroup$ Usually we're not interested in individual cloud distances, though, but rather their statistical space density in some distance range. $\endgroup$
    – pela
    Commented Feb 27, 2015 at 12:06

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