I was reading this (perhaps offbeat) "anti-big-bang" article,


and the author comments "a recent catalog of objects having very large redshifts shows that among 109 quasi-stellar objects for which both absorption and emission lines could be measured, the value of the absorption redshift of a given object is always different from the one measured in emission for the same object."


  1. I had trouble Googling this. What is a particularly good "example object" of this phenomenon (if any), perhaps where I can see the issues arising discussed?

  2. Is there a prosaic or generally accepted "conventional" explanation?

BTW I've just realised the article is perhaps from 1988 (it's somewhat confusingly also dated 2012) - so this may be completely out of date.


I had a quick look at the paper - it's mostly nonsense. The intrinsic light from a quasar is completely dominated by its emission line spectrum and a mostly featureless continuum. The observed wavelength of the emission lines compared to the rest wavelength give the true redshift of the quasar.

Absorption lines in quasar spectra are predominantly due to foreground gas clouds at lower redshifts than the more distant quasar source. Therefore the absorption lines do evidence a lower redshift, but that's because they are due to gas that is unassociated with the quasar.

Here's a picture from a Hamburg Sternwarte description of these absorption lines, that illustrates the situation. Absorption lines are formed due to absorption of the quasar continuum by gas clouds (or maybe even much fainter normal galaxies) A, B and C, which are at progressively lower redshifts (often multiple absorption systems can be found and can be used not only to place the distance of the intervening cloud/galaxy but also say something about its chemical composition). Spectrum D shows how the measured spectrum at the telescope will contain a highly redshifted quasar's light with superimposed absorption bands due to the lower redshift clouds.

The formation of absorption lines in quasar spectra

  • $\begingroup$ Outstanding! To be clear, with "astronomical spectroscopy", in general, are you measuring the light (etc) coming off objects, or light that has happened to travel through something (such as a gas) i.e. as in the image A B C. Or indeed, "both of those"? $\endgroup$ – Fattie Oct 10 '14 at 14:13
  • $\begingroup$ Heh someone seemed to knock off that image on the wiki page .. en.wikipedia.org/wiki/Absorption_spectroscopy $\endgroup$ – Fattie Oct 10 '14 at 14:14
  • $\begingroup$ @Joe - you are talking about "radiative transfer". Absorption lines appear due to light travelling through something else. In a star of course, the something else is the star's own atmosphere. Here, the gas clouds do not have anything to do with the quasar. $\endgroup$ – ProfRob Oct 10 '14 at 14:45
  • $\begingroup$ Ahh! So if we look at Star ABC (a simple nearby star with nothing in between), at the absorption lines .. we're looking at the absorption by the star's atmosphere. Brilliant. $\endgroup$ – Fattie Oct 10 '14 at 15:09
  • $\begingroup$ In the situation in the diagram. This may be naive but couldn't you just go off center, and see if the "ABC" remain, but the quasar's features would disappear? Further reinforcing that ABC are, indeed, just features near us? (Or, is everything involved "too small" to be able to nudge to one side? ie ABC are just as apparently small as the distant target.) $\endgroup$ – Fattie Oct 10 '14 at 15:12

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