'Cosmological Redshift' (with regard to cosmological objects such as galaxies, quasars etc) refers to the observed lowering of spectral frequencies caused by the high speed motion of astronomical objects away from an observer. Now routinely measured, it's taken to mean that all objects in the universe are moving away from each other. That's to say the universe is expanding. (hypothesised as due to the effect of).
As the list of red-shifted objects grew, it was noticed, in the late 1960s and early 1970s, that the amount of red shift sometimes appeared to be 'quantised' (i.e. bunched-up values rather than a smooth distribution) - with the implication that the velocities at which objects recede tends to be grouped around certain speeds. (example)
Since then, some data compilations appear to confirm that the effect is real - while other surveys imply that there is no such effect.
If the effect is real (and not simply due to errors in measurement) there is currently no explanation for its existence.
If it's not real, there is also no explanation regarding how errors in measurement ('artefacts') consistently appear to show the quantised variations.
Further reading Phys.Part.Nucl.Lett.4:5-10,2007
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