Introduction to this series
URL to the video
Please read the introductory blog to the series (linked above) first if you haven't yet done so to get the correct context for this blog.
In this episode Dr. Humphreys take a good amount of time to explain the red shift phenomenon. I have no issues with this, apart from the fact that it could have been more clear that the red shift is an indication of the receding of an object (relative to us) due to the expansion of space-time. It is not a measurement of distance but a heuristic to distance due to Hubble's law. Not that Humphreys state the opposite but that could have been made more clear.
He presented a histogram showing that galaxies tend to cluster around specific rates of red shift (and hence distance). His explanation is that there are "spherical shells" of galaxies around our own galaxy and that those shells are not present from other positions in the universe. This would put the Milky Way galaxy and hence, the earth in a special place and likely the center of the universe. Again, Dr. Humphreys hastens to say that it's not necessarily the earth that is at the center, but that our Milky Way galaxy is near the center, within a million light years.
The first paper that noted the clustering was by William Tifft in 1973 where he recorded the red shift of 200 galaxies. This was repeated by Guthrie and Napier in 1992 with 83 galaxies and in 1997 with 250 galaxies. Am I alone in finding the sample set extremely low?
The first thought that crossed my mind when I heard the presentation was: has he taken clusters and super clusters into account, especially since the data seems to concern very local galaxies (within appr. 20 Mly)? It seems I wasn't too far off. The notion of clustering as a cause for redshift periodicity was part of a 1987 proposal to explain the phenomenon.
Aside from that, I mentioned the low sample set. It might have been that in the 1970s there was not that great a sample set to use, but now there are at least two. There's the 2dF Galaxy Redshift Survey (2dFGRS) that contains 220,000 good quality spectra of galaxies (380,000 objects in total) and the Sloan Digital Sky Survey (SDSS) which contains the spectra for over a million objects. In 2002 Hawkins et al used the 2dFGRS data and found no clear quantization of redshifts and in 2005 Tang and Zhang used both the 2dF and the SDSS data and found no clear periodicity.
Judge for yourself. The SDSS data has been plotted into a movie which does a 3D fly through of the galaxies. Can you spot the shells? I can't.
Dr. Humphreys used the balloon analogy to show why cosmologists do not suppose we are at the center even though further away galaxies recede from us faster (as the Hubble law states). He did a pretty poor job of explaining the concept. The balloon analogy works better when a finite but unbounded universe must be demonstrated, but for the apparent centrality of galaxies, it doesn't work as well. A better way to visualize this is to use two dot patterns, one of which is expanded. The dots represent galaxies and even though this is a two-dimensional representation, it is not hard to imagine a three-dimensional analogy as a loaf with raisins that expands though the working of baker's yeast.
As the image demonstrates, if space-time itself expands, from the viewpoint of any galaxy, the close neighbors do not recede as fast as the further galaxies. This confirms perfectly with what is actually witnessed.
Humphreys did not present that much evidence for a centrality in the Universe and what he presented does not corroborate with the data that we have. There is no reason to assume the existence of a centrality in the universe and even if their is one, there is no data to confirm that the earth is anywhere near such a center and enough data to give the impression that it's not.
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