# Talk:Angular size redshift relation

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## Article

Something unusual is going on with this article. The equation is described as a relation between angular size and redshift (hence the name), but redshift z does not occur in the equation. The graph looks nice, but unless the units are explained, the graph is unrelated to the algebra. Are the units of angular size "arc-seconds" or "kiloparsecs per arc-second" or what? As a non-astronomer, it's unclear to me what's going on here. Flying Jazz 13:08, 21 May 2006 (UTC)

The distance and the redshift are related to each other - but in a non trivial way that depends on the geometry of the universe and how you define distance. I included links to redshift and angular diameter distance to try and make this clear. Redshift is often used as a unit of distance in Cosmology. The units of angular size are kiloparsecs per arc-second - it is written on the graph but is rather small - and requires the knowledge that " is an abbreviation for arcseconds. It is explained in the summary of the graph. The graph is related to the algebra in the case of small distances/redshifts only. Hopefully someone can add the maths which explains the full shape of the graph. Hjb26

## Notation

The notation in the formula is not fully explained. What is $Q_o$? --Booberfish 18:42, 8 August 2007 (UTC)

## Note for history buffs

Hi everybody, I just added such a section since there might be guys and gals interested in history and in knowing how "Einstein's universe" comes out comparing itself with the more contmporary picture of the universe. First of all "Einstein's universe" is much simpler since there is no expansion in it since in 1917 people had no idea that the universe might be expanding. The picture was solidly static (actually "stationary", since some celestial objects were moving anyway) and the possible cosmological redshift was only a theoretical possibility, easily predictible, but not observationally verified yet. It had to wait until 1929 for Hubble to turn his attentio to it. And even now there are guys who maintain (like truly yours) that it is enough to apply Einsteinian gravitation to explain accelerating expansion of the universe without necessity of inventing "lack of conservation of energy", "dark matter", "quasars at cosmological distances", and "real expansion" (since the apparent suffices).

Of course guys like me are in minority but historically, everybody who noticed some new thing (even in old theories) was in minority for a while. And I'm in no hurry. It's enough for me that I know how to calculate various parameters that astronomers have to observe to know their values. Of course observations are good since they confirm Einsteinian predictions for "Einstein's universe". So far 5 of them and I hope that the angular size redshift relation will be the sixth. I'm just working on it since it turned out to be a non trivial problem that can't be done analitically. Luckily we have computers now.