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marcus
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Apr5-07, 03:34 PM
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This has not been observationally verified yet, as far as I know, but is expected to be so by the next generation of galaxy surveys (Hellaby 2005). It is part of the standard mainstream cosmology picture and it would be quite astonishing if it wasn't confirmed as soon as the angular sizes of these very distant things are reliably measured. Keep posted . It is a prediction of the standard model.

Astronomers measure angles in "arcseconds". One second of arc is 1/60 of an arcminute which is 1/60 of a degre. So an arcsecond is 1/3600 of a degree.

As an example of galaxy size---Milky diameter is 30 kiloparsec but a large elliptical galaxy might have diameter 100 kiloparsec.
think of a ruler 100 kiloparsec long the size of a large galaxy.

Here is the angular size of that ruler---the angle it makes in the sky---at various distances away from us, indexed by redshift

redshift z       size in arcseconds
1.4                11.76
1.5                11.71
1.6                11.688
1.7                11.689
1.8                11.71
1.9                11.75
You can see that out past z = 1.6 the angular size of stuff is getting BIGGER the further away it is.

Do you have any questions about this? Maybe knowledgeable people (Wallace, hellfire, cristo?) will help explain why this happens, if there are questions. SpaceTiger already gave a clear explanation of the effect, in a thread last year.

===============
Suppose you understand how the effect works and why "further looks bigger" beyond z = 1.6, but you just want to add some data to the table. How do calculate the arcseconds?

You go to Wright's CosmoCalc
http://www.astro.ucla.edu/~wright/CosmoCalc.html
and plug in a redshift like 1.4 and it tells you 8.502 kpc per "
the (") sign stands for arcsecond, so that means 8.502 kiloparsecs per second of arc
so that means 0.08502 of a 100 kiloparsec ruler for every arcsecond
so the whole 100-kiloparsec ruler is 1/0.08502 arcseconds, which is 11.76 arcseconds. That's already in the table , but if you want you can calculate some others and add to the list.
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