Further looks bigger beyond z = 1.6

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marcus
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Main Question or Discussion Point

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 :smile:. 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

Code:
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 :smile:, but if you want you can calculate some others and add to the list.
 
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Nereid
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One difficulty with this is the lack of a ruler.

The high-z galaxies we see are not only a long way away, but are also much younger.

How to disentangle the effects of galaxy evolution from those of the GR universe (in terms of actual astronomical observations)? There are a couple of very clever ideas I've read, but I doubt they'd be sufficiently accurate to be used to test different cosmological models.
 
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marcus
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...

How to disentangle the effects of galaxy evolution from those of the GR universe (in terms of actual astronomical observations)? ...
glad to see you took note and have some thoughts about this, Nereid!

did you read Hellaby's proposal?
he says "next generation of automated galaxy surveys"
any ideas how to explicate what he is proposing?
 
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Nereid
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glad to see you took note and have some thoughts about this, Nereid!

did you read Hellaby's proposal?
he says "next generation of automated galaxy surveys"
any ideas how to explicate what he is proposing?
I've not; do you have an arXiv reference?
 
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marcus
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http://arxiv.org/abs/astro-ph/0603637
The Mass of the Cosmos
Authors: Charles Hellaby

"We point out that the mass of the cosmos on gigaparsec scales can be measured, owing to the unique geometric role of the maximum in the areal radius. ...We recommend the determination of the distance and redshift of this maximum be explicitly included in the scientific goals of the next generation of reshift surveys. The maximum in the redshift space density provides a secondary large scale characteristic of the cosmos."


6 pages, 9 graphs in 3 figures.

Mon.Not.Roy.Astron.Soc. 370 (2006) 239-244
 
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Nereid
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Of course, knowledge (or assumptions) about true diameters and absolute luminosities of sources, and their z evolution, is essential, and at large z this is a significant uncertainty.
Oh what depths there are in such a simple sentence!

The good news is that the vast amount of data that LSST and Pan-Starrs will produce should certainly allow at least some constraints to be put on the ideas in the paper, in terms of feasibility ... of the top of my head, I don't recall what the relevant targets are, for each of these giant surveys, relevant to the kind of analyses Hellaby suggests (e.g. how complete is coverage expected to be, around z = 1.6? how accurately and reliably would galaxy diameters be able to be estimated, in this redshift range?)
 
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marcus
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Thanks for the good news!

I would not wish Hellaby to be a whit less forthcoming.:biggrin:

I hope and expect the appropriate statistical study will be undertaken, and will be an very challenging one to tackle. Astronomers are remarkably skillful at statistical inference these days. If it is undertaken, I look forward to it being an exciting study and one that catches the public attention.
 

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