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My understanding of the expansion of space itself is very shaky, but if space itself is expanding, then shouldn't very very distant objects appear magnified?

For example these James Webb Space Telescope (JWST) galaxies from like the year 400M.

Perhaps the distance at which that effect would become noticeable is past the observable horizon.

I guess a related question would be What was the scale of space back then compared to now?

user1271772
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orion elenzil
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1 Answers1

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Yes. And they are! This is called the "Angular diameter distance turnaround" (or turnover). In the usual model for expansion $\Lambda CDM$, it is at a redshift of about 1.5 or about 15 billion light-years (corresponding to a light travel time of about 10 billion years, or about 4 billion years after the big bang, the distance is greater due to the expansion of space) I'm using rounded values here, because the actual distance is quite sensitive to the exact parameters of expansion.

A galaxy from 400 million years after the big bang (a distance of 32 billion light-years) would look as big as a galaxy that is 2.7 billion light-years away. (These figures from Ned Wright's calculator)

It's illustrated in this xkcd comic. You can see nearby "galaxies" are large and bright, more distant galaxies are smaller, very distant galaxies are large, dim, and red. More details about this phenomena are answered by Understanding The Turnover Point of Angular Diameter Distance

James K
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  • "And they are!" ...are you saying this has actually been observed? – user541686 Aug 21 '23 at 03:42
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    @user541686 … I mean, yes? “the most distant galaxy we’ve ever observed, GN-z11, actually appears twice as large as a similarly sized galaxy that’s only half the distance away from us.” – fyrepenguin Aug 21 '23 at 05:33
  • @fyrepenguin: Fascinating, thank you. It sounds like this basically proves inflation? – user541686 Aug 21 '23 at 07:25
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    No. Nothing to do with inflation. It is further evidence for the expansion of the universe (but there's no shortage of evidence for that) "Inflation" is the very rapid expansion of space which seems to have occured in the first billionth of a billionth of a second after the big bang – James K Aug 21 '23 at 07:42
  • Nick Lucid does a great job of explaining it in video form: https://www.youtube.com/watch?v=nSJtzn2H3Do – YetiCGN Aug 21 '23 at 10:33
  • Also on Wikipedia. – J.G. Aug 22 '23 at 07:40