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Is it plausible that a possible extraterestrial scientist from a galaxy 13 bilion light years from us sees this part of the universe nothing else than just background cosmic radiation... and even more intriging in the oposite direction he sees an enormous portion of the universe filled with galaxies and stars that are not visible to us due to space expansion greater than the speed of light regarding Earth?Ofcourse his sight in the oposite direction finishes also at a CBMR signal.

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Yes. It is completely plausible that the universe is infinite in spatial extent, despite the fact that any observer can observe only a finite part of it. In fact, current standard cosmological models generally assume that it is infinitely large, because observations are consistent with zero spatial curvature. (There is nonzero spacetime curvature.) You can read about a $k=0$ Friedmann universe.

If the universe actually has positive spatial curvature, then it has finite size. But the spatial curvature has to be so small to match observations that the whole universe would be much larger than the part we can see.

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  • $\begingroup$ Current models assume it's infinitely large because it's an easy approximation, not because picking an arbitrarily large size contradicts with observation. While observations don't favor infinite or finite/large, there are obvious problems with there being an infinite universe and an initial singularity. $\endgroup$
    – Señor O
    Commented Sep 6, 2020 at 0:28
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    $\begingroup$ @SeñorO what obvious problems? The Big Bang is a singularity no matter the shape of the universe. $\endgroup$
    – Javier
    Commented Sep 6, 2020 at 0:39
  • $\begingroup$ @Javier so is the idea then that the bing bang singularity existed at a particular point inside an already infinite volume of "empty" space? $\endgroup$
    – Señor O
    Commented Sep 8, 2020 at 21:13
  • $\begingroup$ @SeñorO Definitely not. $\endgroup$
    – G. Smith
    Commented Sep 8, 2020 at 21:43
  • $\begingroup$ @SeñorO see physics.stackexchange.com/questions/136860/… $\endgroup$
    – Javier
    Commented Sep 8, 2020 at 21:45
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No, it is not plausible that the extraterrestrial scientists would see such differences when they look in opposite directions.

The Friedmann universes describing our expanding space are derived under the assumption that the universe is homogeneous and isotropic. In other words, it is the same wherever you are and in whichever direction you look. In this respect, it doesn't matter if the universe is infinite or finite.

Because you mention galaxies 13 billion light years apart, and that happens to be the approximate age of the universe, I think you probably believe that is the size of the observable universe. However, due to the universe's expansion, that is not the case. It's more like 42 billion light years in radius. (See the answer to this question for a good explanation: Size of the universe)

I'm also assuming that you are asking what someone at the edge of our observable universe would see, not realizing that we are also at the edge of their observable universe. Again, the answer is that observers anywhere in the universe would all see the same sorts of things (ignoring small-scale variations, of course).

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  • $\begingroup$ I wasn't clear enough so I must add that he doesn't see the CBR only from our part of the universe but he also has his shell of the observable universe of 13 B light years. $\endgroup$ Commented Sep 5, 2020 at 21:55
  • $\begingroup$ @ Krešimir Bradvica I don't quite understand your comment. Are you assuming that the extraterrestrials can see things from two distant vantage points? It doesn't actually matter though. Under homogeneous & isotropic assumptions, the two views should be the same. $\endgroup$
    – D. Halsey
    Commented Sep 5, 2020 at 22:10

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