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I get this question a lot:

Where is the center of the Universe?

I see this question on the internet all of the time:

Where is the center of the Universe?

Now I have a question regarding all of the answers that I always either give or read: The question linked is the last question and answer that I saw before posing this question here, and is in no way the same question, as this question asks how the concept of a centerless universe can work, whereas the question below asks about how a universe can be perceived to be centerless even when it might not be.

Is the Universe centerless? Or does it simply appear to be because of our perspective?

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This is about to be a bad example, but please bear with me.

If you were a bird floating in the ocean, among a myriad of other birds, and you let time pass such that every bird floated away from the other birds in some random fashion. Would you, at some later time when everybirdy was roughly separated from one another, say that based upon your observations of the present conditions, and some evidence that at one point all of the birds were together, that the ocean is centerless? Or could you assume that because of your position in the flock, that from your perspective you can't tell where the center is, nor do you think that from a different birds position would you be able to tell? Creating the illusion of a centerless ocean.

Obviously this requires you not be at or close to the very edge of the flock.

Edit: Also see examples in KenG's posed answer and comments.

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  • $\begingroup$ Also: astronomy.stackexchange.com/questions/8802/… $\endgroup$
    – ProfRob
    Nov 18, 2016 at 19:12
  • $\begingroup$ This question is nothing like the link you provided @RobJeffries... The assumption here is that all known physical concept are obviously legit, and now take a step back and look at the universe from a "different perspective." Do you come to the same conclusion? or do you see a different universe? Not centerless! ... say what?... $\endgroup$
    – LaserYeti
    Nov 20, 2016 at 4:00
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    $\begingroup$ You might try asking over at math stack. There's probably a theorem somewhere stating that every closed physical manifold has a center, in the classical sense, but that info is not going to be easy to find in human readable format. $\endgroup$
    – Wayfaring Stranger
    Nov 20, 2016 at 15:17

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The first principle of cosmology is called the "cosmological principle", which asserts that the universe is more or less the same everywhere spatially, but changes with time. This is a fundamentally new way to think of the history of the universe, and has revolutionized our ability to understand long-range observations. We do not, however, know that this principle is true, we only know it is both useful and consistent with all observations. So we adopt it as a working hypothesis, and all of cosmology follows. Some day we may discover it is inadequate, but until then, we will continue to use it. The same could be said for other principles, like conservation of energy-- we don't know that's true either, but it's useful and consistent with observations. The cosmological principle rules out that the universe has a unique center.

Your question is whether the apparent lack of a unique center stems from our location not being special enough to be able to tell that it does have such a center (say, by encountering a boundary). Ironically, the cosmological principle turns that question on its ear, and points to the possibility that one's perspective always puts themselves at the apparent center. So we would say the illusion of having a center is all about perspective-- not the illusion of not having a center!

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  • $\begingroup$ I understand the purpose of the cosmological principle to suggest ultimately that our universe is the "same" anywhere, on a large enough scale (the universe is the same for all observers). Yet the universe being homogeneous and isotropic does nothing to dispute the possibility for a center. I tub of yogurt can be homogeneous and isotropic, but it clearly has a center, right? I guess that is my point regarding the perspective. Are we in a position where, because of our position in the observable universe, we cannot ever determine where the center is/or could be? $\endgroup$
    – LaserYeti
    Oct 20, 2016 at 4:39
  • $\begingroup$ Or is there the possibility that there truly is no center? And if so, then how can that actually be? $\endgroup$
    – LaserYeti
    Oct 20, 2016 at 4:39
  • $\begingroup$ There would be no center if the universe were spatially infinite, or if it were finite but closed, and those are more or less the only possibilities allowed by the cosmological principle. A closed universe is often described as being like the surface of a sphere in a lower-dimensional analog, which has no center on that surface. $\endgroup$
    – Ken G
    Oct 22, 2016 at 4:20
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    $\begingroup$ @LaserYeti "I understand the purpose of the cosmological principle", it doesn't seem so. The only purpose is to try to predict the results of future observations based on past observations, as accurately as possible. Real actual results, affecting you, your family, your cat. Does your hypothesis predict some new interesting observation (i.e. is it falsifiable)? Standard scientific method. $\endgroup$
    – kubanczyk
    Nov 18, 2016 at 21:44
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    $\begingroup$ @LaserYeti The cosmological principle is an assertion that is actively tested. There are occasional reports of small violations on large scales (e.g. in the cosmic microwave background). A flat but finite universe would have a centre. An infinite universe or a closed universe does not have a centre. We are obviously not anywhere near an edge of a finite universe since the cosmological principle holds to a very high degree of accuracy for the observable universe and we cannot see beyond the observable universe. $\endgroup$
    – ProfRob
    Nov 20, 2016 at 19:50

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