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Is it possible that the universe does have a center after all, but we just cannot see it because it already fell beyond the event horizon of our observable universe? If not, how do we know this for sure if we cannot observe it?

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    $\begingroup$ In inflation, everywhere is the center $\endgroup$
    – DKNguyen
    Aug 15 at 2:11
  • $\begingroup$ @DKNguyen Of course, but are inflation and having a center mutually exclusive? Couldn't we have both? EDIT: By "center" I mean a relative point of reference around which all the matter of the universe would for example rotate as a whole similar to a galaxy. $\endgroup$
    – mae
    Aug 15 at 2:19
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    $\begingroup$ Does this answer your question? Did the Big Bang happen at a point? $\endgroup$
    – Sandejo
    Aug 15 at 4:34
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    $\begingroup$ @mae It is possible that the universe far beyond the observable universe is filled with flying green pigs and fluffy pink unicorns. We cannot rule this out for sure. But such untestable speculation is not part of science. $\endgroup$
    – gandalf61
    Aug 15 at 8:07
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    $\begingroup$ @mae You are speculating about conditions far beyond the observable universe and which have no measurable effect on the observable universe. These speculations are, by definition, inherently untestable. Your "unobservable" hypothesis only becomes science once you can propose a way to test it i.e. a way in which it makes a difference to something that we can, in principle, observe (in which case it is no longer unobservable). $\endgroup$
    – gandalf61
    Aug 15 at 10:27
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Science is all about predictive power. It is entirely possible that the laws of physics are completely different from the ones we know. The universe could be managed by tiny deamons and are just waiting for someone to sound a trumpet before the walls come down. However, there's no evidence to suggest we can make predictions in this way.

What we can say is that every observation we have made is consistent with the universe having no center. If we make predictions based on this assumption, we have a curious tendency to be right.

There's nothing that prevents there from being a "center" elsewhere, if the laws of physics still resulted in the same set of observations that we see. We tend to ignore this because the results are more complicated, and they don't provide any better predictions.

To borrow from Russel's Teapot, I can predict that balls fly through the air in a (roughly) parabolic arc. I can also predict that balls fly through the air in a parabolic arc and there is a teapot orbiting around Jupiter. Unless I can make observations around Jupiter, the second theory doesn't add any more predictive capability, so we can side step it entirely.

In the case of the idea that the universe has no center, we can stick to that simplistic notation until someone finds out how to observe something outside of the observable universe. Obviously this phrasing has some drawbacks...

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    $\begingroup$ +1 for Russell’s teapot. $\endgroup$
    – gandalf61
    Aug 15 at 4:37
  • $\begingroup$ I'm not disagreeing, I just find it very disappointing to think that we are just fish in the pond who will never know what goes on on dry land because the only evidence we can ever collect is strictly limited to the water we live in. This makes me feel really depressed. Are there at least any attempts to try to figure out what was already lost to the event horizon or have we just given up? $\endgroup$
    – mae
    Aug 15 at 6:06
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    $\begingroup$ All that being said, this is only science we are talking about. Consider: a Lawyer will not write poetry in a legal proceeding, but that does not mean poetry does not exist. Science is only one way to explore reality. It just happens to be a way that is so astonishingly effective at explaining the world around us that we sometimes forget that it is is just one way, with its particular limits regarding evidence and hypotheses. Whether or not the creativity of the human spirit is bounded by science is a question we can't answer yet. $\endgroup$
    – Cort Ammon
    Aug 15 at 6:40
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    $\begingroup$ However, we can make some reasonable assumptions about what's over the horizon. Eg, the density of matter can't be radically different just beyond our particle horizon, because we'd see evidence of it on this side. $\endgroup$
    – PM 2Ring
    Aug 15 at 7:20
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    $\begingroup$ And here's an "over the horizon" answer I wrote a couple of years ago on our sister site: astronomy.stackexchange.com/a/31795/16685 We have quite a few questions on this topic over there, you may enjoy browsing them. $\endgroup$
    – PM 2Ring
    Aug 15 at 7:44
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We cannot know “for sure” that the universe beyond the observable universe is the same as the part of the universe that we can observe. But “for sure” is not a benchmark that science tries to achieve (or, indeed, can achieve).

The principle of Occam’s razor tells us that if we can think of several possible explanations for observable facts, we should favour the one with the fewest untestable assumptions. In this case, a cosmological model in which the universe expands uniformly in all directions and has no unique centre is better than one that assumes a centre of expansion that is so far away we can never observe it or detect any evidence of it.

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There are things we can observe with great precision which furnish evidence that your scenario is incorrect, as follows.

If it were true that the universe has an expansion center, then the part of the universe we inhabit would look different to us in different directions: we would see younger structures in one direction (toward the center) and older structures in the opposite direction (away from the center). This is not observed.

In addition, the younger structures toward the center would have their spectra red-shifted less and the older structures away from that center would be red-shifted more. This is not observed.

In addition, the cosmic microwave background would be strongly anisotropic in a manner that matched the spectral shifts. This is not observed either.

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  • $\begingroup$ All of these assumptions apply to the observable universe. If I take 10 grains of sand from a 500 mile sandy beach and analyze them, I would never be able to determine where the beach begins or ends. I would conclude that the beach is infinite in all directions whether the beach was expanding or not. $\endgroup$
    – mae
    Aug 15 at 2:27
  • $\begingroup$ Regarding red-shifting. What if the effect was cancelled out by a local movement? We don't even know how likely this is because we have no idea what percentage of the universe we are observing. $\endgroup$
    – mae
    Aug 15 at 2:32
  • $\begingroup$ The beach analogy is meaningless. Our telescopes can "see" the beach in all directions to immense distances, and have uncovered no evidence of any center. Try posting your question on the astronomy SE. $\endgroup$ Aug 15 at 2:32
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    $\begingroup$ Our telescopes see a unknown portion of the beach. For all we know it could be a 10^-100000th of a percent of the universe as a whole, which is a pretty terrible sample to be drawing any statistical conclusions from. $\endgroup$
    – mae
    Aug 15 at 2:34
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    $\begingroup$ @mae I think your sand analogy shows you're not really taking time to read or think about the answer. I appreciate your question here but you've got to understand this is something people have thought and asked about a lot and we've gathered a lot of evidence to get to where we are. $\endgroup$
    – Señor O
    Aug 15 at 4:20
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The universe doesn't have a center, so this question is not well-posed.

If your question is "can the universe have a center anyway?" then you would be challenging the Big Bang, which is something that (as of 2021) is very much in the fringe.

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