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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?

mae
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    In inflation, everywhere is the center – DKNguyen Aug 15 '21 at 02:11
  • @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. – mae Aug 15 '21 at 02:19
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    Does this answer your question? Did the Big Bang happen at a point? – Sandejo Aug 15 '21 at 04:34
  • @Sandejo No, it does not. That question assumes that the big bang is what causes and determines a center. – mae Aug 15 '21 at 06:12
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    @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. – gandalf61 Aug 15 '21 at 08:07
  • @gandalf61 You don't know whether it's untestable. A lot of things in history were thought to be untestable until someone found a way to test them. Are they still speculation? – mae Aug 15 '21 at 09:33
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    @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). – gandalf61 Aug 15 '21 at 10:27
  • @gandalf61 You also do not know whether the conditions far beyond have any measurable effect on the observable universe. You're just speculating yourself. – mae Aug 19 '21 at 01:49
  • @mae This is not a symmetric situation. If you want your hypothesis to come under the heading of science then it is up to you to propose a practical way in which it can be tested. – gandalf61 Aug 19 '21 at 08:13
  • @gandalf61 Certainly, I would never claim any of it to be valid without compiling and providing compelling evidence first (which does not exist at this point). I'm just saying it's something that could be worth exploring. – mae Aug 19 '21 at 10:16

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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...

Cort Ammon
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    +1 for Russell’s teapot. – gandalf61 Aug 15 '21 at 04:37
  • 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? – mae Aug 15 '21 at 06:06
  • @mae I'm not sure which event horizon you're thinking of, but Hawking put a lot of effort into looking at what could come out of a black hole. As for giving up, I don't think anyone has given up... other than to be certain that things which work a certain way will work a certain way. I wouldn't try to go faster than light with things that travel at the speed of light. But there's some fun questions that can arise in Quantum Physics which leaves the door open -- just in a decidedly non-classical way. – Cort Ammon Aug 15 '21 at 06:37
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    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. – Cort Ammon Aug 15 '21 at 06:40
  • @CortAmmon Thank you. To clarify, by "event horizon" I am referring to of the edge of the observable universe due to expansion being greater than the speed of light. – mae Aug 15 '21 at 06:42
  • @mae There are various cosmological horizons. – PM 2Ring Aug 15 '21 at 07:13
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    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. – PM 2Ring Aug 15 '21 at 07:20
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    And here's an "over the horizon" answer I wrote a couple of years ago on our sister site: https://astronomy.stackexchange.com/a/31795/16685 We have quite a few questions on this topic over there, you may enjoy browsing them. – PM 2Ring Aug 15 '21 at 07:44
  • @PM2Ring Thank you! This is very insightful indeed! – mae Aug 15 '21 at 09:15
  • @mae "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 " This is a common attitude , when they people first realise, how much we DONT know. All i can say to help you deal with that disappointment , is Feynman's quote. " I think it's much more interesting to live not knowing than to have answers which might be wrong. I have approximate answers and possible beliefs and different degrees of uncertainty about different things, but I am not absolutely sure of anything and there are many things I don't know anything about. " – silverrahul Aug 15 '21 at 14:13
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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.

gandalf61
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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.

niels nielsen
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  • 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. – mae Aug 15 '21 at 02:27
  • 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. – mae Aug 15 '21 at 02:32
  • 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. – niels nielsen Aug 15 '21 at 02:32
  • who says we have no idea? – niels nielsen Aug 15 '21 at 02:32
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    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. – mae Aug 15 '21 at 02:34
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    @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. – Señor O Aug 15 '21 at 04:20
  • @SeñorO I appreciate you playing the devli's advocate but until we know everything about the universe "people thinking and asking about it a lot" isn't a valid argument, because they clearly haven't done it enough. And my analogy of the observable universe is correct. Perhaps you also did not think about it enough. – mae Aug 15 '21 at 06:27
  • @mae No, you're just suffering from Dunning-Kruger effect. Your analogy about grains of beach on the sand shows a tremendous lack of understanding about things like redshift, cosmological constant, etc. – Señor O Aug 15 '21 at 17:03
  • @mae Now when I talked about ""people thinking and asking about it a lot", I wasn't making an argument as to why their interpretation is correct but rather why you're having such a hard time understanding why your assumptions are wrong - you've come up with a bunch of childish intuitive ideas instead of reading what other people have researched about the topic. Learn more, do better. – Señor O Aug 15 '21 at 17:04
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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.

Allure
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