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If all we can observe are things inside the observable universe, how do we know that anything even exists outside this boundary? I can see four ways of solving this problem.

1) We wait a while, the observable universe should get 'larger', so we should be able to observe more. I don't think this is practical though, since telescopes have only existed for a hundred years or so, whereas the age of the universe is many degrees larger. Also, galaxies are expanding outwards, so they will probably escape the observable universe horizon before we can observe them.

2) We go to another planet, and the 'centre' of the observable universe there is different, so in effect we can 'see' further then we could before. This is also impractical, we can't travel large enough distances for this effect to even have an inkling of an effect.

3) No way of knowing. All the matter that is within the observable universe could be all that there is, and we would have no way of knowing if anything even exists outside.

4) Theoretical calculations. Well scientists seem to be able to calculate (from redshifts?) the amount of dark matter, dark energy, and matter in the universe. That would mean that we would gain an understanding of what SHOULD be outside our observable universe (though we may never see it). This seems the most logical, but I have no idea how these calculations are carried out, any insight would be appreciated.

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    $\begingroup$ It's much more simple than any of that. The observable universe is all we can ever see and nothing beyond it matters BY DEFINITION. In an accelerating universe the observable universe actually keeps getting smaller, because the past keeps disappearing by redshift. Moving to another positions doesn't do anything except to undo a little bit of the redshift at one end, at the expense of more redshift on the other, it never brings things back that are beyond the event horizon. $\endgroup$
    – CuriousOne
    Jan 3, 2015 at 6:57
  • $\begingroup$ "None of that matters" Surely it matters but we simply can't see it? Why does none of it matter? "past keeps disappearing by redshift" How does the past disappear by redshift? Do you mean galaxies 'crossing' the particle horizon so that information from them can no longer be observed? Or do you mean redshifted to the point that the information they send is close to unobservable? And if I say quantum tunnel instantaneously 10 billion lightyears in a particular direction, surely I receive more information, that has supposedly dissappeared? Or does none of this make sense? Thanks for any help $\endgroup$
    – QCD_IS_GOOD
    Jan 3, 2015 at 7:13
  • $\begingroup$ If there is no causal relationship with "something", then "something" does not matter. For all you know there could be fairies and garden gnomes throwing one giant party beyond the event horizon, there is no way to test that scientifically. We are simply not invited to that party. What we see of the universe is enclosed by an event horizon that acts exactly the same way as that of a black hole. Stuff that falls trough gets redshifted all the way to oblivion and we can never see its future beyond that point in time. Your personal ideas about "quantum tunneling" are just that, personal ideas. $\endgroup$
    – CuriousOne
    Jan 3, 2015 at 7:18
  • $\begingroup$ The expansion of the region where the metric of General Relativity (or of the Einstein-Cartan Theory, which reduces to GR in vacuum) has been physically observed to apply (either in a non-multiversal universe, or in a "local universe" causally-separated from the others within a multiverse) differs from the relative motion of objects within it, and it's been my understanding that the reappearance of objects that have left our currently-observable region may, consequently, occur, under circumstances that vary between models. This possibility has been a source of acceleration-based controversy. $\endgroup$
    – Edouard
    Jul 12, 2021 at 21:50

2 Answers 2

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As it happens this issue has just been mentioned in the Science Fiction Stack Exchange. The simple answer is that we cannot know that there is anything outside the observable universe. The best we can say is that it seems likely.

We approximate the universe using the a spacetime geometry called the FLRW metric. This is based on the assumption that the universe is the same everywhere - technically that it is homogeneous and isotropic. If the universe is the same everywhere then obviously it's the same beyond the bits we can see, but remember that this is just an assumption.

It's possible to construct a universe that looks like an FLRW universe locally but contains no matter outside some boundary. This metric is called the Oppenheimer-Snyder metric and was devised as an approximate description of a collapsing star forming a black hole. However we can reverse the time direction and the metric then describes matter emerging from a white hole. As long as the boundary is farther away than the edge of the observable universe we would not be able to tell the difference between an Oppenheimer-Snyder universe and an FLRW universe.

However the Oppenheimer-Snyder construction is a rather artifical one. It is created by welding together a patch of spacetime described by the FLRW metric and a patch described by the black hole (Schwarzschild) spacetime. While it's technically possible for this geometry to describe our universe it would take a deity with a rather peculiar sense of humour to arrange the universe in this way. The FLRW universe seems much less contrived and therefore more likely.

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  • $\begingroup$ Artificiality's less evident in Nikodem Poplawski's torsion-based model, which is based on gravitational collapse of large rotating stars (for which there's much astrophysical evidence), but uses Einstein-Cartan Theory (developed by its authors in 1929, after discovery of particulate spin), rather than 1915's GR. As his model's past- as well as future-eternal, state and ecclesiastically-connected universities, in those parts of the world where creationistic fundamentalism prevails, are not favorable to it. Preprints of his many papers on the model can be found by his name on the Arxiv site. $\endgroup$
    – Edouard
    Jul 12, 2021 at 21:07
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What we do know is that the universe is expanding at an accelerating rate. (you've been incorrect in formulating that the observable universe gets larger with time, the observable universe actually gets smaller, with more and more galaxies and stars getting red-shifted to the point of invisibility.) So to claim that there is nothing more than the observable universe is incorrect because since our observable universes magnitude (apparent) has been decreasing, more and more galaxies are out of sight, so we can technically prove that there are galaxies which were once visible, and currently are not, and hence there exist objects outside the "observable" universe.

I hope this has been clear enough.

Edit: enter image description here

The radius of the observable region would remain somewhat same, but the objects which are to be "observed" move out of it due to space expansion, and hence the observable objects reduce, hence causing a decrease in the size of the observable universe, if we agree on the definition of the word.

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    $\begingroup$ I don't see how this works at all. Space-wise, our observable universe should be INCREASING in size "because light and other signals from these objects has had time to reach the Earth since the beginning of the cosmological expansion.", and if the time Earth has been around is longer, then the limit is being pushed. If galaxies and stars are accelerating beyond the particle horizon, I don't see this as 'the observable universe is getting smaller', rather 'its getting larger, but galaxies are being spread out at a faster rate', or am I wrong? $\endgroup$
    – QCD_IS_GOOD
    Jan 3, 2015 at 7:09
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    $\begingroup$ @JoshuaLin: The invisible universe may increase in size all it wants, the point is that the visible universe is decreasing because of the redshift at the local event horizon. What you see is what you see and what's beyond (or not) does not have any causal effect on us and we can't have any causal effect on it. $\endgroup$
    – CuriousOne
    Jan 3, 2015 at 7:14
  • $\begingroup$ What exactly is the local event horizon? And why does anything beyond not have any causal effect on us? $\endgroup$
    – QCD_IS_GOOD
    Jan 3, 2015 at 7:16
  • $\begingroup$ @JoshuaLin I've added a little something to the answer in answer to your comment. $\endgroup$
    – Hritik Narayan
    Jan 3, 2015 at 7:18
  • $\begingroup$ It's the farthest you can see both in space and in time. $\endgroup$
    – CuriousOne
    Jan 3, 2015 at 7:18

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