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I watched an interview with Neil deGrasse Tyson on The Late Show with Stephen Colbert where he briefly mentioned a disturbing thought. It is something that I have never thought of before, and the idea borders on the eerie.

The idea is this (and here comes a mouthful): If all our knowledge of the Universe is based on observations of distant galaxies (and Milkyway stars), and we accept that the Universe is expanding at such an accelerating rate that at some point in the future only our nearest stars will be visible to whoever inhabits Earth, and that then those future inhabitants will need to explain the Universe based only on what they can observe, then it is possible that we are today trying to explain the Universe based on largely incomplete data.

Like, imagine that there is much more stuff out there, stuff that could help us explain some of the scientific "mysteries" such as dark energy/matter, but it is beyond our horizon. We don't know what is missing from our view.

And so I guess the question is this: How does this affect the validity of our current models of the Universe?

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  • $\begingroup$ I do not think this affect our current model(s). It is rather pointing to the fact that cosmology as a whole is not really falsifiable and has a special status in science. Is the "beyond" that makes this. And perhaps the "behind" at start. Unless those would be removed. $\endgroup$ – Alchimista Jun 6 at 10:36
  • $\begingroup$ @Alchimista There's no special status. Cosmology is based on the observable universe. Even the Big Bang just talks about how the space comprising the observable universe appeared 15 billion years ago - it doesn't say anything about the rest of the universe (if any). It is absolutely falsifiable, and many cosmologies have been utterly falsified, both in the past and in modern times (the Big Bang cosmology has endured many challengers so far). There are some hypotheses that talk about what was before the Big Bang, or around the Big Bang etc., but those are not accepted theories/models. $\endgroup$ – Luaan Aug 14 at 7:14
  • $\begingroup$ Put it as you like. It is right what I say. "Cosmos as whole. .... " @Luaan $\endgroup$ – Alchimista Aug 14 at 8:22
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It well might be the case that universe beyond our observable universe is filled with pink flying elephants, but this just seems highly unlikely.

Our current models are usually based on the fact that universe is isotropic, ie there’s nothing special about our observable universe vs everything else there is. Obviously there are smaller and larger deviations, but on larger scales these are thought to cancel out. This means the simplest position here is to assume all of universe looks more or less the same.

While we can’t currently / no longer observe what is going on on the other side of horizon, there is some evidence supporting above view. Firstly, cosmic microwave background measurements seems to support isotropic universe. Secondly, inflation models seem to provide explanation for isotropic universe via early causal connection beyond our current observable universe, and as our early universe model is able to explain many other things to great detail, it gives some confidence to this claim as well.

Since early universe models do support the view universe outside our observable universe is pretty similar to what we can see now, this helps us to rely on observable universe for overall model building.

Also, there does not seem to be any large mass concentrations outside the observable universe, since these would be seen as bias for movement of galaxies and galaxy groups, which also supports that the rest of universe is at least very close to what we can observe directly. This gives further confidence that our current models are probably not affected much by things beyond our currently observable universe.

However, since everything outside our observable universe is now causally not connected to us, anything that might be going on over there does and will not affect us - thus it is not necessary nor valid to include any of that in our models either.

To summarize my answer: no, there’s probably no effect on universe beyond the observable horizon that would need to affect our models.

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  • $\begingroup$ Nice answer but I suspect the meaning of the quote is deeper. Imagine a very remote future civilisation developing somewhere. They could develop a cosmology based solely on their galaxy or local group or whatever left around them and see just empty space beyond. Perhaps to imagine that beyond their observable universe there are lots of galaxies would be seen (not identical of course :) ) as to think of pink elephants, or at least of normal elephants. $\endgroup$ – Alchimista Jun 6 at 10:29
  • $\begingroup$ They would still be able to make the same observations I mentioned plus they would still be able to see other galaxies (faint and unreachable, but they’d see them, like we see distant galaxies receding at superluminal speeds). They would be perfectly able to build cosmology similar to ours (in terms of laws and equations etc) but they might give it different semantic meaning (eg they might be compelled to think more in multiverse terms). $\endgroup$ – tuomas Jun 6 at 12:02
  • $\begingroup$ Not really. It is just matter of how far the future is. Often is said that the cosmology epoch is "now"! $\endgroup$ – Alchimista Jun 7 at 10:17

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