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I've read multiple physicists talk about stuff (e.g. galaxies, black holes, etc.) "existing" outside the observable universe, but it's unclear to me in what sense these things "exist". By definition things outside the observable universe are spacelike separated, hence casually disconnected, from us. And since two spacelike separated events can always be made simultaneous by taking the right reference frame, we can always pick a reference frame in which any given event outside the observable universe "hasn't happened yet". It seems to me the more accurate thing to say is that the observable universe is "growing" and that "outside the observable universe" is a collection of events that "haven't happened yet". What do physicists mean when they talk about things existing outside the observable universe?

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    $\begingroup$ Could you try making your question more precise, in the sense that if you are concerned about "existence" then you must at least supply a physical definition for it. I suggest a pragmatic approach to it, so things that we detect exist. The rest is hypothetical, or theoretical work which also has value on its own $\endgroup$ – ohneVal May 7 at 15:08
  • $\begingroup$ I guess part of the point of the question is asking if there is a meaningful way to interpret statements about the existence of objects beyond the observable universe. My first/immediate interpretation of "existence" is as you say: things we can detect are precisely the things that exists. Yet under this interpretation it seems to me that we cannot say there are things that exist outside the observable universe, as they are by definition undetectable at the present. $\endgroup$ – Keefer Rowan May 7 at 15:32
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    $\begingroup$ I have never seen the other floors of the building I am in right now, nor have I heard anybody mention them or their contents. They are even separated in a spacelike way from me, so even their simultaneous state is undefined. Yet even if I never ever visit them, I think most people would say it is unreasonable for me to question their existence or the existence of people in them. This is because they are a natural extension of my normal observable world. Same thing for the parts of the universe outside the observable universe. $\endgroup$ – Anders Sandberg May 7 at 15:42
  • $\begingroup$ Of course, we can only guess these things are out there based on the laws of physics that we know. But I want to add, that doesn't mean that there is no meaning on discussing them. Perhaps I am not getting your point with the first sentence on your comment @KeeferRowan $\endgroup$ – ohneVal May 7 at 15:44
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    $\begingroup$ @RodneyDunning All I'm saying is that your definition of "simultaneous" is exactly the definition of "spacelike-separated." That doesn't mean it's meaningless; rather, it's redundant, and it's not clear why you wouldn't just use the usual term "spacelike-separated" and reserve "simultaneous" for the usual frame-dependent concept (having $\Delta t=0$ in the frame you're currently in). $\endgroup$ – probably_someone May 7 at 18:02
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I've read multiple physicists talk about stuff (e.g. galaxies, black holes, etc.) "existing" outside the observable universe, but it's unclear to me in what sense these things "exist".

You ask what the meaning of "exist" is when it is not observable, or more specifically in this case, spacelike separated. I assume that you are not going down the purely empiricist worldview when you ask this question, and are instead talking about the definition of "exist".

I see no problem with the existence of objects that are spacelike separated from me. I honestly question whether anyone does. Sure, one cannot make observations about specific objects, you know, that black hole or that galaxy, but does anyone really think that galaxies can't exist beyond the cosmological horizon, or that even asking that is somehow a non-sequitur?

From a physical standpoint, inflationary cosmology suggests that the cosmological horizon is purely an observational construct, and has no physical basis. Adding that to big bang cosmology results in the statement that the physical universe is dramatically larger than the observable, but it all has a common physical basis. For that reason, there is a very good reason to believe there are objects just like our own on the other side of the imaginary sphere.

There may be some conflation of "event" and "exist", which may be reasonable in GR but I'm not sure works in a wider context.

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  • $\begingroup$ You seem to be confusing "spacelike separated" with "causally disconnected". A Sun flare event is spacelike separated from the Earth, but we can see the flash after 8 minutes. In contrast, we can never see or be influenced by anything beyond the particle horizon. The Lambda-CDM model for a flat space states that for the observable universe, the Big Bang did happen at a point in an infinite space and does not prove that anything exists outside of the light cone of this point. $\endgroup$ – safesphere May 7 at 18:53
  • $\begingroup$ @safesphere At the moment the solar flare happens, the worldline of the Earth is spacelike separated from the event of the solar flare. As time passes (how much time this takes depends on your particular reference frame), the worldline of the Earth becomes lightlike-separated from the event (which is when we can see it), and then immediately afterward becomes timelike-separated from the event. The terms "time/light/spacelike-separated" measure the interval between a point in spacetime (in this case, the solar flare) and another point in spacetime (a point on Earth's worldline). $\endgroup$ – probably_someone May 8 at 11:41
  • $\begingroup$ @safesphere Events (i.e. points in spacetime) that are spacelike separated from each other are indeed causally disconnected; one cannot send a beam of light (or anything else) from one to trigger the other. $\endgroup$ – probably_someone May 8 at 11:45
  • $\begingroup$ @safesphere Also, can you provide a source for your claim about the $\Lambda$-CDM model? Nothing I've ever heard about it has supported what you're claiming here. $\endgroup$ – probably_someone May 8 at 11:48
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IMO there is confusion (also in the comments) between objects, things and events. To me is meaningless to say that an event exists. It may be spacelike separated from myself writing this comment, but become - the event - timely separated tomorrow, then observable.

When we speak of "observable universe" we aren't thinking of events but of objects (e.g. galaxies) which have a continuous existence in time and in some past time could have emitted light now arriving to us.

You might usefully consult wikipedia for "cosmological horizon".

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