Existence of things outside the observable universe

Question

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?

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 — ohneVal, May 07 '19 at 15:08
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. — Physical Mathematics, May 07 '19 at 15:32
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. — Anders Sandberg, May 07 '19 at 15:42
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 — ohneVal, May 07 '19 at 15:44
@AndersSandberg That's not really a fair comparison because there are spacetime events located on the other floors that aren't spacelike separated from you, so there is a meaningful sense in which those events exist. I would say that the events that are spacelike separated from you don't really exist to you at that moment any more than the future exists to you at that moment. So in this sense things outside the observable universe "don't exist to us yet". — Physical Mathematics, May 07 '19 at 15:47
@ohneVal I'm not trying to say it's meaningless to discuss spacelike separated events, just as it's not meaningless to discuss future events. The question is whether there is a way in which we can say the universe is (talking in the present tense) larger than the observable universe or whether we must say the universe will be larger than the observable universe. It seems to me people are talking about events outside the observable universe as if they already happened, but there is no sense in which they have happened yet. — Physical Mathematics, May 07 '19 at 15:51
@AndersSandberg "the other floors of the building [...] are even separated in a spacelike way from me" - This statement has no physical meaning. Spacelike separation applies to spacetime events, not to objects in space. — safesphere, May 07 '19 at 16:21
The trouble with denying the existence of things outside of the observable universe is that every location in space has a different observable universe, so you have to deny the reality of others' observations. — D. Halsey, May 07 '19 at 16:27
@safesphere Couldn't the simultaneous existence of another floor and your floor be taken to be simultaneous events, spacelike separated? — Rodney Dunning, May 07 '19 at 16:29
@KeeferRowan "events that haven't happened yet" - and also will never happen. This is a clear definition of non-existence, but you are unlikely to get a straight answer on this site filled with common misconceptions. Your question is discussed in more details here: https://physics.stackexchange.com/questions/456071/could-the-spatially-flat-universe-start-small - "any other "point in this space is not in our past light cone, is causally disconnected from our observable universe, and cannot influence us in any way other than by *contributing to the global space curvature being flat.*" — safesphere, May 07 '19 at 16:34
@safesphere - I agree, but the OP clearly seems to use spacelike separation to claim that there is also a profound causal separation. — Anders Sandberg, May 07 '19 at 16:36
@RodneyDunning "simultaneous existence of another floor" - This also has no physical meaning. Simultaneity is relative. — safesphere, May 07 '19 at 16:37
@safesphere "Simultaneity is relative." Does this statement have a physical meaning? To say that two events are simultaneous seems comprehensible to me, in the context of physics. It says that there is a reference frame in which $\Delta t = 0$ between those events. Surely there is a physical meaning there. — Rodney Dunning, May 07 '19 at 16:44
@AndersSandberg That point about your office floors seems fairly irrelevant here, no? I think it's completely accepted that events occurring on other floors can have impacts on the physical reality of your own floor. You've likely at very least experienced this in the form of sound. You yourself even point out that they are "a natural extension of my normal observable world". Would that then not obviously exclude events beyond the observable universe, because all natural extensions of our observable world don't extend beyond the observable universe? — JMac, May 07 '19 at 16:46
@RodneyDunning But that's completely redundant with the two events being spacelike separated, so it doesn't really add anything. Any two spacelike-separated events have a reference frame where they occur at the same time, just like any two timelike-separated events have a reference frame where they occur at the same place. — probably_someone, May 07 '19 at 16:48
@KeeferRowan Please also note that the concept of the "observable universe" is an artifact of the official Lambda-CDM model of cosmology, which is problematic and cannot survive without "inventing" non-observable reality like "dark energy". There is no experimental proof whatsoever that the entire universe in not observable. It is observable in some other models, like the Milne model. (I am not claiming that the Milne model is correct, it is not. I am just stating that whether or not the entire universe is observable depends on the model and has not been experimentally verified.) — safesphere, May 07 '19 at 16:48
@probably_someone I'm on floor $N$. A firecracker explodes next to me at $t = 0$ on my watch. A second firecracker explodes on floor $N + z$. When the exact spacetime coordinates of the second explosion are brought to me, I find that $\Delta t = 0$ between these events. Or, $\Delta t > 0$ but less than the time needed for a photon to cover the distance between the events. What kind of separation do we have? Spacelike, lightlike, or timelike? Why is this 'meaningless' and/or not relevant to the OP's question? If I'm reasoning incorrectly (which is likely), I want to know where is the mistake. — Rodney Dunning, May 07 '19 at 17:01
@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). — probably_someone, May 07 '19 at 18:02

score 3 · Answer 1 · answered May 07 '19 at 17:34

3

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.

answered May 07 '19 at 17:34

Maury Markowitz

8,201

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. – safesphere May 07 '19 at 18:53
@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). – probably_someone May 08 '19 at 11:41
@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. – probably_someone May 08 '19 at 11:45
@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. – probably_someone May 08 '19 at 11:48

score 1 · Answer 2 · answered May 07 '19 at 16:37

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".

Existence of things outside the observable universe

2 Answers2