Doesn't there have to be more than one observable universe?

[Imager]

Again, there's as many observable universes as there are observers.

A couple of question to check if I am understanding what is being said:

Assuming an infinite universe, is there any upper limit to the number of observable universes? I say there isn't. The way I am thinking there would be an infinite amount 92 billion LY bubbles across the universe.

Assuming a finite universe, wouldn't the observable universes be forced to overlap each other? I say yes.

And a thank you to Brandersnatch for the many great answers he has given over the years!

 

[kimbyd]

Assuming an infinite universe, is there any upper limit to the number of observable universes?

Maybe, though it depends a bit upon what you mean.

In the quantum view, the number of non-identical observable universes may be finite (but excessively large). For instance, if the cosmological constant really is a constant, then there is a finite number of possible observable universes based upon the horizon size set by that cosmological constant. If the cosmological constant can actually take some finite number of values values, then you get a finite number of possible observable universes for each of those values, for a total number that remains finite. Though this may only work if $\Lambda=0$ is not possible (I think $\Lambda=0$ may be infinite, even in the quantum case, while I'm pretty sure any other values are finite, with the caveat that I'm not certain about negative values).

In the non-quantum view, the number is definitely unlimited (because any observer location will result in a different observable universe, and in the non-quantum view there are an infinite number of observer locations even within a finite region). This actually leads to problems with theoretical predictions: you can't calculate certain kinds of probabilities in an infinite universe (this is known as the "measure problem").

Assuming a finite universe, wouldn't the observable universes be forced to overlap each other? I say yes.

No need to assume a finite universe. Any observer within our observable universe will have its own observable universe which overlaps our own.

The question, rather, is whether there can be any regions which are entirely disconnected, where there can be no overlap between the two regions. That is unknown. It depends entirely upon the physics which resulted in the low-entropy state which occurred early in our observable universe.

 

[Bandersnatch]

The question, rather, is whether there can be any regions which are entirely disconnected, where there can be no overlap between the two regions. That is unknown.

Isn't this pretty much a given in the concordance model? I always thought these entirely causally disconnected observers are spaced every ~130 Gly or so (twice the comoving event horizon distance).

 

[kimbyd]

Isn't this pretty much a given in the concordance model? I always thought these entirely causally disconnected observers are spaced every ~130 Gly or so (twice the comoving event horizon distance).

What I meant is whether or not there can be any distinct regions (not necessarily just observable regions) which have no overlap anywhere.

Certainly it's possible for there to be non-overlapping observable regions in the concordance model, just by being far enough away from one another. But you could, at least as a mental exercise, connect any two observable regions by selecting a set of observers between the two regions, where each observer overlaps with the other, eventually reaching the far-away observer.

My question is: is there a way for that not to be possible? Can there be regions entirely disconnected?

The reason why the physics of the early universe may hold the answer to this question, even though it can never be tested directly, is that many models demand exactly this. For instance, if the start of our universe was a quantum vacuum fluctuation in another region of space-time, observers in that space-time would see what appears to be a microscopic black hole which pops into existence then rapidly evaporates, while observers within our space-time see billions of years of history. These two space-times are entirely disconnected, and there is no way to bridge the two.

Right now we have no way to know if this is possible or not. And we don't know if we'll ever be able to learn enough about the universe to say whether or not this is possible (since it is, by construction, impossible to directly test).

 

[Chronos]

No observer that resides in a region of space visible to us can ever report 'seeing' anything that is not also visible to us. By the time a message about a 'new' event can reach any other observer, the light from that event will also have reached them, no matter where in space it occurs. This is simply due simply to the finite speed of light. You can never 'see' any event that is older than your observable universe and your observable universe is always identical in size to that of any remote observer's observable universe at the same time [age of the universe]. This also suggests any even that is causally disconnected from one observer must also be causally disconnected from all other potential observers.

 

[PeterDonis]

No observer that resides in a region of space visible to us can ever report 'seeing' anything that is not also visible to us.

To make this more precise: at the event on our worldline where we receive a report from some other observer about something they have seen, we must also be receiving (or have already received) light signals directly from the something they report having seen. In other words, any event that is in the past light cone of the observer that reported to us when they sent the report, is also in our own past light cone when we receive their report.

However, the following is also true: on any spacelike slice of constant time in the universe, the events on the worldlines of two spatially separated observers where they intersect that spacelike slice will have different past light cones, and therefore, at those events, the "observable universes" of the two will be different.

In other words, the precise referent of the term "observable universe" depends on which event on which observer's worldline you pick. You can make the "observable universes" of the same two observers the same or different by appropriate choices of events.

 

[kimbyd]

To clarify, the hypothetical connection I was proposing wasn't intended to suggest that you could use it to communicate beyond the observable universe. Rather, it's just a hypothetical construction to illustrate connectedness.