If all of the galaxies we see rotate the same way, are we “looking down” from a pole and seeing only those with the same rotation we have, as opposed to a more equatorial view that would be evenly split.
In other words, your observable universe is different than mine and that's both spheres we're in the middle of. That suggests the universe itself probably isn't a sphere.
Just because something is untestable today doesn’t mean it will be for all time. However, the untestability problem has started to creep much more deeply into cosmology and high particle physics in particular - our technology and models aren’t staying enough ahead to provide a lot of fertile testable ground.
This is generally true, but such ideas are kept outside the realm of science until they are. In this case specifically, all our knowledge points to this remaining untestable as it would require FTL travel which is on par with violating conservation of energy or time travel. It even allows solving the halting problem (Turing machine in timeloop until it halts, you outside of the timeloop can then check if the Turing machine in the timeloop ever left it).
It is entirely possible that there are things which are true which science cannot verify because of the underlying philosophy by which science operates. Things that exist outside of the observable universe, if FTL travel is truly impossible, would fall outside the realm of science.
>The mere fact that two observers have different observable universes indicates it is indeed an illusory artifact.
Do they? The nature of the observable universe is that, if you can communicate with someone else, any information they can receive and pass on to you is part of your observable universe as all information travels at the speed of light or slower. If they can receive information and cannot pass it on to you, they are not part of your observable universe any longer and no longer exist (exception if FTL interactions are discovered). Thus the only observers that exist in a way you can interact with, can make any testable hypothesis concerning, and thus can be considered by science, are observers in your observable universe.
The observable universe is defined as natural light reaching us. It says nothing about repeaters. If someone id at the edge of your observable universe they could still send you a message. They’re observable universe would necessarily include light that wouldn’t reach you due to expansion. Of course, it is possible we’re within a black hole or some other weird space time geometry in which the universe folds in on itself in which case it is possible the observable universe is the universe. I’m not saying that’s impossible since we don’t know. I’m simply stating my Bayesian priors based on my understanding of the evidence collected so far about the CMB and what it and the theoretical models we have suggest. It doesn’t make other theories less valid, it just means where I’d make a wager if I had to. As you say, right now it’s not capable of being a scientific theory and it’s a stretch to even be called a hypothesis.
Still, this is just a reversion to our natural state where we have philosophical ideas grounded in the best knowledge we have trying to find ways to unlock the secrets, not unlike ancient Greeks. We might succeed or we might not but I still think it’s a scientific pursuit grounded in the scientific method in some way. For example, we have no way of really confirming whether our models are correct about estimating the distance to stars. Still, we think it’s true enough because it works locally. Science is and always has been a fuzzy endeavor of truth seeking and only local models of simple interactions have a “nonexistent” amount of error.
I don't think it does once accounting fully for relativity (and assuming perfect sensors, so the idea of information being too redshifted to be detected isn't a factor until that information no longer exists within the universe, and immortality of participants, and near light speed travel).
Say Alice and Bob can communicate X years apart at near C speed. At any time, Alice can jump on a spaceship and reach Bob in ~X years. Therefore, anything in Bob's observable universe at that time counts as also being in Alice's observable universe.
If the distance is so far apart that one day space will expand too fast, then there is a moment where Alice stops being able to travel to see Bob ever again. At that moment, Bob's observable universe is now distinct for Alice's, but they also can no longer communicate.
Bob could get on a spaceship going near C away from Alice and access information that is outside of Alice's observable universe, but Bob only crosses that barrier when he moves far enough away from Alice that he exits her observable universe. (Technically I think you get some sort of infinite sphegettification of Bob leaving as he crosses Alice's observable universe's event horizon, where he never fully leaves and sends back photos that become more and more red shifted until their wavelength equals the diameter of the observable universe.)
So Bob has an observable universe that is different from Alice's, but it is predicated on him exiting from Alice's observable universe to access it. Either he accesses it and stops existing to Alice, or he doesn't access it and eventually it falls out of what he can possible access. The only part he can access and still communicate back to Alice is the part within Alice's universe. It is a bit like a superpower to turn invisible only when no one is looking... sorta kinda...
Given enough time, Alice and Bob either drift together until they clearly share the same observable universe, or eventually drift apart to the point expansion of space shifts them into two separate unrelated observable universes and they stop existing relative to each other.
I’m willing to concede I don’t know enough about the actual math of cosmology and relativity to say. How certain are you in your reasoning? I’m willing to admit either case could be possible and neither is a testable prediction at this time but maybe my first principles reasoning is outright flawed?
There is also the matter of possible verses actual light cones. Assuming we had a ship that could go near C, imagine Alice doing three things. Alice v1 stays home. Alice v2 goes racing off near C to the left. Alice v3 goes racing off near C to the right. Each of these will have different light cones, but Alice v0 who hasn't made a decision could make any one of these decisions and thus all three light cones are in her possible light cones if she chooses to pick each action. Eventually each Alice will be so far away from each other that expansion of space splits their light cones into entirely separate ones from that point on.
>How certain are you in your reasoning?
Not at all. This is just based on my understanding of the very basics. The reason I'm sticking to it like I am is because, if I'm wrong and someone can point out where, it becomes a really good learning opportunity for me. And if I'm not wrong (at least given the layman level of detail of the conversation), the better I can explain my reasoning the better someone else might gain new ideas from it.
The important part is that at any given point on the elastic strip, both sides are getting further away. Everything else is getting further away.
You might think if A-B-C-D are points on the tape and A-B are expanding and C-D are expanding, then B and C must be squished together, but the distance between them is also expanding. You have infinite elastic, but you also have infinite room to stretch it (even along the direction it already occupies). You now have A--B--C--D.
It's tempting to think about that stretch from the point of view of the floor/table beneath the elastic, in which case some parts of the elastic move faster than others as they stretch, but if you always think from a point on the elastic, then the speed of the rest of the elastic depends on how far away it is. Stuff twice as far away moves away twice as fast. Stuff infinitely far away moves away infinitely fast. That's true for every point on the elastic. No bunching up.
But then, what is the universe? One way to think of it is to imagine that every galaxy has at least one intelligent species with their own observable universe. The universe is the sum of all observable universes. The very nature of how to sum them together, what it means to combine multiple sets of thing which include items that don't exist relative to other items in the set, is a question we can't really answer yet. Because of this, even a question like the size of the universe is unknown, and even the question of if more of the universe exists outside of the observable universe isn't simple to answer and gets into the nature of what it means to exist. If someone exists in the universe, but not in the observable universe, it becomes an instance of Russell's Teapot.