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The question is natural, but I think there is a bit more that we already 'know' (well, we concluded with some confidence) to answer this question. (a) Just bec

The question is natural, but I think there is a bit more that we already 'know' (well, we concluded with some confidence) to answer this question. (a) Just because you might be able to move in one direction forever, does not mean the space you're in is infinite. You may imagine the universe as the skin, the surface of a balloon that is expanding: the surface is finite, but still you can move in one direction on the surface forever -- you will wrap around at some point (if you're fast enough to overcome the expansion speed). Now imagine this as a 3D surface of space on a higher dimension balloon for the universe. This is quite a plausible way of imagining this intuitively (as long as you don't ask questions about the balloon space), particularly because it also makes futile the question whether we are close to the edge or close to the center, which bugged me for a while -- we are somewhere on the surface of that imagined balloon, and there might not be a center or an edge, despite being finite. (b) The universe is expanding. In fact, it has expanded already so much that it is far larger than its age, so to say: it's 13.7Gy old, but 45Gly (IIRC) across, most likely. The universe is so large, that light that starts at the farthest spots now will never reach the Earth, because the universe expands that light away from us, so even with the best sensors and a very long time to wait, we will never see from Earth light and thus images from what is far away now. We are quite lucky, though: the oldest light that reaches us now is from the beginning of the big bang (photons started their 'normal' way of travel ~400ky after the BB, and that's what we can actually see!), so theoretically, we do see now every part of the universe at some point in the past (well, unless something obscures our view, and obviously in non-optimal resolution). But this view will fade away, and new light from those farthest places will not reach us in the future anymore (a couple of Gy from now) -- it will be getting darker and darker for us. The farthest places are already completely out of reach, we can only see their past and we will never be able to go there -- a bit like the inside of a black hole is out of reach, but, well, with the boundary around us. (c) When moving at the speed of light, you will never ever reach the spot again that you started at, even with a wrap-around in the balloon model, because the expansion is going on, and even speeding up, so the speed of light is just too slow to keep up with the expansion. (d) Practically, it is of course completely impossible to reach speeds close to the speed of light, for various reasons, e.g., energy needed and harmless cosmic rays becoming gamma rays, etc.etc. Different story entirely. It is good that we have stories to imagine this without the practical restrictions. So the whole question is kinda futile. But interesting. The universe is incomprehensibly huge.

patrickthebold 5y ago
Is there evidence for the finite balloon option? I've always kind of struggled with understanding what people mean when they say "the universe is expanding". My simple mental model is that the 'universe' is topologically R^3, simply going on forever like in a math class, but there's a finite amount of matter, and so there's a giant ball where all the stuff is. And some people might call that ball where all the stuff is 'the universe' and _that_ is expanding into the infinite R^3. Of course, if that's the case some matter is on the edge and some in the center. Then there's the option of R^3 and homogeneous with and infinite amount of stars. I _think_ this can be disproven by arguing the night sky would be bright. And there's the balloon bit where the universe is homogeneous with a finite (but growing) volume and a fixed amount of matter getting less and less dense. But, I'm curious if first option has been disproved in favor of the third. (And yes there's the theory of relativity so R^3 is wrong, something something spacetime which I never quite got.)
- beeforpork 5y ago
  Well, 3D is not really 'wrong', there are still 3 space coordinates. And 1 time coordinate, which behaves a little weirdly when moving. But surely, we we can talk about what the 3D space part might look like. > Is there evidence for the finite balloon option? Well, we cannot know into what the universe might be embedded, so this is a visualisation of a 3D space that has no center, no edge, and is finite. Just like the surface of a sphere, a 2D manifold embedded into a 3D manifold. But that sphere shape is really meaningless, because it is an unobservable structural properly from the embedded manifold itself, i.e., that shape property is outside of our universe. For simplicity, we just imagine a hyperballoon surface. For the finite 3D manifold part: I think that people are searching for pattern repetitions in the night sky in different directions to maybe see something. :-) For me, it is the most satisfying option of bringing together 'finite' and 'everything moves away from us' (=expanding universe) and getting rid of 'are we in the center?' and 'how is it like close to the edge?' kind of questions. > Of course, if that's the case some matter is on the edge and some in the center. Yeah, this one bugged me. It cannot really be the case, why would we be in the center? Our existence is weird enough, but right in the center? And what would it be like 'near the edge'? It just makes no sense. It means the visualisation cannot be good. And again, it seems weird to need a 3D space to contain the universe -- why not just 'open' the universe and take up all that space it expands into? No, the big bang created that space, so it does not expand into something of the same properties or dimensions. But we can observe properties: 'finite' is quite certain, because whichever direction we look, the farthest away we see the remnants of 400000y after the big bang when the photons started moving normally. We seem to see the whole thing, in principle. 'everything we see moves away from us' is also certain, because whichever way we look, far away galaxies are red-shifted, i.e., they all move away from us. The two things fit perfectly the 'balloon surface in 3D' mental model: regardless where we are on that balloon surface, we'd observe the two above phenomena. There is no center, there is no edge, and expansion is easy to visualise (blowing up the balloon).

patrickthebold 5y ago

Is there evidence for the finite balloon option? I've always kind of struggled with understanding what people mean when they say "the universe is expanding". My simple mental model is that the 'universe' is topologically R^3, simply going on forever like in a math class, but there's a finite amount of matter, and so there's a giant ball where all the stuff is. And some people might call that ball where all the stuff is 'the universe' and _that_ is expanding into the infinite R^3. Of course, if that's the case some matter is on the edge and some in the center. Then there's the option of R^3 and homogeneous with and infinite amount of stars. I _think_ this can be disproven by arguing the night sky would be bright. And there's the balloon bit where the universe is homogeneous with a finite (but growing) volume and a fixed amount of matter getting less and less dense. But, I'm curious if first option has been disproved in favor of the third. (And yes there's the theory of relativity so R^3 is wrong, something something spacetime which I never quite got.)

beeforpork 5y ago
Well, 3D is not really 'wrong', there are still 3 space coordinates. And 1 time coordinate, which behaves a little weirdly when moving. But surely, we we can talk about what the 3D space part might look like. > Is there evidence for the finite balloon option? Well, we cannot know into what the universe might be embedded, so this is a visualisation of a 3D space that has no center, no edge, and is finite. Just like the surface of a sphere, a 2D manifold embedded into a 3D manifold. But that sphere shape is really meaningless, because it is an unobservable structural properly from the embedded manifold itself, i.e., that shape property is outside of our universe. For simplicity, we just imagine a hyperballoon surface. For the finite 3D manifold part: I think that people are searching for pattern repetitions in the night sky in different directions to maybe see something. :-) For me, it is the most satisfying option of bringing together 'finite' and 'everything moves away from us' (=expanding universe) and getting rid of 'are we in the center?' and 'how is it like close to the edge?' kind of questions. > Of course, if that's the case some matter is on the edge and some in the center. Yeah, this one bugged me. It cannot really be the case, why would we be in the center? Our existence is weird enough, but right in the center? And what would it be like 'near the edge'? It just makes no sense. It means the visualisation cannot be good. And again, it seems weird to need a 3D space to contain the universe -- why not just 'open' the universe and take up all that space it expands into? No, the big bang created that space, so it does not expand into something of the same properties or dimensions. But we can observe properties: 'finite' is quite certain, because whichever direction we look, the farthest away we see the remnants of 400000y after the big bang when the photons started moving normally. We seem to see the whole thing, in principle. 'everything we see moves away from us' is also certain, because whichever way we look, far away galaxies are red-shifted, i.e., they all move away from us. The two things fit perfectly the 'balloon surface in 3D' mental model: regardless where we are on that balloon surface, we'd observe the two above phenomena. There is no center, there is no edge, and expansion is easy to visualise (blowing up the balloon).