Yes, right now there is a galaxy far far away that is just crossing that threshold. When we look up into the night sky, we will not be able to see it ever again. In the far far future, we will be back to having only one galaxy in the observable universe, and future astronomers will have a hard time telling there was a big bang.
And that is definitely the case. As they get farther away, the get dimmer, but they also get red shifted. That’s why the James Webb telescope will not be operating in visible light, but infrared, to see these further away galaxies that are shifted out of the spectrum that hubble can see.
This goes back to my point earlier though: in theory as time progresses we will be able to observe more and more galaxies as the observable Universe gets bigger faster than the the volume of space it contains. However practically it becomes harder to view galaxies due to red shift.
This Penrose diagram of the evolution of the Universe is probably the easiest way to see what is going on (note the distance measure is comoving which means that expansion is suppressed and the time measure is conformal which means its been re-scaled to map onto flat spacetime)
The “speed limit” only applies locally, to movement through space. The recession velocity of distant galaxies attributable to the expansion of space can and does exceed c.
I can understand if the so called Big Bang started from an infinite region that the current universe could be infinite with no spatial bound. I have a hard time with a finite Big Bang inflating to an infinite size.
I agree, if infinity means anything, it’s hard to imagine how it could arise from anything that was ever finite in the past. When I described the ants-on-a-stretchy-band model as infinite, it’s not intended to be an argument that the universe is infinite, more that I think maybe it helps dispel certain misconceptions about the expansion of space if you can sort-of get your head around how things would work if it were infinite. But part of a correct conception of the expansion of space under the Big Bang model is that it should only work (looking back in time) up until just before we reach the start of the universe. The starting point itself is undefined in the model.
The Big Bang is often portrayed as an “event”, some well-defined thing that actually happened. That’s misleading. In fact, what we have is a Big Bang model, which describes everything after the starting point pretty well, but not the starting point itself. Looking backwards in time, the “moment of creation” of the universe is the state that the contraction of space tends towards, the limit at which the model breaks. So the Big Bang is often popularly taken to mean the moment of creation; but, ironically, the Big Bang model describes everything except the moment of creation.
I’m sure you’re aware that there are other models (inflation) that are hypothesized to govern what happens when (looking backwards) the simple Big Bang model breaks down near the beginning of the universe.
Concur with yourself and Riemann on this - the idea of something growing larger until it is infinite, does not make any sense in the physical world.
There is never a moment where something that physically exists is so big, that adding just a little more to it (or even doubling it, or whatever) will render it infinite in size - the concept there is just ‘bigger’, which is easy for the man in the street to casually conflate with infinity, but not by any means the same thing.
The most grasp-able concept of a universe with no edges, for me, is the ‘finite but boundless’ one - where the curvature of the universe itself means there are no edges, but it is possible to travel in (what appears to be) a straight line indefinitely - like the surface of the Earth, or the ‘universe’ inside the old-school Asteroids arcade game (which is topologically the surface of a toroid - things that go off the top of the screen appear at the bottom, etc).
The trouble with all of those analogies is that in likening the finite-but-boundless universe to a finite-but-boundless entity inside our own experience, they open up the question “yes, but what’s outside of that?” - and accepting the answer “there is no outside” (or maybe “there is no valid answer to the question ‘what is outside?’”), is hard.
There’s a simple, traditional answer to OP’s question which nobody has given yet.
Imagine a spherical balloon. It has no edges; when you inflate it it gets bigger. It is a 2-D topology called the 2-sphere; any creatures living on the balloon would be in ‘Flatland’, not even knowing their world was curved if it was very big. The interior of the spherical balloon is called a 3-ball. But we needn’t posit a 3rd dimension to cope with the 2-sphere: it is complete in itself.
If the universe is finite, it is a 3-sphere, the 3-D analog of the 2-sphere. This may be hard to visualize but just think of the inflating balloon and imagine there’s another dimension! (The surface of a 4-D ball is a 3-sphere, but this mayn’t help visualization and, anyway, as with the 2-sphere that extra dimension is unneeded.)
Yes, it’s an oft-used analogy. In my experience, it’s not usually helpful for people trying to get to grips with the expansion of space, because in order to “get” what corresponds to what in the analogy, you almost need to understand what’s going on in the first place. If you think the universe is an expanding balloon with edges, it’s a little difficult to be given an analogy which consists of an expanding balloon with edges as a tool to understand that the universe is not an expanding balloon with edges!
Why must it be a simple sphere? E.g., this article describes a candidate topology which is the quotient of a 3-sphere by a group of order 120. Now, surveys of the sky and of the cosmic density parameter are supposed to put constraints on such possibilities. Perhaps a cosmologist could explain some of the theoretical issues involved (quantum gravity?) that determine the shape of the universe when it is formed, and as it evolves?
I was thinking about pointing out the same analogy, but didn’t for much the same reason.
However, thinking about it, there is a useful analogy when talking about edges. Compare a flat earther with a round earther. The flat earther either believes that the earth is infinite in extent but flat, or flat with an edge. Most flat earthers don’t suggest the earth is infinite, but has an edge. Comparing their ideas with a spherical Earth helps close the gap with the ideas of the possible natures of the universe. In particular it is possible to see how an Earth with no edge can be possible - either because it is infinite, or because it has some difficult to grasp additional dimensionality (I mean really, the people on the other side would fall off. Idiots!!)
One can describe the question of the expanding universe in 2D, the ideas don’t need 3D. So adding a third dimension to an expanding plane, and thinking about what that might mean is possibly a useful way of building up. We could make a mobius universe as well. All sorts of fun.
Humans are pretty used to their daily lives sitting on a world that is for all intents flat, so the built analogy should not need much work.
I think you may be right - and I think another one like that is the ‘rubber sheet’ analogy of gravity by spacetime warping - it’s a fun visualisation, but it doesn’t explain anything (mainly because it still requires gravity to make things roll into the dents in the rubber sheet)
Yes, I think we ran into that exact problem in a recent thread. I think I was the one who brought up the analogy, in response to a question about why, if gravity is really “curved space”, the path of an object depends on its speed. The analogy tells you what happens when a moving object passes a massive body, but it doesn’t really give any sound intuition about why.
If the universe is neither infinite nor shapeless, then (whatever its shape is) there must be at least an edge or a face that can be described as standing rather at its periphery. I guess the OP refers to this peripheral zone. I don’t know, but there may be shapes that allow for the periphery to manifest everywhere. But the universe is considered infinite, which makes the discussion about its center or periphery even more difficult, I think.
Where is the periphery of planet Earth? You could walk all over the world for billions of years and never reach an edge. Yet the Earth is finite. If you keep walking in a straight line eventually you find yourself back where you started.
So we can imagine a Universe where if you took off in your space rocket nearly as fast as light in a straight line, you would eventually after billions of years find yourself back where you started. You could travel trillions of years and never find an edge.
But you probably couldn’t so in our universe, because the universe is expanding faster than things can actually travel. By the time you would have arrived back where you started, where you started is no longer there. Imagine trying to walk around the world if the Earth were expanding like a balloon, if the expansion is faster than your walking speed you will never make it back to your starting point no matter how many millions of years you walk.
Because the speed of light is the fastest speed there is, and because of the Hubble constant, you could never circumnavigate our Universe, even if our Universe turns out to be finite.
And it could turn out that the Universe is infinite in which case there’s no boundary either.
I don’t know if there’s any way even in theory to distinguish between a universe that’s really really really big and the really distant portions are causally separated from us and an infinite universe where the distant portions are causally separated from us. Seems to me it would appear the same either way.
Perhaps you are considering only “shapes” that we have intuitive experience with in normal life. The universe can certainly be finite but without edges, and that’s relatively easy to illustrate in a two-dimensional example, the surface of a sphere. If you’re a 2-D being living on the surface of a sphere, there are no edges to your universe, but it’s finite in extent.
A 3D universe could, in principle, be something with an edge, of course - but I’m not aware of any models that take that idea seriously.
And see septimus and DPRK comments above (and I now see I was multiply ninja-ed).
