The analogy that's usually used to help people wrap their heads around this is to think of the 3D universe as the curved 2D surface a balloon. As the balloon expands, every point becomes farther away than every other point. A bug walking on the surface of the balloon could go anywhere on the balloon and never hit an edge. That makes sense.
But also in this analogy if the bug travels long enough in what to it seems a straight line, it will wind up back where it started. Does this hold true for the universe as well?
Let's say the universe stopped expanding tomorrow, and didn't shrink. If someone got in a spaceship and traveled in (what seems to them) a straight line for long enough, would they wind up back where they started?
If not, how can the universe have no edge? How can a you always see another galaxy straight ahead, in a universe with a finite number of galaxies? The universe doesn't just create new galaxies. You have to start recycling galaxies at some point.
I have recently wondered about this. When we talk about the big bang, the mostly-uniform CMB, accelerated expansion, etc. aren't we always axiomatically presuming the correctness of the cosmological principle (which, in a hand-wavy way says the universe plays the same game everywhere). I would think all we can say confidently is that the principle is working out in our locale.
Imagine a hotel with infinite rooms and there is a guest in each room. A bus arrives carrying infinite guests and you must assign each of these guests to a room. One solution is to ask each guest to move two rooms over and assign the new guests to the odd number rooms.
I am not a physicist, but i do visualize the universe expansion as such an infinity paradox.
> But also in this analogy if the bug travels long enough in what to it seems a straight line, it will wind up back where it started. Does this hold true for the universe as well?
That’s indeed a very important question. The shape of the universe is a subject of study in cosmology.
The property you mention relates to its curvature (hyperbolic, flat or spheric). Experimental data shows that the observable universe is flat which means the universe is either flat or sufficiently big for the observable universe to appear flat as a local area.
> If not, how can the universe have no edge?
What we see of the universe indeed as an edge. Light takes time to travel. It means that the farther you look the earlier you look and we can’t look farther than the beginning of time.
"Experimental data shows that the observable universe is flat", agreed. However the research put a maximum bound on it's non-flatness. So it's low, but it's impossible to prove it's perfectly flat.
Is it true that if the universe is flat, then either it has to have an edge (meaning a place where you see galaxies in one direction and nothing in the other), or it has to have an infinite number of galaxies?
The thing I don't get with that analogy is the ballon IS expanding into something - eg if you put the balloon in a cup, it won't expand (unless you have REALLY strong lungs :-) ).
Also, curious where the extra energy comes from to keep the 'vacuum energy' (zero point energy?) from going down - IIRC, its remained constant even though the universe is expanding
This article might help: "Energy Is Not Conserved" by Sean Carroll [1]. Here are a couple of paragraphs to give you the gist of it.
> But many people have just this reaction. It’s clear that cosmologists have not done a very good job of spreading the word about something that’s been well-understood since at least the 1920’s: energy is not conserved in general relativity. (With caveats to be explained below.)
> The point is pretty simple: back when you thought energy was conserved, there was a reason why you thought that, namely time-translation invariance. A fancy way of saying “the background on which particles and forces evolve, as well as the dynamical rules governing their motions, are fixed, not changing with time.” But in general relativity that’s simply no longer true. Einstein tells us that space and time are dynamical, and in particular that they can evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved.
> Also, curious where the extra energy comes from to keep the 'vacuum energy' (zero point energy?) from going down - IIRC, its remained constant even though the universe is expanding
Energy is ill-defined in a global sense: it's the conserved quantity associated with time-translation symmetry, but there's no single choice of "time translation" that can be extended to all of spacetime.
> But also in this analogy if the bug travels long enough in what to it seems a straight line, it will wind up back where it started. Does this hold true for the universe as well?
It's possible, but the experimental evidence have indicated that it does not - the 2D analogy that more closely matches the measurements is "infinite flat sheet", not "curved surface of a sphere."
Infinite means infinite number of galaxies, right? I watch How the Universe Works religiously. Most of the astrophysicists on that show don't seem to think the universe is infinite.
At this point even if there was an edge you couldn't get to it because points move apart faster than the speed of light and we can't go faster. The points themselves aren't actually moving (rather space is being created in between) so there's no speed limit violation.
Yes but even in these analogy the balloon is still expanding into something. There is space around the balloon and when the balloon expands, there is less of that surrounding space.
does the universe have a finite number of galaxies? We see the 'observable' universe. The universe is even larger than what we can see, but that light can never reach us.
Is there an "edge" in the sense that, if you lived there and looked in your telescope in one direction, you see galaxies, but if you turned it around, you'd see nothing?
There is no edge, no. In some theories Space warps around on itself, but it would be more like a möbius strip. But the edge of our observable universe is just when the Big Bang happened, and before that we can’t detect any light, so it looks like an edge. If you were there the universe would looks much the same as it does from our own vantage as it’s self similar.
Why a Möbius strip? That would imply that if I travelled around the universe and back to Earth, everything would be like it had been reflected in a mirror.
Yeah this is what I don't understand. In order for this not to be true, space would have to be curved back in on itself like the surface of a balloon. Meaning you could eventually wind up back where you started. But do we know for sure this is how the 3D space in our universe works?
The 'big bang' is both a point in time, and in space. The boundaries of the universe are the light cone of the big bang.
It is conceivable that our 'universe' is just one regional phenomena within a much larger universe. Multiple such 'universes' in the same physical space could seem to be isolated until they expand to the point of overlap and begin to collide.
As shown in that figure, a level 1 multiverse is simply whatever other areas exist beyond our universes light cone. There could very well be 'stuff' out there that we are expanding into!
> The boundaries of the universe are the light cone of the big bang.
That "boundary" is not a spacial boundary, but simply the farthest distance we can "see".
The universe is expanding fast enough that any given space-time point can only "see" a finite region of the greater universe.
But it is all connected. We can see objects far away, whose citizens (if they exist) can see beyond our visible boundary, and so on.
And our area is not expanding "into" other areas. Think of the expansion as equally inflating every area, including our own. Pushing distant areas to be more distant. All in a smooth way across the region of the universe we can see and beyond.
Interesting. Would that larger universe also be expanding and of so, what would it be expanding into? Maybe the larger universe exists in a larger-larger universe?
Maybe the universes that are expanding inside a larger universe would not overlap but repell when they encounter each other, i.e. push each other out of the way instead changing the shape of a universe.
The article says it’s NOT a point in space btw and that it happened everywhere simultaneously. This is an interesting idea I wasn’t aware if and trying to wrap my head around it now :)
A great visualization, is to imagine that space remains unchanged, but everything in it is shrinking at the same rate.
The shrinking people don't perceive they are shrinking, or that anything else is shrinking. But the distances between things not held together by significant forces (like chemical or gravitational bonds) appear to be growing and growing.
And space isn't expanding >into< anything. It is just relative changes between bond forces and space, creating the experience of more space. There is no impact of this expansion "outside" the universe (that we know of, of course).
I would add the connection to GR. In a Newtonian world, there would be nowhere for matter to expand into, and our intuition would be correct. But in General Relativity, space itself can grow or shrink like a fluid.
Universe is expanding in the sense that the "amount" of space is growing: there was so much space between those two stars and a year later there is more of it. Where does this space come from? And why is it only space that's expanding, why not time?
> When we say we live in an expanding universe, we’re saying that the distances between galaxies, on average, grows with time. And that’s it.
How could this be due to the expansion of space, which should expand our galaxy, and my foot-long ruler, to the same extent that it expands the empty space between our galaxy and the next?
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But also in this analogy if the bug travels long enough in what to it seems a straight line, it will wind up back where it started. Does this hold true for the universe as well?
Let's say the universe stopped expanding tomorrow, and didn't shrink. If someone got in a spaceship and traveled in (what seems to them) a straight line for long enough, would they wind up back where they started?
If not, how can the universe have no edge? How can a you always see another galaxy straight ahead, in a universe with a finite number of galaxies? The universe doesn't just create new galaxies. You have to start recycling galaxies at some point.
As as I know, we don’t know. As I understand it we can see ~13.9 billion light years away but not further, and so we don’t know what’s past it.
I’d love if an expert chimed in.
This still requires the balloon to expand into 3D space.
It's an analogy that works in our experience that we can visualise.
There is no reason to suppose that the universe requires something "outside" it to expand "into". It could just... get bigger.
I am not a physicist, but i do visualize the universe expansion as such an infinity paradox.
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That’s indeed a very important question. The shape of the universe is a subject of study in cosmology.
The property you mention relates to its curvature (hyperbolic, flat or spheric). Experimental data shows that the observable universe is flat which means the universe is either flat or sufficiently big for the observable universe to appear flat as a local area.
> If not, how can the universe have no edge?
What we see of the universe indeed as an edge. Light takes time to travel. It means that the farther you look the earlier you look and we can’t look farther than the beginning of time.
Also, curious where the extra energy comes from to keep the 'vacuum energy' (zero point energy?) from going down - IIRC, its remained constant even though the universe is expanding
> But many people have just this reaction. It’s clear that cosmologists have not done a very good job of spreading the word about something that’s been well-understood since at least the 1920’s: energy is not conserved in general relativity. (With caveats to be explained below.)
> The point is pretty simple: back when you thought energy was conserved, there was a reason why you thought that, namely time-translation invariance. A fancy way of saying “the background on which particles and forces evolve, as well as the dynamical rules governing their motions, are fixed, not changing with time.” But in general relativity that’s simply no longer true. Einstein tells us that space and time are dynamical, and in particular that they can evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved.
[1] https://www.preposterousuniverse.com/blog/2010/02/22/energy-...
Energy is ill-defined in a global sense: it's the conserved quantity associated with time-translation symmetry, but there's no single choice of "time translation" that can be extended to all of spacetime.
That, and stretching a surface intuitively imparts momentum on things on top, whereas it's not the case here.
It's possible, but the experimental evidence have indicated that it does not - the 2D analogy that more closely matches the measurements is "infinite flat sheet", not "curved surface of a sphere."
It is conceivable that our 'universe' is just one regional phenomena within a much larger universe. Multiple such 'universes' in the same physical space could seem to be isolated until they expand to the point of overlap and begin to collide.
This image from Max Tegmark illustrates this idea well: https://space.mit.edu/home/tegmark/multiverse.jpg
As shown in that figure, a level 1 multiverse is simply whatever other areas exist beyond our universes light cone. There could very well be 'stuff' out there that we are expanding into!
That "boundary" is not a spacial boundary, but simply the farthest distance we can "see".
The universe is expanding fast enough that any given space-time point can only "see" a finite region of the greater universe.
But it is all connected. We can see objects far away, whose citizens (if they exist) can see beyond our visible boundary, and so on.
And our area is not expanding "into" other areas. Think of the expansion as equally inflating every area, including our own. Pushing distant areas to be more distant. All in a smooth way across the region of the universe we can see and beyond.
Maybe the universes that are expanding inside a larger universe would not overlap but repell when they encounter each other, i.e. push each other out of the way instead changing the shape of a universe.
The shrinking people don't perceive they are shrinking, or that anything else is shrinking. But the distances between things not held together by significant forces (like chemical or gravitational bonds) appear to be growing and growing.
And space isn't expanding >into< anything. It is just relative changes between bond forces and space, creating the experience of more space. There is no impact of this expansion "outside" the universe (that we know of, of course).
This seems to come from a Rust programmer.
Every C programmer knows that the universe expands into void and the size of void is machine and inplementation dependent.
There is no "into" without the concept of "space".
The universe is space (and time).
Thus it's not expanding into anything as there is no space outside of the universe.
QED.
Every second we have another second* of time in most of the universe.
* May be less time near large amounts of mass.
How could this be due to the expansion of space, which should expand our galaxy, and my foot-long ruler, to the same extent that it expands the empty space between our galaxy and the next?