5 ms·

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 ex

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.

puffybuf 4y ago
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.
fknorangesite 4y ago
> 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."
- suzzer99 4y ago
  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.
benreesman 4y ago
Disclaimer: I’m not a cosmologist or astronomer just a fan. 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.
- findalex 4y ago
  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.
rhn_mk1 4y ago
I think the balloon analogy is used because people have an easier time imagining that rather than an infinite surface. That, and stretching a surface intuitively imparts momentum on things on top, whereas it's not the case here.
- floxy 4y ago
  Doesn't dark energy add momentum?
  - rhn_mk1 4y ago
    Why would it?
    - floxy 4y ago
      I thought dark energy was the reason the non-gravitational-bound galaxies are accelerating away from each other? If they are accelerating, that would be a change in velocity, which seems like a change in momentum. ?
      - rhn_mk1 4y ago
        The name "acceleration" is just as misleading as "expanding" here, but maybe useful. Our galaxy is non-gravitationally-bound to a lot of other galaxies, and we "accelerate" away from them. Yet we don't really feel that. We also obviously can't accelerate away from all of them, when they are in random directions. That's because the "acceleration" describes the change of distance, without changing position. Without changing position, there's no change of momentum. That "acceleration" is a consequence of the expansion/stretching described in the article.
        floxy 4y ago
        Why is it called dark energy if energy isn't changing?
JumpCrisscross 4y ago
> analogy that's usually used for this is to think of the 3D universe as the curved 2D surface a balloon This still requires the balloon to expand into 3D space.
- deleted 4y ago
  [deleted]
- kneebonian 4y ago
  The idea would be by analogy we're expanding in a higher dimensional space.
- goalieca 4y ago
  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.
- MattPalmer1086 4y ago
  But it does not require there to be higher dimensions for the universe to expand into. 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.
WastingMyTime89 4y ago
> 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.
- suzzer99 4y ago
  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?
  - WastingMyTime89 4y ago
    I’m not entirely sure of that to be honest with you. I seem to remember than counterintuitively you can embed a flat torus in 3D but it goes further than my knowledge. It means it’s not a “small” hypersphere, that’s for sure.
- sliken 4y ago
  "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.
mech422 4y ago
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
- consilient 4y ago
  > 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.
  - mech422 4y ago
    Hmm...This goes beyond what PBS space-time taught me about it :-) My understanding was we have more space (eg space is expanding) but the 'vacuum energy' is staying constant.. If I understand what you're saying, that statement depends on the time frame you measure against ? edit: s/your/you're/
    - consilient 4y ago
      No, it's just not meaningful. The vacuum energy density is (likely) constant. The total energy of the universe is simply not a thing.
      - mech422 4y ago
        Hmm..ok - thank you. I'm gonna have to work on that more, atm I'm having a hard time comprehending how volume can increase without density decreasing. To google! Thanks!
        consilient 4y ago
Eddy_Viscosity2 4y ago
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.
karmakaze 4y ago
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.

puffybuf 4y ago

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.

fknorangesite 4y ago

> 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."

suzzer99 4y ago
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.

benreesman 4y ago

Disclaimer: I’m not a cosmologist or astronomer just a fan. 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.

findalex 4y ago
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.

rhn_mk1 4y ago

I think the balloon analogy is used because people have an easier time imagining that rather than an infinite surface. That, and stretching a surface intuitively imparts momentum on things on top, whereas it's not the case here.

floxy 4y ago
Doesn't dark energy add momentum?
- rhn_mk1 4y ago
  Why would it?
  - floxy 4y ago
    I thought dark energy was the reason the non-gravitational-bound galaxies are accelerating away from each other? If they are accelerating, that would be a change in velocity, which seems like a change in momentum. ?
    - rhn_mk1 4y ago
      The name "acceleration" is just as misleading as "expanding" here, but maybe useful. Our galaxy is non-gravitationally-bound to a lot of other galaxies, and we "accelerate" away from them. Yet we don't really feel that. We also obviously can't accelerate away from all of them, when they are in random directions. That's because the "acceleration" describes the change of distance, without changing position. Without changing position, there's no change of momentum. That "acceleration" is a consequence of the expansion/stretching described in the article.
      - floxy 4y ago
        Why is it called dark energy if energy isn't changing?

JumpCrisscross 4y ago

> analogy that's usually used for this is to think of the 3D universe as the curved 2D surface a balloon This still requires the balloon to expand into 3D space.

deleted 4y ago
[deleted]
kneebonian 4y ago
The idea would be by analogy we're expanding in a higher dimensional space.
goalieca 4y ago
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.
MattPalmer1086 4y ago
But it does not require there to be higher dimensions for the universe to expand into. 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.

WastingMyTime89 4y ago

> 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.

suzzer99 4y ago
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?
- WastingMyTime89 4y ago
  I’m not entirely sure of that to be honest with you. I seem to remember than counterintuitively you can embed a flat torus in 3D but it goes further than my knowledge. It means it’s not a “small” hypersphere, that’s for sure.
sliken 4y ago
"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.

mech422 4y ago

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

consilient 4y ago
> 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.
- mech422 4y ago
  Hmm...This goes beyond what PBS space-time taught me about it :-) My understanding was we have more space (eg space is expanding) but the 'vacuum energy' is staying constant.. If I understand what you're saying, that statement depends on the time frame you measure against ? edit: s/your/you're/
  - consilient 4y ago
    No, it's just not meaningful. The vacuum energy density is (likely) constant. The total energy of the universe is simply not a thing.
    - mech422 4y ago
      Hmm..ok - thank you. I'm gonna have to work on that more, atm I'm having a hard time comprehending how volume can increase without density decreasing. To google! Thanks!
      - consilient 4y ago

Eddy_Viscosity2 4y ago

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.

karmakaze 4y ago

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.