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So, imagine the box is filled with hydrogen gas. The space inside the box gets bigger. But protons don't, they're little packets of quarks that hold each other

So, imagine the box is filled with hydrogen gas. The space inside the box gets bigger. But protons don't, they're little packets of quarks that hold each other very powerfully. You'd have to expand space very, very, very fast to rip those quarks apart from each other, and once you did, you would have a bunch of bare quarks, not hydrogen. The quarks aren't getting bigger, since they appear to be indivisible and essentially pointlike, and even if they were made of something smaller, those things will hold tight to each other, too. A nice gentle expansion will just expand the space in between the protons. This scales up to forces in between atoms, molecules, and bits of matter. It's only at the very large scales (larger than galaxies) that all the forces that are holding the stuff together starts getting overwhelmed by the expansion of space and it starts to pull things apart. The expansion is so gentle that even relatively weak gravity can hold galaxy clusters together even as space grows. In other words: space gets bigger, but matter doesn't. It more or less stays the same size.

galaxyLogic 7y ago
Well so in other words "space" is NOT expanding. It's particles of matter that are moving away from each other. Right? So fine, but then why would we say that space is expanding, that would seem to imply we know there is some edge of space which is moving away from us, but what evidence do we have that space has such an edge? Saying that "space is expanding" seems to hold the assumption that space has a (limited) width and height.
- cygx 7y ago
  Specific volume is increasing. If space was finite (but still without boundary - think eg surface of a sphere), so would total volume.
- Certhas 7y ago
  No space really is expanding. Think of it this way: You have a rubber band and you pull it apart, if you place two beads on the rubber band the distance between the beads will increase (even though they don't move relative to the rubber). But if you connect the two beads by a strong spring, then their distance will stay the same, despite space expanding. The force of the spring makes them move (relative to the rubber), counteracting the expansion of space between them.
  - shpeak 7y ago
    Such counteract action will make lot of side effects, which can be measured. For example, trajectories of separating or oscillating bodies will be slightly curved. Do you have any experimental support for this theory?
    - DavidSJ 7y ago
      This isn’t Certhas’s personal theory. It’s part of standard cosmology, called the metric expansion of space. That term should get you pointed in the right direction if you want to learn more about the evidence for it.
      - shpeak 7y ago
        It's too complex to explain, I will not understand it. Right?
        urbanjunkie 7y ago
        Literally Google "metric expansion of space" Dickhead
        Certhas 7y ago
        It's not, but it might take more work than reading a few HN comments.
    - Certhas 7y ago
      The effect of the expansion of the universe on bound systems is far to small to be observed. Two objects 1 meter apart would move at a speed of 10^-17 m/s. Utterly unobservable. But we know space is expanding by looking at things that are super far away from us and that are not bound.
      - shpeak 7y ago
        We can directly measure distance to stars which are near us, using trigonometry and waiting half of year for Earth to make half of distance around Sun. Distance to thousands of stars is already measured with ±5% precision.[0] Yes, it's true that at least some stars, which are near to us, are moving away from us. However, it can be explained in number of ways, without inventing of Bing Bang or other epic events. We cannot measure distance to super far away stars directly. Period. 10^-17 m/s is very high speed. Distance to Moon is 0.385E15 mm, and it can be measured at sub-millimeter accuracy, so this effect can be spotted. However, it will violate Conservation of Energy principle: no force applied, but job is done. 0: https://www.youtube.com/watch?v=i6GhsYrU5WQ https://www.youtube.com/watch?v=i6GhsYrU5WQ
- jjgreen 7y ago
  Mandatory: https://xkcd.com/895/ https://xkcd.com/895/
- ses1984 7y ago
  If things were moving away from each other, there would have to be something that set them in motion away from each other. What made all far apart galaxies appear to move away from each other? Expanding space doesn't necessarily imply edge of space. What if your universe isn't flat, what if it's a torus? It could hypothetically have a finite volume but no edge. There's also no reason an infinite universe couldn't have local expansion everywhere.
- lmilcin 7y ago
  Space is expanding but gravitational attraction and other forces are many orders of magnitude stronger on short distance. While the space within the box expands the forces that attract molecules of the box are such that it will not change its shape until the expansion gets much, much stronger. Think of the changing pressure. If the box is tight and rigid and the pressure outside the box increases, the box will roughly stay the box until the pressure overcomes the the forces between the molecules that build the walls. Same with space. If you could measure, the increase in the volume of space would put a constant pressure on every molecule to try to rip it from other molecules. The fact that the rate of change of space and the distances between molecules are so small means that it is not evident that it will ever be possible to devise an experiment to show this on very short distances. We can only see this on very long timescales and between objects that are very far from each other because all other forces on those scales have diminished so much that only then the increase of volume of space can dominate.
- roywiggins 7y ago
  Every cosmologist since Hubble who has held that space is expanding, no, they don't assume the universe is finite. The opposite, in fact.
dieppe 7y ago
Thanks for this reply. I guess I now understand what people mean by "space is expanding". If I understand correctly, space is what supports matter? Like a table supports dishes for instance (well, except that space is also "inside" particules) — and so if the table grows, the dishes don't. I never actually even thought of what "space" means before. I thought that when we talked about space, matter was included in that (in the sense that quarks or whatever is the smallest — known? — particle take some "space" ; they're not points that can't be measured — or are they? ^^'). Anyway, it sparkled back an old interest of mine for astrophysics (well, I read one book from Hubert Reeves when I was a kid — and loved it). Thanks again :) (oh, as an added bonus: I guess we have no idea _why_ space is expanding, right? Because, I mean, it's a bit mind boggling...) Edit: to be a bit clearer; I thought space was just void that "always" existed and extended indefinitely. Like, there was nothing except a small point with all the energy in the universe ; "everywhere" else a bit nothingness. Then, bim, the big bang, and matter was just booming away and making stars and planets and then galaxies and all that was just floating in this void/nothingness that was always there, never changing, still extending indefinitely. And that was why we were saying the universe was expanding — because galaxies were still speeding away from each others due to the big bang ; like an explosion with no friction perpetuing indefinitely. Now I guess I'm really confused about what "space" is...
- roywiggins 7y ago
  Quarks and electrons act like they're actually points. Nobody can be totally sure, but the standard math says they are, and nobody has managed to smash them apart into something smaller or measure them as anything other than "smaller than anything we can measure." They might be small loops of energy instead (this is part of string theory), but if they are, they'd be so small it would be beyond known science to prove it. It's easier to prove something has a size (by measuring it) than to prove the opposite. Every instrument has some margin of error, so you'd never be able to exclude there being some size. The weird thing about the Big Bang is that it's really all of space that banged, and to a first approximation, space and time as we know it didn't exist before a singularity at the beginning of the expansion. It's very possible that's wrong (there may have been a "parent universe" or a "big bounce") but to a first approximation it seems to be that there may not have been a "before" the Big Bang. https://en.m.wikipedia.org/wiki/Expansion_of_the_universe https://en.m.wikipedia.org/wiki/Expansion_of_the_universe
naruhodo 7y ago
Space is filled with virtual particles that are constantly popping into and out of existence. As space expands, do these particles keep appearing in a given volume at the same rate? Or is the density of virtual particles declining as the volume of space increases?

galaxyLogic 7y ago

Well so in other words "space" is NOT expanding. It's particles of matter that are moving away from each other. Right? So fine, but then why would we say that space is expanding, that would seem to imply we know there is some edge of space which is moving away from us, but what evidence do we have that space has such an edge? Saying that "space is expanding" seems to hold the assumption that space has a (limited) width and height.

cygx 7y ago
Specific volume is increasing. If space was finite (but still without boundary - think eg surface of a sphere), so would total volume.
Certhas 7y ago
No space really is expanding. Think of it this way: You have a rubber band and you pull it apart, if you place two beads on the rubber band the distance between the beads will increase (even though they don't move relative to the rubber). But if you connect the two beads by a strong spring, then their distance will stay the same, despite space expanding. The force of the spring makes them move (relative to the rubber), counteracting the expansion of space between them.
- shpeak 7y ago
  Such counteract action will make lot of side effects, which can be measured. For example, trajectories of separating or oscillating bodies will be slightly curved. Do you have any experimental support for this theory?
  - DavidSJ 7y ago
    This isn’t Certhas’s personal theory. It’s part of standard cosmology, called the metric expansion of space. That term should get you pointed in the right direction if you want to learn more about the evidence for it.
    - shpeak 7y ago
      It's too complex to explain, I will not understand it. Right?
      - urbanjunkie 7y ago
        Literally Google "metric expansion of space" Dickhead
      - Certhas 7y ago
        It's not, but it might take more work than reading a few HN comments.
  - Certhas 7y ago
    The effect of the expansion of the universe on bound systems is far to small to be observed. Two objects 1 meter apart would move at a speed of 10^-17 m/s. Utterly unobservable. But we know space is expanding by looking at things that are super far away from us and that are not bound.
    - shpeak 7y ago
      We can directly measure distance to stars which are near us, using trigonometry and waiting half of year for Earth to make half of distance around Sun. Distance to thousands of stars is already measured with ±5% precision.[0] Yes, it's true that at least some stars, which are near to us, are moving away from us. However, it can be explained in number of ways, without inventing of Bing Bang or other epic events. We cannot measure distance to super far away stars directly. Period. 10^-17 m/s is very high speed. Distance to Moon is 0.385E15 mm, and it can be measured at sub-millimeter accuracy, so this effect can be spotted. However, it will violate Conservation of Energy principle: no force applied, but job is done. 0: https://www.youtube.com/watch?v=i6GhsYrU5WQ https://www.youtube.com/watch?v=i6GhsYrU5WQ
jjgreen 7y ago
Mandatory: https://xkcd.com/895/ https://xkcd.com/895/
ses1984 7y ago
If things were moving away from each other, there would have to be something that set them in motion away from each other. What made all far apart galaxies appear to move away from each other? Expanding space doesn't necessarily imply edge of space. What if your universe isn't flat, what if it's a torus? It could hypothetically have a finite volume but no edge. There's also no reason an infinite universe couldn't have local expansion everywhere.
lmilcin 7y ago
Space is expanding but gravitational attraction and other forces are many orders of magnitude stronger on short distance. While the space within the box expands the forces that attract molecules of the box are such that it will not change its shape until the expansion gets much, much stronger. Think of the changing pressure. If the box is tight and rigid and the pressure outside the box increases, the box will roughly stay the box until the pressure overcomes the the forces between the molecules that build the walls. Same with space. If you could measure, the increase in the volume of space would put a constant pressure on every molecule to try to rip it from other molecules. The fact that the rate of change of space and the distances between molecules are so small means that it is not evident that it will ever be possible to devise an experiment to show this on very short distances. We can only see this on very long timescales and between objects that are very far from each other because all other forces on those scales have diminished so much that only then the increase of volume of space can dominate.
roywiggins 7y ago
Every cosmologist since Hubble who has held that space is expanding, no, they don't assume the universe is finite. The opposite, in fact.

dieppe 7y ago

Thanks for this reply. I guess I now understand what people mean by "space is expanding". If I understand correctly, space is what supports matter? Like a table supports dishes for instance (well, except that space is also "inside" particules) — and so if the table grows, the dishes don't. I never actually even thought of what "space" means before. I thought that when we talked about space, matter was included in that (in the sense that quarks or whatever is the smallest — known? — particle take some "space" ; they're not points that can't be measured — or are they? ^^'). Anyway, it sparkled back an old interest of mine for astrophysics (well, I read one book from Hubert Reeves when I was a kid — and loved it). Thanks again :) (oh, as an added bonus: I guess we have no idea _why_ space is expanding, right? Because, I mean, it's a bit mind boggling...) Edit: to be a bit clearer; I thought space was just void that "always" existed and extended indefinitely. Like, there was nothing except a small point with all the energy in the universe ; "everywhere" else a bit nothingness. Then, bim, the big bang, and matter was just booming away and making stars and planets and then galaxies and all that was just floating in this void/nothingness that was always there, never changing, still extending indefinitely. And that was why we were saying the universe was expanding — because galaxies were still speeding away from each others due to the big bang ; like an explosion with no friction perpetuing indefinitely. Now I guess I'm really confused about what "space" is...

roywiggins 7y ago
Quarks and electrons act like they're actually points. Nobody can be totally sure, but the standard math says they are, and nobody has managed to smash them apart into something smaller or measure them as anything other than "smaller than anything we can measure." They might be small loops of energy instead (this is part of string theory), but if they are, they'd be so small it would be beyond known science to prove it. It's easier to prove something has a size (by measuring it) than to prove the opposite. Every instrument has some margin of error, so you'd never be able to exclude there being some size. The weird thing about the Big Bang is that it's really all of space that banged, and to a first approximation, space and time as we know it didn't exist before a singularity at the beginning of the expansion. It's very possible that's wrong (there may have been a "parent universe" or a "big bounce") but to a first approximation it seems to be that there may not have been a "before" the Big Bang. https://en.m.wikipedia.org/wiki/Expansion_of_the_universe https://en.m.wikipedia.org/wiki/Expansion_of_the_universe

naruhodo 7y ago

Space is filled with virtual particles that are constantly popping into and out of existence. As space expands, do these particles keep appearing in a given volume at the same rate? Or is the density of virtual particles declining as the volume of space increases?