5 ms·

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

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
      - Certhas 7y ago
        What's your point? Do you want me to explain the basics of GR and Cosmology, including the vast amounts of evidence we have for everything in HN comments? Do you think you are pointing out subtle errors in reasoning that none of the tens of thousands of physics students since the early twentieth century spotted? You don't have the decency to try to learn the basics but presume to lecture me/expect explanations? Take a course. Show a little humility. Even reading Wikipedia thoroughly would have informed you that it's galaxies, not stars that confirm the expansion of the universe. The expansion (not the acceleration of the expansion) is beyond doubt. Conservation of Energy is not a priory defined for the question at hand because in GR you can not just add the energy at different points in space. Measuring the perturbation to the moon's trajectory from expansion would require knowing all parameters that enter the trajectory to this accuracy, not just the average distance. This is all basic if you want to learn, but you seem to have a different agenda...
- Koshkin 7y ago
  > relative to the rubber This makes me uneasy about your analogy. It’s like saying “relative to space” which is nonsense. Also, why would space limit itself to only expanding outside a physical body?
  - roywiggins 7y ago
    You can't make an electron bigger, so it can't be stretched out. Other point particles are the same. The forces that hold physical stuff together are much more powerful than the gentle expansion of space, so we don't see people and planets dissolving.
    - Koshkin 7y ago
      Indeed you can, if you realize that the place occupied by the "point particle" is described by its wavefunction.
      - roywiggins 7y ago
        I know! But when you poke it, it still acts like a particle and the wavefunction localizes again, right? It doesn't exactly "puff up" like a billiard ball. Electrons don't really get bigger when they are put into states where their position is less localized. But the expansion of space does "stretch" light- or at least, the wavelength gets longer, so it's not not happening.
  - deleted 7y ago
    [deleted]
  - Certhas 7y ago
    I never said it is only expanding outside. Also moving relative to space is not nonsense. Motion is not just relative to other objects in space. This what Newton's bucket thought experiment showed. Edit: the pearl is made from around bound by electric forces, so you can repeat the same argument for inside the pearl. You can repeat it all the way down until you have to admit that the approximations inherent in the concepts useful to describe the world at distances from nano meters to mega parsec eventually break down.
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.

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
    - Certhas 7y ago
      What's your point? Do you want me to explain the basics of GR and Cosmology, including the vast amounts of evidence we have for everything in HN comments? Do you think you are pointing out subtle errors in reasoning that none of the tens of thousands of physics students since the early twentieth century spotted? You don't have the decency to try to learn the basics but presume to lecture me/expect explanations? Take a course. Show a little humility. Even reading Wikipedia thoroughly would have informed you that it's galaxies, not stars that confirm the expansion of the universe. The expansion (not the acceleration of the expansion) is beyond doubt. Conservation of Energy is not a priory defined for the question at hand because in GR you can not just add the energy at different points in space. Measuring the perturbation to the moon's trajectory from expansion would require knowing all parameters that enter the trajectory to this accuracy, not just the average distance. This is all basic if you want to learn, but you seem to have a different agenda...
Koshkin 7y ago
> relative to the rubber This makes me uneasy about your analogy. It’s like saying “relative to space” which is nonsense. Also, why would space limit itself to only expanding outside a physical body?
- roywiggins 7y ago
  You can't make an electron bigger, so it can't be stretched out. Other point particles are the same. The forces that hold physical stuff together are much more powerful than the gentle expansion of space, so we don't see people and planets dissolving.
  - Koshkin 7y ago
    Indeed you can, if you realize that the place occupied by the "point particle" is described by its wavefunction.
    - roywiggins 7y ago
      I know! But when you poke it, it still acts like a particle and the wavefunction localizes again, right? It doesn't exactly "puff up" like a billiard ball. Electrons don't really get bigger when they are put into states where their position is less localized. But the expansion of space does "stretch" light- or at least, the wavelength gets longer, so it's not not happening.
- deleted 7y ago
  [deleted]
- Certhas 7y ago
  I never said it is only expanding outside. Also moving relative to space is not nonsense. Motion is not just relative to other objects in space. This what Newton's bucket thought experiment showed. Edit: the pearl is made from around bound by electric forces, so you can repeat the same argument for inside the pearl. You can repeat it all the way down until you have to admit that the approximations inherent in the concepts useful to describe the world at distances from nano meters to mega parsec eventually break down.

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.