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Sorry for also asking a space 101 question, but since the fabric of space is expanding, does that mean that things are getting bigger as well? Naively, it would

Sorry for also asking a space 101 question, but since the fabric of space is expanding, does that mean that things are getting bigger as well? Naively, it would seem that way. In your balloon example, the dots would get bigger.

ddevault 6y ago
Local factors (gravity, strong & weak nuclear force) easily overcome the expansion of space. So things stick together.
chowells 6y ago
Not in practice. In practice, this provides a very tiny force pulling subatomic particles apart in an atom's nucleus, or pulling atoms away from each other in a molecule, or pulling a celestial body's molecules away from each other. It makes no difference at all, because that force is absolutely dwarfed by the strong nuclear force, electromagnetism, or gravity, respectively. Systems that are coupled together by those forces, up to and including galaxies, are impacted only very minutely by this expansion. For the expansion to have a significant impact on relative distances, the distances involved have to be so extreme that there's no real other interaction between the objects you're measuring from.
- hashmymustache 6y ago
  Does that expansion provide potential energy by creating space between attractive bodies? If so how do you account for conservation of energy?
  - chowells 6y ago
    Yes, it will create some additional potential energy. That doesn't contradict strict conservation of energy - it just adds another energy source that has to be accounted for when balancing everything. It might contradict the aphorism "energy is never created or destroyed", but I'm not sure that was ever science. It's more fun to think about it in terms of thermodynamics. Can that extra energy be used to reverse the second law of thermodynamics? I'm guessing not - it's so diffuse that it's probably impossible to use it to reduce entropy. The math for actually doing those calculations is well beyond what I know, though, so I'll just say that one's no more than a guess.
- mekkkkkk 6y ago
  Great explanation! Thanks.
ben_w 6y ago
Just to add to your question: The usual response of “it only affects space between galaxies not inside galaxies” is deeply unsatisfying, and feels like it must be an oversimplification. What I think is meant, but I absolutely want someone wiser to confirm or refute, is that the rate of expansion is proportional to distance, and therefore so small inside a single galaxy it can be ignored. What I don’t know is: can any orbits still be stable in an expanding universe? (In theory I could simulate the orbits question easily; in practice, last time I tried to do astrophysics maths I made an elementary mistake).
- thaumasiotes 6y ago
  > What I think is meant, but I absolutely want someone wiser to confirm or refute, is that the rate of expansion is proportional to distance, and therefore so small inside a single galaxy it can be ignored. I am not that person. But I feel like this can't work -- if all space is expanding equally, in proportion to its own extent, then we would perceive no expansion at all. If two points start out 1 light year apart, and over time 1 light year expands to be 10% longer than it was before... and also, everything in the universe expands to be 10% larger in all spatial dimensions... then the distance in meters between the two points hasn't changed, because meters grew too.
  - saagarjha 6y ago
    You'd notice a slower speed of light, wouldn't you?
  - johncolanduoni 6y ago
    The problem with that model is that as was previously said, the electromagnetic, nuclear, and gravitational forces easily countermand the expansion on non-inter-galactic scales, to the point of not being measurable on the scale of a meter. Ultimately the molecular structure of your meter-stick is determined by various structure constants that aren’t changing, so the expansion can’t affect it unless it starts ripping bonds apart.
    - thaumasiotes 6y ago
cygx 6y ago
No, things aren't getting bigger. Like other gravitational effects, locally, spatial expansion would manifest as a pseudo-force, but be too weak to overcome the nuclear and electromagnetic forces that keep matter bound. However, Friedmann cosmology is only a large-scale approximation. A more realistic model is swiss-cheese cosmology, which comes with a different metric within the 'holes', and spatial expansion might indeed not act on galactic scales (though I would image it to have an indirect effect as boundary conditions at the edge of the hole have to be matched; I'm no expert on this part of the story, though).
taotau 6y ago
I came up with the analogy of 2 pairs of independent dancers on a large checkerboard floor. Lets say the tiles are 10cm^2 and the two couples start out 10 tiles apart. Every second, the tiles on the floor push away from all their neighboring tiles by one tile width and new tiles appear in their place. The tiles are small enough that the dancers are easily able to adjust their step/grip on eachother to simply maintain their dance routine. However, even tho the dancers can stay together, the couples drift apart at an ever increasing rate. They start 10 tiles apart, after one second the couples will be 20 tiles apart from eachother, 40 tiles apart after 2 seconds, 80 tiles apart after 3 seconds (a new tile is being inserted between each pair of existing tiles). My tiling math is probably slightly off, but I think this illustrates the exponential increase in expansion. The dancers are quarks, atoms, molecules, planets, galaxies, etc. Their grip on eachother is big enough to overcome the relatively small adjustment in the space that was created between them. A few questions using this analogy arise. Do the new tiles actually appear between the dancers, or is it more a case of the force of friction the dancers are applying to the floor prevents those tiles between them from moving apart ? So when 'new space' is being created, is it created around strongly bound things and the things simply slide around through this new space to maintain their current configuration, or do the 'strong' forces prevent new space being formed around them ? If the new space is not being created, what happens to the energy that would have gone into the creation of the new space ? Does it simply get applied to the nearest point where it can squeeze in more space ? If the space is being created and the dancers pull themselves together to overcome it, where does this extra energy come from ? As for the big rip, I always thought it was the equivalent to the dancers finally getting tired and not being able to hold onto eachother hence giving in to the expansion of space. ie. It happens around the heat death of the universe when everything has dissipated its kinetic energy so much that the energy of the expansion is more than is left in the system to keep things together.

ddevault 6y ago

Local factors (gravity, strong & weak nuclear force) easily overcome the expansion of space. So things stick together.

chowells 6y ago

Not in practice. In practice, this provides a very tiny force pulling subatomic particles apart in an atom's nucleus, or pulling atoms away from each other in a molecule, or pulling a celestial body's molecules away from each other. It makes no difference at all, because that force is absolutely dwarfed by the strong nuclear force, electromagnetism, or gravity, respectively. Systems that are coupled together by those forces, up to and including galaxies, are impacted only very minutely by this expansion. For the expansion to have a significant impact on relative distances, the distances involved have to be so extreme that there's no real other interaction between the objects you're measuring from.

hashmymustache 6y ago
Does that expansion provide potential energy by creating space between attractive bodies? If so how do you account for conservation of energy?
- chowells 6y ago
  Yes, it will create some additional potential energy. That doesn't contradict strict conservation of energy - it just adds another energy source that has to be accounted for when balancing everything. It might contradict the aphorism "energy is never created or destroyed", but I'm not sure that was ever science. It's more fun to think about it in terms of thermodynamics. Can that extra energy be used to reverse the second law of thermodynamics? I'm guessing not - it's so diffuse that it's probably impossible to use it to reduce entropy. The math for actually doing those calculations is well beyond what I know, though, so I'll just say that one's no more than a guess.
mekkkkkk 6y ago
Great explanation! Thanks.

ben_w 6y ago

Just to add to your question: The usual response of “it only affects space between galaxies not inside galaxies” is deeply unsatisfying, and feels like it must be an oversimplification. What I think is meant, but I absolutely want someone wiser to confirm or refute, is that the rate of expansion is proportional to distance, and therefore so small inside a single galaxy it can be ignored. What I don’t know is: can any orbits still be stable in an expanding universe? (In theory I could simulate the orbits question easily; in practice, last time I tried to do astrophysics maths I made an elementary mistake).

thaumasiotes 6y ago
> What I think is meant, but I absolutely want someone wiser to confirm or refute, is that the rate of expansion is proportional to distance, and therefore so small inside a single galaxy it can be ignored. I am not that person. But I feel like this can't work -- if all space is expanding equally, in proportion to its own extent, then we would perceive no expansion at all. If two points start out 1 light year apart, and over time 1 light year expands to be 10% longer than it was before... and also, everything in the universe expands to be 10% larger in all spatial dimensions... then the distance in meters between the two points hasn't changed, because meters grew too.
- saagarjha 6y ago
  You'd notice a slower speed of light, wouldn't you?
- johncolanduoni 6y ago
  The problem with that model is that as was previously said, the electromagnetic, nuclear, and gravitational forces easily countermand the expansion on non-inter-galactic scales, to the point of not being measurable on the scale of a meter. Ultimately the molecular structure of your meter-stick is determined by various structure constants that aren’t changing, so the expansion can’t affect it unless it starts ripping bonds apart.
  - thaumasiotes 6y ago

cygx 6y ago

No, things aren't getting bigger. Like other gravitational effects, locally, spatial expansion would manifest as a pseudo-force, but be too weak to overcome the nuclear and electromagnetic forces that keep matter bound. However, Friedmann cosmology is only a large-scale approximation. A more realistic model is swiss-cheese cosmology, which comes with a different metric within the 'holes', and spatial expansion might indeed not act on galactic scales (though I would image it to have an indirect effect as boundary conditions at the edge of the hole have to be matched; I'm no expert on this part of the story, though).

taotau 6y ago

I came up with the analogy of 2 pairs of independent dancers on a large checkerboard floor. Lets say the tiles are 10cm^2 and the two couples start out 10 tiles apart. Every second, the tiles on the floor push away from all their neighboring tiles by one tile width and new tiles appear in their place. The tiles are small enough that the dancers are easily able to adjust their step/grip on eachother to simply maintain their dance routine. However, even tho the dancers can stay together, the couples drift apart at an ever increasing rate. They start 10 tiles apart, after one second the couples will be 20 tiles apart from eachother, 40 tiles apart after 2 seconds, 80 tiles apart after 3 seconds (a new tile is being inserted between each pair of existing tiles). My tiling math is probably slightly off, but I think this illustrates the exponential increase in expansion. The dancers are quarks, atoms, molecules, planets, galaxies, etc. Their grip on eachother is big enough to overcome the relatively small adjustment in the space that was created between them. A few questions using this analogy arise. Do the new tiles actually appear between the dancers, or is it more a case of the force of friction the dancers are applying to the floor prevents those tiles between them from moving apart ? So when 'new space' is being created, is it created around strongly bound things and the things simply slide around through this new space to maintain their current configuration, or do the 'strong' forces prevent new space being formed around them ? If the new space is not being created, what happens to the energy that would have gone into the creation of the new space ? Does it simply get applied to the nearest point where it can squeeze in more space ? If the space is being created and the dancers pull themselves together to overcome it, where does this extra energy come from ? As for the big rip, I always thought it was the equivalent to the dancers finally getting tired and not being able to hold onto eachother hence giving in to the expansion of space. ie. It happens around the heat death of the universe when everything has dissipated its kinetic energy so much that the energy of the expansion is more than is left in the system to keep things together.