5 ms·

This is a well-known phenomena. The usual image is to consider ants traveling on an expanding balloon. The speed of light is the limit of how fast the ants ca

by btilly 8y ago

This is a well-known phenomena. The usual image is to consider ants traveling on an expanding balloon. The speed of light is the limit of how fast the ants can walk. The expansion of the universe is the expansion of the balloon. And now you see how two distant ants can be carried away from each other faster than they can walk towards each other. If you understand manifolds and general relativity, this analogy is surprisingly exact.

mehrdadn 8y ago
I've heard of raisins on a cupcake instead (I hate raisins on cupcakes btw), I assume since it's a 3D example. It's meant to show how it's possible for everything to be getting farther away from each other without anything being at the center.
- jandrese 8y ago
  Cupcakes have a center.
  - mehrdadn 8y ago
    That wasn't the point. The cupcake represents space. The things are the raisins which are generally not in the center despite moving away from each other.
  - jmts 8y ago
    This is physics, so it is best to assume an infinite cupcake.
- btilly 8y ago
  I haven't heard the cupcake version. It does have something to recommend it, though raisins don't crawl. So that may make it harder to visualize the difference between how fast you're expanding versus how fast you're moving versus local space. Which is the whole point.
  - mehrdadn 8y ago
    Good point, I guess they're good for illustrating different things :)
  - vanderZwan 8y ago
    What I find confusing is that the space inside the ants/raisins should expand as well. Why do we not experience this expansion locally?
    - function_seven 8y ago
      Space is expanding everywhere, but the matter that resides within it is bonded to other matter, or gravitationaly attracted together, so it holds. Back to the balloon analogy, the surface of the balloon is expanding, even under the ant's feet. But the ant doesn't grow.
      - acqq 8y ago
        And since the even the whole groups of galaxies are gravitationally bound, it seems that the galaxies, including our own galaxy, are not "stretching" but remain the same size. And, based on the same principle, everything inside of them also, including us. So back to the cupcake analogy, the raisins are the whole galaxy groups. Also: https://medium.com/starts-with-a-bang/this-is-why-we-arent-expanding-even-if-the-universe-is-d8c770531082 https://medium.com/starts-with-a-bang/this-is-why-we-arent-e... "As physicist Richard Price once put it, “Your waistline may be spreading, but you can’t blame it on the expansion of the universe.”" https://www.newscientist.com/article/dn8082-cosmic-expansion-is-not-to-blame-for-expanding-waistlines/ https://www.newscientist.com/article/dn8082-cosmic-expansion... https://arxiv.org/abs/gr-qc/0508052 https://arxiv.org/abs/gr-qc/0508052
      - caf 8y ago
        Here "bonded to other matter" means "electrostatically attracted together".
    - gregdunn 8y ago
      It's not strong enough to overcome the forces keeping us together. But that might not be the case for forever: https://en.wikipedia.org/wiki/Big_Rip https://en.wikipedia.org/wiki/Big_Rip
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by btilly 8y ago

mehrdadn 8y ago

I've heard of raisins on a cupcake instead (I hate raisins on cupcakes btw), I assume since it's a 3D example. It's meant to show how it's possible for everything to be getting farther away from each other without anything being at the center.

jandrese 8y ago
Cupcakes have a center.
- mehrdadn 8y ago
  That wasn't the point. The cupcake represents space. The things are the raisins which are generally not in the center despite moving away from each other.
- jmts 8y ago
  This is physics, so it is best to assume an infinite cupcake.
btilly 8y ago
I haven't heard the cupcake version. It does have something to recommend it, though raisins don't crawl. So that may make it harder to visualize the difference between how fast you're expanding versus how fast you're moving versus local space. Which is the whole point.
- mehrdadn 8y ago
  Good point, I guess they're good for illustrating different things :)
- vanderZwan 8y ago
  What I find confusing is that the space inside the ants/raisins should expand as well. Why do we not experience this expansion locally?
  - function_seven 8y ago
    Space is expanding everywhere, but the matter that resides within it is bonded to other matter, or gravitationaly attracted together, so it holds. Back to the balloon analogy, the surface of the balloon is expanding, even under the ant's feet. But the ant doesn't grow.
    - acqq 8y ago
      And since the even the whole groups of galaxies are gravitationally bound, it seems that the galaxies, including our own galaxy, are not "stretching" but remain the same size. And, based on the same principle, everything inside of them also, including us. So back to the cupcake analogy, the raisins are the whole galaxy groups. Also: https://medium.com/starts-with-a-bang/this-is-why-we-arent-expanding-even-if-the-universe-is-d8c770531082 https://medium.com/starts-with-a-bang/this-is-why-we-arent-e... "As physicist Richard Price once put it, “Your waistline may be spreading, but you can’t blame it on the expansion of the universe.”" https://www.newscientist.com/article/dn8082-cosmic-expansion-is-not-to-blame-for-expanding-waistlines/ https://www.newscientist.com/article/dn8082-cosmic-expansion... https://arxiv.org/abs/gr-qc/0508052 https://arxiv.org/abs/gr-qc/0508052
    - caf 8y ago
      Here "bonded to other matter" means "electrostatically attracted together".
  - gregdunn 8y ago
    It's not strong enough to overcome the forces keeping us together. But that might not be the case for forever: https://en.wikipedia.org/wiki/Big_Rip https://en.wikipedia.org/wiki/Big_Rip

deleted 8y ago

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