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I understand that the inter-galactic space is expanding but galaxies themselves are not. What is happening to the space within a galaxy? Is it fixed by the gravity of the galaxy or is it expanding and moving out around it?

Using the coins on a balloon analogy, is the balloon surface under the coins (galaxies) stuck to the coin and constrained or is it free to expand out, slipping underneath the coins leaving them in the same place (and the same size)?

Kyle Kanos
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JohnnyD
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  • I'm not asking about matter, which does not expand. But how the space around it changes. – JohnnyD Aug 13 '14 at 16:33
  • It is not an exact duplicate, but I think the answers there should answer your question (the first answer even talks about galaxies and the balloon analogy and seems to address what you are thinking of directly) – ACuriousMind Aug 13 '14 at 16:34
  • Also see What is the difference between matter & spacetime?. Although this has been closed as a duplicate of the question ACuriousMind linked, it is relevant to your question about space sliding past the coins. – John Rennie Aug 13 '14 at 16:50
  • Space isn't a substance that can move, flow, or slip. If you change the question so that it doesn't assume space is a substance that can move, then the question becomes equivalent to others such as this one: http://physics.stackexchange.com/questions/70047/can-the-hubble-constant-be-measured-locally –  Aug 13 '14 at 16:52
  • There's a limitation to the analogy for sure but mass can deform space-time so it is dynamic even if it has no substance. So, reading the answers to the other question, no gravity (nor anything else) does not stop the expansion of space within galaxies but it is so small as to be negligible. – JohnnyD Aug 13 '14 at 17:53
  • I heard a talk by Kip S. Thorne, in which he gave a rather useful analogy for a black hole event horizon as the edge of a spacetime waterfall. I found this paper in arxiv: http://arxiv.org/pdf/gr-qc/0411060.pdf. Without any endorsement for its quality, it may give you a few insights. Of course, if you understand black holes as deep waterfalls from which there is no escape, once you are over the cliff, one could argue that the analogy for galaxies would be something like shallow lakes, which may answer your question in the affirmative. Yes, spacetime would be "caught" in there. – CuriousOne Aug 13 '14 at 19:17

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