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One idea from cosmology is that galaxies are all moving away from each other. Naively this could be taken to mean that they are expanding into something. Popularly this could be seen as them moving outwards into an area of completely empty space at the 'edge' of the universe.

However, if the universe is infinite and unbounded and uniformly populated, there is no empty volume for the galaxies to move into. Therefore logic dictates that the space between galaxies must be expanding.

Let us suppose this is true. Then

  1. Is it also the case that the space within the galaxies is expanding.

  2. What about the space in our solar system.

  3. Where do we stop? Under this theory is the space inside an atom expanding?

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    $\begingroup$ Related: physics.stackexchange.com/q/2110/56299 and links therein. $\endgroup$ – HDE 226868 Nov 3 '15 at 0:38
  • $\begingroup$ The space-time expansion theory is a model based on General Relativistic equations which works on a large scale where the matter distribution appears homogenous. On a smaller scale attractive and electromagnetic forces become important, and on an atomic scale nuclear forces become significant. These forces are far stronger than magnetic constants. I highly recommend reading the link posted by user HDE226868. $\endgroup$ – Cicero Nov 3 '15 at 1:05
  • $\begingroup$ Hey Chas ... this is a perhaps related question physics.stackexchange.com/questions/223610/… $\endgroup$ – Fattie Dec 12 '15 at 15:20
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However, if the universe is infinite and unbounded and uniformly populated, there is no empty volume for the galaxies to move into. Therefore logic dictates that the space between galaxies must be expanding.

Your premise is faulty. Just because something is infinite doesn't mean it can't expand. For example, although it's intuitively obvious that the (infinite) set of even integers is half the size of the (also infinite) set of all integers, a classic proof shows that the set of even integers is the same size as the set of all integers.

Getting back to the cosmos, immediately after the Big Bang the universe was both enormously dense AND infinite. In the time since then it's expanded greatly, and its density has decreased greatly, but it's still infinite.

A number of current theories postulate that [space is granular], but that's at a much, much, much smaller scale than what you're proposing.

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  • $\begingroup$ Thanks for answering. I don't see the fault in my premise. It assumes that an infinite universe with those properties is expanding and draws a conclusion about how it must be expanding. So your second sentence is criticising something I didn't say. $\endgroup$ – chasly - supports Monica Nov 3 '15 at 1:26
  • $\begingroup$ Well, we disagree on that. $\endgroup$ – Daniel Griscom Nov 3 '15 at 1:50

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