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  • $\begingroup$ In Woody Allen's "Annie Hall" this question comes up; the answer? "Brooklyn is not expanding!" The reason is that it is spacetime that is expanding; objects that have any kind of binding energy are being held together by forces. Photons expand, so would cosmic sound waves. But Brooklyn is held together by motherly love. See the scene at youtube.com/watch?v=5U1-OmAICpU $\endgroup$ Commented Feb 13, 2016 at 20:40
  • $\begingroup$ Hi. @PeterDiehr and Sushant23 . But why Brooklyn doesn't? If on the small scale we agree on the posted answer, that we cannot see the expansion since everything is expanding, then why see it on the big scale? Is it because it expands faster on the big scales but on the small the speed is the same for all objects? Thanks. $\endgroup$ Commented May 28, 2016 at 17:37
  • $\begingroup$ @ConstantineBlack: the expansion is equivalent to a very weak force - local binding forces always overwhelm it: atoms, molecules, people (eg, not a valid excuse for the waistline!), planets, solar systems, and galaxies. But you can see it over very great distances - hence the red shift due to cosmic expansion is a good proxy for distance, though other proxies are used to set the distance scale. See Hubble's Law $\endgroup$ Commented May 28, 2016 at 17:46
  • $\begingroup$ @PeterDiehr Thanks for the fast response. I find it conceptually wrong to admit that expansion is a force such that you can use an equation like Newton's or any argument at least saying that: the total force on the object is expansion + other_forces so that the result in small scales is not-expansion. It' s more reasonable to either say that experiments say this or that or that in small scales, the expansion rate is the same for all objects( even inside the galaxy??) so that we don't observe it. Am I losing something here? Thanks. $\endgroup$ Commented May 28, 2016 at 18:01
  • $\begingroup$ @ConstantineBlack: make this a new question, and we'll provide an answer that is hopefully correct at all levels. $\endgroup$ Commented May 28, 2016 at 18:24