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If you have a particle that is indivisible (e.g. electron), we assume the forces holding it together would prevent it from expanding. If the forces holding the indivisible particle together were weaker than that due to cosmic expansion, wouldn't the particle itself also expand in volume as well? Also, if there was a particle that lacked any internal forces, what exactly happens? (I realize this may violate the definition for a particle, but I'm trying to understand how everything [since "things" occupy space] should expand as long as they don't have internal forces preventing this expansion.)

Now, by wave-particle duality, there is an associated wave function in the position space denoting probabilities of detecting a particle somewhere. Due to expansion, would not cosmic expansion affect (however minimally) the probability associated with detecting a wave at a particular position?

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  • $\begingroup$ Hi, I have never read that an elementary particle contains forces holding it together. If may well be true, but I don't think we have any experimental evidence for another type of force affecting experimental results. The size of an electron ( or rather the lack of it), makes it very difficult to examine. $\endgroup$
    – user108787
    Sep 22, 2016 at 6:06
  • $\begingroup$ Related: physics.stackexchange.com/q/2110/2451 and links therein. $\endgroup$
    – Qmechanic ♦
    Sep 22, 2016 at 6:46
  • $\begingroup$ @CountTo10 Thank you for the response. That makes sense. I was assuming there was some intrinsic internal structure but cannot of course yet prove that for electron. So if something is a point particle and lacks any internal forces, would not expansion influence the very definition of that point's spatial width (i.e. $x = 0$ itself)? If it was found that point particles have a minimum spatial width, wouldn't the "point particle" expand as well in such a case? $\endgroup$
    – Mathews24
    Sep 22, 2016 at 13:28

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The reason for a particle being indivisible, may (or does) not have anything to do with the strength of any forces holding it together. In the case of the electron, our current understanding is that it is a point particle. In other words, it does not have any size (or volume) associated with it. In this sense it is literally a mathematical point. Therefore, there are no internal forces necessary to hold it together. Moreover, a point does not expand, even if the space in which it is embedded expands. Such an expansion would indeed expand the wave function of that particle. As a result, the length scales, such as the wavelengths, of the particle would become longer. This is what happened to the cosmic background radiation, for instance.

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  • $\begingroup$ The one point I just don't quite understand yet is: "In the case of the electron, our current understanding is that it is a point particle. Therefore, there are no internal forces necessary to hold it together." I realize this is conjecture, but with a point particle, does it not occupy space? Would not the space (however infinitesimally small) it occupies expand itself? To my understanding, the only reason matter does not expand is because of forces preventing it. But for a point particle, if it lacks any analogous phenomena preserving its internal structure, what's to stop it from expanding? $\endgroup$
    – Mathews24
    Sep 22, 2016 at 13:44
  • $\begingroup$ Moreover, I just don't quite understand how a point can't expand if the entire space itself is expanding. You say: "a point does not expand, even if the space in which it is embedded expands"—but is not a space simply the collection of all points within it? If we say a point cannot expand, what is then expanding? $\endgroup$
    – Mathews24
    Sep 23, 2016 at 2:42
  • $\begingroup$ @Mathews24, no it does not occupy any space. It is a point in the true sense of the word. See the edit. $\endgroup$
    – flippiefanus
    Sep 23, 2016 at 4:12

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