This is a very simple question which is similar to some others. Since we say all of space is expanding this must also include the space between atoms and constituents. Of course at the small scale I have just mentioned expansion would hard to measure. My question is not why does matter not expand along with space but is there any other effect on matter due to this expansion, such as thermal energy created from a slight friction?
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1$\begingroup$ Related but not necessarily the same question (and the answers are not developed well) : Is there a friction or retarding force in creating spacetime? $\endgroup$– sammy gerbilCommented Jan 31, 2020 at 19:55
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1$\begingroup$ @sammygerbil - thank you for the help. You sense I am after something specific but I probably need to provide some type of relevant example to make the issue clearer. I'll be back! $\endgroup$– ClockCommented Feb 1, 2020 at 10:39
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1 Answer
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Is there friction between matter and expanding space.
No. In the absence of non-gravitational forces, matter simply travels on geodesics through spacetime, whether spacetime is expanding or not. This is essentially a postulate of General Relativity and we have no experimental evidence that it is wrong.
Geodesic motion is the curved-spacetime equivalent of “straight ahead, with constant velocity” in flat spacetime.
Is there any other effect on matter due to this expansion?
Yes. The most dramatic effect of spacetime expansion on matter is that sufficiently rapid expansion can create matter as particle-antiparticle pairs. For details see this paper by Leonard Parker, one of the discoverers of this process in the 1960’s.
The effect is the gravitational analog of pair creation by a sufficiently strong electric field in quantum electrodynamics.
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$\begingroup$ @sammygerbil I've strengthened the second part of my answer in response to your feedback. $\endgroup$– G. SmithCommented Jan 31, 2020 at 20:22
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1$\begingroup$ And now I have reversed the order to address the first question first. $\endgroup$– G. SmithCommented Jan 31, 2020 at 20:29
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$\begingroup$ I don’t understand your question. We are aware of non-gravitational forces, such as electromagnetism. We observe that charged particles in electromagnetic fields do not travel on geodesics, while uncharged matter does. So I don’t see anything tautological. $\endgroup$– G. SmithCommented Jan 31, 2020 at 20:42
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$\begingroup$ @G.Smith - Thank you for the fine explanation. The paper is mostly about how the author approached his work in this respect so it will take me some time to sieve out the gold but I'll try to get back to you if I need any clarification. For now, you clearly state that uncharged matter travels on geodesics so I think my question is well answered. $\endgroup$– ClockCommented Feb 1, 2020 at 10:37