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Just what the question says. As the universe expands, and space expands, do objects like planets ever expand? Does the space INSIDE of those objects expand?

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    $\begingroup$ Things that are bound by some force (planets bound by gravity, etc) stay the same. Things that are unbound will drift apart. $\endgroup$
    – Florin Andrei
    Mar 21, 2019 at 17:30
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    $\begingroup$ Nice question, probably it was asked already many times :-) The important thing is: we can't measure anything absolutely. All measurement compares something to another thing. Thus, if everything would expand, we couldn't measure it. How? All of our measuring devices would also expand. $\endgroup$
    – peterh
    Mar 21, 2019 at 17:40
  • $\begingroup$ Closely related: astronomy.stackexchange.com/questions/11726/… $\endgroup$
    – PM 2Ring
    Mar 21, 2019 at 18:45
  • $\begingroup$ See also: astronomy.stackexchange.com/questions/104/… $\endgroup$
    – mtewes
    Mar 22, 2019 at 11:30
  • $\begingroup$ I think it's the latter, Keith. $\endgroup$
    – John Duffield
    Mar 22, 2019 at 17:49

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Things that are bound by other forces will not be seperated by the expansion of the Universe. For instance, planets do not expand due to cosmic expansion. The solar system does not either because it is held together by gravity. However, the space between galaxies or between galaxy clusters does expand.

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Think of the universe as a partially inflated balloon. If you decrease the air pressure of the air outside of the balloon then the balloon inflates. The density of the air in the balloon has just dropped because the mass is the same but it is taking up more space. The air molecules have spread farther away from each other but the molecules themselves don't expand. This is the same thing that happens to everything in our universe as it expands. The space between the components of particles has increased.

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    $\begingroup$ So yes or no, ..."do things like planets expand also?" balloon and raisin cake analogies usually avoid answering this directly. A planet drawn on a balloon would expand, a raisin in a cake won't. $\endgroup$
    – uhoh
    Jan 30, 2020 at 6:06
  • $\begingroup$ So it will start off at the subatomic level, atom's components will spread apart, then as the expansion of the universe speeds up eventually, theoretically, planets will, more explode once you reach this stage, expand. $\endgroup$
    – Daosof
    Jan 31, 2020 at 4:21

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