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Is my understanding correct?

Weren't it for the internal resistance forces between particles in a solid object, the expansion of the Universe would continuously increase the distances between all of those particles, gradually "pulverizing" the solid. The size of a bar of steel remains constant though, and the manner in which it remains constant is through internal resistance forces, which imply internal stresses are present.

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  • $\begingroup$ Related: physics.stackexchange.com/q/2110/2451 $\endgroup$
    – Qmechanic ♦
    Commented Oct 8, 2021 at 20:37
  • $\begingroup$ Thank you for the reference. So just as gravity holds astronomical objects together in a galaxy, do electromagnetic forces within a solid hold it together against the expansion of the Universe? And therefore there really are internal stresses developed within the solid? $\endgroup$
    – jvf
    Commented Oct 8, 2021 at 20:43
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    $\begingroup$ There’s a lot of confusion about this question out there. The correct answer is that the part of the expansion of the universe due to dark energy does stress solids (under the default assumption that it just comes from a cosmological constant), but the rest of it doesn’t. That’s why you’ll often see conflicting answers. $\endgroup$
    – knzhou
    Commented Oct 8, 2021 at 20:44
  • $\begingroup$ Is the part of the expansion due to dark energy you refer to the acceleration of the expansion? $\endgroup$
    – jvf
    Commented Oct 8, 2021 at 20:56

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We don't fully understand this expansion actually. This is an effect measured from the CMB and from cepheids, so it's true for light travelling in vacuum. If there is a cosmological constant, it stretches the vacuum, but actually it is really a weak thing. It gets strong due to the super large scale of the universe. When you are in the cosmic webs, filaments, there the gravitational potential dominates over the effect of the cosmological constant. There the effect is extremely small, non measurable, maybe even zero depending on the true effective coupling between gravity and matter and lambda.

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