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This question already has an answer here:

Given the Universe is expanding. Therefore everything within the U is expanding, in all dimensions, subatomic to cosmic. Then all metre sticks are expanding. The question: How can we measure U expansion with metre sticks that are also expanding? Alice only knew she had grown ten feet tall by comparing herself to her surroundings.

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marked as duplicate by BMS, Brandon Enright, ACuriousMind, Kyle Kanos, John Rennie Sep 6 '14 at 5:22

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Space is expanding. However, nearby atoms (e.g. those in a metre stick) are not moving away from each other because the inter-atomic forces restore them to their original positions.

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Similarly, as the space between the earth and the sun increases (at an insignificant rate), the gravitational force restores the earth and sun back to their equilibrium distance.

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  • $\begingroup$ Great visual explanation for this common (and reasonable) question. $\endgroup$ – BMS Sep 5 '14 at 23:44
  • $\begingroup$ This is true, but only for a constant rate of expansion. In an increasingly accelerating universe the newly created space does have a non-trivial effect, hence the possibility for a "big rip" event. $\endgroup$ – CuriousOne Sep 6 '14 at 0:42
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    $\begingroup$ Also we should note that the current expansion rate is about $2\times 10^{-18}/\mathrm{second}$. This tiny expansion is completely swamped by thermal vibrations that are many orders of magnitude greater. $\endgroup$ – Mark Adler Sep 6 '14 at 4:21

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