There's another question on physics.SE whose answer, if I have understood it correctly, explains that the farther the points are in space the faster they are moving away from each other.

Actually, the apparent speed with which two of the points on the circle in a distance $D$ of each other would move relative to each other will be $v = H_0 D$ where $H_0$ is the speed the balloon itself is expanding.

This means that even the ruler sitting on my desk is expanding, just that the expansion is very very slow owing to the ruler's small size relative to the cosmic scales. However, if everything is scaling then everything should apparently be of the same size to an observer in such an expanding space. The size million pants quote from the show "The Big Bang Theory" comes to mind.

To be able to counter that, there're some phenomena that we can observe which are scale invariant. Red-shift would be one such phenomenon. But is red-shift the only way we can tell that the space is expanding? Is there a logical flaw in my thinking? I know that red-shift is a result of the Doppler effect - there's bound to be a decrease in the frequency of a signal, as perceived by an observer, emitted by a source which is moving away relative to the observer.

  • $\begingroup$ Even Hubble's law is only an argument for the expansion of space if combined with the Copernican principle (that we are not at a special point in the Universe). $\endgroup$
    – ProfRob
    Oct 5, 2014 at 10:03

1 Answer 1


There are a few other reasons why we think the universe is expanding. Wikipedia lists and explains them far better than I will ever be able to.

One thing though, your ruler is not expanding. The intermolecular forces are making sure the distance between molecules stays the same. Empty space is expanding, expanding essentially the distances between weakly interacting objects. Objects we see everyday are not subject to this expansion due to the forces keeping them together being way too big for the expansion to affect them.

  • $\begingroup$ When you say that the ruler is not expanding, is not expanding at all or is it expanding so slowly that it can't be measured? That's the primary import of my question. $\endgroup$ Apr 1, 2014 at 20:27
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    $\begingroup$ not at all. It is bound $\endgroup$
    – Jim
    Apr 1, 2014 at 20:27
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    $\begingroup$ Space between molecules is expanding but the forces between them is stopping them from increasing their distance between each other. What I'm trying to say is: the slow expansion of space is overcome by the strong forces between molecules $\endgroup$
    – PhotonBoom
    Apr 1, 2014 at 20:31
  • $\begingroup$ And to put it another way, expansion is so weak locally that even gravity is (currently) able to work so much faster that our galaxy is unaffected and even the galaxies in the Local Group are overcoming it. $\endgroup$
    – Michael
    Aug 29, 2016 at 1:43

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