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According to space.com the universe is expanding with 75km/1Mpc/1s (megaparsec), which comes down to $\beta=2.43\times10^{-18}/s$, a relative rate per second.

To put this in perspective: The average distance between Earth and Sun is $149\,600\,000\, km$. Multiplying this by $\beta$ and one year, I get roughly $11.5\,m$, which means every year the Sun moves away from earth by over $11\,m$ on average, just because of the expansion of the universe (amazing enough as to think I messed up the orders of magnitude, so please check). But this is not the point. I would like to understand the nature of this expansion better.


  1. Wikipedia says the expansion could be measured in principle with a ruler, say a piece of metal of a given length. Is that really true or does the space inside the ruler expand with the same rate such that by counting how often the ruler fits between Earth and Sun, we would see no difference?
  2. What if we measure instead with a beam of light (ignoring relative motion for the moment) and measuring how long it takes to bounce back from Sun. Will the time be longer or is space rather like a checker board on a rubber sheet and independent how large you stretch the sheet, 8 rectangles are 8 rectangles and the light moves from rectangle to rectangle, not noticing that they are all stretched out.
  3. In (2) above, what would be the right way to measure time anyway? Let a beam of light bounce between two mirrors and count? Huh, but the distance between the two mirrors expands also, while one beam of light goes to the Sun and back, the bouncing one bounces always the same number of times independent of how much space is stretched between to measurements.

3 questions in one post may be a bit too much, so a general explanation how the expansion of space relates to (a) the size of physical objects, e.g. the typical diameter of atomic particles (1) above, and to (b) speed, wave length and frequency of light would be great.


marked as duplicate by Rob Jeffries, ACuriousMind, Jim, Qmechanic Feb 13 '15 at 18:08

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ Well, according to these other question's answers, Wikipedia is correct in saying we can measure it with a rule, because the ruler does not expand along with space. But generally I found the answers quite helpful. $\endgroup$ – Harald Feb 13 '15 at 14:13
  • $\begingroup$ Yes, I beg wikipedia's pardon. I did not read carefully enough. Yes, effectively the ruler has a fixed length that we can use to judge the expansion on large scales. $\endgroup$ – Rob Jeffries Feb 13 '15 at 14:22
  • $\begingroup$ Marking this as duplicate of physics.stackexchange.com/q/2110 and physics.stackexchange.com/q/70047 among others. $\endgroup$ – Rob Jeffries Feb 13 '15 at 14:23

Space does expand, but on "small" scales, gravity holds objects together:

Here, "small" means the scales of planets, the Solar system, our galaxy, and even the nearest handful of galaxies. Thus, the distance to the Sun does not increase, The Milky Way doesn't expand, and Andromeda doesn't recede from us (in fact it approaches us).

In principle (ignoring technical details such as obscuring dust and gas, absorbing mirrors, and, in particular, that even a laser beam slowly diverges), you could point a light beam toward a very distant galaxy (say, 10 million lightyears away), hit a mirror, and receive the reflected beam 20 million years later, plus the extra time needed because the distance has increased slightly in the meantime.

If in the meantime you have replaced your laser with a mirror, the beam will bounce back and forth with an ever-increasing period, until eventually the accelerated expansion of the Universe makes the galaxies recede faster from each other than the light beam can catch up with.

You could say that space is "tied" to the masses. That's a fundamental prediction of general relativity. So, you should not think of the expansion on small scales actually happening, but objects being pulled together at the same rate so that their distance doesn't increase. Rather you should think of expansion itself being slowed down or halted by the mass of these objects.

The same thing has happened to the Universe as a whole for most of its life: After inflation, the Universe continued expanding, but the gravity of its constituents gradually slowed down its expansion. However, "recently" (a few billion years ago), space had expanded so much that its energy density is now dominated by dark energy, which unfortunately has the opposite effect, accelerating the expansion exponentially.

(unfortunately in the sense that our Universe won't collapse and start all aover with new, interesting physics, which I think would be beautiful, but instead wil expand forever, making it more and more dilute, cold, and boring)

  • 1
    $\begingroup$ Hmm, I am not yet sure I have the answer I am after: ok, physical objects (including the solar system and individual galaxies) do not expand, because they are held together by forces. But does the space in between expand (I am tempted to say "thin out":-) or not? Maybe the right question is: is the expansion a homogenous phenomenon all over the universe or does it happen only between galaxies? $\endgroup$ – Harald Feb 13 '15 at 14:28
  • $\begingroup$ Okay @Harald, I understand. I started elaborating here, but my answer became too long, so instead I'll edit my answer. Hang on! $\endgroup$ – pela Feb 13 '15 at 14:41
  • $\begingroup$ How do we know for sure this is true? The numbers seem very tiny compared to what could be practically measured. $\endgroup$ – Jiminion Jun 3 '15 at 14:29

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