# How do we know the universe is expanding, and not that its contents are shrinking?

Is there a physical reason why not to think that instead of space expanding, all physical constants and parameters are shrinking (including of course the instruments we use to measure the constants) and space is static, or is it a case of Occam's razor?

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The expansion of the universe is happening at large-scales. This means that, if you choose two galaxies, they are moving away from each other at a speed proportional to their distance. The space between the Sun and the Earth, for instance, is not expanding. In fact, some galaxies close to us appear blue-shifted due to their peculiar velocities. The physical constants must be very complicated functions of space and time to mimic such a phenomenon. The almost-FRW universe yields the expansion quite naturally. So, in a sense, you are right that it is because of Occam's razor, but, isn't all of "established" physics due to Occam's razor? :)

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Thanks for the answer, but by Hubble's law the Earth-Sun space (assuming it is a constant) is expanding 0.36 $\mu m/s$. Of course the fact that the Earth-Sun distance varies due to their movements in space, but that's irrelevant. –  Alyosha Oct 31 '12 at 10:38
I think that Hubble's Law cannot be applied to such short length scales. The reason we use an almost-FRW metric is because, empirically, we observe that matter is distributed like that. But, this applies only to large scales. So, Hubble's law cannot be applied to the Earth-Sun region. It is a "coarse-grained" law, not a fundamental one, because there are perturbations in the energy density. –  contrariwise Oct 31 '12 at 17:45
When you say 'matter is distributed like that', do you mean that we do not observe precisely enough to see whether or not the FRW metric holds or is it that our observations are accurate enough AND they tell us that the FRW metric is not fully implemented (if that's the right word) in our universe? –  Alyosha Oct 31 '12 at 18:06
If the FRW metric is to hold exactly, there would be no perturbations in the energy density - in particular, you and I wouldn't exist. So, the fact that the FRW metric doesn't hold on very small scales is beyond doubt. If you look at how clusters of galaxies are distributed and how the CMB is distributed, you notice that the perturbations in the energy density are very small. –  contrariwise Oct 31 '12 at 21:55
continued: For instance, the temperature fluctuations in the CMB are one part in $10^5$. So, we can deduce that the distribution of energy on very large scales is almost isotropic. This is the evidence that our observable universe is almost-FRW. –  contrariwise Oct 31 '12 at 21:55