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Originally Posted by jbrosenberg
If massive objects are not expanding, then what happens to the "inner space" that exists within all matter, i.e. most solid matter is empty space, and if this is not expanding how is it different from the space between galaxies, etc.?
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The expansion is an effect related to gravity. On the largest scales, electromagnetism and other forces don't play much of a role, so gravity is king. On smaller scales, gravity or electromagnetism or other forces work to keep collections of particles together. So on the large scales, there can be an expansion that is not related to behaviour that we see more locally.
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One thing that was interesting to me, was that the cosmic background radiation is slowly shifting to longer wavelengths (e.g. lower energy). Does this mean that all transmitted electromagnetic photons lose energy overtime? Is it something we could measure? Could we detect a loss of energy of a lightsource between point A and point B? (I'm guessing it would be a rather difficult experiment, indeed)....
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This loss of photon energy over transmission time is something that is used in cosmological measurements. Over great distances, it's impressive. Over small distances, it's not so big.