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The diffence between expanding universe and light slowing?

  1. Jan 24, 2012 #1
    I am under the impression that the slowing of light over time would make the universe appear to be expanding. Is it possible that the permittivity or permeability of space has been decreasing over time giving the illusion of an expanding universe?

    Is there any way to test the differences and rule out the slowing of light. In fact, if we were to assume that the speed of light was decreasing (in a vacuum), would it even be possible to detect a decreasing speed of light, given our instruments, metrics and clocks would all slow down proportionally?
  2. jcsd
  3. Jan 25, 2012 #2


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    The difficulty here is that the speed of light is intimately related to the relative strength of the electric and magnetic forces which hold matter together. So if you change the speed of light, you also change the structure of atoms and molecules. The structure of atoms appears to be nearly identical across the entire observable universe. There may be some tiny changes for really far-away galaxies, but those are so far using rather error-prone measurements that have yet to be independently confirmed. And regardless, the changes in the structure of atoms are so tiny that they wouldn't result in any noticeable change in the speed of light.
  4. Jan 25, 2012 #3
    I think he means that the specific light travelling to us through deep space is being slowed, creating an impression of expansion, rather than suggesting the maximum speed of light in general has decreased since the big bang.

    I'm only making a guess, but I don't think light with a lowered top-speed would, by itself, account for the observed red shifting. Also, a decrease in the permittivity of space would most likely require an increase in its density, leading to the opposite effect of expansion which would blue shift the light.
  5. Jan 25, 2012 #4


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    There's also the point that space is far, far too transparent to be filled with any sort of medium that would allow any sort of change in the speed of light due to a change in permittivity.
  6. Jan 25, 2012 #5
    I thought Light red-shifted because of a change in frequency as it passes through expanding space, not that photons slow down.
  7. Jan 31, 2012 #6
    Correct shifty, OP is just saying what if.....
  8. Feb 1, 2012 #7


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    The speed of light is intimately tied to the alpha [fine structure] constant, as noted by Chalnoth. The alpha constant is vital to stellar fusion and even a modest change could disrupt synthesis of elements like carbon, or even prohibit stellar fusion in the first place. We also know from spectral lines of quasars that alpha has not changed to any noticeable extent for about 10 billion years. So we have good reason to believe c has been fairly constant throughout most of the history of the universe.
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