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Q & A: cosmic expansion and wavelength of light

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Most recent answer: 06/28/2015
Q:
This question is in regard to the expanding universe question. The point has been made in the answers to several questions that expansion does not affect atoms, molecules, etc. However, in discussion of the cosmic background radiation, the point is often made that as the universe expanded, the wavelength of this radiation increased in wavelength. Was the increasing wavelength the result of space's coordinate system expanding? If so, how can the expansion of space affect something with such a small wavelength but not the particles that make up my body?
- Scott Cassady (age 43)
Bowling Green
A:

Great question! For other readers, I think you're referring back to this thread:  .

Things like atom sizes are set by small-scale effects, mainly the strength of the electric force, the mass of the electrons, and Planck's constant. The expansion of the universe doesn't change those. Only the acceleration of the expansion acts like a stretching force on an atom, but the effect is truly tiny.

Here's what's different about those light waves. There's no particular forces setting a characteristic wavelength for them. There can be light waves of any size. You can sort of think of it as the fields just traveling along with the typical behavior of space, unlike the fields in an atom that hold together because of the interactions. Here's one way to see it. Say that the universe happens to be finite and filled with electromagnetic waves going all directions. As the universe expands, those waves are still going to fill the whole thing, since no region is special. The number of wavelengths for one wrap-around won't change, since that would mean a sudden break somewhere in the wave. So the wavelengths must get longer. If the universe happens to be infinite, the argument is a little harder to make but the local behavior is the same.

Mike W.

 


(published on 06/28/2015)

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