Tag Archives: redshift

No redshift in a box

I previously talked about a thought experiment which would supposedly show that "expansion of the universe" is fundamentally different from just "everything moving away from each other". However, discussion at the Physics Forum has convinced me that light would not be redshifted over time in a perfectly reflecting mirrored box despite the light being stretched along with the expansion of space.

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A cosmologically redshifting puzzle in a box

In the comments for another blog post about cosmological distances, I suggested a thought experiment that would highlight how the redshift from distant galaxies is not due to a Doppler shift, that is, light from very distant galaxies is not redshifted because of the movement of the galaxies, but rather because the intervening space itself is expanding:

If someone managed to trap light when the universe first became transparent, about 300,000 years after the Big Bang, inside a perfectly reflecting mirrored box such that the light is contained in perpetuity, what would have happened to the wavelength of the trapped light by now?

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