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Question about red/blue shift

  1. Jul 25, 2012 #1
    I was wondering if something could be redshifted to the point that all of the EMR it was emitting was outside of the visible spectrum, thus not being able to be viewed.

    I googled it and found some people discussing it, there answer was essentially no, because the object would be emitting all sorts of different wavelengths of EMR so that as some of it is redshifted below the wavelength of visible light, some wavelengths that were initially above the visible spectrum are then redshifted into the visible spectrum of light, etc.

    But if that's the case, why does redshift happen at all?

    Or, assuming things cannot be redshifted completely out of the visible spectrum, is this because that objects that are far enough away to be moving quickly enough to cause it (metric expansion of space) are so far away that the light has never reached us, anyway?
  2. jcsd
  3. Jul 25, 2012 #2


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    Staff: Mentor

    Consider the cosmic microwave background, for example: At time of formation, it was a (nearly) perfect black body with a temperature of ~3000K, and emitted a lot of visible and infrared light with smaller contributions in other frequency ranges.
    Now, this light got redshifted by a factor of ~1000, and we see a blackbody spectrum corresponding to a temperature of ~3K: Lots of microwaves, with smaller contributions in other frequency ranges. There might be some photons which had an extremely high initial energy and got shifted to the visible light now, but their fraction is so extremely small that you will never see one.

    For a practical measurement of redshift, the common method is to observe spectral lines. Their relative distance and absolute position is known from experiments on earth, so if you identify them and measure their (shifted) position in a spectrum, you can calculate the redshift.

    There are objects so far away that light has not reached us (and probably never will), indeed. They are outside the observable universe.
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