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Doppler effect vs expansion effect on wavelength

  1. Dec 22, 2013 #1
    Could you please clarify for me how much each of these contribute to the red shifting of light from distant objects? It seems to me that red shifting of light from near by objects i.e. within our galaxy would be affected more by the Doppler effect whereas for intergalactic objects the red shifting of light would be affected more by the expansion of the universe, eg. Light originating from some place billions of light years away and billions of years ago would get stretched out by the expansion of the universe and appear to be red shifted as if it were caused by the Doppler effect.

    Thank You! :smile:

  2. jcsd
  3. Dec 22, 2013 #2


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    Staff: Mentor

    You are correct. The redshift from objects within our galaxy is solely due to their motion through space relative to us, as expansion does not affect bound objects like galaxies. Once you get a few hundred million lightyears away expansion starts to become the dominant form of redshift.
  4. Dec 22, 2013 #3


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    Gold Member

    Here is a paper with some additional information:

    “The kinematic origin of the cosmological redshiftâ€
    By Emory F. Bunn and David W. Hogg

    "A common belief about big-bang cosmology is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must be viewed in terms of the stretching of space. We argue that, contrary to this view, the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that it is always valid to choose a coordinate system that is locally Minkowskian. We show that an observed frequency shift in any spacetime can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a suitable family of observers along the photon’s path. In the context of the expanding universe the kinematic interpretation corresponds to a family of comoving observers and hence is more natural."

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