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Red Shift, Blue Shift

  1. Jun 9, 2014 #1
    I apologize if this is a dumb question, or if I am not remembering things correctly, but ...

    ... wasn't one of the earliert objects (a Cepheid?) Hubble detected as red-shifted located within the Andromeda Galaxy? If so, and since we're on a "collision course" with the Andromeda, why didn't Hubble see this as a blue-shift?

    As a follow on queston ... on a more local scale, aren't the objects detected seen with a mix of red and blue shifts, since cosmic expansion (or the cosmological redshift) isn't quite enough yet to overcome all doppler shifts from local relative velocities? Then at a larger scale, when it is, the majority of objects are seen as redshifted?

  2. jcsd
  3. Jun 9, 2014 #2

    D H

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    Hubble discovery M31-V1 was the key to resolving the "island universe problem": Was the Milky Way synonymous with the universe, or did the universe comprise a bunch of "island universes" (now called galaxies)? It was Hubble's work on assessing distances that resolved the issue.

    Hubble did not measure the velocities to those Cepheid variables. That was the work of Vesto M. Slipher. Hubble combined Slipher's velocity measurements with his own distance measurements to arrive at an initial version of Hubble's law. M31-V1 is coming towards us, so it's blue-shifted rather than red-shifted.
  4. Jun 9, 2014 #3
    If it was it might be because the Andromeda galaxy is rotating and moving towards us quite slowly, so parts of it might show blue shift and parts red shift. Note that just because an object is moving towards you it doesn't necessarily mean it's on a "collision course". Think of Hayley's comet.
  5. Jun 9, 2014 #4


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    1) Do you have a source for this? I'm not sure, but there could potentially be a red-shift if the Cepheid were rotating away from us within the Andromeda galaxy as the galaxy as a whole was coming towards us. I don't know the relative speeds involved though, so I can't be sure.

    2) Local galaxies will have (average) blue/red shifts based on local motions. You are correct in thinking that the expansion of the universe only takes over for a larger scale. Exactly at which point this happens I'm not sure, but certainly larger than our local group. Even the Virgo super cluster, which is ~30 megaparsecs in size is not uniformly moving away from us I don't think.
  6. Jun 9, 2014 #5
    Thanks DH! That clears it up for me. The Cepheid gave a distance determination that cleared up the "island universe" thing. The red-shift, blue shift thing came later in the way one would expect. Got it! ;-)
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