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I Andromeda and the cosmological redshift

  1. May 15, 2018 #26

    JMz

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    Distance to M31 is 800 kpc, right? So we would get the earlier 55 km/s estimate.
     
  2. May 15, 2018 #27
    OMG you're absolutely right, I misplaced a decimal point! Sorry!
     
  3. May 19, 2018 #28

    JMz

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    'S OK. I have never in my life misplaced a decimal point. Or a minus sign. Hah! ;-)
     
  4. May 21, 2018 #29
    This is logically flawed: "without the expansion of the universe" there would be no Milky Way, no Andromeda. No galaxies at all.

    Now, you can look at the current situation of where these two galaxies are and how they move, disregarding their deep past. In this case, "expansion of space" or lack thereof is simply a choice of coordinates. You can choose one where "space expands" or one where it does not. Both are equally valid. Physical observables, such as relative velocity of these two galaxies, are independent from coordinate choice. Ergo, "we should have been seeing" the same approach velocity.

    Physically observable changes are only seen for the case where expansion of space is not constant - in particular, when it is increasing, as appears to be the case. But this (1) has very little effect on the distance scale of Andromeda;
    and (2) the effect is very small anyway - if you want to directly measure (as opposed to inferring it by comparing observed data to cosmological models w/o dark energy) that expansion of space is increasing, by showing that distant galaxies' redshifts are increasing [actually, that "they decrease slower than they should"] - this would require very precise and very long-baseline measurements. IIUC such measurements are not within our capabilities yet.
     
    Last edited: May 21, 2018
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