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Redshift Confusion

  1. Dec 9, 2016 #1
    1. The problem statement, all variables and given/known data
    I encountred a question that says : Assume that gravitational interactions between galaxies in groups lead to random galaxy velocities of 300 km/s At what distance would the redshift predicted by Hubble's law dominate over that produced by individual velocities? and what is the redshift at that distance?
    I don't understand whats the difference between the 2 redshifts mentioned here, so I am confused on how to approach this

    2. Relevant equations
    V= h_0 * D
    where D is the distance and the hubble constant is 2.18*10^-18 s^-1

    V /c = Δλ/λ
    3. The attempt at a solution
    I tried substituting in the numbers and got D= 1.36*10^20 km and v/c=0.001.
  2. jcsd
  3. Dec 9, 2016 #2


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    Science Advisor

    I think you did it right. There is a smoothly increasing velocity moving away from us faster and faster as the distance increases (the "Hubble flow"), and superimposed on this is a random set of what are called "peculiar velocities". You are asked to determine how far away you need to look before the smooth Hubble velocities begin to dominate. A couple of comments: (1) I would put the distance in units more used by astronomers, like parsecs or Megaparsecs in stead of km, and (2) you gave a velocity, but you were asked for the redshift z.
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