Velocity and redshift

  1. What is the formula used to convert the measured redshift into a velocity?, not the approximated formula for low speeds v=cz , but the more general and accurate one.

    Thanks.
     
  2. jcsd
  3. atyy

    atyy 9,939
    Science Advisor

  4. Do you want the answer for special relativity or cosmology or both?
     
  5. For cosmology, the one used to get a velocity from the redshift and plug it in the Hubble Law formula.
     
  6. I think , this is the one

    v=[((1+z)^2-1)/((1+z)^2+1)]c=Ho*D

    c=light speed constant
    Ho=Hubble constant
    D=distance
    v=velocity
     
  7. George Jones

    George Jones 6,363
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    Gold Member

    No, this isn't correct. See section 3 from

    http://arxiv.org/abs/astro-ph/0310808.

    It is fairly easy to derive equation (1) from this paper.
     
  8. I don't think it is correct. For zero density universe it is:

    [tex]v=H_{0}D[/tex]

    [tex]D=(c/H_{0})ln(1+z)[/tex]
     
  9. The one I wrote is exactly equation (2) from that paper.

    This is not exactly what I wanted. I asked for the way to translate from z to velocity for high z or at least >1, this must be a very common formula for cosmologists, I'd say.
    The formula I used maybe is not correct for the Hubble law but I'm interested in the first part, express v as a function of z, is that so difficult?
     
  10. Ok, I see what you mean, after looking at the paper and the formula again, I see what you mean, but according to some cosmologists the formula that doesn't give superluminal velocities is alright too, and anyway this is a cosmology debate that I find artificial and tiresome and I don't really wanna get into it , I think it's been discussed enough in these forums, just remember that people as prestigious as David Hogg supports the view of cosmological redshift as Doppler.
     
  11. George Jones

    George Jones 6,363
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    Yes, but this is not the correct equation to use for cosmology.
    This expression and the expression that TrickyDicky gave in post #5 are both true in special relativity, i.e., in an empty universe. The conventions used for distance, however, are different in posts #5 and #7, and this leads to differing expressions for speed.
     
  12. Yes, for empty universe [tex]D=(c/H_{0})ln(1+z)[/tex] gives distance that goes into Hubble's law. Equation (1) you pointed at is general one, and [tex]\dot{R}[/tex] would depend on particular values of [itex]\Omega_{\lambda}[/itex] and [itex]\Omega_{m}[/itex] you choose.
     
    Last edited: Oct 8, 2010

  13. What debate?

     
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