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A question about Friedmann Equations

  1. May 27, 2009 #1


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    Hello everyone,

    Its been 4 years now since I posted, Last time i was asking for school homework and now I'm about to finish my physics master degree with 1 exam left in cosmology.

    As I'm sure you are aware, the k term in the Friedmann equation represents the curvature term k = 0, -1 & +1. I have a few queries about this.

    Why is this term restricted to these values? Is a continuous k not valid under the FRW model?
    Secondly, more generally, does this k represent the "shape of the universe" and is different from the curvature in GR?

  2. jcsd
  3. May 27, 2009 #2

    George Jones

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    Yes, but coordinates in FRW models that allows this can always be rescaled so that k only takes the values -1, 0, 1.
    k represents the curvature of the spatial hypersurfaces that result for each fixed value of cosmological time, not the curvature of spacetime.
  4. May 28, 2009 #3


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    Yup. However, today we often don't worry about measuring k, but instead Omega_k, which most certainly does take on a continuous value. Also, I may be mistaken, as it's been a while since I looked at this, but I think that the freedom to rescale the coordinates is already taken up by the convenient choices we make in order to apply the FRW equations to measuring the expansion (t = 0 at the false singularity, t = 1 today).
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