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Homework Help: Onset of Universe acceleration

  1. Oct 31, 2015 #1
    1. The problem statement, all variables and given/known data
    Assume the cosmological model with H0=72,Omega_M=1-Omega_lamda=0.3,(so dark energy with w = − 1) and a flat universe.)
    a) Find the redshift z at which the universe starts accelerating (that is, when it transitions from decelerating to accelerating).
    b) How long ago did this happen?
    2. Relevant equations

    3. The attempt at a solution
    I don't quite understand the accelerating here. Does it mean that the expanding rate begins to accelerate? How should I start this problem?
  2. jcsd
  3. Oct 31, 2015 #2


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    Accelerating expansion means that the second derivative of the scale factor ##a(t)## is positive.
  4. Nov 1, 2015 #3
    I see. Should I use the deceleration formula?
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