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Homework Help: Rate of continuous creation

  1. May 2, 2016 #1
    • Thread moved from the technical forums, so no HH Template is shown.
    Estimate the rate of continuous creation required to keep the density of the universe constant at 10-26kg/m3. Express your answer in protons/year/km3.

    Assuming a spherical matter-dominated Friedmann universe, we know from solving the fluid equation that ρ ∝ 1/a3 (where ρ is the energy density and a is the scale factor).

    I believe the result of the continuous creation is that ∂ρ/∂r = 0 - hence the change in density due to expansion is cancelled out by the change in density due to the continuous creation:
    ∂ρexpansion/∂r = -∂ρcreation/∂r
    Thus I am expecting that the continuous creation's energy density will vary as ρcreation ∝ a3.

    However I do not know how to progress this any further, such that any advice, suggestions or solutions would be greatly appreciated! Note: I do not know what the correct answer is!
  2. jcsd
  3. May 2, 2016 #2

    Simon Bridge

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    You need to know how the scale parameter ##a## changes with time.
  4. May 3, 2016 #3
    And so this is the Hubble parameter then?

    H = 1/a × da/dt
  5. May 3, 2016 #4

    Simon Bridge

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    That seems a reasonable place to go... a lot depends on your coursework to date.
  6. May 4, 2016 #5
    OK thank you! :-)
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