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I Matter +lambda universe

  1. Oct 15, 2017 #1
    for the matter+lambda universe why is Ω[Λ,o]=1-Ω[m,0]?
     
  2. jcsd
  3. Oct 15, 2017 #2

    PeterDonis

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    Where are you getting this equation from? Do you have a reference?
     
  4. Oct 15, 2017 #3

    kimbyd

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    Because the sum of the ##\Omega## terms is defined to be equal to one, and a matter+lambda universe has only matter and a cosmological constant term.
     
  5. Oct 16, 2017 #4
    will you please explain why is the net Ω is equal to 1,and net Ω consists of which components
     
  6. Oct 16, 2017 #5
    will you please explain the reason for being total omega to be 1
     
  7. Oct 16, 2017 #6

    PeterDonis

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    @Das apashanka , you marked this thread as "A", indicating a graduate level knowledge of the subject matter. The questions you are asking indicate that you don't have that background; accordingly, I have changed the thread level to "I".
     
  8. Oct 16, 2017 #7

    kimbyd

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    There's no deep meaning. The parameter is just defined that way.
     
  9. Oct 16, 2017 #8

    George Jones

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    It isn't, necessarily.

    This only is true for spatially flat matter/Lambda universes, i.e., it is not true for general matter/Lambda universes.
     
  10. Oct 16, 2017 #9

    kimbyd

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    Whichever ##\Omega## values you include in your model, their sum is always identically equal to one. That's how they're defined: as density fractions. If you have matter, cosmological constant, and curvature, then it's:

    $$\Omega_m + \Omega_\Lambda + \Omega_k = 1$$
     
  11. Oct 16, 2017 #10

    George Jones

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    Of course. As you say, this is true **by definition**. But this is not what the original poster asked about. A question equivalent to the OP's question is "Why is ##\Omega_k = 0##?"

    One possible answer is that the OP is looking at the (very close to) spatially flat observed universe at a time when ##\Omega_r## is negligible. One nice thing about this universe is that there is an explicit expression for the scale factor.

    If, however, this an exercise in theoretical cosmology, there is no reason for

    $$\Omega_m + \Omega_\Lambda = 1$$

    in a matter/Lambda universe, which is what the OP asked about. So, what Peter asked is necessary,

     
  12. Oct 16, 2017 #11

    kimbyd

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    That's more or less the point I was getting at. There's no deep reason here. It's just the way the terms are defined.

    But yes, there might be a more important question that is being missed here, so we might be able to offer better guidance if the broader context of the question were presented.
     
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