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Astronomy cafe

  1. Jul 7, 2014 #1
    I found an equation relating hubbles constant and time!It is highly simplified but helpful!


    da/at = (2/3)*a0* (1/t0) * (t/t0)^-1/3

    then dividing by a = a0 (t/t0)^2/3 you get

    H(t) = H0*(t0/t)
  2. jcsd
  3. Jul 7, 2014 #2


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    This is not simplified -- it is the explicit Hubble parameter as a function of time in a matter-dominated Robertson-Walker universe. It follows from the Friedmann and continuity equations of general relativity.
  4. Jul 7, 2014 #3


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    Maybe the simplification is in the fact that it does not always give the right numbers, because our real universe has not been and will not be all the time matter-dominated. It has been radiation dominated for a while in the past, and then kind of a balanced mix for a while. And lately it is gradually shifting over from matter dominance to Lambda-dominance.

    The main trouble, though, is what you see if you follow the link. The author refers to this as "the speed of expansion of the universe".

    The equation quoted is about H(t). It is not about "the speed of expansion of the universe." People often get confused, it seems to me, when they are told that the universe has a speed of expansion:

    1. then they get the idea that it must have a definite known size (else how could it have a speed of expansion?)

    2. and they get the idea that this imagined speed must be increasing because they have heard "acceleration" mentioned. (whereas H(t) has been and is expected to continue decreasing).

    So they get a bunch of misconceptions that IMHO take root primarily because of careless language.
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