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Hubble+/- 4.3%, is the constant constant ?

  1. Aug 20, 2008 #1
    Hubble+/- 4.3%, is the constant "constant"?

    Why is the Hubble constant assumed to be "constant"? Just to keep things simple a la Occam's razor? Or is there a lot more to it than that?

    With further increases in the accuracy of its measurement, and a few centuries to make repeated measurements, is anyone placing bets yet on its value turning out to NOT be a constant rate of acceleration?

    I.e. the rate of acceleration is either increasing or decreasing at a constant rate.
    Or, the rate of acceleration is a function of some as yet undetermined factor that changes subtly over time.
     
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  3. Aug 20, 2008 #2

    mgb_phys

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    Re: Hubble+/- 4.3%, is the constant "constant"?

    The light from more distant objects left at an earlier time, so plotting distance against redshift (ie rate) you are measuring the expansion at different times.
     
  4. Aug 20, 2008 #3

    marcus

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    Re: Hubble+/- 4.3%, is the constant "constant"?

    Cosmologists do not assume that the Hubble parameter is constant over time. In fact the standard model of the universe requires that H(t) change with time in a certain way. So when they calculate they take account of that.

    Monsters, where did you get the idea that H(t) is supposed to be constant over time? Nobody who knows what they are talking about would tell you that. It must be the bad verbal terminology.

    H0 is the symbol used for the value of H(t) at the PRESENT time.

    The Hubble law is that v(t) = H(t)D(t). At any given moment of time, the current recession speed at that moment is equal to the distance D(t) at that moment, multiplied by the current value of the Hubble.
    For convenience you write it v = H0 D, where everybody is supposed to understand that here v and D are the present recession speed and distance. The type of distance is that measured by observers at rest with respect to CMB, same with time also---that as measured by observers at rest with respect to CMB. If that doesnt mean anything don't worry, it is a technical detail about how distances and the present moment are defined.

    If you want to see how H(t) has changed over the (billions of) years. Try out Morgan's calculator in my signature. If you put in a redshift like z= 10 the calculator will tell you what the Hubble was back then when the light was emitted, that we now see redshifted by that amount.
    You can easily find out that H has changed by a hundred fold or more over the course of history.
     
    Last edited: Aug 20, 2008
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