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Why is the modern value of Hubble's constant so different from today's measured value

  1. Nov 14, 2011 #1

    s3a

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    Why is the modern value of Hubble's constant so different from today's measured value? At first I thought it was because the Hubble constant is related to density via:

    ρ_c = 3H^2/(8*π*G)

    but in the past 2011 - 1921 = 90 years, I don't think the density could have changed that much since it's a negligible amount of time in cosmological scales.

    So this means, it boils down to measurement improvements but what specifically?

    I would REALLY appreciate it if someone could explain and elaborate on this for me!
    Thanks in advance!
     
  2. jcsd
  3. Nov 14, 2011 #2
    Re: Why is the modern value of Hubble's constant so different from today's measured v

    Why is the value that Hubble originally calculated different than the one today.
    for starters:
    Improved instumentation.
    A better understanding of stars - originally Cephoid stars were thought to be of one type
     
  4. Nov 14, 2011 #3

    s3a

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    Re: Why is the modern value of Hubble's constant so different from today's measured v

    Thank you for your answer!

    Just to say though, the "Improved instrumentation" is the typical vague answer I got (which I wasn't satisfied with) but fortunately for me, I found the answer to that after extensive research.

    As for "originally [Cepheid] stars were thought to be of one type," I didn't know that and found that informative.

    Thanks again!
     
  5. Nov 14, 2011 #4

    s3a

    User Avatar

    Re: Why is the modern value of Hubble's constant so different from today's measured v

    Thank you for your answer!

    Just to say though, the "Improved instrumentation" is the typical vague answer I got (which I wasn't satisfied with) but fortunately for me, I found the answer to that after extensive research.

    As for "originally [Cepheid] stars were thought to be of one type," I didn't know that and found that informative.

    Thanks again!
     
  6. Nov 14, 2011 #5
    Re: Why is the modern value of Hubble's constant so different from today's measured v

    You are right, that is pretty vague.
    The list could be several pages long.

    Optical telescopes in space - example Hubble, Cobe
    Radio arrays.
    Infrared instrumentation.
    Since more of the electromagnetic spectrum received from space can be recorded and analyzed nowadays, the data from radio waves, to visible light, to x-rays from an object, can be compared for agreement.

    Better understanding of mass/luminosity for stars, supper novas.
     
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