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H naught?

  1. Jun 21, 2014 #1

    chasrob

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    I read at Space dot com that astronomers have made a precision measurement of Hubble's constant of 68 km/s/million light years and their (I'm pretty sure) paper says so.

    222 km/s/Mpc? Isn't that triple the ~70 that most others have claimed?
     
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  3. Jun 21, 2014 #2

    Bill_K

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    I was confused by that too. I think the value given in the space.com article is just a typo. In the paper, they don't mean the current Hubble constant, rather what it was at an earlier epoch, H(z = 2.34).
     
  4. Jun 21, 2014 #3

    marcus

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    Sloppy pop journalism. the Hubble expansion rate has been declining so it was greater in the past and they have measured what it was in the past (at z = 2.34)

    "...we find DA(z=2.34)=1662±96(1σ) Mpc and H(z=2.34)=222±7(1σ) kms−1Mpc−1…"
     
  5. Jun 21, 2014 #4

    marcus

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    Let's see how well their figure of 222 agrees with the standard cosmic model in Jorrie's calculator.

    If I paste this into google:
    1/(222 km/s per Mpc)

    I get that the Hubble time (back in the z=2.34 day) was 4.4 billion years (according to Delubec et al).

    On the other hand if I but S = 1+z = 3.34 into Lightcone
    I get that the Hubble time back then was 4.13 billion years. Close enough.
     
  6. Jun 21, 2014 #5

    marcus

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    Jorrie's Lightcone calculator, in the default, uses the two model parameters 14.4 and 17.3 billion years as present and eventual Hubble times.
    But you don't have to stay with the default, you can try varying. Those correspond to the 2013 Planck mission estimates, but it lets you select WMAP estimates (see the button at the top) which are 14.0 and 16.5

    I happened to go to the WMAP option and then split the difference between 14.0 and 14.4 so I typed in and looked the case 14.2 and 16.5 billion years.

    It matched the Delubec et al 4.4 billion year Hubble time (i.e. their 222 km/s per Mpc @ z=2.34

    [tex]{\scriptsize\begin{array}{|c|c|c|c|c|c|}\hline R_{0} (Gly) & R_{\infty} (Gly) & S_{eq} & H_{0} & \Omega_\Lambda & \Omega_m\\ \hline 14.2&16.5&3300&68.9&0.741&0.259\\ \hline \end{array}}[/tex] [tex]{\scriptsize\begin{array}{|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|r|} \hline a=1/S&S&T (Gy)&R (Gly)&D_{now} (Gly)&D_{then}(Gly)&D_{hor}(Gly)&V_{now} (c)&V_{then} (c) \\ \hline 0.299&3.340&3.0038&4.4009&19.410&5.811&10.562&1.37&1.32\\ \hline \end{array}}[/tex]

    So we could interpret Delubec et al as saying that TODAY Hubble rate could be slightly larger than what Planck mission says. Namely it could be the reciprocal of 14.2 billion years instead of 14.4.

    A smaller Hubbletime corresponds to a larger Hubble rate. Anyone who wants to see what 14.2 corresponds to can paste this into google:
    "1/(14.2 billion years"
    and get 2.23160314 × 10-18 hertz and then convert that to km/s per Mpc
    If I paste this thing into google, to do the conversion:
    "2.23160314 × 10^(-18) hertz in km/s per Mpc"
    I get 68.86 km/s per Mpc

    So the Delubec et al result doesn't really seem much at variance with what we already thought. :biggrin:
     
    Last edited: Jun 21, 2014
  7. Jun 22, 2014 #6

    chasrob

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    So roughly 8 billion years ago H was 222 km/s/Mpc? As I understand it--and I may be wrong--there was a period of accelerated expansion before this. Would this mean when the acceleration ended, H reached a maximum and thereafter slowed? Is there any way to guesstimate this maximum of H (even very roughly) in km/s/megaparsec?
     
  8. Jun 22, 2014 #7

    chasrob

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    Whoops, I'm in error. The acceleration is in the future, so my query is meaningless. Last night was pretty rough on me.:tongue:
     
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