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What's the Dark Matter Density in Universe?

  1. Jul 20, 2015 #1
    In wikipedia says Physical baryon density: ##Ω_bh^2=0.02230±0.00014## and
    Physical dark matter density:##Ω_ch^2=0.1188±0.0010##
    Matter density:##Ω_m=0.3089±0.0062##
    so If we collect baryonic matter density and dark matter density we cannot get matter density

    https://en.wikipedia.org/wiki/Lambda-CDM_model
     
  2. jcsd
  3. Jul 20, 2015 #2

    Orodruin

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    Notice the appearance of h^2 in the values you quote for DM and baryon abundances.
     
  4. Jul 20, 2015 #3
    Lets suppose I want to calculate dark matter mass/baryonic matter mass ? What should I do
     
  5. Jul 20, 2015 #4
    Whats the meaning of ##h^2## in here
     
  6. Jul 20, 2015 #5
    You ignore the h2 which tells you the result scales with H0. Substituting h~0.7 you get the correct results.
     
  7. Jul 20, 2015 #6
    I got it thanks.
     
  8. Jul 20, 2015 #7

    Orodruin

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    ##h## is the Hubble constant H today divided by 100, i.e., ##h \simeq 0.6780\pm 0.0077## (PLANCK 2013).
     
  9. Jul 20, 2015 #8
    I add them and I get 0.287 not 0.3089
     
  10. Jul 20, 2015 #9

    Orodruin

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    What did you use for ##h##? Using 0.678 gives me 0.3069.
     
  11. Jul 20, 2015 #10
    I used 0.49
     
  12. Jul 20, 2015 #11

    Orodruin

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    You mean you used ##h^2 = 0.49##? For ##h = 0.678## you will get ##h^2 = 0,46##. You should however note that ##h## also comes with an error. The reason that the abundance is given in ##\Omega h^2## is that this quantity is better bounded.
     
  13. Jul 20, 2015 #12
    Yeah I used h2=0.49
     
  14. Jul 20, 2015 #13

    Chalnoth

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    To get the right answer, you have to use the value of ##h## that was used to measure those parameters. As ##h = H_0 / 100 km/s/Mpc##, and ##H_0 = 67.74 km/s/Mpc## in that data set, ##h = 0.6774##. Use that number, and it will work. There will be some small differences, due to the fact that these numbers aren't published with full accuracy. But it'll be well within the errors.
     
  15. Jul 20, 2015 #14
    Finally.Thank you
     
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