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Dark Matter you say?

  1. May 25, 2007 #1
    I've heard much talk of dark matter and the universe being more massive (according to calculations) than it should be given the amount of observable matter in it. Or something to that effect.

    But Im thinking that since blackholes pretty much consume matter and gain mass and are unobservable to us other than their gravity's affect on the surrounding matter, that they would be my first guess as to where that extra mass is. Especially since - I would assume - there have been blackholes consuming whatever matter presented itself to it for billions of years now. i could see the mass of the amount of matter consumed to be quite massive by now.

    Why have they been ruled out and Dark Matter theorized?
  2. jcsd
  3. May 26, 2007 #2


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    Welcome to these Forums FrigginGenius!

    That is a good suggestion that actually has not been completely ruled out.

    It has been discussed before on PF here and here.

    The problems with the hypothesis that Dark Matter (DM) comprises of Black Holes (BHs) are:

    1. These BHs would have originally formed from ordinary baryonic matter but there is not enough baryonic matter in the mainstream [itex]\Lambda[/itex]CDM model to make all the BHs that would be required.

    Measured as a fraction of the critical density, the amount of DM is determined to be [itex]\Omega_{DM}[/itex] = 0.23, whereas the anmount of baryonic matter that could be produced in the standard model Big Bang Nucleosynthesis (BBN) is only [itex]\Omega_{b}[/itex] = 0.04.

    2. These BHs would gravitationally lens more distant stars and quasars in their line-of-sight and these lensing events should have been discovered.

    3. You have to explain how, in the early 'dark age' universe, the material formed such BHs and not stars, i.e what was the Initial Mass Fraction (IMF) of the first 'star' formation?

    For the hypothesis to stand up it would require:

    1. GR to be be modified to give a slower expansion rate in the BBN epoch so more baryonic matter could have been produced.

    2. The BHs to be of the right mass so they might not yet have been discovered,
    hence the ~102 - 104 [tex]M_{sun}[/tex] range.

    If they are Primordial BHs of mass less than [tex]10^{14}~g[/tex] they would have shown Hawking evaporation radiation, if there were lots of 1 ~102 [tex]M_{sun}[/tex] BHs then their microlensing events would have been detected, if they were more than 104 [tex]M_{sun}[/tex] then they would disrupt globular clusters and other large structures.

    3. An epoch of Population III stars, which would have been massive and which could have left behind a population of BHs and high metallicity gas and dust.

    Last edited: May 26, 2007
  4. May 26, 2007 #3

    Thanks for the welcome and the info Garth. This is the first forum I've posted on. Well, second but this one looks much more interesting. :smile: I'll try to make a habit of doing a search before posting from now on.

    See you around
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