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Dark Matter and black holes

  1. Dec 13, 2004 #1
    If dark matter has mass, interacts with gravity, and is pervasive then why isn’t it gathering at points of mass and turning all the suns into black holes?
  2. jcsd
  3. Dec 13, 2004 #2
    I have also wondered this too. In most descriptons I read, it seems to act totally different from normal matter, why? In a recent programme I watched, a scientist made an announcement that DM in globular clusters was at a much higher density that spiral galaxies, why aren't we seeing DM blackholes and other measuable effects? It only seems to add to the 'average' mass of galaxies without even producing localised anomalies.
  4. Dec 13, 2004 #3
    Because dark matter is weakly interacting with other dark matter. It doesn't act like normal matter.

    Take two large masses of normal matter, separate them, and let them go. They will gravitate towards one another and hit, eventually forming a large single mass.

    Take two large masses of dark matter, separate them, and let them go. They will gravitate towards one another and pass through each other, slow down, stop, and repeat. At least this is what it looks like it does.

    Dark matter doesn't really clump. If you look at the dark matter distribution for galaxies you see a smooth increase in density radially with no clumping up anywhere. It basically forms the same system you would get with regular gas if the atoms passed right through each other.
  5. Dec 13, 2004 #4


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    You pretty much summed it up silverpig. DM is both dark and mysterious. It is so anti-social it does not even interact with it's own kind. The major role of DM in modern cosmology was to provide the gravitational seeds needed to initiate stellar and galactic formation in the early universe. Without DM, it is thought galaxy formation would have taken longer and fewer would have formed, perhaps much fewer.
  6. Dec 14, 2004 #5
    Ah. OK. Could it fall into stable orbits around massive objects or is the reason we do not observe the effect of dark matter’s gravity locally because it has a uniform density throughout the galaxy?

    As the solar system was forming would you expect dark matter to go through the same "sieving" process as the visible matter, heavier elements ending up close to the sun? Is there a ring of dark matter orbiting the sun?
  7. Dec 14, 2004 #6
    When a cloud of normal matter collapse by its own gravity then its temperature increased by a large amount. Since normal matter can interact with radiation so it is easy for it to cool by radiative colling. This cooling enhance further gravitational collapse. Since dark matter does not interact with rdaition so it can not cool and this heat prevents it from further collapsing. Conclusion is that dark matter clusters less stronger than normal matter.
    Last edited: Dec 14, 2004
  8. Dec 14, 2004 #7
    Interesting, and I find the concept of some sort of “super insulated sun” amusing but…
    If dark matter cannot interact with itself, why would a dense collection of it cause an increase in temperature? In fact, if dark matter is so weekly interacting, could several particles exist at the same point in space simultaneously?
  9. Dec 14, 2004 #8
    In genearl when we talk about temperature then we mean kinetic energy. When a dark matter cloud collapse then its potential energy increases
    (in magnitude, it becomes more negative). Since the total energy must remain same so the kinetic energy also increases. As long as dark matter particles are moving fast they can not cluster much. Radiative cooling is the only way by which they can slow down their motion, which is not possible for dark matter.

    Dark matter is considered to be collisionless, means in dark matter binary collisions are rare. Either more than one dark matter particles can exist at the same place depends on their nature. For example if you take neutrino as dark matter particle, then they can not exist at same place because they are fermi particles.
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