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  1. #11
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    Quote Originally Posted by throneoo View Post
    Neutrinos could be dark matter if they don't interact through the weak force........however......we would need a tremendous amount of them to fill the missing mass...?
    Oh, I had forgotten about neutrinos. The issue with neutrinos is slightly complex but I'll try to explain:

    Neutrinos are very light. Light particles tend to move very fast. If you imagine kicking a ping pong ball and a basket ball with the same energy, the ping pong ball will move faster. So, if neutrinos were dark matter that's what we'd call "Hot Dark Matter". Remember that "Hot" means "particles move fast". The problem with Hot Dark Matter is that it makes it more difficult for galaxies to form. If most of the gravity is in neutrinos, because they move fast, they tend to "wash out" the galaxies and really get in the way of galaxy formation.

    The alternative to "Hot Dark Matter" is "Cold Dark Matter". That means heavy particles. If the particles are very heavy, they move very slowly, so they tend to gather together into clumps and those clumps become a catalyst for galaxy formation.

    In summary:

    Hot Dark Matter = Light particles = Hard to make galaxies.
    Cold Dark Matter = Heavy particles = Easy to make galaxies.

    So the standard dark matter model is the "Cold Dark Matter" model, usually called CDM.
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  3. #12
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    Quote Originally Posted by Dustworm View Post
    Regarding these sneaky WIMP's, could relativistic jets be used as a tool
    to detect them?
    Honestly, I don't know.
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  4. #13
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    Interesting indeed. And lets not forget about the super massive black holes in the center of galaxies, i bet they could catch a lot more.

    Regards


    Dustworm

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    Quote Originally Posted by DanielC View Post
    The other number that could be wrong is the 1/10,000 which I just guesstimated, but I suspect that if anything that number might be too low. I'm going to run a proper simulation to get the correct value, but I'm waiting for something else to compile first.
    I just got around to running the simulation. The correct value for the Sun is 1/4000. In other words, out of every 4000 WIMPs that go through the Sun, one gets captured.
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    Why can't a black hole be the guilty criminal? No one knows when it was formed or how much matter it's consumed. There's no meter at it's EH saying how heavy it is.
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  8. #16
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    Quote Originally Posted by Dudelabowski View Post
    Why can't a black hole be the guilty criminal? No one knows when it was formed or how much matter it's consumed. There's no meter at it's EH saying how heavy it is.
    I'm not sure what you are trying to say here: We know how much mass a black hole has consumed - that's just the black hole mass. We can estimate the density of black holes in the galaxy from microlensing events. Furthermore, black holes are made of baryonic matter and we can put independent constraints on the total amount of baryonic matter in the universe (the density of baryonic matter is reflected in the Hydrogen vs Helium ratio of the universe -- if you have more baryonic matter, you get more fusion and more helium).
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    This thread is amazingly interesting.
    Thanks DanielC.
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    Quote Originally Posted by Dustworm View Post
    Interesting indeed. And lets not forget about the super massive black holes in the center of galaxies, i bet they could catch a lot more.

    Regards


    Dustworm
    I'm pretty sure they would - not least because if I understand DanielC's point right the capture is proportional to area of the capturing body, and in a black hole the event horizon's area is proportional to the square of the mass, so for a single star it's true that WIMP captured would be fewer in the black hole than for the burning star, but for a supermassive blackhole at the centre of a galaxy or star cluster things could be quite different.

  12. #19
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    Quote Originally Posted by oldmancoyote View Post
    I'm pretty sure they would - not least because if I understand DanielC's point right the capture is proportional to area of the capturing body, and in a black hole the event horizon's area is proportional to the square of the mass, so for a single star it's true that WIMP captured would be fewer in the black hole than for the burning star, but for a supermassive blackhole at the centre of a galaxy or star cluster things could be quite different.
    I guess so... I checked and you are right that the area of the EH goes with the square of the mass. So one large black hole would capture more WIMPs than many small ones... My concern is that I don't see how you could test this observationaly. Black holes don't give you a lot of information (mass, spin and magnetic field are basically it), so I don't see how you could tell whether a BH has swallowed a lot of WIMPs or not.
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    Yep - unless there is some other WIMP-specific quantity that is preserved...

    Going to the other extreme - is the proportionality of the rate of capture to the area maintained independent of the density of the capturing body? What I mean is do we expect the capture rate of something like VY CMa to be the same as (say) that of a main sequence star? Or if you want to generalise the question, what is the interaction mechanism that allows capture? Purely gravity? Or some other interaction?

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