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  1. #31
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    Quote Originally Posted by kencrowder View Post
    Another troubling thought results from the comparison of "capture" by the sun vs bh. Capture begins by gravitaional binding. The bh has a much greater reach. Thus the probablility of a bh absorbing a WIMP depends on more than just the radius of the eh.
    A black hole does not have any more reach than a star of the same mass. If you magically replaced the Sun with a black hole with the same mass, Earth's orbit wouldn't notice.

    Ignoring the effect of gravitational waves (which is negligible for 99.9999999999999999% of cases) particle orbits have constant energy. A particle that comes toward a star or BH from "infinity", will follow a parabola or hyperbola, have a close encounter, and go back to "infinity". The only way you can capture this particle is if you make it lose energy, like, through a collision.

    If I assume the WIMPS in our galaxy are each "bound" by the central hole, directly or indirectly,
    The central black hole only has 0.002% of the mass of the galaxy. So stars and particles in the galaxy are normally bound "to the galaxy" rather than to the black hole.

    they will all eventually decay into it unless absorbed by collision with some star.
    I'm not sure how to respond to this... for all intents and purposes, orbits do not decay... If you want to be pedantic, then technically all orbits emit gravitational radiation, but this is completely negligible... For any star to decay into the central black hole it would take gazillions of times longer than the age of the universe, by which point the galaxy has probably largely disintegrated by gravitational scatter between stars.


    I would expect that capture would occur as soon as they are gravitationally bound.
    To become gravitationally bound to a star, a WIMP has to lose energy. For a BH it's a bit more complicated, but it does have to get fairly close to the event horizon.


    I'll call that radius R. In my thinking, R is much greater for a bh and thus it would eventually grab far more WIMPS.
    Which radius?

    Anyway, a BH does not have stronger gravity than a star of the same mass.

    But since r is greater for the sun, collision would happen sooner if orbit decay rates are similar.
    In every sense that matters, orbits don't decay. The only case where an orbit decay is even measurable is in a double pulsar, where there are two neutron stars orbiting each other... By measuring the pulse rates very accurately we atomic clocks, we can detect a small decay in the orbit. The two neutron stars will collide in 85 million years.

    But this is a very extreme example, and it is totally unrelated to WIMPs.
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  3. #32
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    As usual, in an attempt to be brief I'm force to not be precise. By the term "orbit decay" I'm thinking of all the many ways an orbiting body may lose energy and thus have a lower orbit. These include, but are not limited to, tidal action, radiative loss, collisions, etc.

    Your discussion on how WIMP collisions effect a transfer of energy was especially helpful. Thanks

  4. #33
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    Quote Originally Posted by kencrowder View Post
    As usual, in an attempt to be brief I'm force to not be precise. By the term "orbit decay" I'm thinking of all the many ways an orbiting body may lose energy and thus have a lower orbit. These include, but are not limited to, tidal action, radiative loss, collisions, etc.

    Your discussion on how WIMP collisions effect a transfer of energy was especially helpful. Thanks
    Ok. I get your meaning now.

    For WIMPs, AFAICT the only way to drop the orbit is collisions with stellar material. They don't radiate, and tides wouldn't be significant for a particle.
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  6. #34
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    Thanks again for the education. But I'm still trying to understand the nature of WIMP interaction with the world.

    Particles do radiate when they lose energy. It results in an EM photon. This only requires an interaction with an EM field. But the weak interaction nature implies to me that Maxwell's equations may not apply here. The interaction might be at a nuclear level only. It may not interact with even a strong magnetic field.

    Can I visualize a collision as being similar to a neutron-neutron collision?

    Neutron stars demonstrate that they do interact with the EM field. Magnetic fields are required to form the jets we observe.

    I guess I'm looking for the characteristic differences which are probably ignorable in a first order approximation. How does the model show scattering occurs between two particles? Are the ignorable characteristics still ignorable in a one on many scatter?

    This may require an explanation that is well beyond the scope of this thread. I'm only trying to indicate where my head is taking me.

  7. #35
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    Quote Originally Posted by kencrowder View Post
    Thanks again for the education. But I'm still trying to understand the nature of WIMP interaction with the world.
    Think of them as neutrinos, but with two changes:

    1. WIMPs are very massive (about 100 proton masses).

    2. WIMPs are their own anti-particle (so they annihilate).


    Particles do radiate when they lose energy. It results in an EM photon. This only requires an interaction with an EM field.
    This is only true for particles that interact by the electromagnetic force. WIMPs, like neutrinos, do not. Emitting a photon is a manifestation of the electromagnetic force. Neutral charge particles do not emit photons.

    Can I visualize a collision as being similar to a neutron-neutron collision?
    With caveats... I guess you could... Neutrons are messy particles. They are made of three quarks, which makes their collisions messy and complex compared to point particles like electrons.

    Neutron stars demonstrate that they do interact with the EM field. Magnetic fields are required to form the jets we observe.
    Neutron stars are not particles. They are very complex objects, which actually include a fraction of protons and electrons. These are responsible for making the neutron star into an electrically conducting fluid that can have a magnetic field.


    I guess I'm looking for the characteristic differences which are probably ignorable in a first order approximation. How does the model show scattering occurs between two particles? Are the ignorable characteristics still ignorable in a one on many scatter?
    I'm not sure what you are asking here. But maybe some of what I said earlier helps clarify things.

    I treat the WIMPs as having elastic collisions with particles in the Sun.
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  9. #36
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    Quote Originally Posted by DanielC View Post
    Think of them as neutrinos, but with two changes:

    1. WIMPs are very massive (about 100 proton masses).

    2. WIMPs are their own anti-particle (so they annihilate).

    I'm not sure what you are asking here. But maybe some of what I said earlier helps clarify things.

    I treat the WIMPs as having elastic collisions with particles in the Sun.

    This is my problem. To have an elastic collision there must be a force involved. If WIMPs aren't made of quarks, why would they interact with the weak field?

    Maybe that's where I'm off. I think of neutron-neutron elastic collisions resulting from weak field forces.

    Of course if it is the strong field, that would help me quite a bit. The collision cross section would shrink to explain the low probability.

    I'm also thinking that the mass is in some way related to the Higgs which we may learn about in the next year or so from CERN. If it is, then your work might provide an independant view of the Higgs.

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    Quote Originally Posted by kencrowder View Post
    This is my problem. To have an elastic collision there must be a force involved. If WIMPs aren't made of quarks, why would they interact with the weak field?
    The weak force is not about quarks. Electrons and neutrinos also interact by the weak force and they are not made of quarks either. Neutrinos can collide with atomic nuclei - this is how neutrino observatories work. The idea is that WIMPs can do the same thing.

    Of course if it is the strong field, that would help me quite a bit. The collision cross section would shrink to explain the low probability.
    No, it's the weak force. And that already has a very low probability. As I said before, WIMPs are like neutrinos but heavy. How often do neutrinos collide with atomic nuclei? Not very.
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    Can we indirectly (or directrly) to see dark matter? Answer is may be. The WMAP satellite in microwave spectrum saw a "haze" coming from electrons near the galactic center:


    May be near the galactic center, the dark matter particles annihilate in pairs and create relativistic electrons and positrons which generate a synchrotron radiation when interacting with the galactic magnetic field and produce the additional gamma rays observed by Fermi





    See WMAP Haze: Directly Observing Dark Matter?

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