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  1. #21
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    Quote Originally Posted by oldmancoyote View Post
    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?
    For a star the issue is more complex because the way capture happens, a WIMP has to collide with a particle inside the star (basically hydrogen) and lose enough energy that it is bound to the star.

    For a black hole the situation is comparatively simple: If the WIMP crosses the event horizon, it's absorbed, otherwise it is not.

    This means that a star will capture a lot more WIMPs than a black hole with the same mass (it is a much larger target). But there aren't any billion-solar-mass stars in the universe, so there's nothing to compare a super-massive BH to.

    As a *very* *rough* estimate, you can assume that:

    WIMPs captured by a star = <const> * R^2 (where R = star radius).
    WIMPs captured by a BH = <const> * 10 * R^2 (where R = BH radius).
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  3. #22
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    Not talking about black holes... if the capture is due to collision/kinetic energy loss and then binding through gravity, in the case of a red hypergiant (e.g. VY CMa), where density of the outermost layers is extremely low, we would expect the capture rate to be lower, and not proportional to the area. Is this correct?

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  5. #23
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    Ok, I wasn't thinking.

    I haven't modeled red giants yet... On the one hand, the outer layers are very long density as you pointed out. On the other, the core of the star is higher density, and the core is where most of the capture occurs. So I don't have a general intuition here, I don't know what to expect.

    I'm busy this week, but I was planning to model red giants some time next week-ish.

    An unrelated problem with VY CMa in particular is that it is a very massive star. Massive stars are disproportionately luminous, so they'd need to capture a lot more WIMPs before the effects of WIMPs (e.g. energy through annihilation) become significant.
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  7. #24
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    Quote Originally Posted by DanielC View Post
    Let's try a rough calculation first:

    ...

    the ratio between these two quantities is:

    (1/10000 * R_Sun^2) / (R_BH^2) = 5.4 x 10^6
    Couple of days ago read article Dark Matter collisions with the Human Body. What is your opinion about this article? Can you calculate estimate collision of Dark Matter with Human Body in the same way how you estimate it with Sun and BH? If estimation 100,000 collisions per year for each human on the planet is correct? If such collision are dangers and effect to health?

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    @DanielC: Thanks! Would you mind posting here once you have run the models?

    When you say "before the effects of WIMPs become significant", you mean in terms of empirical detection of "this is what is happening", or in some other sense?

    Apologies for all the silly questions - it's great to have someone who is actually doing current research on the topic!

    @Astroval: shades of Philip Pullman...

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  11. #26
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    Quote Originally Posted by astroval View Post
    Couple of days ago read article Dark Matter collisions with the Human Body. What is your opinion about this article? Can you calculate estimate collision of Dark Matter with Human Body in the same way how you estimate it with Sun and BH? If estimation 100,000 collisions per year for each human on the planet is correct? If such collision are dangers and effect to health?

    The article looks correct. All the numbers, tables and formulae that I saw look right (WIMP cross sections, masses, collision rate, etc). I'm sure the calculation is fine. And 100,000 collisions pear year sounds like nothing.

    Perhaps a more interesting calculation is to figure out how much energy a WIMP has compared to, say, a cosmic ray. Take a WIMP mass of 100 GeV / c^2 (typical estimate is 60). WIMPs should be travelling at around 200 km/s. That means that it's kinetic energy is about:

    E = 1/2 * m * v^2 = 50 GeV / c^2 * v^2

    E = 50 GeV * ( 200,000 / 300,000,000 )^2

    E ~ 20 keV

    This is a very low energy compared to a cosmic ray which is at least 10 MeV.
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    This is all quite new to me so I need some clarification. When we say "captured" it normally means bound in some way. Thus the WIMPs in our galaxy are already bound by the gravity of the galaxy. To me that is "capture."

    When a WIMP crosses the eh of a bh, I don't question the probability that it can't escape. That is a space-time issue.

    But what happens when a WIMP interacts with the mass of, say, the sun? That's a very different "capture." Can fusion take place? Can the collision make the WIMP larger? If so, does it retain its WIMPiness? I haven't seen evidence of mass disappearing in a model due to WIMP collisions.

    Or does the WIMP give up so much energy that it simply stays inside a star? That would imply a hotter interior resulting from WIMP absorbtion. That would, in turn, affect the observations on the surface. Is this where the theory is headed?

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    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. If I assume the WIMPS in our galaxy are each "bound" by the central hole, directly or indirectly, they will all eventually decay into it unless absorbed by collision with some star. The r^2 term only applies to collisions.

    I would expect that capture would occur as soon as they are gravitationally bound. 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. But since r is greater for the sun, collision would happen sooner if orbit decay rates are similar.

    What my thinking suggests is that the model needs to start at the time a WIMP begins an orbit. It then needs to have a time line for decay and probability of collision.

  15. #29
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    Quote Originally Posted by kencrowder View Post
    This is all quite new to me so I need some clarification. When we say "captured" it normally means bound in some way. Thus the WIMPs in our galaxy are already bound by the gravity of the galaxy. To me that is "capture."
    A WIMP is bound to the galaxy, but that doesn't mean that it is bound to a star. The Earth is bound to the Sun. A WIMP that is bound to the Sun would be in some sort of orbit. In practice it would probably be a very elongated orbit.


    Quote Originally Posted by kencrowder View Post
    But what happens when a WIMP interacts with the mass of, say, the sun? That's a very different "capture."
    This will be clearer if we don't use the word "interact" which is very vague... A WIMP can interact by the weak nuclear force and gravity. The weak nuclear force basically means that a WIMP can collide with a particle in the Sun (e.g. a proton). After the collision, the WIMP can lose some speed. If it loses enough speed, it will be bound to the sun - aka. it will be in some elongated orbit that crosses the Sun.

    Can fusion take place?
    No.

    Can the collision make the WIMP larger?
    A WIMP is a particle. A particle can't just get larger.


    I haven't seen evidence of mass disappearing in a model due to WIMP collisions.
    Nobody is proposing that mass disappears.


    Or does the WIMP give up so much energy that it simply stays inside a star?
    That is closer to the truth. Though a typical first WIMP orbit would be very elongated, so the WIMP would go ways out of the star, perhaps about as far as the orbit of Jupiter, and then back down into the Sun. It's a very eccentric orbit.

    After a number of orbits, by chance, a WIMP would collide again, and then it would lose additional energy.... With each collision, the WIMP orbit would shrink... After just a handful of collisions (a few hundred years) the WIMP orbit would be entirely inside the Sun. And after a few more, the WIMP orbit would be entirely inside the core of the Sun.

    In this way, WIMPs would quickly accumulate right in the middle of the Sun.

    That would imply a hotter interior resulting from WIMP absorbtion. That would, in turn, affect the observations on the surface. Is this where the theory is headed?
    It wouldn't make the interior hotter. First, the energy deposited is insignificant compared to the energy you get from fusion. But even if it wasn't, the fact is that the Sun is always just the right temperature to keep itself from collapsing due to gravity. It works this way:

    * Imagine that you magically compress the Sun a little, or make it a bit hotter... That would increase the pressure inside the Sun, causing it to expand... The expansion of the Sun would then bring down the temperature and pressure.

    * Imagine that you magically expand the Sun a litle, or make it colder... That would decrease the pressure in the Sun. The reduced pressure would no longer be quite enough to hold the Sun's material against gravity, so the Sun's gravity would make it compress again.

    Either way, the Sun goes back to where it was. This is basically why stars are stable. Stars are in a stable equilibrium. If for any reason they shift a little to one side, the way they respond brings them back to the equilibrium point.

    The main way that WIMPs could affect a star is that WIMPs can meet each other at the centre of the star, where they aggregate. When they collide, they would annihilate, because the theory is that WIMPs are their own anti-particles... In this way, even a small mass of captured WIMPs could add a meaningful bit of energy to a star. The upshot is that the star would not need to burn as much fuel to hold itself up against gravity, leading the star to live longer.

    In brief, the effect we would like to find is a star that seems to have evolved too slowly.
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  17. #30
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    Quote Originally Posted by oldmancoyote View Post
    @DanielC: Thanks! Would you mind posting here once you have run the models?
    I guess I could... but I'm not sure how interesting it would be... I can give you the high-level results already:

    1. Stars capture 1000 times more WIMPs than previously thought.

    2. This is interesting, but still not enough. We would be very lucky to find a star that has captured enough WIMPs to matter, and it would have to be in the galactic centre.

    3. Binary stars capture fewer WIMPs than single stars. The effect of the two points of gravity causes WIMPs to be scattered around and lost.

    I began the project hoping that binary stars might capture more WIMPs, but fearing that they might now... The only way to find out is to do a full simulation and see what happens. So that's what I'm doing.


    When you say "before the effects of WIMPs become significant", you mean in terms of empirical detection of "this is what is happening", or in some other sense?
    I mean in terms of having any meaningful effect at all on the star... The main effect of WIMPs is that, through matter-antimatter annihilation (WIMPs are thought to be their own antiparticles) they'd deposit energy into the star, causing it to burn fuel more slowly. So the star would evolve more slowly. So when I talk about the effect of WIMPs being significant, I'm mainly thinking in terms of WIMP annihilation producing enough energy to contribute something to the star's energy budget.
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