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B Dark matter variations on Earth

  1. Aug 21, 2017 #1
    Are there variations of dark matter concentration on earth based on months or seasons of the year?
     
  2. jcsd
  3. Aug 21, 2017 #2

    phinds

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    Hard to say since no dark matter has ever been detected directly and the density in our solar system is very low even by the standards of dark matter. As I recall, the general estimate is that in the entire solar system there is enough dark matter to make up the mass equivalent of a modest sized asteroid.
     
  4. Aug 21, 2017 #3
    The sun is the most massive object in the solar system and the most gravity.. does it mean all the dark matter has gravitated into the sun?
     
  5. Aug 21, 2017 #4

    phinds

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    No, it's spread out. Even if it fell into the sun, it would just pass through and go out the other side
     
  6. Aug 21, 2017 #5
    You mean the sun gravity is not enough to attract any dark matter even those with stronger coupling to matter of our universe?
     
  7. Aug 21, 2017 #6

    Drakkith

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    No, he means that dark matter falls into the Sun and passes right through, emerging on the other side with the same speed that it started with. This is because the speed it gains falling into the Sun is then given back up as it moves further out of the Sun's gravity well. Imagine a pendulum. Lift it up and then drop it and the pendulum first accelerates downwards, gaining speed until it reaches the bottom before decelerating as it begins to swing upwards. If it weren't for friction and air resistance, the pendulum would oscillate forever, moving back and forth to the same height on each upswing. Dark matter would do the same thing except that it is already likely moving too fast to be captured into any sort of orbit or oscillatory motion by the Sun. It still accelerates while falling towards the Sun and decelerates while moving away, but this results in zero net change in speed.
     
  8. Aug 22, 2017 #7
    you mean only a black hole can really capture dark matter? or how many sun masses before it can capture dark matter or put it in orbit?
     
  9. Aug 22, 2017 #8

    Chronos

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    Dark matter is evidently no better at interacting with other dark matter than ordinary matter. See, for example, the bullet cluster. Only a black hole has the ability to actually entrap dark matter. Even then it can only capture particles that fall directly in. DM cannot be slowed allowing it to spiral into a BH as does ordinary matter Simulations suggest only a tiny fraction of BH mass can be attributed to DM..
     
  10. Aug 22, 2017 #9
    Does Sun leave a tail moving through dark matter?
     
  11. Aug 22, 2017 #10

    phinds

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    Sun doesn't move through dark matter. Dark matter moves through sun. No tail.
     
  12. Aug 22, 2017 #11

    Drakkith

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    That's hard to say, as both dark matter and the Sun are moving with respect to each other and with respect to the galaxy overall. I'm sure that the Sun moves through dark matter that's in the process of falling in towards the center of the galaxy, but I don't think this would form a tail at all. However I admit that my knowledge of this topic is extremely limited.
     
  13. Aug 23, 2017 #12

    mfb

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    Dark matter didn't form a rotating disk like the regular matter. It is more like a big cloud of gas with dark matter particles moving around randomly. The Sun moves through this cloud, but as the particle motion is random and nearly uniform before, it is random and nearly uniform afterwards as well.

    Due to Earth's orbit, we move through the cloud at different speeds for different times of the year. In principle this could be measurable as yearly variation of the detection rate - assuming we find dark matter in the lab in the first place.
     
  14. Aug 24, 2017 #13
    Does not follow.
    Suppose that the nearly uniform random motion were slow relative to movement of Sun relative to dark matter. In that case, the dark matter ahead of Sun could be approximated as an array of particles stationary with respect to dark matter frame.
    Now, the particles that move directly the centre of Sun are not deflected. They accelerate as they fall into Sun, then decelerate after passing Sun, to the exact same speed and direction as before.
    But particles that move past Sun are deflected. Since they have no way to confer energy to Sun in a frame where Sun is stationary, nor angular momentum, they follow hyperbolic orbits (if outside Sun). Their speed after passing the Sun is the same relative to Sun but in a different direction.
    Which means that in the dark matter frame 1) the particles that passed near Sun acquire a substantial crisscrossing velocity they did not previously have, and 2) they also acquire a component of velocity towards Sun´s movement they also did not previously have.
    Also: since Sun is conferring momentum to dark matter, Sun must itself be slowed down.
    Meaning that the mass distribution as perturbed by passage of Sun must contain some sort of tail attracting Sun from behind.
    Correct?
     
  15. Aug 24, 2017 #14

    mfb

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    The particles are not slow relative to the Sun, they have a similar speed.
    Yes they get some momentum on average, but the number of particles getting a significant momentum change is tiny.
     
  16. Aug 25, 2017 #15
    do all kinds of dark matter interact with gravity? Is there no dark matter that doesn't interact with gravity?
     
  17. Aug 25, 2017 #16
    Everything that has a mass interacts with gravity.
     
  18. Aug 25, 2017 #17
    is there no dark matter that has no mass that doesn't interact with gravity? yet say it only interacts with matter...

    or are there no dark matter that interacts with matter yet doesn't have any mass nor gravity?
     
  19. Aug 25, 2017 #18
    All dark matter particles are massive, as far as I know (WIMPs). And gravity and weak interaction are the only interactions via which dark matter interacts.
    So now let's take again your questions, one by one:
    (Answers: as far as I know, and based on what I stated in these two posts of mine)

    1.
    Yes

    2.
    No
    (but your question is not syntactically correct [it has two negations])

    3.
    No
    (again, the first part of the question is not syntactically correct)

    4.
    No
    (but again no need of the first "no" in your question)
     
  20. Aug 25, 2017 #19
    So the definition of dark matter is it has mass and interacts with gravity...

    If there is a substance that interacts with matter but has no mass and doesn't interact with gravity.. then what is this substance called? And what laws of physics makes it not possible to exist?
     
  21. Aug 25, 2017 #20
    Not the definition. Dark matter gets its name from the fact that it is not seen (thus doesn't interact electromagnetically), but still has a mass to interact gravitationally. You can kind of call that as the definition. (For more see e.g. https://en.m.wikipedia.org/wiki/Dark_matter)

    It's just that the prevailing theory of DM claims that it is composed of Weakly Interacting Massive Particles (WIMPs) ...
    "The most widely accepted hypothesis on the form for dark matter is that it is composed of weakly interacting massive particles (WIMPs) that interact only through gravity and the weak force."
    (from the above source)

    But note that WIMPs (and thus Dark Matter) haven't been directly observed yet. Just inferred by their gravitational effects on the galaxies ... (etc.). Experiments are still working on it.

    While you take a look at those, I will address your other question (which is a legitimate one).
     
  22. Aug 25, 2017 #21
    If as "no mass" you mean zero mass all the time ... , then no there is no such a thing.
    But in Elementary Particle Physics, [at least] two particles (sets of particles) are known to be massless (in the relativistic sense though): the two gauge bosons, i.e. photons and gluons. Gravitons would be as well, but they haven't been discovered yet. [All these three mentioned [sets of] particles are force carriers (for EM, strong and gravitational forces, respectively). You might hear of them also as field particles.]
    But these particles never stay at rest. They always move with the speed of light. Thus, although they have zero rest mass (or invariant mass), they always move at c , carrying energy and momentum (the term "relativistic mass" used to be used, but I think it's outdated ...). So there is no contradiction or problem at all!

    But that may not be exactly what you had in mind for "no mass" ... is it? However that's how valid Physics nowdays would address it.

    Note: also note that field particles also do interact gravitationally, although in the contemporary definition are "massless". E.g. photons (always moving at c) are "bent" by gravity (their trajectory ...) .
     
    Last edited: Aug 25, 2017
  23. Aug 25, 2017 #22
    Ok let me correct my syntactically incorrect questions. So let me rephrase it.

    Are there certain species of dark matter where it has no mass at rest and only has relativistic mass if it is accelerated to near speed. Or based on your last message where only gauge bosons have no mass... Are there gauge bosons which have no mass and can stay at rest and only have mass if it is accelerated to near light speed? If not possible. What symmetry or laws of physics disallow such particle or gauge bosons or whatever? And btw. why can't gauge bosons stay at rest? What gauge symmetry group can make gauge bosons stay at rest?
     
  24. Aug 26, 2017 #23

    Drakkith

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    There are no models where this kind of dark matter particle is hypothesized.

    There are not. All massless particles move at c at all times while in a vacuum.
     
  25. Aug 26, 2017 #24
    I don't think so. All WIMPs are supposed to be massive particles, and 'massive' means they have a non-zero rest mass. But (theoretically) what you're asking could be at least a legitimate question, but not within the standard model type of DM theory (e.g. WIMPs).
    No!
    [Gauge bosons always move with the speed of light.]
    Having a massless particle that can stay at rest is no particle at all [IMO]. (what kind of part of matter would that be?)
    For symmetry reason 'Lorentz Invariance' came to my mind first, but I think it's something deeper. E.g. Einstein stated the two Principles of Special Relativity, the first being the constancy of the speed of light. But why does light (and all field particles) move always at that speed? (in vacuum) I don't think anyone has fully answered that, but rather it's a principle and a law of physics (verified or supported by experiment).
    For the same reason light can't (answered above).
    None! I think it's not a matter of gauge symmetry but rather relevance to a fundamental principle and law of physics. For one thing, if what you say happened, Lorentz Invariance and Special Relativity would be violated! ...

    Some of the above questions are fully legitimate (as long as you keep them as questions [and answers] (Q&A), and not as potential theories) and it's good that you asked them.
     
    Last edited: Aug 26, 2017
  26. Aug 26, 2017 #25
    Going back to the question whether the sun can capture dark matter.. two astrophysicists have been studying this.. who else? what do you make of this (can you find any flaw?):

    http://iopscience.iop.org/article/10.1088/2041-8205/722/1/L95/meta

    "
    Abstract
    Although helioseismology has been used as an effective tool for studying the physical mechanisms acting in most of the solar interior, the microscopic level and the dynamics of the deep core are still not well understood. Helioseismological anomalies may be partially resolved if the Sun captures light, non-annihilating dark matter particles, a currently discussed dark matter candidate that is motivated by recent direct detection limits. Once trapped, such particles (4-10 GeV) naturally fill the solar core. With the use of a well-defined stellar evolution code that takes into account an accurate description of the capture of dark matter particles by the Sun, we investigate the impact of such particles in its inner core. Even a relatively small amount of dark matter particles in the solar core will leave an imprint on the absolute frequency values of gravity modes as well as the equidistant spacing between modes of the same degree. The period separation for gravity modes could reveal changes of up to 3% for annihilating dark matter and of up to 20% for non-annihilating dark matter. This effect is most pronounced in the case of the gravity dipole (l = 1) modes."

    go to the site for the complete article...
     
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