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Dark Matter density given specific location

  1. Aug 10, 2015 #1
    I am doing research and I need to find the dark matter density of Galaxies or dust clusters (It can be any type of thing) which the distance from earth will be ≅4000 Mpc.Here the picture
    Think the radius of sphere.

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  2. jcsd
  3. Aug 10, 2015 #2
    I'm not sure how you would go about it from the Earth as a reference point, however the NFW profile will probably do from galactic center.

  4. Aug 10, 2015 #3
    Edit I don't think that formula will work at 4000 Mpc. I'm pretty sure it won't, as the galaxy is only 30 kpc. Sounds like your looking for the average dark matter density.

    https://en.m.wikipedia.org/wiki/Lambda-CDM_model which the Planck 2013 can be found here
    Last edited: Aug 10, 2015
  5. Aug 11, 2015 #4
    I looked my equations and I saw a mistake.Thanks for replying again.The distance will not be 4000Mpc.I am working on it If I find the right solutiion I will gonna ask again
  6. Aug 11, 2015 #5


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    Dark matter mapping is typically conducted via gravitational lensing
  7. Aug 11, 2015 #6
    Is there any dark matter where no baryonic matter.I mean are we observe dark matter only around the matter ? Or dark matter can be anywhere(In empty space which theres no baryonic matter around)
  8. Aug 11, 2015 #7


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    It's the other way round - matter tends to accumulate where (the more frequent) dark matter is. Baryonic matter accumulations tend to be more concentrated, so just outside galaxies (and even outside the disk of spiral galaxies) you have a significant dark matter density but a low density of baryonic matter.
  9. Aug 11, 2015 #8


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    Just to add to what mfb said, there's nothing to say that there is not some cluster of dark matter somewhere that has little or no baronnic matter in the cluster. I'm not at all suggesting that this is likely, just that I'm not aware of any reason why it would not be possible. If there were such a cluster it would be pretty much impossible to detect although if it were a big enough cluster and we knew exactly where to look for it, we might see it through gravitational lensing.
  10. Aug 11, 2015 #9
    Dark matter causes the structure of matter, not the other way around. I tend to think of galaxies not as swirling pockets of stars but as a vast cloud of dark matter with a small pocket of matter bobbing around inside of it.
  11. Aug 11, 2015 #10
    If thats true (matter goes where theres dark matter) then I have a question (This question can be also ask when BM attracts Dark matter-which I learned thats not possible- ).My question will be how can dark matter and baryonic matter can be same place without any interaction.

    I want to give an example consider a massive star and a planet.And If planet is close enough to sun then the planet will be fall into the sun.So sun here DM and planet is BM.Dark matter curves space-time and makes a dense place.As this place DM attracts BM.So they must be interact somehow but theres not interaction ? Why I think there must be an interaction.Cause theres a force of gravity right.If earth pulls meteorite,meteorite should fall into earth.So If BM fall this dense area then İt mmust be contact somehow with dark matter.

    I hope I clarify myself
  12. Aug 11, 2015 #11


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    Both attract each other.
    We have 5 times more dark matter than baryonic matter. It's like attaching an RC car to an actual car via a string. Sure, the RC car has its own motor, but the system will go in whatever direction the car moves.

    Where is the problem? They interact via gravity and maybe via the weak interaction, but not via anything else. The weak interaction is negligible, and the interaction via gravity is relevant at the scale of galaxies only.

    The planet that falls into the sun is stopped there via the electromagnetic interaction. A dark matter particle will just move through the star and continue to fly through space, just with a slightly different direction due to gravity.
    A tiny fraction of dark matter particles could interact via the weak interaction - that's what experiments on Earth try to see.
  13. Aug 11, 2015 #12
    It's possible for dark matter to not even interact with itself.
  14. Aug 11, 2015 #13
    In general we wont expect any interaction but somehow we expect a little bit interaction which we want to see in experiments
  15. Aug 11, 2015 #14
    I'm not sure if scientists expect it or not, it's just something that we could theoretically observe, so they are checking on it. The weak interaction would cause the dark matter to decay, creating antimatter, which will produce a gamma ray photon of a very specific energy.

    No one said dark matter is not expected to interact with anything, they only said it doesn't seem to interact with matter and the electromagnetic forces, which are a small fraction of the universe.
  16. Aug 11, 2015 #15
    How can this decay can be possible ? If DM decays anti mattter then we have to seen it already.But we didnt observe such a thing.I am wrong ?
  17. Aug 11, 2015 #16
    Why it should be matter it cant be an energy ? A energy which never interects with matter ? Thats the reason why we cant see it ?
  18. Aug 11, 2015 #17
    Not sure, probably some assumed symmetries and conservation of certain quantum numbers. I'd not be the best to explain why this might be expected.

    Because dark matter is known because of it produces gravity, only things with mass produce gravity. Remember that matter is energy.

    There is still way too much that isn't known to give you any good answers, when you are almost completely ignorant of something, all possibilities are valid. The whole thing could be an illusion caused by our formulas being wrong. Could be a fleet of Death Stars. All we can say is what it's not: It's not neutrinos, it's not black holes...
    Last edited: Aug 11, 2015
  19. Aug 11, 2015 #18
    A mystery
  20. Aug 11, 2015 #19
    Thank you guys
  21. Aug 11, 2015 #20


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    Well, at least gravitationally it has to.

    There are four main ways dark matter can be detectable:
    - it could decay with a very long lifetime (much longer than the age of the universe - after all, it is still there) to known particles like photons, electrons, muons, quarks and so on. The decay has to be mediated by the weak force or some yet unknown mechanism, as the dark matter particles do not couple to the strong and electromagnetic interaction directly. The detection would happen via telescopes and particle detectors in space.
    - it could annihilate with other dark matter particles. Not a decay in the strict sense, but it gives a signature very similar to decays. The "effective lifetime" depends on the dark matter density then, but it has to be much longer than the age of the universe again.
    - it could interact with matter in underground detectors. The most likely interaction is elastic - we would just see a "kick" for some particle in the detector without a visible cause.
    - it could be produced in particle accelerators. The detectors wouldn't detect the dark matter directly, but it would be notable as an imbalance in the observed transverse momentum of the visible particles (simplified: we see the recoil with known particles on one side, but not the produced dark matter going to the other side)

    What do you mean with "if DM decays anti matter"?

    Why does it have to be a banana why can't it be yellow? Matter has energy, this is not "one or another".
    This is not true. Energy is the source of gravity, not matter. Mass has energy, but light and motion has energy without mass.
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