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What is dark matter?

  1. Dec 21, 2013 #1
    My Google searches are result in either something too complicated, or confusing non scientific journal articles.

    I always thought black holes were what people were referring to by dark matter. And I also was under the impression that small black holes should have all evaporated by now if they ever existed.

    Dark matter is some new kind of particle? What's the justification for adding a new particle? I read something that said we could be certain dark matter doesn't exist in supervoids because an overall lower temperature was measured there. But if dark matter isn't emitting light, how can it emit heat (I can't imagine things colliding much in regions that large)?
     
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  3. Dec 21, 2013 #2

    phinds

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    Dark matter is an unknown form of matter whose presence has been shown beyond dispute in at least a couple of different ways. It is called "dark" matter because as you apparently already know, it does not absorb or emit electromagnetic radiation. It only interacts with normal matter and itself gravitationally. This has serious ramifications. If does not clump and form stars or planets because dark matter particles (if indeed it is even made of particles, which does seem likely) do not collide with each other. It exists primarily in halos at the outer areas of galaxies.

    It has zero to do with black holes. The leading candidate for dark matter is WIMPS.

    Black holes evaporate so slowly that they will be the last large objects to exist in the universe.
     
  4. Dec 21, 2013 #3

    ZapperZ

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    Zz.
     
    Last edited by a moderator: Sep 25, 2014
  5. Dec 21, 2013 #4

    adjacent

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    TheWikipedia article is really good.You can get a lot of information from there.

    However,
     
  6. Dec 21, 2013 #5
    Dark matter can't emit a photon of any wavelength? If it only interacts through gravitational forces, where does the heat come from?

    If it interacts with itself gravitationally, why doesn't it form clumps?

    Is it possible for their to be a light cloud of dark matter throughout our solar system and we don't know it?

    What is dark energy?

    I'm sorry for all the questions, but this just doesn't make any sense!

    edit: To make the heat question more clear, I'm speaking of the thing I read that said supervoids in space lack even dark matter because of lower temperatures.
     
    Last edited: Dec 21, 2013
  7. Dec 21, 2013 #6

    phinds

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    There is no heat associated with dark matter.

    Because it ONLY interacts gravitationally. Regular matter particles have friction among themselves, dark matter does not. This is why dark matter creates halos at the outer areas of galaxies --- if it move into the center, it just keeps going and comes back out to the halo and because of the equations of such motion, it spends most of its time in the halo.

    no, that would, by definition, not BE dark matter

    whole different topic. Look it up
     
  8. Dec 21, 2013 #7
    Check out my edit for the heat question!

    And with dark matter present in our solar system, I didn't mean "light" as in luminescent, I meant it as in a small amount. If there's so much of this substance, why is it only present at the edges of galaxies?

    Is this similar to the idea of a graviton?

    Thank you very much for your time!
     
  9. Dec 21, 2013 #8

    phinds

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    Has nothing to do with gravitons.

    Small amounts of light emitted by something rules it out as dark matter.

    I already explained why it is MAINLY (not ONLY) present at the outer regions (not edges) of galaxies.

    The fact, if true, that some regions don't have dark matter has nothing to do with any heat associated with dark matter because there IS no heat associated with dark matter.
     
  10. Dec 21, 2013 #9
    Oh, wow. I was thinking in terms of stars and rings around planets, but with no interactions they go all the way out the the edge of a galaxy. That's pretty amazing.

    Are there any more basic properties?

    1. It doesn't interact with anything outside gravitational forces.
    2. It carries mass.
    3. It does not emit light.

    I don't get how something can carry mass and not bump into things. That just.. doesn't make sense? Is there something I'm missing?

    edit:

    I think I'm confused with exactly what the four fundamental forces do. If we take a planet's worth of hydrogen and break it up into components of electrons, protons, neutrons, then give them random motion in close proximity, what happens?

    With all four forces present, a planet and maybe some rings will eventually form, right?

    What happens when we take away one particular fundamental force and keep the other three? Is this a good approach to understanding why dark matter is behaving like it does?
     
    Last edited: Dec 21, 2013
  11. Dec 21, 2013 #10
    Look at Neutrinos. they have mass but rarely interact with other particles. Thinking of particles "bumping into each other" is not the most accurate way of visualising these events. Interacting is probably a better way. Or so I believe.
     
  12. Dec 21, 2013 #11

    phinds

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    No, you're not missing anything, that's just the way it is.

    There are numerous things in quantum mechanics (the very small) and cosmology (the very large) that don't "make sense" to us because they are totally outside the realm of the things that we have evolved being familiar with. Throwing a spear to hit a moving animal leads to LOTS of things for example, but NOTHING leads to any intuitive feel for the double slit experiment.
     
  13. Dec 21, 2013 #12

    Bobbywhy

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    Dark matter may “clumpâ€. There have been many dark matter collisional cross-section studies. One recent paper, “A STUDY OF THE DARK CORE IN A520 WITH HUBBLE SPACE TELESCOPE: THE MYSTERY DEEPENS†describes their research and its puzzling results.
    From the conclusion:
    “Dark matter self-interaction cross-section must be at least _6 _ larger than the upper limit _1cm2g􀀀1 determined by the Bullet Cluster observation. Therefore, it is difficult to attribute the feature to dark matter self-interaction without falsifying the weak lensing analysis of the Bullet Cluster (Clowe et al. 2006), which does not show any significant mass clump between the two dominant mass peaks.
    Despite our solid confirmation on the presence of the dark core, we conclude that it is yet premature to single out the most probable cause of the dark core from the above scenarios.â€

    http://arxiv.org/pdf/1202.6368.pdf
     
  14. Dec 22, 2013 #13

    sophiecentaur

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    It is so nice to read a thread which is 13 whole posts long yet contains a civilised interchange of ideas with not a hint of cranky magic / Teslaism or a groundless 'but surely'. Perhaps it's too early in the life of Dark Matter for the BS bug to have found it.
    It's what PF is all about.
     
  15. Dec 25, 2013 #14

    Bobbywhy

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    Oh, sophiecentaur! I agree that here in our PF there is great opportunity for learning and spreading scientific knowledge.

    Is there some irony in your invocation of "the BS bug"? Is it possible you are a "secret DM denier"?

    Here are two articles on the same "LUX" experiment results just released. They've not detected one single "WIMP"! Might DM consist of axions? Or, some other unnamed "particle"? The mystery continues...and the deniers continue to look for alternate explanations to explain the observations.

    https://www.sciencenews.org/article/candidates-dark-matter-particles-bite-dust
    http://www.scientificamerican.com/a...means-search-may-soon-become-more-challenging
     
  16. Dec 25, 2013 #15

    sophiecentaur

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    Oh ye of little faith . No hint of irony in my message. Give me dark matter every time.
     
  17. Dec 25, 2013 #16

    Chronos

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    In the Dirac-Milne model, dark matter and dark energy are not required - re: http://arxiv.org/abs/1110.3054, Introducing the Dirac-Milne universe. On the downside, it requires antimatter with antigravity properties. Observational and theoretical constraints suggest that is improbable. While WIMPs are the leading DM candidate, other candidates remain viable, such as neutralinos, sterile neutrinos, axions, and gravitinos.
     
  18. Dec 26, 2013 #17
    I was just reading this on Dark Matter...
    http://en.wikipedia.org/wiki/Local_supercluster#Dark_matter

    Gave me some better insight into the thought process of the idea. But I think I disagree with it. I believe near field gravitational lensing versus far field gravitational lensing could be used as an explanation for the disparity in mass to light ratios.


    Like... I think of the Earth's orbital speed around our Sun... Then I think of how the Orbital speed of the Sun is about 8 times that around the center of our galaxy. And I then I go back to thinking of some other cosmological models, like the plasma universe for example. I don't particularly subscribe to it - but I think of radiation pressure, gravitational lensing, and how everything interacts on a solar level, galactic level, universal level... and it just kind of all makes sense without the need for dark matter to me.
     
    Last edited: Dec 26, 2013
  19. Dec 26, 2013 #18

    Drakkith

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    The problem is that when you get right down to it, the other cosmological models simply don't make as much sense as dark matter. By that I mean that they require even more changes to modern physics than dark matter does.
     
  20. Dec 26, 2013 #19

    Chronos

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    DM could consist of a variety of particle, just like baryonic matter. A mix of warm and dark DM particles of various species appears possible. We already know of one DM particle called the neutrino. They are incredibly difficult to detect because they rarely interact with other particles. They can easily pass through several million miles of lead - re: http://www.newton.dep.anl.gov/askasci/phy05/phy05165.htm. We have been able to confirm their existence only because they are believed to be the most abundant particle in the universe. Neutrinos would be considered hot dark matter, so we know they cannot be the only particle in the DM zoo.
     
  21. Jan 8, 2014 #20
    The link you mentioned above is so much helpful.
    Thanks for such a nice post!
     
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