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What is dark matter?

  1. Aug 19, 2009 #1
    What is dark matter? How was dark matter formed? Any replies would be greatly appreciated.
     
    Last edited by a moderator: Aug 19, 2009
  2. jcsd
  3. Aug 19, 2009 #2
    Simply put, dark matter is matter that is dark. hahaha!!!

    A bit more details would probably be helpful. Let's start from the beginning. By looking at galaxies around us, we can see them spinning around a center. The speed of rotation depends on the amount of matter in the galaxy. The more matter the faster the spinning. Nothing too complicated up to now. Problem comes here. Calculating the rotational speed of galaxies from the only mass we see does not explain the speed observed in the telescops. Two solutions are possible: 1. our laws of phyiscs are wrong, 2. some matter is hidden somewhere. Of course, we could never imagine that we made a mistake drawing the laws of physics, therefore we had to look into matter that is not seen (therefore dark matter).

    Does that explain your question???
     
  4. Aug 19, 2009 #3
    What about ...Dark matter is matter that cannot be detected by its emitted radiation but whose presence can be inferred from gravitational effects on visible matter such as stars and galaxies. Estimates of the amount of matter in the universe based on gravitational effects consistently suggest that there is far more matter than is directly observable.
     
  5. Aug 19, 2009 #4
    That's the beauty of this subject. By definition of "dark", dark matter does not emit any radiation.

    Now the gravitational pull is the reason why we started wondering about dark matter. Since then, we came a long way. We know that there are many candidates that could explain this missing matter. First, let's not forget the massive objects (starts) at the end of the life, like white dwarfs, neutron stars, black holes don't emit radiation (or very few). They partly explain this rotational overspeed. Therefore, scientists attention turned to find what is the rest. You have the choice, between neutrinos, WIMP (weakly interactive massive particles) and many more. Since this is not my field, I can only give few details.

    Your next comment might be on the neutrinos, thinking "how can such small particles explains missing mass of the universe???" My answer would be quite simple. Of course one does make that much of a difference in our Universe. But remember that more than [tex]10^{11}[/tex] particle pass through every kg of your body every second of your life.

    To make the little story complete. These neutrinos have no electric charge (no electric field, and don't interact with matter, except for direct collision), very little mass (from what I remember less than 1/1000 the mass of the electron). They seem to be very good candidate for this dark matter.

    Cheers
     
  6. Aug 19, 2009 #5

    George Jones

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    The most massive neutrinos (there are three flavours) have mass less than 1/1000000 the mass of the electron (page 396 of the second edition of Introduction to Elementary Particles by David Griffiths).
    Neutrinos likely account for only a small fraction of dark matter mass. Also, neutrino dark matter cannot account for structure formation in the early universe that leads to the galaxies and clusters of galaxies that we observe. Neutrinos move too fast to allow this to happen.
     
  7. Aug 19, 2009 #6
    Thank you for the clarifications on neutrinos. I gave the numbers from the top of my head.

    You might be right to say that neutrino account for only a fraction of the dark matter. From my understanding, we are just at the beginning of this field, and discoveries will most likely enlighten us in the near future.

    Facts are that dark matter seems to be out there. We just need to find the right place to look for it.
     
  8. Aug 19, 2009 #7
    Im the beginner in the physics and it is simply interesting to me
    Forgive for a silly or simple question

    tnx



    http://www.u-n-i-v-e-r-s-e.com/the_Universe.html" [Broken]
     
    Last edited by a moderator: May 4, 2017
  9. Aug 19, 2009 #8
    I have a question that regards to some of the comments posted above - According to string theory dark matter might possibly be a higher vibration of the superstring -Since string theory claims that us three dimensional beings can only see the lowest vibration of the superstring (e.g atoms, light) then dark matter might be the next set in vibrations - how popular is this string theory interpretation of dark matter, for I noticed it was not mentioned above? Is the reason neutrinos are so seemingly elusive to us the fact they have a higher vibration or are neutrions seperate from string theory altogether?
     
  10. Aug 20, 2009 #9

    Chronos

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    Dark matter is like neutrinos, we know its there but is mighty hard to directly detect. Most scientists doubt it is a Baskin-Robbins collage of neutrinos, rather suspecting it is a fundamentally different family of particles [I suspect there is more than one flavor, as is the case with neutrinos]. I usually generally avoid string discussions. The music is lovely, but, there are no lyrics.
     
  11. Aug 24, 2009 #10
  12. Aug 24, 2009 #11

    Chronos

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    Expanding on [repeating?] what George said, neutrinos travel nearly at the speed of light. This is not conducive to large scale structure formation. Dark matter appears to travel around the same speed as ordinary matter.
     
  13. Aug 24, 2009 #12

    ideasrule

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    I haven't read the paper, but it seems to address why the galaxies are accelerating away from us, and not why galaxies have the observed rotation curve.

     
  14. Aug 24, 2009 #13

    George Jones

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    Right, it discusses (the possible non-existence of) dark energy, not dark matter. See also the thread

    https://www.physicsforums.com/showthread.php?p=2319754#post2319754.
     
  15. Aug 24, 2009 #14

    DaveC426913

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    It might be more more intuitive to see it as a form of matter (it could be much like protons and electrons for all we know) that simply does not interact with photons - neither absorbing them nor emitting them.

    If it does not intereact with EM radiation, then it is invisible to all our sensory apparati yet still interacts normally with gravity.
     
  16. Aug 24, 2009 #15
    What kind of technology would we need to positively detect dark matter and go beyond inferring its existence?
     
  17. Aug 24, 2009 #16

    DaveC426913

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    Why, a Dark Matter Detector of course.:tongue: (Go head. Ask what a DMD is and how it works.)



    Seriously. You do realize that, since we don't know what it is or why we can't see it, there is no way of knowing what it would take...
     
  18. Aug 25, 2009 #17

    Chronos

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    Particle physics is the current search method. Even dark matter particles have a probability of interacting with normal matter, or other dark matter particles, if you observe a sufficient number of collisions.
     
  19. Aug 25, 2009 #18

    George Jones

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    I think that if the LHC finds evidence of supersymmetry, the case for non-baryonic dark matter will be greatly strengthened.
     
  20. Aug 25, 2009 #19
    Yes, dark matter particles don't emit electromagnetic radiation, but surely there are other ways of positively identifying them. What is known from their interactions with particles that do emit radiation?
     
  21. Aug 26, 2009 #20
    The only way of identifying dark matter is through indirect effect. Like when a neutrino makes a direct hit with a nucleus, we can only measure the recoil of the nucleus and deduce that it was made by a neutrino.

    If there would be a direct way of detecting dark matter, it would become "visible" in some way, and could not be called "dark" matter anymore.

    Cheers
     
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