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Simple question on Dark Matter

  1. Mar 30, 2010 #1
    Hi All,

    I would like to know what is dark matter.


  2. jcsd
  3. Mar 30, 2010 #2
  4. Mar 30, 2010 #3

    Vanadium 50

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    That's not correct. That's dark energy - maybe.

    Dark matter is matter that we can see only via it's gravitational effects. It does not interact strongly with ordinary matter.
  5. Mar 30, 2010 #4


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    The cosmological constant has nothing necessarily to do with dark matter. Furthermore, dark energy may or may not be a cosmological constant.

    I think questions as broad as "What is dark matter" are best researched on one's own outside the forum. When you have a basic understanding, come back and ask more specific questions. Then we'd be happy to help.
  6. Mar 31, 2010 #5


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    When you just want to get started on a topic, and don't yet know much, Wikipedia is almost always a good place to do that.
  7. Mar 31, 2010 #6
    Well Chalnoth is right.
    Wikipedia is really a great database. Actually the thing is even I didnt know much about dark matter myself. Wiki helped!!

    Here's the definition from wikipedia:
    For more info go to
  8. Mar 31, 2010 #7
    Ok Thanks for redirecting me to Wikipedia. But now I would like to ask (only to those who is interested in giving answers, of course) if this emission criteria for defining dark matter may be interpreted as celestial bodies far enough and emitting so small an amount of radiation (planets, for instances) that we cannot detect it due to scattering or absortion in the midway.
    It seems that there is another possibility, namely: a different patern (species) of matter.

    Thank you all

    Best Wishes

  9. Mar 31, 2010 #8
    So I am inclined to think that questions as broad as
    what is time?
    what is space?
    what is energy?
    what is momentum?

    may also be first looked up in wikipedia, am I correct?

    Best wishes, Mr. Censor-148

  10. Mar 31, 2010 #9


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  11. Mar 31, 2010 #10


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    This was an early proposed solution, but detailed investigation showed that it didn't work. In particular, dark matter has a very different distribution from the normal matter: normal matter can lose energy by radiating, and so collapses into things like stars and galaxies. By contrast, dark matter loses very little energy with time, and so doesn't tend to collapse nearly as much as normal matter.

    There's also the issue that before the emission of the cosmic microwave background, the normal matter experienced pressure because it could interact with the photons, while the dark matter did not. This different behavior leads to exceedingly different signatures in the CMB, and because of this we are very sure that it's not just a matter of the dark matter being normal matter we can't see: it actually has to be stuff not made out of protons, neutrons, and electrons.
  12. Mar 31, 2010 #11


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    And the simple answer is – No one knows, it's a mystery!

    (And that's why they tell you to go investigate it yourself! :wink:)

    I guess you did read http://en.wikipedia.org/wiki/Dark_matter" [Broken], and knows that it started with the "missing mass" in the orbital velocities of galaxies in clusters, including the rotational speeds of galaxies.

    The best proof for the existence of Dark Matter is the Bullet Cluster:

    http://imgsrc.hubblesite.org/hu/db/images/hs-2006-39-a-xlarge_web.jpg" [Broken]

    This picture shows the formation after the collision of two large clusters of galaxies (the most energetic event known in the universe since the Big Bang).

    Here's a video from NOVA scienceNOW, explaining The Dark Matter Mystery:

    https://www.youtube.com/watch?v=<object width="480" height="385"><param name="movie" value="http://www.youtube.com/v/nJN2X3NrQAE&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6"></param><param [Broken] name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/nJN2X3NrQAE&hl=en_US&fs=1&rel=0&color1=0x006699&color2=0x54abd6" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="480" height="385"></embed></object>

    The Large Hadron Collider that started (world record) collisions at 7TeV yesterday, hopefully will https://www.physicsforums.com/showthread.php?t=390908".

    (Chalnoth, if you read this – sorry for taking so long to https://www.physicsforums.com/showthread.php?p=2648574#post2648574".)
    Last edited by a moderator: May 4, 2017
  13. Mar 31, 2010 #12

    The current leading candidate for dark matter is supersymmetry (SUSY) particles, called neutralinos.

    In supersymmetry models, all Standard Model particles have partner particles with the same quantum numbers except for the quantum number spin, which differs by 1/2 from its partner particle. Since the superpartners of the Z boson (zino), the photon (photino) and the neutral higgs (higgsino) have the same quantum numbers, they can mix to form four eigenstates of the mass operator called "neutralinos".

    The exact properties of each neutralino will depend on the details of the mixing (e.g. whether they are more higgsino-like or gaugino-like), but they tend to have masses at the weak scale (100 GeV - 1 TeV) and couple to other particles with strengths characteristic of the weak interaction. In this way they are phenomenologically similar to neutrinos, and so are not directly observable in particle detectors at accelerators.

    As a heavy, stable particle, the lightest neutralino is an excellent candidate to comprise the universe's cold dark matter. In many models the lightest neutralino can be produced thermally in the hot early universe and leave approximately the right relic abundance to account for the observed dark matter. A lightest neutralino of roughly 10-10000 GeV is the leading weakly interacting massive particle (WIMP) dark matter candidate.

    In particle physics, supersymmetry (often abbreviated SUSY) is a symmetry that relates elementary particles of one spin to other particles that differ by half a unit of spin and are known as superpartners. In a theory with unbroken supersymmetry, for every type of boson there exists a corresponding type of fermion with the same mass and internal quantum numbers, and vice-versa

    If supersymmetry exists close to the TeV energy scale, it allows for a solution of the hierarchy problem of the Standard Model, i.e., the fact that the Higgs boson mass is subject to quantum corrections which — barring extremely fine-tuned cancellations among independent contributions — would make it so large as to undermine the internal consistency of the theory. In supersymmetric theories, on the other hand, the contributions to the quantum corrections coming from Standard Model particles are naturally canceled by the contributions of the corresponding superpartners. Other attractive features of TeV-scale supersymmetry are the fact that it allows for the high-energy unification of the weak interactions, the strong interactions and electromagnetism, and the fact that it provides a candidate for Dark Matter and a natural mechanism for electroweak symmetry breaking.

    Other candidates are called supersymmetry weakly interacting massive particles or SUSY WIMPS.

    http://en.wikipedia.org/wiki/Supersymmetry" [Broken]
    http://en.wikipedia.org/wiki/Neutralino" [Broken]
    http://en.wikipedia.org/wiki/Weakly_interacting_massive_particle" [Broken]
    Last edited by a moderator: May 4, 2017
  14. Mar 31, 2010 #13


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    Thanks Orion1 for a thorough explanation.
    (Maybe DM is only 'mysterious' if you don't know what you are talking about... :smile:)

    Correct me if I'm wrong, but as I understand you there is some 'incompatibility' between SUSY and the Higgs boson (mass)?

    Does this mean that if the LHC finds evidence for SUSY, we will not find the Higgs boson, and vice versa?

    If I understood you wrong (there is compatibility SUSY/Higgs) – Does the Higgs boson interact with DM to give it mass? And if so – Will we then have an indirect 'link' to DM through Higgs? And if so – Why does DM interact with the Higgs boson, and no other boson? If not so – what gives DM mass?

    (Interesting times... LHC -> Higgs -> SUSY -> DM -> GUT -> Extra Dimensions -> Strings=GR=QM=TOE)

    There are a lot of abbreviations out there, and I'm happy no one came up with one for "Particles with Unbroken SuperSymmetrY"... My God, life can be tough enough for a little WIMP between Black Holes and MACHOs! :biggrin:

  15. Mar 31, 2010 #14


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    Oh, no, not at all. SUSY models most definitely include Higgs bosons.
  16. Mar 31, 2010 #15


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    Hi Chalnoth! What's up (with "planck s":wink:)!

    So what do you say about this:
    Does the Higgs boson interact with DM to give it mass?
    And if so – Will we then have an indirect 'link' to DM through Higgs?
    And if so – Why does DM interact with the Higgs boson, and no other boson?
    If not so – What gives DM mass?

    (Edit: It's real late here, I'll be back tomorrow, GN)
    Last edited: Mar 31, 2010
  17. Apr 1, 2010 #16


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    Well, since we don't know what the dark matter particle is, we obviously can't say for sure. However, that said, in quantum field theory, even SUSY, there remains a fundamental problem: if you insert a non-zero fundamental mass for any particle in the theory, it leads to a mathematical contradiction. This means that all masses must arise from interactions. For single particles, that interaction is modeled by an interaction with one or more Higgs fields. For composite particles (like protons and neutrons), it's a complex combination of interactions between the Higgs fields and the binding energy of the component particles.

    Finally, if you think it strange that DM would interact with the Higgs and not other bosons, consider this: of the confirmed bosonic interactions, quarks interact with photons, gluons, and W/Z bosons. Electrons interact with only photons and W/Z bosons. Neutrinos only interact with the W/Z bosons.
  18. Apr 1, 2010 #17


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    Thanks Chalnoth, interesting answers as always.

    I know that the Higgs boson is expected at LHC, but also that prominent scientist like http://vimeo.com/4062801" [Broken] – "Well, I think it'll be a lot more exciting if we don't find it."

    I have absolutely no clue about the complicated math behind all this. But I do know there are some 'difficulties' in getting all 'pieces in place', like the measured cosmological constant, that is smaller than the calculated quantum field vacuum energy by a factor of 10-120. And we don't know what DM really is. And 90% of the mass in nucleons comes from quantum fluctuations (virtual particles), etc.

    To me this looks like there must be some 'BIG Answers' to dig out from the quantum field vacuum... it looks like all is 'connected' through this 'spooky stuff'... DE, DM, mass, etc...? Or is this wacky? Could DM get most of its mass from quantum fluctuations, like nucleons??

    Okay, but isn't it 'weird' that DM does not interact directly with itself (except through gravity)? Electrons and photons (must?) do. Well, maybe neutrinos don't...
    Last edited by a moderator: May 4, 2017
  19. Apr 1, 2010 #18


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    Well, yes, in part because it would point us in an entirely new, unexpected direction of high-energy physics. Finding new information is always interesting, but finding things that nobody expected are often more interesting.

    Though I should mention that there do exist some specific models that have no Higgs, I'm not familiar with them, and as far as I know they tend to be rather less well-motivated than SUSY.

    Sadly I worry that the energy available at the LHC may simply be dramatically insufficient to say much about dark energy, or even about much of high-energy physics. As for dark matter, the LHC just won't be good at either producing or detecting such particles, so it's somewhat unlikely that we'll see them.

    The mass of nucleons, on the other hand, is, I believe, a rather well-understood consequence of quantum chromodynamics.
    Last edited by a moderator: May 4, 2017
  20. Apr 1, 2010 #19


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    If dark matter is a WIMP -- a weakly interacting particle, then it interacts through the weak force as well as gravitationally. Thus, it would couple to W/Z as well as with itself through neutral currents.
  21. Apr 1, 2010 #20


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    So far as I know, the "weak" in WIMP doesn't specifically relate to the weak nuclear force. Rather it's just a statement that its interactions with itself and other matter, whatever they may be, are rather weak. We don't yet know what those interactions are: they could be the weak nuclear force, they could be something else. But they aren't electromagnetism or the strong nuclear force (as with those interactions they'd simply interact too strongly with normal matter).
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