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Dark matter

  1. Apr 7, 2007 #1
    An easy one.

    How does dark matter clump together,is it possible for dark matter to form a sun.
  2. jcsd
  3. Apr 7, 2007 #2
    If it formed a sun it would no longer be dark. I think dark matter is not different from that which we observe its' just that with out lite or some form of radiation hitting it and reflecting back to a detector it's presents can't be observed except of course for it's gravitational affect which is in fact how we know it exist. -Robert
  4. Apr 7, 2007 #3
    I thought i they believed dark matter was formed by some form of exotic matter.

    Is it possible for exotic matter to form a sun , and if so would we be able to tell the difference
    Last edited: Apr 7, 2007
  5. Apr 7, 2007 #4


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    Well yes and no! :smile:

    When cosmologists are talking about Dark Matter (DM) in the standard model it is "different from that which we observe". That model puts a stringent limit on the amount of ordinary matter in the universe. If we accept that the total density of the universe is the critical or closure density that produces a flat space universe then the amount of ordinary or baryonic matter, made of protons, neutron and electrons, is limited to 4% of the total.

    However there seems to be another 23% of the total that forms dark haloes around galaxies and in galaxy clusters, which causes spiral galaxies to have 'flat' rotation profiles and galactic clusters to be gravitationally bound. This DM also gravitationally lenses light from distant quasars and it is also necessary to help the large scale structures such as galactic clusters to condense quickly in the first place, so does seem to be there.

    However you are partially correct as we do not see the 4% ordinary matter, we only observe 0.03% of the total density in the form of stars and interstellar gas and dust.

    So we see 0.03% of the total, but there is 13X that in the form of unseen ordinary baryonic matter, as you said, then there is another ~6X that in the form of unseen unknown non-baryonic matter.
    The remainder is thought to be Dark Energy. You will find lots of posts about that in these Forums!

    Last edited: Apr 7, 2007
  6. Apr 8, 2007 #5
    Thanks, I'll have to look farther into this. -Robert
  7. Apr 12, 2007 #6
    what is the Higgs boson?
  8. Apr 12, 2007 #7
    One odd thing reg. DM. Why is it not found on earth or the moon? I assume it may not be in the solar system. -Robert
  9. Apr 12, 2007 #8


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    We don't have much of an idea of what dark matter is (if it really does exist, which is not established beyond all reasonable doubt) however we know what some of it's properties must be if it does exist. One important property is that dark matter is collisionless. By this we mean that the 'particles' of dark matter (if it is made up of some kind of particle) do not interact with the kinds of particles we are made of, and don't even interact with each other!

    This means that there most certainly could be dark matter hanging around in the solar system, inside and outside the earth, moon, your bedroom and even you! Since it doesn't emit or absorb light or interact with the atoms in your body there is no way to know. Since the orbits of the planets are pretty happily described by Newtonian gravity with no dark matter needed (which is unsurprising since the orbits of the planets were used to come up with the Newtonian gravitational force law) we don't think there is a lot of DM around these parts, but there could well be small amounts.
  10. Apr 12, 2007 #9
    Sorry but I find that comparison one step from insulting the magnificent work of Isaac Newton.
    Newton developed a theory, we cannot say that from current cosmology, which is currently more like, as pointedly verbalized in wikipedia, "a parameterization of ignorance".

    Unfortunately too many express an arrogance and certainty about the existence of dark matter and dark energy, as if it is derived from the model rather than made up to make it fit with experiment.
  11. Apr 12, 2007 #10
    Leaving aside the issue of dark energy, for the moment, physicists tend to believe the existence of dark matter because pretty much any extension of the standard model that you can possibly write down predicts the existence of particle that don't interact through any of the standard model interaction. This is, pretty much by definition, dark matter.

    Take a simple example. The standard model has only left-handed neutrinos. So, what would happen if we added right-handed ones? Well, neutrinos have no color charge; so they are [tex]SU(3)[/tex] singlets. They also have no hypercharge, making them [tex]SU(1)_Y[/tex] singlets. This leaves just the [tex]SU(2)_L[/tex] interaction; but, no right handed particles interact under that - hence the "L." So, a right-handed neutrino would not interact through any standard model interaction. It could, however, mix with the left-handed neutrino mass states (in fact, this is used as a way to explain why the active neutrinos have such small masses).

    Now, could right-handed neutrinos be dark matter? Probably not; but that's not the point. The point is that it is completely reasonable to expect there to be particles that are totally unaffected by the standard model interactions, allowing them to act as dark matter.

    As for dark energy, a great many types of field theory models have vacuum energies that could drive exponential expansion. Usually, the problem is less in finding something to act as dark energy as it is finding a way to make [tex]\Lambda[/tex] as small as it is measured to be.
  12. Apr 12, 2007 #11


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    Wow, I cannot fathom how that could possibly be construed as insulting to Newton??? Newton showed that a 1/r^2 force law would lead to the orbits that are seen and explained the mechanism behind Keplers laws. But he couldn't have done this by sitting in a box, observations were made first, then the beautiful theory was found to describe them. This is a process we call science and Newton was one of, if not, the best practitioner of it that ever lived. Again I am astounded by any suggestion that my simple explanations could possibly taken as an insult???

    I've never seen anyone ever state they are certain of the existence of DM or DE, let alone arrogantly.

    I would point out that science is nothing but a procees in which things are made up to fit experiments. We do not derive things from nothing and hope that the natural world agrees. This is what Aristoteleans did, they thought empiricism was flawed since only things that and deduced a priori are valid. It was the battle of Galileo, Newton and others to overturn these century old ideas and develop empirical science based on observations and experiment.
    Last edited: Apr 13, 2007
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