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Dark Matter

  1. May 10, 2010 #1
    The thing we call dark matter its really just dark gravity. we don't know if its matter or something else, for example:
    -in ST it could have something to do with parallel universe
    -in QM maybe some quantum fluctuation in the higgs field
    -in GR it some matter that doesn't interact with anything similar to neutrino (only neutrino gives little energy in water molecules)
    There could be three explanations of what causing the fast rotation of the stars in the galaxy(because the results with stars in the galaxy are by far different when compering the orbital and the brightness method):
    -There is Dark Gravity
    -The gravity is defined wrong
    -We can't use the solar system orbital method with galaxy orbital method
    So can anyone tell me what is most likely to be because i don't know really the advanced math into this just the basic and what are the evidence so far.
  2. jcsd
  3. May 10, 2010 #2
    At this point, the evidence says probably not. By looking at quasar microlensing, we are starting to get maps of dark matter, which suggests that it really is matter.

    I'll try to find a review paper that lists what we know. Most of it doesn't involve particularly advanced math. A lot of it involves, well if it were X, we'd see Y, but we don't see Y, so it can't be X.
  4. May 11, 2010 #3


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    The main problems with this are:
    1. The Bullet Cluster.
    2. Before the CMB was emitted, normal matter and dark matter would have behaved fundamentally differently, and we can see this signature clear as day on the cosmic microwave background as a difference in amplitude between the even and odd harmonic peaks.

    There are more reasons, but these, to me, are the most significant.
  5. Jun 9, 2010 #4
    If you define 'normal matter' as that which can be seen, ie. stars, planets, gas etc., then this did not exist before the C.M.B.R.
  6. Jun 10, 2010 #5


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    In this context by 'normal matter' one means 'baryonic' matter (everything except dark matter). Normal matter includes all stable electromagnetically charged particles (electrons, protons ,etc), and the neutral (and ionized) atoms created from nucleosynthesis. So basically everything besides gravitationally bound structures and the heavy elements produced in stellar nucleosynthesis was around before the CMB was generated.
  7. Jun 10, 2010 #6
    hubblesite announced some time ago that dark matter only let anti-matter escape and this huges baryonic masses have exsited for 3000 billions of years, so my sugestion is, elemtary my dear Watson, that this anti-matter, repulsed from DMs gathered, untill the critical mass was obtained, then colapsed, exploded and by that turned into matter, so if that is true we are then anti anti matter!this idea came very simple with the help of complex numbers, they are imaginary, so are our thoughts, maybe our brain do create anti-matter?Are we different of the rest of our universe?If we are made like the universe, then the missing energy should be H²O in a different state(supraconductivity),H²O surprises us all the time, it reacts different than all other molecules.
    I think we are in a microcosmos and sun is a watermolecule and the earth an electron?
  8. Jun 10, 2010 #7


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    All of this is right. Except the H2O. It's H2O.
  9. Jun 23, 2010 #8
    there is dark matter because through gravitational lensing..we see things made bigger but we are unable to see what is causing it
  10. Jun 28, 2010 #9
    As far as I know the leading theory is that Dark Matter is matter. More specifically Weak Interacting Massive Particles... WIMPs. WIMPs are particles that carry a large amount of mass (causing an equally large amount of gravity), but have either little or no other interactions with normal (baryonic) matter.

    There are other theories such as gravity leaking in from another universe in the multiverse, but WIMPs are currently the best fit into the standard model of the universe.
  11. Jun 28, 2010 #10

    One problem with cold dark matter is that the hypothetical problem it requires doesn't fit in with standard model of particle physics.
  12. Jun 28, 2010 #11
    I know several predictions made by the CDM theory don't fit with the observable universe, but I'm not sure which hypothetical problem you're talking about. Also I'm not really sold on CDM yet, but from what I've read it fits the best so far.

    My knowledge on the subject is fairly limited though.
  13. Jun 28, 2010 #12


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    They don't necessarily have to be individually massive. There merely have to be enough of them that the total mass density is a few times higher than that for baryons. Some dark matter models have dark matter particle masses that are quite low, but there are just enough of them that the total density is quite high (axions, for instance).

    I believe you're referring to some of the potential solutions to the problem of the accelerated expansion (as in, alternatives to dark energy), not dark matter. As near as I can tell, the alternatives to dark matter are two-fold:

    1. MOND: Modified Newtonian Dynamics. This really isn't a theory. It's basically just curve-fitting. The proposal is that gravity falls off differently after some critical radius. I imagine the hope among those pushing it was that they would eventually find an underlying theory that would explain this.

    2. STV Gravity: scalar-tensor-vector gravity. This is a proper theory, in that it presents a physical model of what gravity is. You can read up more on it here. Within the field, most people consider this proposal to be vastly more complex than dark matter, and it isn't clear that the same parameters that allow STV gravity to explain galaxy rotation curves, for instance, fit with other observations.
  14. Jun 28, 2010 #13
    Could you explain this?
  15. Jun 29, 2010 #14


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    Well, what I meant is that before the CMB was emitted, normal matter and dark matter behaved fundamentally differently from one another. It's not that they don't behave differently now, but rather that the difference between normal mater and dark matter is visible as a signature on the CMB.

    The signature comes about because normal matter interacts with radiation, and thus experiences pressure. This means that when normal matter fell into a potential well before the emission of the CMB, it would tend to bounce back out. Dark matter, on the other hand, had no interaction with radiation, and so just falls in.

    When we look at the power spectrum of the CMB, the first peak (the largest scale peak, at about one degree on the sky) represents a sound wave that has had just enough time to go through half an oscillation: it is matter that has clumped together in overdense regions. Both dark matter and normal matter contribute to this peak. The second peak, at half a degree, is an oscillation length that has had time to go through a full oscillation. It is matter that fell into the overdense regions, then bounced back. Only normal matter can do this, so the more dark matter you have, the smaller the second peak is with respect to the first.

    This pattern continues to smaller angular scales with odd peaks continuing to have the contribution from both matter and dark matter, while even peaks only have the contribution from normal matter.
  16. Jun 29, 2010 #15
    Thanks, Chalnoth
  17. Jun 29, 2010 #16


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    The observational necessity for dark matter trumps theoretical objections. That is how science works. Dark matter is not going away anytime soon.
  18. Jun 29, 2010 #17
    That statement seems devoid of content. From a theoretical perspective, of course this disconnect between observation and prediction (that we all conveniently call dark matter, since the effect is basically as if additionally there were matter that isn't lit) isn't going to be swept under the rug next week. Perhaps by dark matter you specifically meant a kind of WIMP? It's probably redundant to say but a variety of theories regarding dark matter are going to be around until one of them gets more corroborating evidence. Even definitively attributing it to a particle wouldn't end this discussion if it still hasn't been located in the new standard model of particle physics. That is how science works. (Just yesterday, I saw consequences of multiple branes being investigated for, among other reasons, explanation of dark matter.. I think the basic idea being that matter there would be gravitationally coupled to our brane but otherwise not permit typical physics/chemistry..)
    Last edited: Jun 29, 2010
  19. Jun 29, 2010 #18
    I wonder if dark matter can be difinitively proven to be a particle, and not some gravitational field correction, by observation of frame dragging effects. Frame dragging is produced only by particles orbiting, is this right? Could we observe frame dragging by subtle characteristics of microlensing? Thanks.
  20. Jun 30, 2010 #19


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    I'd say what we know about dark matter is quite a bit more specific than that. First, we can be pretty certain it's a particle outside the standard model of particle physics. None of the fundamental standard model particles fit, and the thermal production of any of the hypothetical composite particles is speculative at best.

    So we are left with one of three possibilities:
    1. The observations are wrong.
    2. Our understanding of gravity in the weak-field limit is wrong.
    3. Our understanding of the contents of the universe are wrong.

    The first possibility is ruled out by the wide variety of independent observations that all agree on this discrepancy. The second possibility is largely ruled out by observations of the CMB and clusters. So the third possibility remains, and it's now just a question of what sort of matter is out there that doesn't interact with electromagnetic radiation. Much of the current work, on the side of the cosmology, comes from looking for dark matter annihilation products, and examinations of early structure formation to nail down the temperature of the dark matter (we know it has to be fairly cold, but how cold, exactly?).
  21. Jun 30, 2010 #20


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    I sincerely doubt that this would be in any way detectable. The problem is that due to their lack of friction, the orbits of the dark matter particles aren't going to be coherent, so the randomized motions of all the particles will largely cancel out and you won't get any appreciable frame dragging in any one direction.

    Instead, if we want to detect dark matter particles, detailed searches for their annihilation products and terrestrial detection are our best bet.
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