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Dark matter/energy

  1. Jul 7, 2009 #1
    Not sure if this subject belongs in Astrophysics, or Cosmology, or even Quantum Mechanics, or somewhere else!

    It is my (weak) understanding that the motions of the greater universe as a whole seems to require more mass than what is observed in order to fit with the known laws of gravity, which apparently are pretty well understood and tested. So the idea that there must be something that pervades all of space, has mass, can carry energy, and at the same time does not interact through any force except gravity with other matter was invented to help resolve the issue. Having the specified properties, the stuff obviously isn't made of the same stuff the visible universe is, so it is assumed that it must be composed of as-yet unobserved particles of a new variety, the WIMPS. And try as they might, the bar keeps getting raised higher and higher for the energy range of where these new particles may be seen by particle accelerators.

    OK, so here's my problem, I guess: That doesn't sound very scientific to me. An observation doesn't fit with a well established theory... a new type of "matter" is hypothesized to bring everything back into line... the properties of this matter become abstract and un-testable very quickly... new and yet more exotic theories and properties and particle types are invented to explain why we have yet to be able to begin to test the first wave of the new theory... more abstraction.

    It seems that it is far simpler to say that either the observation that caused the confusion in the first place is flawed, or the theory that is used to describe the problem is flawed, or the theory has properties that are not apparent on the scales that we have "proved" them, or all of the above. So, can our observation be off, and if so, why? Could it be that the fundamental theories of gravitation that we know and love can have effects that may not be apparent at the scale of our little (solar system-sized) experimental window?

    P.S.: I know this is very scientific of me, but what I'm driving at is that the theory of dark matter just doesn't "feel" right. My mechanical intuition, down in my gut, says something is bad. I know that means nothing in reality, but other strange theories, a little closer to home, such as QM, "feel" like they are at least aimed in the right direction. Dark matter has a certain grit to it... Anybody get what I mean by that?
     
    Last edited: Jul 7, 2009
  2. jcsd
  3. Jul 7, 2009 #2
    There is more than simply anecdotal evidence to support DM. Check out the paper called "Can Dark Matter Take a Bullet" (or something) its about the bullet cluster dynamics as the poster child for DM. Also DM does very well, compared with things like MOND, at explaining the WMAP data (peaks and harmonics etc).

    EDIT: Sorry the paper i was thinking of actually talked about MOND taking a bullet, but my other comments about how the cluster relates to DM are valid: http://en.wikipedia.org/wiki/Bullet_Cluster
     
    Last edited: Jul 8, 2009
  4. Jul 8, 2009 #3
    hi decay product....
    i hope that you are liking to know about the dark matter....i recommend you to read "angels and demons" by dan brown..or atleast watch the flim...it explains a lot about this DM...they have explained that it possible to produce DM.....
     
  5. Jul 10, 2009 #4
    My understanding is that dark matter/energy isn't so much a theory, it's more like a hypothesis.

    We don't know what it is or how it "works" so it's more like informed speculation than a good working theory. The word "dark matter" is a sort of placeholder for something deeper and more insightful.
     
  6. Jul 10, 2009 #5
    is Dan Brown a well known cosmologist? I've never heard of him in cosmology related discussion before.

    as a novelist he is not only unbelievably bad, but also known to distort and fabricate 'facts' for the purposes of his stories.
     
  7. Jul 10, 2009 #6
    Not to mention that he talks about antimatter not dark matter in his books.
     
  8. Jul 10, 2009 #7
    Frankly I agree with you. This dark matter/dark energy nonsense fits the universe to our flawed explanation of how it works, assuming the laws and making preposterous conclusions, rather than finding a simple explanation for how the universe actually does work. The mistake we are making is assuming that our vantage point is not impaired, but we are clearly overlooking something just as fundamental as a curved rather than flat Earth or a haleocentric rather than paleocentric solar system. I promise you that at the galactic level the acceleration discounted in our assumption of purely circular orbits throws away a highly significant figure. What we have observed are that stars are not spinning fast enough to maintain orbit around a galaxy and that galaxies are moving farther away from each other. These are the supposed proofs of missing matter and energy. But maybe galaxies aren't as stable as we've assumed, and what's really happening is that all the material is spinning around as it's sucked into a black hole in the center. If our existence is being slowly compressed then our metersticks are getting shorter and shorter and so of course it would look like the intergalactic distances are increasing. Just as the local gravitational field affects the perceived energy level of light, change in the local gravitational field could shape our perception as well. By assuming that our metersticks are immutable we have to contrive all of these crazy hypotheses about how the rest of the universe functions in order to understand the changes that we see, but the simpler theory is that the changes are only perceived due to local criteria that we have thus far failed to take into account.
     
  9. Jul 10, 2009 #8

    Janus

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    It is not that the galaxies don't rotate fast enough, it is that they rotate too fast. This alone could be explained by a large mass at the center (supermassive black hole), but the rotation curves (how the rates of rotation change as you move outward from the center) don't match this. The rotation curves points to this extra mass being distributed in a halo around the galaxy. There are additional observations beyond this that support the DM idea, such as the Bullet Cluster observation.

    As far as dark energy is concerned, the simple fact that the galaxies are receding is not the evidence that point towards it, this observation doesn't need DE. What prompted the idea of DE, was the disclosure that this expansion is accelerating; that the rate was slower in the past than it is now. The difference is between watching two rocks traveling upwards. Rock 1 is traveling upwards but loses speed as it does so due to gravity. (what we expect to see with simple Universe expansion) Rock 2 travels upward with ever increasing velocity (what we actually see in the Universe). In the second case, something is apparently pushing the rock upwards against gravity, and this is where the idea of DE comes in.
    As pointed out, the spin rate of galaxies is the too fast, not too slow. But even if this was the case, as we look out into the Universe, the further a galaxy is away, the younger we see it. If galaxies where not fairly stable, we would see a steady progression as we look from further galaxies to nearer in terms of compaction.
    Here you are just replacing(a not well understood mechanism driving the expansion of the Universe(DE) with a unexplained mechanism altering our meter sticks. I fail to see how the second is any better or more reasonable than the first.
     
  10. Jul 10, 2009 #9
    the thing that gets me in the dark stuff ideas
    is 2/3 dark matter 1/3 dark energy [ruffly] with a bit of real matter [3 to 5 %]
    as the total mass of the universe
    but how does dark energy count as total mass
    esp as it is tearing apart space but counts as mass???
    there is also the whole bit of dark stuff not clumping
    but gathers outside a galaxy close in to effect orbits
    but not inside a galaxy or inside a star or planet
    and is thought not to effect planets orbits around stars
    just stars orbits around the galaxy ???
     
  11. Jul 10, 2009 #10

    malawi_glenn

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    DecayProduct:
    I think there is plenty of evidence that either the current understanding of gravity is not correct or that there are invisible/Dark matter. Then what we can do is to make theories which explains data and can make new predictions and falsifications and then hope for the best.

    Why is proposing "new kind of matter" less scientific than proposing "new kind of gravity"?
    I simply don't get the point...
    and the theories for Dark Matter candidates have been very fruiful, Supersymmetry for instance, have really taken us far in quantum gravity and string theory etc. and thus solves "more" problems than what e.g. modified Newtonian gravity does.

    And you didn't mention your thoughts on Dark Energy, although you did write it in the title of this topic.

    rat b: That is the ENERGY DENSITY composition of the universe you are talking about, mass = energy in theory of relativity :-)

    davilla: so you think you know cosmology better than professional cosmologists? Well done, I admire you, when can I read your peer-reviewed articles? Will you invite me to the Nobel Prize party? :-D
     
  12. Jul 10, 2009 #11
    I really don't know, for sure, because I'm not good at posing questions in such a way as to convey what I mean. So, bear with me as I try to articulate my feeling on the subject.

    I don't think it is such a stretch to assume that gravity (or something else we've considered) may be flawed, in light of the fact the actual mechanism behind gravity is unknown. We know alot about its effects, and we know that it is a property of mass, but that actual how is completely unknown and the subject of several competing theories. All of which give interesting results, but haven't quite hit the mark.

    On the other hand, matter, and its production, is pretty close to home, and of all of the high energy experiments out there have yet to see a single particle of dark matter pop out. I'm not saying that it won't happen with new energies available, but seeming limits have been set in the past only to have new energies say "nope, not here either". Cosmic ray observations haven't turned up anything either.

    It seems like theories based on observations and interactions will naturally have at least some predictions that resemble the world from which they sprang. String theory has some pretty interesting results, but more and more people are starting to think that maybe it is the wrong path to follow, even if some cool tools have come around from it. Super Symmetry is another theory which has some interesting properties, but we have yet to see a single electrino.

    I think that inventing dimensions and whole families of particles with new relations and forces, etc. is more complicated than falling back to first principles and seeing if we may have missed something closer to home. I mean, there is no solid theory of why a shower curtain blows into the shower no matter what the water temp is, or how water gets to the tops of the tallest trees. I know these have nothing to do with the subject at hand, but they are examples of gaps in our knowledge of physics on the scale of a bathroom or tree, let alone the universe as a whole.
     
  13. Jul 11, 2009 #12
    Oh, thank God! For the past day or two I thought for certain we were all doomed. Thank you for your reply. I think I'd rather be hurled out into intergalactic space than pulled into the event horizon. On the other hand, watch what you wish for, right?

    Couldn't this be explained more simply, for instance by the presence of intergalactic matter above and below the plane, in the sphere around the galactic center? This is a cold baryonic explanation of dark matter.

    Again, cold baryonic matter would account for most of the mass of the galaxies not interacting electromagnetically.

    Galaxies do not have to be stable in composition to look consistent going back into the past. They could be hurling matter into intergalactic space, and at the same time attracting unbound matter, for instance that intergalactic matter in the halo above and below the plane, in an organic equilibrium.

    I'm not pushing forward any particular theory so much as pushing forward the belief that there must be a better theory. What's called "dark energy" should probably just be called "dark mystery" because we don't know if it's energy or the half-life of a graviton that's causing it.

    Again, maybe it only appears to be slower in the past, due to something that hasn't been taken into account. The Earth appears to be flat, the Sun appears to spin around us, and the older universe appears to be expanding less rapidly. What have we missed?

    Don't be silly. I know my place. I don't know enough physics to edit the relevant wiki articles or defend myself against bullish attacks. I'm only saying that I agree with the first poster, that this solution smells rather fishy. There isn't much dark matter present in our own solar system, just the neutrinos, so its existence is a tough swallow. But then again, so is the whole of quantum physics. You have to have almost a religious faith to understand it. There is the observation, which is science, and the explanation, which at this point is mere speculation, bordering on superstition, until the mechanism is better understood. You should know well enough that there is no complete model, so I am entitled to every doubt.
     
  14. Jul 11, 2009 #13

    malawi_glenn

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    Why is not the mechanism of gravity known? It is General Relativity, what we are searching for is Quantum Gravity and all those enterprises takes Einsteins Gravity as the boundary condition, just as GR takes newton as boundary condition, so therefore proposing that we need a new mechanism for gravity at scales MUCH MUCH lower than plank scale is like saying that GR is crap ^^

    Matter is not close to home, we have explored a TINY region of energy/mass as compared with the planck mass/scale where quantum gravity effect should play role. We don't even know many fundamental things about matter at the scale that we HAVE explored.
     
  15. Jul 13, 2009 #14
    Neutrinos are not dark matter. By definition dark matter is something the make up of which we do not know, neutrinos on the other hand are relatively well understood.

    I'm not sure that faith leads to understanding. I could ramble on some more but that sort of thing belongs in the philosophy forum so i shall refrain =). Depending on how strict you make your conditions you could say that there is no complete model for anything. We could, in the realm of possibility, make a discovery tomorrow that would invalidate all of current physics. The point is physics is an observational discipline. The facts are what you measure, the model is what you use to describe the things you have not yet been able to measure which you can later verify. As far as science is concerned dark matter is a valid model/hypothesis because it, more or less, fits the bill to describe what it needs to describe. As for what it is, well that's where the creative modeling comes in. As for testing that model, well that depends what dark matter is ultimately made up of (if it exists). If it's something that is not found naturally on earth and only exists in the depths of space we will have a hard time verifying any proposed hypothesis =P
     
  16. Jul 13, 2009 #15

    malawi_glenn

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    We know pretty much about these "dark mysteries", we know their properties very well.

    We call it dark matter since we know that is has to be matter-like, we have it's equation of state etc, but what this matter is we don't know. It might be non-baryonic, cold baryonic etc what ever, but it is not a "mystery". Same hold for the Dark Energy, it contributes to the energy density of the universe, not the "mystery" density. You are kinda like making straw man arguments..

    And half life of graviton, yeah, you are waaay out from physics one can tell, so why pursuing that "we don't know anything"? You are the one who does not know anything, so why should that be valid for professional physicists? (the graviton is massless and hence stable, it has nothing it can decay into..)


    Now don't you think that "mystery" in a quite religious concept? ;-)
     
  17. Jul 13, 2009 #16
    I haven't put my view in on this discussion, so here goes:

    1) Yes, there is 'unseen' matter that plays a significant role in our observations of the universe.

    2) So far, there is NO conclusive evidence that this matter must have any different properties than ordinary baryonic matter. Even the Bullet Cluster analysis cannot assign anything unusual. All it is, is a gravitational field that we can't see the source -- hence 'dark'. Personally, on this one I'm leaning to a simple explanation: its the 'core' material of both clusters. The highly compacted matter is relatively small in size and simply did not 'collide' with anything. There much higher mass was hardly slowed. So they just kept going while the clouds and other matter interacted. Normal super-compacted baryonic matter.

    3) The universe is really big -- this means that much of what we are labeling 'dark matter' may actually be more than one phenomenon.

    4) On dark energy. So far it ONLY seems to be needed IF you believe the models up to that point were right. Its justification is based on a departure from the model. That is 'acceleration' only shows up as needed if the other answers (including dark matter) were right.

    5) Some simple data fits that do not use the General Relativity model also do not show an acceleration.

    6) I have no doubt that General Relativity can be used to model and obtain good results in various gravity situations. The question is though -- is it being properly applied to the universe as a whole?

    7) Frankly, simply because such questions exist I don't think GR is being properly applied. In fact, I beginning to think that it does NOT play a major role in defining the expansion of the universe.

    8) I believe that GR CAN be properly applied and get better results. One area that needs to be revisited is the metric. The metric assumes spherical symmetry.

    9) For example, a cylindrical symmetry produces lower (than spherical) energy orbits for the same mass. If these exist, they would be favored by thermodynamics (higher entropy). These orbits also would have a different velocity profile. Perfect cylindrical symmetry gives orbits of constant velocity (as seen in galaxies) -- that is the orbital velocity is NOT a function of the radial distance.

    10) IF this dependence on symmetry for the gravitational field can be justified on a galaxy level scale, it would seem the assumed spherical nature of the universe (as used by FRW metric General Relativity) needs to be re-examined.

    NOTE: This is ALL my personal speculation. Far from being 'mainstream'.
     
  18. Jul 15, 2009 #17

    Chronos

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    Who asserted the universe is spherical? . . . every observer within the universe. That should suggest some of them may be deceived.
     
  19. Jul 17, 2009 #18
    I said what's called "dark energy" should probably just be called "dark mystery". I never said dark matter wasn't matter. And you're accusing ME of strawman?

    I don't know what that means. To me energy is the capacity to do work. Does gravity have energy? I wouldn't have thought so, and if gravitational attraction doesn't scale then that alone could account for acceleration, if it is as we observe.

    We don't know that the graviton even exists, yet you're already so certain about how it works. You continue to attack me and claim to understand this so much better, but you can't understand the physical universe just by the knowledge of one model that may not hold. That's why I call it religious belief, because you refuse to consider alternatives when it is apparent that there is incomplete information.
     
  20. Jul 17, 2009 #19
    You tell us. You're the know it all.

    I will have to frame this.

    Do you hear yourself?

    There can't be a graviton, otherwise gravity couldn't get out of a black hole. You claim to know about 'particles' that don't exist!
     
  21. Jul 17, 2009 #20
    Einstein and de Sitter each had spherical universe propositions. I don't think either one asserted it. As a science adviser, I am surprised that you don't know that.
     
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