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Why 'dark matter'?

  1. Mar 13, 2007 #1
    I have two questions. They are related, but different enough that I think it makes sense to raise them in two separate threads.

    The first concerns 'dark matter'. The NYT article is a good example of what is bothering me:


    (Please excuse the long-winded nature of my question. I am having difficulty formulating it concisely.)

    As I understand gravity (which is only in simple terms), it is an observed phenomenon with no underlying rationale. Newton and Einstein have described it mathematically and experiments agree with their descriptions extremely well. They did, that is, until careful examination of the behavior of large assemblages of objects, such as galaxies, revealed a substantial discrepancy with their predictions.

    As a naive cosmologist, but en experienced scientist, my first instinct would be to suspect that the math simply doesn't account for some behavior that is associated with extended groupings of massive bodies, including large numbers of stellar objects and unknown numbers of black holes or other super massive objects not yet identified.

    Why postulate the existence of a form of matter that interacts with the matter we can detect only by gravitational influence? I am aware that there have been attempts to reformulate the mathematics of gravity to explain the discrepancy, but the few I have looked at (and can understand a bit) do not seem to be fully baked. Has the 'scientific community' concluded that the current lack of success in describing the observations by altering the mathematical formulations implies that no such description is likely or even possible?

    The postulate of dark matter seems even less appealing to me, sort of like the classic Sidney Harris cartoon showing an intricate formalism interrupted by, "Then a miracle occurs", from which it continues on to the conclusion. I must be missing something important.

    What is it about non-baryonic particles that is so compelling that it causes very intelligent analytical thinkers to make enormous extrapolations about the nature of matter in the universe - with hardly a sign of the tentativeness I would expect of such a departure from direct experience? Is it the result of their familiarity with the question? Or is it just the nature of the popular press to take the idea and run with it, neglecting the rigorous inclusion of caveats?

    Included in my thoughts are cases where mathematical conjectures that seem intuitively obvious, or at least plausible, exist for decades, even centuries before someone finds a way to prove them. Why be dismayed that a mathematical reformulation of the relationship between mass and gravity that accurately describes what we are observing in non-local environments hasn't been found yet?

    On the other hand, and here's where my question really gets convoluted, if the 'beauty' of the current gravitational theory is just too magnificent to alter, then what's so dumfounding about the properties of these new postulated particles? What, exactly, do we know about conventional gravitational attraction between 'normal' matter that we do not know about these new hypothetical particles? Isn't it all a mathematical construct to quantitatively describe what we observe? There is no first principle that requires the mutual attraction of objects based on mass. We just observe that it occurs and do our best to describe it in a way that is useful. Right? Or am I missing something again?
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  3. Mar 13, 2007 #2


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    Welcome mwsund to these Forums!

    Why Dark Matter?

    Cosmologists measure the density of various components of the universe relative to the 'critical' or 'closure' density, which is equal to

    [tex]\frac{3{H_0}^2}{8\pi G}[/tex]

    as a measure of [itex]\Omega[/itex], this is equal to about 10-29gms/cc.

    So the density of a component x is given by:

    [tex]\Omega_x = \frac{8\pi G\rho_x}{3{H_0}^2}[/tex]

    In terms of this critical density, the density of all visible matter, stars and luminous gas is only 0.003.

    However Big Bang nucleosynthesis in the standard model predicts a density of about 0.04 baryonic matter to produce the correct helium, deuterium and other light element abundances. So already a density of 0.037 of this baryonic matter must be in some unseen form, ~12X the visible mater.

    The density of galactic clusters, gained both from their gravitational dynamics (orbital velocities) and from their gravitationally lensing distant quasars 'behind' them is 0.27 (latest best estimate). Also this density is required to make large scale structures form in the early universe.

    Hence about 0.23 of the mass of the universe must be in some form of exotic non-baryonic matter, it is this that is often called ''Dark Matter'.

    Other alternative theories such as MOND (MOdified Newtonian Dynamics) suggest this DM does not exist, it is as you suggest an artifact of modelling the universe using ordinary Newtonian dynamics. Another alternative is the “Freely Coasting” Cosmology model which expands linearly. In this model the BBN continues for much longer yielding a higher baryonic density of ~0.2, in other words the DM is there but it is baryonic, the question is in what form does it exist today to be so invisible?

    The total density obtained from the WMAP CMB power spectrum is 1.0 or just larger, which means there is another 0.73 to account for, this is the Dark Energy, the subject of your second question!

    Last edited: Mar 13, 2007
  4. Mar 13, 2007 #3
    Thank you for your quick reply. As I'm not well-versed in the mathematics of cosmology I may have misunderstood your response, but aren't you assuming one current theory to be the correct explanation? Part of my question is more along the lines of, if observation and theory are in conflict, why do we assume the theory is correct and doubt the observation?
  5. Mar 13, 2007 #4


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    The standard model is not in conflict with the observation.

    However, a criticism of the standard [itex]\Lambda[/itex]CDM model is that it achieves this agreement with observation only by introducing first Inflation, which requires an Inflaton field, then DM, which requires a non-baryonic particle(s) and now DE to make it concordant.

    Although the theory does cross check in several ways, e.g. DE is required both for cosmic acceleration and to make up the total cosmic density to [itex]\Omega[/itex]~1, the model can indeed be questioned as the Inflaton/Higgs particle, the DM particle and DE have not been discovered in the laboratory even after decades of looking. They may even prove to be a modern form of adding 'epicycles' to "save the appearances", as in the Ptolemaic theory.

    That is why I have included two possible alternatives in my post #2.

    Last edited: Mar 13, 2007
  6. Mar 13, 2007 #5
    Not everybody assumes that!

    But any new theory must either be simpler to calculate or must predict additional testable conditions to be worthy of being replaced by an old one.

    Only the best and most creative people in science engage in critical thinking about existing theories, the rest just follows, dogmatically, existing theories, and become uncomfortable by even the smallest hint of an alternative view.

    New, credible, theories and ideas seldom come from people who think conformally, on the contrary it scares them, it could harm their professional status, they prefer to keep existing theories and rather consolidate observational discrepancies by conjuring up "dark" or even untestable things.
    But note that they still could be right in the long term! :smile:

    It is not the best and most creative theoretical scientists that resist change, it is the levels below that. And science education is most resistant to change!

    Not that there is anything wrong with that, it is perfectly understandable!
    And checks and balances do need to be made when it comes to the new theories, but obviously dogmatic resistance just slows everything down.
    Last edited: Mar 13, 2007
  7. Mar 13, 2007 #6


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    To me the freely coasting "alternative" seams not to be an alternative at all, because it seams not to provide a mechanism to produce density perturbations. May be I did not correctly understand that paper, but I see no mechanism for the generation of perturbations there. I cannot imagine any mechanism for the generation of perturbations out of an homogeneous background other than inflation.
    Last edited: Mar 13, 2007
  8. Mar 13, 2007 #7


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    The mechanism is assumed to be primordial quantum fluctuations in the density and hence gravitational fields.

    In A Concordant “Freely Coasting” Cosmology we read

    What FCM does not have is a mechanism to deliver the strictly linear expansion. Kolb suggested 'k-essence', today known as DE, but with such properties as to cancel out the deceleration and acceleration of the universe.

    SCC provides such a mechanism, although SCC is a k = +1 model whereas FCM is a k = -1 Milne model.

  9. Mar 13, 2007 #8


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    Quantum fluctuations are not density perturbations. For example, you may have quantum fluctuations of a scalar field in Minkowski vacuum without any real perturbation in the energy density. A mechanism is needed that makes a conversion between them, and this mechanism will heavily determine the shape of the power spectrum. Thinking about it I could imagine that something like this might work without inflation, but, anyway, the paper does not explain this, does it? To me this seams to be a crucial point.
    Last edited: Mar 13, 2007
  10. Mar 13, 2007 #9
    I wish I could have articulated the issues as concisely as you did. Thanks. It would certainly be helpful if the existence of alternative models were acknowledged in popular accounts.

    (Would it be fair to characterize the state of the model as being similar to, 2+2=3 is not in conflict with the rules of decimal arithmatic as long as you assume that there is a -1 that just hasn't been discovered yet?:wink: )
  11. Mar 13, 2007 #10
    In the way of alternatives to lambdacdm, does anyone have a link to any successful alternatives with/out their take on dark matter?
  12. Mar 13, 2007 #11
    So, it is so much easier to force agreement with an existing theory than to replace or modify it, until a theoretical breakthough comes along attempts to make an existing theory conform to observation will attract most of the attention. Is that the current state?
  13. Mar 13, 2007 #12


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  14. Mar 13, 2007 #13
    Addressing a couple of parts of the original question:

    Dark matter is assumed to function exactly the same under gravity as normal matter, with the only differences being in particle masses. In this regard, at least, there's nothing at all special about dark matter.

    From a particle physicists perspective, one might almost say that dark matter is less special that baryonic matter. After all, the various baryonic and leptonic particles can interact with each other in a whole variety of ways that dark matter cannot. After all, that's why we call it "dark." "Dark," here, doesn't mean "mysterious" or "unknowable." It just means that it doesn't interact with light (or, for that matter with the other force-carrying particles of the standard model).

    As for the reason that physicists tend to like the idea of non-baryonic dark matter, it largely comes down to the fact that there are several places in our understanding of particle physics that we expect to find such particle - although the question of whether or not these various types of particles are dark matter is still an open one. The most popular candidates for dark matter are axions and neutralinos. Axions are generally thought to be needed in Quantum Chromodynamics (the theory of the strong nuclear force) to keep the theory invariant under CP-symmetry - an invariance which the phenomena QCD describes are observed to respect. Neutralinos are the supersymmetric partner of neutrinos. Supersymmetry is thought to exist for a whole plethora of reasons, too long to discuss here.

    There, of course, are not the only non-baryonic particles that are thought to exist. Our understanding of the mechanism that both generates neutrino masses and makes them as ridiculously small as they are requires heavy sterile neutrinos - neutrino-like particles that don't interact under any of the standard model interactions. It has been suggested that a somewhat lighter sterile neutrino that the one responsible for this mechanism could also, potentially, act as dark matter.

    So, there is plenty of reason to think that non-baryonic matter exists, if only because much of the theory we use fails without it.
  15. Mar 13, 2007 #14
    How convenient!

    Look at it this way:

    We have GR and discover it is not in accordance with experiment. So then we assume the existence of some dark matter, so that GR is again in accordance with experiment. We conveniently make sure dark matter is not observable in any way except through GR. And then we make claims that we have proven it because, lo and behold, we measure some big structure in space and make the calculations and, guess what, the dark matter makes it all in accordance with GR!

    That "logic" has about the same level of integrity as the average creationism argument.

    Now it might be true, that dark matter exists, don't get me wrong, I am making no claims to the contrary here, but do you really think everybody will take the above mentioned "logic" as gospel?

    Forgive me for getting a bit concerned here that such arguments might smell a bit fishy.
    Now if we could have some peer reviewed references that QCD in some way predicts or rationalizes that about 23% of all the matter in the universe is supposed to be dark matter then that would help.
    Last edited: Mar 14, 2007
  16. Mar 13, 2007 #15
    I'd go further than that, Parlyne. The name "dark matter" just refers to the fact that standard laws of physics are thought to match observations only if we add an unknown ("dark") extra quantity of mass ("matter").

    Some people think this means there really is something out there (some perhaps-new thing that has mass, is distributed in the way dark matter needs to be, and just doesn't produce light for us to observe directly), some people think the standard mathematical model of the universe is mistaken (and that "dark matter" approximates the correction term describing the difference between the actual universe and our naive model, implying either the laws of physics need correcting or else we still just haven't applied them correctly).

    The mistake to avoid is thinking "dark" means "magical" and then presuming that physicists are all making crazy assumptions about the universe before obtaining experimental support. Then we're stuck with crackpots thinking their own ideas are more plausible than (their misinterpretation of) the crazy mainstream literature. Similar situation with dark energy. It's just a correction term: we may not know the reason for it, but we certainly can measure and study its properties in a rigorous manner, so we need a name by which to communicate news about it, and the choice of name reflects only how much (or little) we knew so far about the reason behind it.
  17. Mar 13, 2007 #16


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    Or, to put it plainly, we call it dark matter simply because it is not bright enough for us to see. It's a pun.

    Planets, for example, can be dark matter. There are quite a few ways we can detect them through their gravitational effects, whereas we cannot directly see them.

    The totality of dark matter may be the kind of stuff we already know, or there may be dark matter made out of stuff we don't know about. But we already knew that tautology. :wink:
    Last edited: Mar 13, 2007
  18. Mar 14, 2007 #17
    Now hold on just one minute. First off, it's the very gravitational profile of dark matter halos directly inferred from the observations which demands that dark matter not interact electromagnetically. If it could, the halos would be much smaller due to energy loss to radiation. Unless dark matter is some kind of composite particle, it can't interact under the strong force, due to the same kind of confinement effects that trap quarks in protons, neutrons, and other hadrons and mesons. Simply put, the fact that dark matter doesn't interact is not an assumption just thrown in to explain why we don't see it directly.

    Second, the fact that a type of particle doesn't interact with any of the standard model gauge fields (gluons, photons, Ws and Zs) does not mean that it has no observable effects on particle physics. If such a particle is thought to be needed in any particle theory, it will generally mean that the particle will have some kind of direct effect on the "matter" fields of the theory (for example, the masses of the sterile neutrinos mix with those of the active ones). These kinds of effects actually make such particles observable, just not easily.

    How would a review article by one of the authors who introduced the axion in the first place do?


    I make no guarantees as to your comprehension of the article. I don't know exactly what your background is; but, this is rather technical.

    I'm afraid you're not too likely to find a freely available online version of any of the original papers which introduced axions, as they're old enough not to be on the arXiv; but, the basic idea is covered here.
  19. Mar 14, 2007 #18
    I agree with MeJennifer to the extent that to maintain a level of progress, scientists will always have to remain total sceptics. All possibilities must be considered. I feel that there's certainly an arrogance about science these days in that if you observe the vast majority of undergradute courses (where everyone starts their career in some fashion) - you'll notice that students are generally only taught the current theory.

    Problems with our standard models are always covered, but generally noted is that academics hope to somehow perturb the model to account for any disagreements. Certainly in the undergraduate material I've explored, other theories are given no credence - and I feel that at this stage in a career, it's essential to teach exploration and questioning to breed creativity.
    Last edited: Mar 14, 2007
  20. Mar 14, 2007 #19

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    The standard take is that the theory of big bang nucleosynthesis, combined with ratios of stuffs we see, rules out dark matter made of "normal stuff', like the protons and neutrons that make up the vast majority of the mass of the stuff here on Earth.

    Of course: not everyone accepts this; the standard take could change.
  21. Mar 18, 2007 #20


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    This might be useful. It was published in the Nov. 2006 issue of Scientific American, and now on-line.

    What are dark matter and dark energy, and how are they affecting the universe?

    Robert Caldwell, a cosmologist at Dartmouth College, explains.
    Personally, I have a problem with theorizing about something I cannot see or detect.

    Recently, I have been seeing articles which claim "Astronomers have announced the first direct evidence of dark matter"! I could swear I saw it in an article at SciAm.com, but I can't find it now. It had an image of the Bullet Cluster, which shows up on other sites.

    Dark matter found in colliding galaxies

    Gravity lens reveals dark matter - 25 August 2006

    Last edited: Mar 18, 2007
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