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Questoin about MOND

  1. Jun 3, 2008 #1
    I've read through a lot of the other MOND vs DM debate threads, but I was wondering whether or not this has been addressed:

    If observed abundances of deuterium place very tight constraints on [tex]\Omega_m[/tex] being roughly .03 or so, wouldn't we have to posit DM as the other .27?

    To me, I think that this data rings the death knell for MOND or MOG much moreso than even the Bullet Cluster data.
  2. jcsd
  3. Jun 3, 2008 #2


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    You are correct. Big Bang Nucleosynthesis, along with the size and position of peaks in the CMB angular power spectrum tell us a great deal about the density of baryons, and this result agrees with what we find from the rest of cosmology, using different methods. This is just one element of the concordance between data sets that makes the current model become known as the concordance model.

    Still, since we have known this for a while and people still play around with MOND there must be someway in which it is thought MOND could wriggle out of this. I think there is some controversy about lithium abundances not agreeing with the others, although since it is the least abundant and therefore hardest to measure perhaps this is not surprising.
  4. Jun 3, 2008 #3


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    No version of MOND appears viable without some amount of dark matter. This does not rule out MOND, but, does make it very unaesthetic.
  5. Jun 9, 2008 #4
    Hi everyone
    Looks like I'm the only MOND guy around at the moment. Gotta fight my corner.

    Of course not. If the inverse gravity law breaks down for galactic scales, then the tight constraints on the friedmann equation's omega values break down with it. I doubt the friedmann equation is even valid for MOND, since friedmann is based on GR, and GR assumes newton's grav law is correct for classical situations ( e.g. galaxy rotation dynamics ).

    MOND doesn't strictly abide assertions made by DM thoeries. It cannot be deemed to have failed when it contradicts DM thoeries.
    Like all science, it can only be judged by observation of nature.

    If you insist on taking the friedmann perspective for the universe expansion, even though it remains to be shown that equations are valid for mond, then it's likely mond makes up for lost mass by higher spatial curvature and higher vacuum density. But for me, to force mond to conform to the friedmann model is crazy - a tenuous analogy at best.

    With respect to what wallace was saying, from the mond point of view, the current big bang model et all is wrong, again because it assumes newtonian gravity for galaxys, and therfore can be ignored. This is a simple way for mond to get round the conflicting aspects of DM vs mond, no need for 'wriggling'!

    What you actually mean is...
    MOND, if used to model the universe at inter galactic scales and greater, fails for the same reason newton's model does, observation shows the mass of the universe binds to itself more than the model predicts. But MOND fails by a lesser amount than newton's model.

    1. MOND is not a model for inter galactic dynamics and larger dynamics. If it fails at these scales, it has little significance for MOND. The fact that all the criticism for mond is based on it's failure when applied to a part of physics it isn't concerned with, is as daft as saying hooke's law is discredited because it fails when applied to quantum mechanics.

    Strictly, this also applies to the bullet cluster. The bullet cluster is going through all manner of abnormal dynamics. MOND was intended for stable galaxies.

    2. If we want to fiddle with mond to get it to work for these large scales outside it's orignal scope, there are variables other than dark matter which can be altered or introduced.

    3. The fact that MOND, a rule for galaxy dynamics, improves on newton's gravity when applied to inter galactic dynamics suggests MOND is a better fit. It's bad logic to say such an improvement is unaesthetic.

    People fixate on dark matter and dark energy as the solution to any disagreement between model and observation. They do this to preserve newton's gravity. But since mond is not bound by newtons grav, there's no need for this narrow focus on hidden mass-energy when using mond.

    At the core of any DM vs MOND debate is always the insistance from the DM group to apply MOND as a universal\general theory whereby MOND fails. MOND isn't a general theory, so such arguments against MOND are not valid. It's really that simple.
    Last edited: Jun 9, 2008
  6. Jun 9, 2008 #5


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    Eeek! I'd avoid describing yourself as 'a MOND guy'. You'd be hard pressed to find someone who'd call themselves a 'LCDM guy' or a 'Big Bang guy' since scientist's would much rather have the perception that that they simply follow the evidence and aren't wedded to a particular model. If the evidence shifts so will there best guess at the correct model. We all know that it doesn't always work exactly like this in practice, people are people after all, but we all strive to the ideal of complete objectivity.

    Calling yourself a 'MOND guy' makes you sound more like a lawyer or advocate than a scientist. I.e. I've take the position of MOND and will find all arguments in favor of that position exclusively regardless of their strength compared to a counter argument. Not a good way to start a post if you want people to listen on a science forum! Just a tip. I'm not saying that this is neccessarily your position, I'm sure your comment was intended to be light-hearted, but you should know how it sounds.

    What cannot be ignored is the data, and much of the constraints in terms of baryonic content from BBN and the CMB do not rely heavily on the FRW model. Certainly BBN doesn't. Obviously the constraints are presented in terms of FRM parameters (Omega baryon, Omega matter etc) but the raw data itself stands and can't be brushed aside. Just saying 'the Big Bang is wrong in the MOND model' without replacing this with the MOND explanation of the early universe data, thus removing from consideration most of the data we currently have, is not a good way of advancing the acceptance of MOND!

    Attempts to extend MOND to a general theory (TeVeS etc) have also failed to compete with LCDM. If we have a choice between a general model that seems to work everywhere pretty well (dark matter) and a phenomenological model that works in only some places (MOND) it is hardly surprising that more attention is given to dark matter. That being said, there is plenty of work being done on finding more universal modified gravity models that explains the totality of cosmological data. None yet can seriously compete with LCDM, but this may not always be the case. We cannot predict the future...
  7. Jun 10, 2008 #6
    Hello? This is spose to be a science forum right?
    Scientic method = Hypothesis, test, conclusion.
    Note : hypothesis. A theory without evidence, which predicts a specific result in a yet unobserved set of conditions.
    In this way, science has the theory preceeding the evidence. Always. The alternative is exhaustively testing all different combinations of conditions, i.e. ' stamp collecting ' until all stamps are collected, which isn't science, but some cataloging of data is always a prerequsite to making a hypothesis.
    So what you're insinuating, that's it's daft to make conjecture without conclusive evidence, is just plainly wrong. Blimey. How long have you been sniping the scientific method? Calling people who make hypotheses lawyers, advocates, or wedded to their belief?

    Do you know DM is a hypothesis with no conclusive evidence?

    I think the reason for not finding people calling themselves a 'LCDM guy' is because it's the current trend to have more formal titles, such as " Chair of the International Advisory Committee for the Workshop series 'Identification of Dark Matter' " - Prof Neil Spooner which means 'dark matter dude'. It's nothing to do with avoiding making hypothesis and only assuming what has been observed aka following evidence.

    Now for the case of MOND, it is not a hypothesis! It used to be, but then some conditions, unobserved at the time of the MOND conjecture, were observed (LSB galaxys), and they agreed with mond predictions. Note that DM hypotheses failed the LSBG rotation test.


    Oh, if only you were involved in physics debate around 100 year ago. At the time a cerain Mr E was working on the principles of a new hypothesis and said " Any relativistic theory of gravity will have to be non euclidean " - effectively saying " The current euclidian based model of the univerise is wrong! ". At the time ( 1906 - 1914 ) he had no theory to replace the newtonian model. You could have said ...
    Or maybe identifying key differences between a proposed model with the current model is a good idea during the construction of a new model.

    Reiterate : Models that extend MOND to larger scales and then fail to match observations do not invalidate MOND.

    Dark matter cheats. And this is the other sore spot between DM vs MOND. DM has failed observation more times than TeVeS. But the DM community inexplicably says it's ok to modify the DM parameters in light of new data. This is NOT scientific method! Those parameters have been altered several times since the inception of DM. This is what I call bad science. Akin to bashing the square peg into the round hole.

    MOND predictions, within the intended bounds, has yet to disagree with observation, and have had conclusive evidence in the form of LSB galaxys.

    I see this topic is going the way of all DM vs MOND topics. If you are interested in MOND and the DM vs MOND debate please visit these sites...


    in particular...

    ...instead of me rehashing whats been said there.
    Last edited: Jun 10, 2008
  8. Jun 10, 2008 #7


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    I'm pretty confused by your points here, so feel free to correct me if I read them wrong, but it seems to me that what you are saying (mainly in a post above) is that MOND predicts galactic dynamics, but on a larger scale although it fails, it fails by less than Newtonian gravity. Well, ok, this is good.. but I don't see how this really gives us anything.. i.e. there is no Cosmology coming from this if you say that MOND cannot be extended. As I said, though, I may just be reading your posts wrong.

    Another thing that I don't really understand about MOND is how it can get structure formation without dark matter. (I admit I've not really read much aboutj MOND, though).
  9. Jun 11, 2008 #8


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    Agreed, Cristo. MOND cannot account for large scale structures without some amount of DM. This point has not been properly addressed by MOND advocates. I have no prejudice against MOND, merely doubts. It is an effective theory - not fundamentally based on the known laws of phyics. I therefore consider it suspect.
  10. Jun 11, 2008 #9
    Where did I say that? I expect MOND can be generalized, but I'm don't know of one that has been verified by data as yet. That's not to say there isn't one. It's hard to know when then are few layman interpretations of the load of grand abstract cosmo theorys around at mo.

    Structure formation: Can you be more specific? Whats there to not understand?
    Mass binds due to gravity, so no suprise that it binds under MOdified Gravity ( which is stronger that gravity ). In newton grav, extra mass ( DM ) is needed to get galaxys to bind. In MOND, the extra acel binds the galaxys.

    Reiterate: People fixate on dark matter and dark energy as the solution to any disagreement between model and observation. They do this to preserve newton's gravity. But since mond is not bound by newtons grav, there's no need for this narrow focus on hidden mass-energy when using mond.

    Reiterate :At the core of any DM vs MOND debate is always the insistance from the DM group to apply MOND as a universal\general theory whereby MOND fails. MOND isn't a general theory, so such arguments against MOND are not valid. It's really that simple.

    I just have done. Twice.

    As an aside let's move onto this curious line chronos takes. He's made statements to this effect before.
    History shows that effective theorys are almost always correct. I find it impossible to understand why anyone could think they are suspect in their correctness. All that has to be done to create an effective theory is to express what is seen. e.g. 'If I let go of a ball, it falls.' No need to invoke any physics at all. The only assumption here is time symmetry. The data is already there. It was the same with MOND.
    In constrast, almost all hypotheses based on physics models fail. When one is found to be correct, it is a big event in science.
    And true to the trend, DM theorys failed repeatedly. Current DM theory is the result of repeated corrections in light of data that didn't previously fit.

    Exercise. Classify these theorys as either effective or derived.
    1. Kepler's laws of motion.
    2. Bohr's 2nd rule for the old quantum theory atom.
    3. Relativistic time dilation
    4. The aether
    5. Newtons 2nd law of motion
    6. Rayleigh jeans radiation law ( UV catastrophe )
    7. Old quantum mutli electron atoms theory.
    8. Planck's theory of radiation
    9. De broglie hypothesis
    10. 1st law of thermodynamics
    I get 5 derived, 4 effective, and 1 intuitive
    The point is just to show theorys that are effective are amongst the most solid in physics, while hypothesis that are perfectly consistent with the rest of known physics can be wrong.

    So it's only logical to suspect derived hypothesis, while being much more sure of effective ones. How Chronos has got them vice versa in his head, is beyond me, and to ponder how is a source of amusement, the world may never know how he's done it!
  11. Jun 11, 2008 #10


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    Ah yes of course!

    History has shown how effective the Ptolemaic theory was - more accurate than that tin pot Copernican idea! :wink:

  12. Jun 11, 2008 #11


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    Ok, perhaps I worded my comment badly: what I meant to say was that this model of MOND does not explain (or even attempt to explain) the behaviour of gravity on intergalactic scales or larger? Thus, it is not a Cosmological model. As I said above, whilst it is good that in certain situations it fails less than Newtonian gravity, I don't see that this can ever rival LCDM.

    So your arguing from a lay-person's perspective?
    I mean how does large scale structure form from initial density fluctuations in the early universe in a model of MOND without dark matter?

    So here you kind of make my point: MOND is only a model of gravity in certain situations, and so cannot rival the standard Cosmological model.
  13. Jun 11, 2008 #12
    It's important to word to right, it's a fine point you see.
    Ok, let's see if we can straighten it out.
    MOND was intended to predict rotation curves of stable spiral galaxys. In this it has agreed with observation.
    It was not intended to predict Inter Galactic dynamics.
    It has failed to predict IGD absolutely correctly.
    But it's done so within a modest degree of error. Enough to make many argue that it's paradigm should be considered for IGD as an alternative to LCDM.
    Compare this with DM.
    DM was intended to predict GRCs and IGD.
    It has always failed to predict GRCs.
    It failed to predict IGD.
    With several modifications, it fits IGD reasonably well, but still has significant probs.

    Can you see with this?
    source : http://www.astro.umd.edu/~ssm/mond/bullet_comments.html

    Lay is a relative term. Most of the top scientists were effectively lay to GR and QM when they 1st appeared. I understand MOND pretty well, but understanding the Grand unifed theorys that derive MOND from principles is no easy task. Can any gurus here do with these principled MOND theorys the same as Feynman did with QM - i.e. convert hard maths of a new theory into common intelligable language? No ? Didn't think so.

    In that case I reiterate : Mass binds due to gravity, so no suprise that it binds under MOdified Gravity ( which is stronger that gravity ). In newton grav, extra mass ( DM ) is needed to get galaxys to bind. In MOND, the extra acel binds the galaxys.

    Subtle correction. MOND only claims\was intended for gravity in certain situation. But it's incorrect prediction outside those situations were still good enough to warrent attention.
    The original MOND is competing outside it's intended use, with a theory which is inexplicably allowed repeated postdiction, and which is spose to work in the domain of MOND and beyond. It does quite well outside, and is far better within.
  14. Jun 16, 2008 #13
    Seems like the mond bashers have acknowledged MOND's usefulness over LCDM in predicting GRCs, therefore justifying MOND. Maybe it was that graph that did it. Hard to tell. Often people simply drop the thread if it doesn't go there way, rather than explicitly acknowledging the other argument.

    Now the main question of the thread's been dealt with, might as well examine Garth's comment. I'm bemused by it. Was it spose to be a neat refutation of my outline of effective theorys being very often correct? If it was, it was pretty flawed.

    Here some advice on refuting effective theorys as usually solid...
    1. Are we sure they are effective theorys? i.e. Were they cut off from the rest of the 'natural science' of the day? Since the scientific era didn't begin properly till roughly 1600, it is difficult to decide. Nat-Sci was wraped in philosophy at the time of Ptolemy. You should pick an effective theory from 1600 onwards really.

    2. Domains. Did the theory fail for it's intended domain? Nearly all physics theorys fail if they are taken outside their domain, but that doesn't invalidate them, except they can't be universal. They can still be sound science.
    Today, ptolemy's geocentric model still has the huge majority domain over other models in terms of application. I don't know of any biology on Earth that doesn't assume the Earth is stationary. Migrating birds might need to model the Earth's rotation. I can think of only space vehicles, and space science that need to model the Earth as moving round the sun. Aerospace and long range ballistics need to assume earth rotation. Nearly every other application or system assumes the Earth is stationary.

    Conclusion : ptolemy geocentric model is still valid. It is not a counter example to effective theorys being almost always correct. It's not even clear that it was an effective theory.

    Also, you used the word effective in a different meaning in your post.
    You used : effective to mean useful. I meant it differently, see above for definition.
  15. Jun 16, 2008 #14


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    Are you talking about me here? If so, I suggest you stop labelling people, and refrain from putting words in my mouth.

    Sorry, but this just doesn't cut it. Are you familiar with mechanisms of structure formation? The point here is not so much the "extra mass" but the fact that the "extra mass" here does not interact under any force other than the gravitational force. I just don't see how MOND gives this, and your repitition of the same, general comments, does not help this.
  16. Jun 17, 2008 #15


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    All fundamentally based theories are 'effective theories'. Not all effective theories achieve mainstream status. That is how the examples you cited graduated from effective to fundamental [i.e., mainstream]. MOND must follow this path to gain widespread acceptance.
  17. Jun 17, 2008 #16
    Good grief cristo. If you honestly mean what you say, it's doubtfull you'll ever understand MOND, never mind stucture formation under MOND grav.
    If, for you, studying MOND means spending time puzzling over how the MOND theory can result in non baryonic, exclusively gravity interacting matter, then.... o_O
    Seems like my comment wasn't general enough, try this...
    MOND doesn't use dark matter! MOND doesn't need dark matter! MOND is opposed to dark matter theorys!

    I'm off.
  18. Jun 17, 2008 #17


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    But you've not given any explanation to back this up. You can shout that 'MOND does not need dark matter' as loud as you like, but note that all I said is that I don't see how MOND can obtain large scale structure without dark matter. I presume you take this phrase as as your theory being "bashed," but all I was asking for was an explanation, which you have not yet given.

    However it may be, as Chronos says, that the professionals don't have an answer to this question, yet.
  19. Jun 18, 2008 #18


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    So much for crackpot theories.
  20. Jun 20, 2008 #19


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    MOND is a bit like epicycles isn't it?

    Postulated as a formulation to fit the observational data that's not explained by theory?

    Not that I am necessarily enamored of Dark matter either as it seems to arrive to our thinking in a similar way, but rather as an explanation for the unexplained for different equations.

    What's all the big fuss then? Ptolemy, Newton, Einstein? Aren't they all steps along the way to ultimately understanding the whole shebang? (Or should I say the whole bigbang?)

    If nothing else MOND looks like a useful engineering tool - that is assuming there was something on that scale we could engineer.
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