Astrophysical ambulance-chasers everywhere got a bit excited this week, and why wouldn’t they? Here are some of the headlines we read:
- Findings Raise New Questions About Dark Matter (redOrbit)
- Dark matter theory challenged by gassy galaxies result (BBC)
- More Evidence Against Dark Matter? (Science NOW)
Wow. More evidence against dark matter? I didn’t know about the original evidence.
Sadly (and I mean that — see below) there is no evidence against dark matter here. These items were sparked by a paper and a press release from Maryland astronomer Stacy McGaugh, with the rather more modest titles “A Novel Test of the Modified Newtonian Dynamics with Gas Rich Galaxies” and “Gas rich galaxies confirm prediction of modified gravity theory,” respectively.
I’m the first person to defend journalists against unfair attacks, and we all know that headlines are usually not written by the people who write the actual articles. But we can legitimately point fingers at a flawed system at work here: these articles are a tiny but very clear example of what is wrong wrong wrong about our current model for informing the public about science.
McGaugh’s new paper doesn’t give any evidence at all against dark matter. What it does is to claim that an alternative theory — MOND, which replaces dark matter with a modification of Newtonian dynamics — provides a good fit to a certain class of gas-rich galaxies. That’s an interesting result! Just not the result the headlines would have you believe.
It’s obvious what happens here. Nobody would read an article entitled “Gas rich galaxies confirm prediction of modified gravity theory” — or at least, most editors doubtless feel, fewer people would be interested in that than in evidence that went directly against dark matter. So let’s just spice up the story a bit by highlighting the most dramatic possible conclusion we can imagine drawing, and burying the caveats until the end. Net result: a few more people read the articles than otherwise would have, while many more people just read the headlines and are left with less understanding of modern cosmology than they started with. Scientists and journalists together have a responsibility to do a better job than this at making things clear, not just making things sound exciting.
But let me take this opportunity to lay out the problems with MOND. It’s a very clever idea, to start. In galaxies, dark matter seems to become important only when the force of gravity is not very strong. So maybe Newton’s famous inverse-square law, which tells us how the force of gravity falls off as a function of distance, needs to be modified when gravity is very weak. Miraculously, this simple idea does a really good job at accounting for the dynamics of galaxies, including — as this new result confirms — types of galaxies that weren’t yet observed back in 1983 when Mordehai Milgrom proposed the idea. Whether or not MOND is “true” as a replacement for dark matter, its phenomenological success at accounting for features of galaxies needs to be explained by whatever theory is true.
Which is an important point, because MOND is not true. That’s not an absolute statement; among its other shortcomings, MOND is not completely well-defined, so there’s a surprising amount of wriggle room available in fitting a variety of different observations. But to the vast majority of cosmologists, we have long since passed the point where MOND should be given up as a fundamental replacement for dark matter — it was a good idea that didn’t work. It happens sometimes. That’s not to say that gravity isn’t somehow modified in cosmology — you can always have very subtle effects that have yet to be discovered, and that’s a possibility well worth considering. But dark matter is real; any modification is on top of it, not instead of it.
Let’s look at the record:
- MOND is ugly. Actually, that’s very generous. More accurately, MOND is not a theory; it’s only a phenomenological rule that’s supposed to apply in a limited regime. The question is, what is the more general theory? Jacob Bekenstein, in an heroic bit of theorizing, came up with his Tensor-Vector-Scalar (TeVeS) theory, which hopefully reduces to MOND in the appropriate limits. Here is the action for general relativity:
And here is the action for TeVeS:
Don’t worry about what it all means; the point is that the theory underlying MOND isn’t really simple at all, it’s an ungodly concatenation of random fields interacting in highly-specific but seemingly arbitrary ways. That doesn’t mean it’s not true, but the theory certainly doesn’t win any points for elegance. - MOND doesn’t fit clusters. Long ago, rotation curves of galaxies were the strongest evidence in favor of dark matter. Very long ago. We know better now, and a mature theory has a lot more hoops it needs to jump through. The nice thing about MOND is that, despite the ugliness above, when you get down to making predictions for large astrophysical objects, there really isn’t any wriggle room: you fit the data or you don’t. It works for galaxies, but when it comes to clusters — you don’t. Not close. Proponents of MOND understand this, of course, and they’ve come up with a clever workaround. It’s called “dark matter.” That’s right — even MOND’s biggest supporters admit that you need dark matter to explain galaxies. Let’s just emphasize that for those who find all this text kind of tedious:
Even with MOND, you still need dark matter.
Some people try to claim that the necessary dark matter could be neutrinos rather than some brand-new particle, and that’s supposed to be morally superior somehow. But there’s no two ways around the conclusion that dark matter is real.
- MOND doesn’t even fit all galaxies. For almost twenty years now we’ve known that MOND fails for a certain type of galaxies known as “dwarf spheroidals.” These are small (thus the name) and hard to observe, so MONDians have come up with various schemes to explain away particular galaxies. That might even be okay — nobody said fitting the data would always be easy, even in the correct theory — except that it’s precisely this kind of extra work that is being scoffed at in the case of dark matter in these recent news items.
- Gravity doesn’t always point in the direction of where the ordinary matter is. This is the lesson of the famous Bullet Cluster (and related observations). The evidence from gravitational lensing is absolutely unambiguous: to fit the data, you need to do better than just modifying the strength of Newtonian gravity. Once again, people try to wriggle out of this in TeVeS and other MONDian approaches. However, the way they do it is by imagining that other fields have energy, which warps spacetime, and therefore a gravitational field. We have a useful phrase to describe new fields whose energy warps spacetime: “dark matter.” MOND-like theories don’t replace dark matter so much as they make it much more complicated.
- MOND doesn’t fit the cosmic microwave background. Saving my favorite for last. One of the coolest things about the temperature anisotropies in the cosmic microwave background is that they are sensitive to the existence of dark matter. In the early universe, dark matter just collapses under the pull of gravity, while ordinary matter also feels pressure, and therefore oscillates. As a result, the two components are out of phase in the even-numbered peaks in the CMB spectrum. In English: dark matter pushes up the first and third peak in the graph below, while suppressing the second and fourth peak. That would be extremely hard to mimic in a theory without dark matter; indeed, this was predicted before the third peak was precisely measured. But now it has been. And…
See that dotted line? That’s the theory with dark matter, fitting all the data. See the solid line? That’s the MOND (really TeVeS) prediction, definitively inconsistent with the data. Can some clever theorist tweak things so that there’s a MOND version that actually fits? Probably. Or we could just accept what the data are telling us.
Having said all that, I’m glad that some people are still thinking about MOND-like approaches. You can still learn interesting things about galaxies, even if you’re not discovering a new law of nature. And dark matter, to be honest, isn’t established with 100% certainty; it’s really more like 99.9% certainty, and you never know.
What’s less admirable is people (mostly outside the professional community, but not all) hanging onto a theory because they want to believe it, no matter what new information comes along. Personally, I think it would be much cooler if gravity were modified, compared to the idea that it’s just some dumb new particle out there. I’ve put some thought into the prospect myself, which helped lead to some productive research ideas. But ultimately the universe doesn’t care what I prefer. Dark matter is real — gravity could also be modified, but there’s no reasonable doubt about the dark matter. So let’s try to figure it out.
Dear colleagues, dark matter has passed away…
But first some words to defend MOND “against unfair attacks”.
MOND is ugly. Ugly is a subjective term. We, scientists, do not care about ugliness but about if the model fits to data, the predictions made… and we know that MOND rocks
http://www.astro.umd.edu/~ssm/mond/mondpred.html
http://www.astro.umd.edu/~ssm/mond/mdlg.gif
http://www.astro.umd.edu/~ssm/mond/mdlg.gif_2
whereas the dark matter models cannot fit the fine data (only give us a rough description) and make no predictions (dark matter fits are done a posteriori)
http://www.astro.umd.edu/~ssm/mond/fit_compare.html
http://www.astro.umd.edu/~ssm/mond/mondvsDM.html
MOND would not be associated with TeVeS (i.e. with ONE attempt to mix GR with MOND). In any case, neither ugliness or elegance are scientific requirements for evaluating a theory as TeVeS.
MOND doesn’t fit clusters. A non-relativistic theory having problems to fit some relativistic data does not look as a problem for MOND, which continues explaining the rest of data very well. Moreover, dark matter models have problems to explain the same clusters as well
http://arxiv.org/abs/0704.0381
http://adsabs.harvard.edu/abs/2010ApJ…718…60L
Even with MOND, you still need dark matter. Only if you want to apply a non-relativistic theory to relativistic regimes and fill the holes between relativistic data and a nonrelativistic model with unobserved dark matter. A more serious alternative is to apply a relativistic theory to relativistic regimes. Then you do not need dark matter at all!
MOND doesn’t even fit all galaxies, How many entries you can find in “Galaxies which are NOT fit by MOND”? (I count exactly zero)
http://www.astro.umd.edu/~ssm/mond/fitroster.html
For almost twenty years now we’ve known that MOND fails for a certain type of galaxies known as “dwarf spheroidals”. But your claimed “fails” are associated to taking certain values for high-uncertainty parameters.
MOND doesn’t fit the cosmic microwave background. As has been said to you above, the first prediction was done using a MOND cosmology. MOND people predicted the correct first and second peaks, whereas dark matter cosmology gave wrong first and second peaks. Only after the final data was known the dark matter community modified parameters in dark matter cosmological models for their posteriori fit to the data. MOND just predicted the correct values before the experiment was run. This part of the history would not be omitted!
About the third peak, well again this is the regime where relativistic corrections enter. Again nobody serious would wait MOND (a non-relativistic theory) to apply to relativistic regimes.
Why is dark matter a dead end?
Because (i) it has never explained the data
http://www.astro.umd.edu/~ssm/mond/fit_compare.html
and because (ii) nobody has found dark matter. Sometimes one read in the news that dark matter has been finally ‘found’, but they only mean that some data has been more or less explained assuming the existence of dark matter. The current claims of observation of dark matter have the same validity that the ancient claims of observation of Vulcan planet.
At the same time as emphasized in Dr. Corda FQXi Essay forum,
http://fqxi.org/community/forum/topic/875
We already developed theoretical alternatives that rule out dark matter (and dark energy). We can already explain observational data that cannot be explained neither by dark matter models, nor by other models as MOND, TeVeS, PCG… Moreover, we can combine both the cosmological and the astrophysical models and then solve some other mysteries, such as why the astrophysical constant a0 used in MOND is numerically close to the Hubble aH. Our new theory predicts a0 = 1/8 aH.
Our new theory gives a correction to GR that explains all the data including cluster mass limits, cosmological data, etc. We can rewrite this theory in GR form plus a fictitious distribution of matter and show that this fictitious distribution of matter is that you call dark matter. Since the expression for this fictitious distribution of matter follows from first principles, we can derive the properties of that ‘matter’. In this way we can explain its anomalous density, its interactions… in direct correspondence with your empirical data, but this matter is not real, the real picture is an novel gravitational interaction beyond GR.
The same about dark energy, it is not real, but a fictitious energy that arises from an extension of GR and, I am sorry to remark this, the concordance between our new theory and observation is excellent, specially when compared with other approaches.
By reasons of size (and because this is a blog, not a formal paper) I have avoided many important details in my previous post. For instance, consider the point (i). The link given above
http://www.astro.umd.edu/~ssm/mond/fit_compare.html
compares the dark matter fit with the MOND fit for the dwarf galaxy NGC 1560.
However, I did not comment that the dark matter fit is the best fit obtained with a two free parameter model, whereas no such freedom exists for the MOND model. As everyone can see still MOND fits the data better…
101-102: Many thanks for putting up these convincing results.
There is, of course, a problem with the fit to the “data” of NGC 1560, which is well documented, but nearly always swept under the carpet by our MOND enthousiasts. The situation is nicely illustrated in Figures 9 and 13 of the recent paper by Gentile et al. on this galaxy (see http://arxiv.org/abs/1004.3421). They show that the galaxy is asymmetric, and that the wiggle, so easily fitted with MOND, but not with the smooth curves thought to represent dark matter models, is only seen on ONE side of the galaxy. This was already clear in the original paper by Broeils (1992). Prudent researchers would thus discard such a galaxy from the discussion, since the non-circular motions are localized, and the averaged rotation curve (with half the wiggle amplitude) is probably not representative of the axisymmetric force field.
On a positive note, Stacy’s paper definitely caught the attention of some pundits, one of them reporting a couple of days ago informally in my institute about an effort to explain the ‘fine-tuning’ of the detected baryon fraction in LCDM models. I think that is the way to go about it: it is no use sweeping problems under the carpet, and selling claims that either dark matter is doing just fine, or that some MOND stuff is convincing, when neither is perfectly the case.
I could not find any fig 13 in the pre-print by Gentile et al. (the above link by Albert does not work and I rewrite it here)
http://arxiv.org/abs/1004.3421
But I can find that they report new data with twice the previous resolution and that the new data continue being better reproduced by MOND than by dark matter models.
As the authors report, only MOND gives both a good fit and reproduction of the wiggle. The Burkert dark matter model gives a good fit but does not reproduce the wiggle, whereas the Navarro, Frenk, & White dark matter model (based in a cosmological ΛCDM framework) FAILS BOTH.
Evidently, the problem is not only in the wiggle, as Albert seems to believe, but that ΛCDM based models once again cannot reproduce the data. The article is very clear “ΛCDM simulations result in a bad quality fit“.
The conclusion for “our MOND enthousiasts” is that MOND continues being this ‘ugly’ model which is able to explain observations which cannot be explained using ‘beatiful’ dark matter models even when using twice (or more) parameters that in MOND models.
“our MOND enthousiasts” are also rather proud of that all the predictions done by our ‘ugly’ MOND have been confirmed (see links in #101-102), whereas the ‘beatiful’ dark matter models did none of those…
Juan (#105): thanks for correcting the link. You will find Fig. 13 on page 11 of the preprint. You have not addressed my concern: at the basis of the derived rotation curve is a velocity field, which is asymmetric. Since the wiggle is only at one side of the galaxy, it is doubtful whether the rotation curve has a wiggle. Once that is clearly understood, it does not matter what you fit to it: you fit models to derived ‘data’, which are derived from more basic data using a wrong interpretation. If you are not familiar with how velocity fields and rotation curves are derived, there is nothing I can do for you: you are just out of your depth.
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106: Albert, from looking at the paper http://arxiv.org/abs/1004.3421 by Gentile et al. it is rather clear that NGC1560 is fit on both sides separately by MOND very well, while the LCDM model (i.e.NFW) fails (the Burkert prifile is not LCDM, it has no theoretical dark-matter basis, but it is a good description of the phantom dark matter halo you would derive if you wrongly interpret the rotation curve in Newtonian dynamics – Burkert profiles are essentially a confirmation of MOND, interestingly).
From Fig.9 it follows that this galaxy is quite symmetric, apart from the one side being more”wiggly” than the other. My interpretation of the data and the MOND calculcations , done without fine tuning!, is that the local velocity field is reproduced by MOND very well, as should be the case if the normal (gas+stars) matter is the sole source of gravitation. NGC1560 does not appear to be highly perturbed at all, that is, the velocity field is not significantly affected by a possible interaction with another galaxy. The galaxy appars to be quite isolated:
http://martingermano.com/N1560.htm
http://www.bautforum.com/showthread.php/105681-NGC-1560-Maffei-galaxy-in-some-IFN
So the point you are trying to make is not clear.
#108. The basic point is that the rotation curve gives you the circular velocity of a test particle at a given radius, independent of azimuthal angle. In NGC 1560, at one side of the galaxy there is a local perturbation which causes a wiggle in the position-velocity curve at that side. This is mainly due to an uneven distribution of neutral gas. It is not clear to me that that wiggle can be interpreted as accurately reflecting the circular velocity there. Hence the ‘true’ rotation curve of NGC 1560 is not necessarily the average of both sides, at least in my opinion. Whether or not MOND fits both sides separately (not even well) is immaterial : a galaxy in equilibrium has one rotation curve (and not two).
Gentile et al. don’t give a chi-squared for the fits, but by eye the Burkert model gives a better fit than MOND. Furthermore, they assume a distance of 3.45 Mpc when fitting DM models, and for MOND they adopt a distance of 2.94 Mpc. The 3.45+-0.36 Mpc distance comes from the tip of the red giant branch method. So there is an inconsistency here – the best MOND fit is for a distance different from the best distance estimate -, as is also known for NGC 3198. Note that on Stacy’s page the comments are the other way around : the Begeman et al. fit for D = 3.0 Mpc was not good enough, so they said the galaxy ought to be at 3.4 Mpc for a good fit! So not all is well here (OK, a 13% problem).
The dip in the wiggle in NGC 1560 coincides with a relative depletion of the HI in the northern part. Interestingly enough, there is an extension in the vertical direction in the HI there, see Figure 3 of the paper of Gentile et al. This is not further discussed in that paper, but it could be related to the uneven gas distribution in the main plane.
As for the Burkert model fitting better than a model using the NFW profile (Figures 10 and 11), this graphically illustrates the ‘core-cusp’ problem in LCDM. It is indeed true that it took observers a lot of time to convince cosmologists that LCDM had that problem : their general attitude was (and partly is) still very close to the attitude displayed above by Sean (attitude of saying DM doing “just fine”, despite all the glaring problems; declaring MOND is ugly, as if 4% baryons + 23% unknown matter + 73% ‘dark energy’ is not messy). In that sense, LCDM was, and continues to be, oversold. But that does not mean that MOND should be oversold as well: it also has subtle problems, such as the one I am pointing out about NGC 1560.
Albert, about NGC 1560, it is still intriguing that the wiggles in the rotation curve and in the HI surface density are at exactly the same position and on the same side of the galaxy. As stated in Gentile et al., it is unlikely that this is due to strong non-circular motions as in NGC 3031. It rather seems to be a true wiggle in the gas tangential velocity for orbits that are only slightly perturbed (but it is true that they are of course not circular stricto sensu). To settle this debate, mildly non-axisymmetric models of NGC 1560 reproducing the HI surface density on both sides and a mock “rotation curve” should be produced both in MOND and within a dark matter halo, and why not also by considering molecular gas in an “HI-scaling” fashion, which would probably produce an excellent fit too, apart from the very outer data points (see the impressive “HI scaling” rotation curve fits in http://arxiv.org/abs/astro-ph/0403154)… but I doubt the dark matter models (even the Burkert-based ones) will succeed… Related to HI scaling, an intriguing possibility to explain the baryonic Tully-Fisher relation and the MOND phenomenology in galaxies would be to investigate the consequences of internal physical factors in self-regulated gravitating disks where “dark matter” would mostly be in the form of molecular gas (i.e. multiplying the HI mass by a large factor, that could be of the order of 15 or even more depending on the galaxy), as advocated by, e.g., http://arxiv.org/abs/astro-ph/0409621 … Of course, if this would be the true explanation of the success of the MOND phenomenology, it would be as unlikely as in the modified gravity hypothesis that XENON100 and other similar experiments will ever ever find a CDM particle…
Just to clarify things, I am (a) not claiming that it is possible to simply replace *cosmological* DM with baryonic DM, we know that this is a priori impossible, (b) not even claiming that it is possible to *fully* replace galactic DM by baryonic DM, which is what I meant with the fact that HI-scaling would have troubles fitting the outermost datapoints of rotation curves (while in addition it also couldnt explain the near-sphericity of the potential obtained from fitting orbits of tidal streams, so you’d still need an additional DM halo or modified gravity in the outermost parts), just that *a lot* of DM in galaxies could actually be in cold gas form, which could (i) help explaining the wiggles a-la-NGC 1560 and (ii) perhaps even ease the understanding of the baryonic Tully-Fisher relation from feedback mechanisms (and incidentally, it would drastically reduce the local non-baryonic DM density at the position of the Sun: no luck for direct detection experiments). I still think this last point about feedback is far from easy, and that “plain MOND” is still by far the easiest way to explain the baryonic TF relation. I am just not closed to the idea of a lot of baryonic DM being present in galaxies in the form of H2… easing the understanding of some observations but somehow also complicating the whole picture, both from the DM and from the modified gravity points of view… Who said it was all gonna be easy?
Albert,
thanks for pointing to the figure in the reference page. In early posts, I did a series of corrections to unfounded attacks on MOND and added information about a hundred of galaxies being perfectly described by MOND as well as about a dozen of predictions done by MOND and that have been verified.
You have only focused in one wiggle around 300” in one galaxy! Still, I commented about this wiggle and about how this would not hide the subsequent difficulties for the ΛCDM model, which, I repeat, has been experimentally falsified in galaxies (ΛCDM systematically fails to reproduce both the velocities and the shape of the observed rotation curves).
In fact, you cite now the “core-cusp” problem in ΛCDM and confirm the attitude of cosmologists as Sean Caroll to ignore empirical evidences and to make unfounded comments against MOND.
You affirm that “by eye” the Burkert model gives a better fit than MOND. I find that you again are missing the whole picture because you are ignoring (i) the fit to the wiggle (curiously the best-fit curve in the Burkert model goes through the region of the wiggle), (ii) the number of FREE parameters used in each fit (TWO in the Burkert model, ZERO in MOND), (iii) that MOND gives better M/L ratio that Burkert, and (iv) that ΛCDM gives a bad fit.
There is not inconsistency about distances but uncertainty. Adopting the 3.45+-0.36 Mpc in MOND reduces the fit quality (chi-square) about a 0.13 and there is lower distances than 2.94 Mpc reported by other methods. The comments in Stacy McGaugh’s page are about the Broeils 1992 data not about Gentile et al. recent data. McGaugh emphasizes that “The CDM fit is not unique” and that “Its prediction is more vague than that of MOND, and misses the mark by a wider margin.” The same conclusions are found by Gentile et al.: “bad fits using the a Navarro, Frenk & White halo” (CDM), “and good fits using MOND”.
Now cosmologists as Sean Carroll would explain how is possible than the ‘beautiful’ ΛCDM model once again fails to explain data, whereas ‘ugly’ models as MOND explain the data (without free parameters).
Ben, as far as I understand it, the bump in the HI causes the wiggle in the expected rotation curve. Unfortunately, the MOND fits never show the contributions of the individual components in MOND (i.e. they show the individual contributions for the Newtonian case).
Juan, NGC 1560 has ‘iconic status’ for MOND, i.e. the ‘remarkable’ ability to fit the wiggle is widely quoted as a good reason to take MOND seriously. As for the hundred other galaxies ‘perfectly well described’ by MOND, I advise you to read
http://arxiv.org/abs/1005.5456 where more counterexamples are discussed. Good distances might indeed tighten the constraints on MOND, since now de facto distance is used as a free parameter in MOND fits, in addition to the M/L ratio. Note that the M/L ratio is not predicted by MOND, contrary to your statement that MOND has ZERO free parameters. M/L ratios are rather uncertain, since the IMF is not very well constrained.
Albert,
MOND ability to fit the wiggle in NGC 1560 is not the reason for which MOND advocates consider “to take MOND seriously”. The reasons are (i) a hundred of galaxies explained by MOND as well as (ii) the dozen of predictions done by MOND and verified up to the date.
Regarding the recent preprint by Gentile et al. they do not claim “to take MOND seriously” because of the wiggle, as you continue to believe, but because they got “good fits using MOND” and “bad fits using the Navarro, Frenk & White halo” (an ‘elegant’ ΛCDM model). Reproducing the wiggle is only one point of an overall score that has given MOND its good empirical status.
As explained in the section 5.2 of Gentile et al., the Burkert dark matter model uses TWO parameters (rho_0 and r_{core} in the eq. 1), whereas MOND uses ZERO parameters (see eqs. 6 and 7). For values of r_{core} = 5.6 kpc and rho_0 = 0.8×10^−24 g cm^−3, the best fit M/L ratio for the Burkert dark matter model is of 2.3. Using ZERO parameters, the best fit M/L ratio for MOND is 0.98. The reference value for the ratio obtained from stellar population analysis is 1.43.
If you take all the points together (chi-squared plus wiggle plus ML ratio plus total number of free parameters…) you will find that MOND works better and, indeed, Gentile et al. affirm that MOND does it better than Burkert model and much much better than ΛCDM (which is empirically falsified).
Regarding http://arxiv.org/abs/1005.5456, the authors claim that “MOND is successful” for roughly three quarters of the galaxies in a sample of 27 dwarf and low surface brightness galaxies and that “This is remarkable, given that MOND is a one parameter fit with only M/L as a free parameter.”
MOND does not adequately explain the observed rotation curves for the remaining one quarter. The authors also emphasize that for the discrepant galaxies poor predictions were expected due to strong uncertainties in inclinations and distances.
If you take a look to their sample, you will find that some of the galaxies (e.g. UGC 5750, UGC 6446) were already covered in the section “Galaxies for which MOND fit is dubious” of a link given in a previous post from mine (http://www.astro.umd.edu/~ssm/mond/fitroster.html). However, UGC 5750 was then listed in that section, whereas now is reported as being in good agreement with MOND predictions.
This sample adds 8 new galaxies to the section of “Galaxies for which the MOND fit is dubious” and 14 new to the section “Galaxies well fit by MOND” and moves UGC 5750. This gives an overall score of about 98-0-18, with the 18 being associated to strong uncertainties.
Juan, having heard talks about MOND since its inception in the 1980s, I can assure you that the wiggle in NGC 1560 plays a prominent role in the presentations of MOND by its advocates since the early 1990s. I have seen that plot presented quite a number of times, and Stacy singles it out in his Web presentation as well. I don’t think my sampling of MOND advocates is biased.
Albert, you are probably right about overinterpretations of the wiggle of 1560, even though as I said it is much more *plausible*, I think, to explain it in mond or with HI-scaling than with a DM halo. But non-axisymmetric models in each of these paradigms are needed in order to prove this: hopefully it will come soon. Concerning showing the individual contributions of the gas and stars, it doesnt really make sense in mond as the theory is non-linear. Finally concerning the stellar M/L ratio being a free parameter, you are totally right (even though one of the strong arguments for mond is that the fitted ones follow well the trend expected from population synthesis models), but the whole point of Stacy’s paper under discussion here was to take away that free parameter by considering gas-dominated galaxies. The fact that mond still works well for these is, I think, striking. But you are right that it is not true to state that mond has, in general, zero free parameters.
#116: So Ben, you are confirming that with MOND one does not get an improved representation over dark matter models since in MOND there are free parameters that can be chosen to improve any fit?
Albert,
In this comment-page I have introduced six links from Stacy website. Only one part of one of the links discuss the wiggle. The other links discuss MOND predictions (all verified), samples of about a hundred of galaxies explained by MOND…
The same about the Gentile et al paper. They discuss the wiggle and more as ML ratios and overall fits.
Albert, Ben, and Question Mark,
MOND has zero parameters. It is the dark matter model which uses “free parameters that can be chosen to improve any fit” (as Question Mark says).
I invite you to read again the section 5.2 of Gentile et al.
http://arxiv.org/abs/1004.3421
Two parameters for Burkert dark matter model (central density and core radius).
One/two parameters for the ΛCDM model of Navarro, Frenk & White (concentration and virial mass). Gentile et al. use cosmological simulations to correlate both (see eq. 3), but due to bad fit to the data they repeated the fit leaving both parameters free (read section 6) and still obtained a bad fit and related difficulties.
Zero parameters for MOND (see equation 6).
Regarding http://arxiv.org/abs/1005.5456, the authors use M/L as a free parameter to correlate velocity with light distribution, not because M/L was a free parameter in MOND. MOND are equations 1 and 2 in that preprint and they have zero parameters.
If you were to repeat their analysis using dark matter models you would use one or two parameters (of the dark halo) plus the M/L ratio of the correlation to light.
I.e. using the Burkert dark matter model you have three times more freedom (rho_0, r_{core}, and M/L) and still cannot match MOND predictions.
Precisely, the fact that MOND has zero parameters is the reason which is so sensitive to uncertainties in distances, whereas the dark matter models can absorb distance uncertainties into the halo parameters.
Above I wrote that in dark matter model you have three times more freedom and still cannot match MOND predictions. A beautiful example of this is discussed in
Extended rotation curves of spiral galaxies – Dark haloes and modified dynamics. Mon. Not. R. astr. Soc. 1991: 249, 523-537. Begeman, K. G.; Broeils, A. H.; Sanders, R. H.
Authors compare three-parameter dark-matter fits (M/L for the visible disk, plus two parameters for the dark matter halo: the core radius and the asymptotic circular velocity of the halo) with one-parameter MOND fits (M/L for the visible disk).
They find that MOND works well and “in some cases better than multi-parameter dark-halo fits.”
I am sorry to say this to dark-matter enthusiasts, but MOND rocks…
@question mark: No, I disagree. The M/L ratio is a “free” parameter but its best-fit obtained in mond from dynamics alone is ususally amazingly similar to what one expects from completely independent models based on stellar populations synthesis (that have nothing to do with dynamics): this has been, and still is, one of the strongest arguments for mond! In addition, this free parameter goes away when fitting gassy galaxies, and that was precisely the point of the whole press release of Stacy. Then, there is the distance, which again can sometimes be a little discrepant when set completely free, but which usually gives very good fits when constrained to lie within the range of distances obtained from various independent methods. So, yes there are free parameters (M/L and distance, and in the case of Stacy, only distance), but you wouldnt call a good fit a fit using a crazy value for these parameters, that are in the end not so free anymore… The point is that mond yields brilliant fits in most spiral galaxies with completely reasonable values of those parameters. This is neither overselling nor underselling what mond does. And again whatever the reason for it, its success should be, in my opinion, understood.
My problem with DM+GR is not that it does not fit the data.
My problem is that the MOND fits the data without needing DM.
Since this simple equation works very well at the galactic scale everywhere. The fits actually keep on improving with more data. There can only be two solutions.
1) DM does not exist at Galactic scale.
2) DM position in space is defined by BMs position in space. This is simply untenable if we believe DM to be separate particles. Also BM is not supposed to interact with DM except by gravitational force. This is totally non-sensible
So the real solution is that DM does not exist at the galactic scale.
I would have had no problem with DM, if MOND did not work so well at galactic scale.
The fact that MOND does not work as well at cluster or higher scales makes no difference. It is probably an indication that some form of DM exists on those scales. I don’t even think that MOND can be enhanced to form a theory. TeVeS is just a toy theory that shows how to build one, but I am pretty sure the encompassing physical theory will be discovered from a totally unexpected direction. The recent Verlinde’s theory of Entropic gravity looks interesting.
I liken MOND to an Empirical law. Any quantum theory of gravity needs to bring out MOND or it is not physical. Since GR in its present form does not predict MOND, it is not physical.
It is as simple as that. Empirical laws must be explained by all physical theories. If Newtons gravitational theory did not explain Kepler’s laws, it would be as useless (at the solar system scale) as GR is presently (at the galactic scale).
anand srivastava,
There is a third solution:
1′) DM does not exist at all.
“The fact that MOND does not work as well at cluster or higher scales…” This is only partially true
http://arxiv.org/abs/0704.0381
and, of course, discrepancies with MOND do not imply existence of DM, (ΛCDM has also difficulties http://adsabs.harvard.edu/abs/2010ApJ…718…60L), but that MOND is being applied outside its range of empirical validity and that a more general theory beyond MOND is needed.
As commented above (#101), MOND (and generalizations of it) can be derived from a truly general gravitational theory. You are right on that “the encompassing physical theory” was “discovered from a totally unexpected direction”!
Our goal was to correct other known deficiencies of GR (see the FQXi Essays cited above) and, as a bonus, we discovered that MOND and its acceleration scale were natural outcomes from the new theory.
If you look to my FQXi Essay, you will discover that the recent theory is much more general than Verlinde’s theory, which is based in a number of approximations and controversial assumptions.
@123
No MOND does not work very well beyond Galactic scales so we cannot conclude that at all. It is certainly possible, but we cannot determine that from the data alone. But that is not important MOND working at Galactic Scales kills DM+GR at Galactic Scales. This means GR needs modification. We don’t know what the correct theory is going to be. I am not a physicist or a mathematics. I am just a software developer, with an interest in the MOND problem. I can only conclude if a model is good based on its applications and what it predicts. Unfortunately I did not see much effort at resolving different problems of GR in the quoted essay. Particularly MOND and Pioneer Anomaly.
anand srivastava,
I agree with most of what you say. Effectively MOND “kills DM+GR at Galactic Scales” and effectively this means that “GR needs modification“.
I did not say that MOND works very well beyond Galactic scales. I said just the contrary and even explained why one must wait discrepancies: “MOND is being applied outside its range of empirical validity“.
I also wrote in this blog how, thanks to a new theory, we do not need DM (neither DE) to explain observations beyond the Galactic scale anymore.
FQXi Essays are limited in size by Contest rules, therefore I could not write about all the advantages beyond GR (my first draft Essay was over the size limit and I was forced to eliminate many interesting stuff! For instance, I gave some additional technical details, on how the new canonical theory goes beyond M-theory and the rest of quantum gravity approaches, in Dr. Tejinder Pal Singh Essay).
In despite of size limitations, details and further info are given in the cited literature and in the technical notes in my Essay. In the technical note in page 9, I explain the assumptions and approximations over the which GR is based and how we derive GR from a more fundamental theory. I explain how well-known problems of GR as “the lack of gravitational energy-momentum-stress tensor“, “spacetime singularities“, “the problem of the systems of reference“, “violation of the usual conservation laws“, and “the impossibility to obtain a consistent quantization of such [geo]metric theory” are absent in the new theory.
Moreover, I have commented in Dr. Corda Essay how we can already go beyond MOND, PCG, TeVeS… explaining data that those theories cannot explain, and also commented I have not still studied Pioneer anomaly enough to say.