What is the current status on MOND

[Tanelorn]

I wondered if MOND is still considered a possible alternative to cold dark matter?
See section titled "Alternatives to dark matter"

http://en.wikipedia.org/wiki/Galaxy_rotation_curveOne other question: when I play the galaxy rotation video it appears to me almost as if the arms are gravitationally bound structures held together like chains. Also matter from beyond the galaxy is falling into the galaxy arms at high velocity keeping their rotational velocity high. Is there any possibility this is actually taking place or is this just an illusion on my part (most likely)?

 

[Chronos]

Since Clowe announced the Bullet Cluster findings, Mordehai Milgrom [MOND founder] authored 21 of the 128 papers on arxiv with the word MOND in the title. Needless to say, the other 107 papers were less unanimous in their support. This suggests interest in MOND among mainstream cosmologists has been in decline.

 

[Vanadium 50]

Let me start by saying I do not believe MOND is the correct description of reality.

That said, there are some really bad arguments used by LCDM proponents, and worse, these are often the ones that are used with the public.

The worst one is "any problem with MOND is an argument in favor of LCDM". We don't accept this line of argument from creationists, so we shouldn't accept it from ourselves. The second worst one is the Bullet Cluster. This is billed as a single-event "smoking gun" which proves LCDM. Problem #1 is that while the aftermath of the collision in the Bullet Cluster looks like one would expect from LCDM, the collision itself does not: the relative velocities are too fast.

Problem #2 is Abell 520, another pair of colliding galaxies where the inferred dark/normal matter distributions are reversed. The same people who argue that the Bullet Cluster proves LCDM, when asked about Abell 520 say "Well, you can't tell anything from a single event, and maybe this is an optical illusion, and blah blah blah".

I would argue that we either decide to weight pro- and anti- LCDM models equally, or admit that part of the appeal of LCDM is prejudice based on how we feel things "should" work. I don't have a problem with theoretical prejudice. I use it myself, because often it is right. But one needs to be aware that one is doing this.

MOND does better than LCDM at some predictions, LCDM at others. Stacy McGaugh of Maryland has a nice comparison here: http://www.astro.umd.edu/~ssm/mond/LCDMmondtesttable.html

In my view, one of the failings of the LCDM community - which is the mainstream - is their refusal to answer the question "If MOND is wrong, why does it work as well as it does?" For example, the Einstein Crystal model explained specific heats at room temperature, but not at low temperature. The Debye model explained both, and explained why the Einstein model worked as well as it did. LCDM proponents should not be saying "we don't have to explain MOND" - they do. It's an empirical fact that spiral galaxies behave as if MOND is correct. A complete theory would explain why.

 

[bcrowell]

MOND does better than LCDM at some predictions, LCDM at others. Stacy McGaugh of Maryland has a nice comparison here: http://www.astro.umd.edu/~ssm/mond/LCDMmondtesttable.html

The trouble with all the tests listed is that they're non-definitive tests based on low-precision astronomical observation.

It would be definitive if dark matter could be detected directly. As far as I know, the most recent review is Hooper and Kelso, http://arxiv.org/abs/1106.1066 . It would also have been encouraging for ΛCDM if the LHC had detected supersymmetry at the electroweak scale -- but it looks like SUSY is dead or dying.

MOND is not a theory, it's a data-fitting procedure. There are various theories that can produce MOND-like effects, their flavor being mostly something like scalar-tensor gravity. Some examples: http://arxiv.org/abs/1003.2185 , http://arxiv.org/abs/1001.1564 , http://arxiv.org/abs/gr-qc/0506021 . These typically make predictions that differ from GR's in solar-system tests. It would be definitive if high-precision solar-system observations falsified GR and supported one of these theories.

 

[Chronos]

I think the problem with MOND is it is an effective theory without a clear theoretical basis. IMO, most researchers are instinctively distrustful of any model that is formulized cut and paste from observational data. While there is nothing wrong with this approach, it is risky to draw conclusion before the underlying physics is fleshed out. Ptolemy's epicycles was a remarkably effective theory, but, failed to lead to a deeper understanding of nature. The same fate may await MOND. It describes the effect, but, not the cause.

 

[Vanadium 50]

The trouble with all the tests listed is that they're non-definitive tests based on low-precision astronomical observation.

It is, however, what we have.

MOND is not a theory, it's a data-fitting procedure.

I think it's at least a little better than that. It does make predictions the most important of which is that the parameter one gets from this data-fitting procedure is universal. This turns out to be the case with galaxies with a wide variety of morphologies and measurements. This turns out not to be the case with clusters of galaxies.

 

[Chalnoth]

I wondered if MOND is still considered a possible alternative to cold dark matter?
See section titled "Alternatives to dark matter"

http://en.wikipedia.org/wiki/Galaxy_rotation_curveOne other question: when I play the galaxy rotation video it appears to me almost as if the arms are gravitationally bound structures held together like chains. Also matter from beyond the galaxy is falling into the galaxy arms at high velocity keeping their rotational velocity high. Is there any possibility this is actually taking place or is this just an illusion on my part (most likely)?

As I understand it, MOND has always been a highly problematic idea. The main issues which I can recall off the top of my head are:
1. Different galaxies tend to have very different rotation curve profiles. This is relatively easy to understand with CDM, but it tends to imply different MOND parameters for different galaxies, which is obviously problematic.
2. Clusters of galaxies and rotation curves for individual galaxies again seem to indicate rather different MOND parameters, while CDM fits cluster behavior quite easily.
3. The bullet cluster is trivially-explainable by CDM, but requires a new species of massive neutrino for MOND to work, and such a species of heavy neutrino is probably ruled out by our CMB observations.
4. MOND theories have always had issues with structure formation in the very early universe, with the structure of the CMB anisotropies easily-understood in the context of CDM, but not so easily with MOND.

(note: This is off the top of my head, and I'm only about 60%-70% sure about numbers 1 and 2, but I'm quite sure numbers 3 and 4 are accurate)

 

[Tanelorn]

Thanks for replies.

I believe that the mass dstribution in a galaxy is much different to what we are used to in for example solar systems. In a galaxy the central black hole is only a very small percentage of the total galactic mass, and so is the central hub. Therefore wouldn't the orbital velocity near the galactic center need to be lower than we might normally expect? If stars moved too fast wouldn't they be forced to move to more distant orbits?

If most of the mass is contained in the outer arms would they tend to be gravitationally bound structures and perhaps tend to stick together, even if the are moving at higher orbital velocity?

Could our estimates for the total mass of ordinary matter be off? eg. I believe that there are now thought to be many very small and large masses out there not orbiting solar systems, all the way from dust up to Jupiter sized planets.

Chalnoth regarding your point 1 above, would not different size, shape, and matter distribution spiral galaxies have different rotational velocities and also rotation curves?

In summary I was just wondering if there arent more mundane explanations to explain at least some of this effect? However I am sure that scientists have already modeled and simulated a spiral galaxy in complete detail on a super computer, much better than I can attempt to visualize. Does anyone know if there might be an amature version of such a simulator out there to play with?

Thanks

 

[mfb]

The worst one is "any problem with MOND is an argument in favor of LCDM". We don't accept this line of argument from creationists, so we shouldn't accept it from ourselves.

Every problem in one theory is an argument in favor of all other theories without that particular problem. The question is: Do you expect so many problems from our lack of knowledge?
Creationism is not a theory. And evolution has not more problems than expected from our lack of knowledge.

It's an empirical fact that spiral galaxies behave as if MOND is correct. A complete theory would explain why.

With just galaxies, I would expect that curve-fitting gives reasonable results if the amount of CDM is somehow related to the amount of visible matter.

 

[Jonathan Scott]

As I understand it, MOND has always been a highly problematic idea. The main issues which I can recall off the top of my head are:
1. Different galaxies tend to have very different rotation curve profiles. This is relatively easy to understand with CDM, but it tends to imply different MOND parameters for different galaxies, which is obviously problematic.

No, that's the really weird thing about MOND. Somewhat amazingly, the same MOND parameter (there is only one) gives a good fit for every galaxy to which it's been applied, regardless of the rotation curve profile, over a wide range of different shapes and sizes of galaxy.

True, it only works on the single galaxy scale, not on smaller or larger scales.

 

[Tanelorn]

Jon, presumbly MOND doesn't work on galaxies which have recently had interaction with other galaxies?
Also would MOND work on a galaxy which has a ring of stars around it such as this one?

http://en.wikipedia.org/wiki/Hoag's_Object

 

[Vanadium 50]

1. Different galaxies tend to have very different rotation curve profiles. This is relatively easy to understand with CDM, but it tends to imply different MOND parameters for different galaxies, which is obviously problematic.

Just the opposite. MOND does as well or better with its one parameter than LCDM does with three. And its not just rotation curves: it gets Tully-Fisher right as well.

2. Clusters of galaxies and rotation curves for individual galaxies again seem to indicate rather different MOND parameters, while CDM fits cluster behavior quite easily.

I agree with the first part, not so much the second. MOND makes a prediction, which fails. LCDM can accommodate the properties of clusters, but it does not predict them.

3. The bullet cluster is trivially-explainable by CDM

But that same explanation says that systems like Abell 520 don't exist. My conclusion is that the trivial explanation is wrong and the true dynamics are more complicated.

4. MOND theories have always had issues with structure formation in the very early universe, with the structure of the CMB anisotropies easily-understood in the context of CDM, but not so easily with MOND.

That I agree with.

 

[Vanadium 50]

With just galaxies, I would expect that curve-fitting gives reasonable results if the amount of CDM is somehow related to the amount of visible matter.

But why is the distribution of Dark Matter in a single galaxy such that it gives a MONDy result, and when you go to a different galaxy you also get a MONDy result with the same parameter? Remember, the Dark Matter distribution in a galaxy does not track the visible matter distribution.

True, it only works on the single galaxy scale, not on smaller or larger scales.

Are there measurements on smaller scales? I thought that smaller objects had accelerations that were too large to see MOND-like phenomena.

Jon, presumbly MOND doesn't work on galaxies which have recently had interaction with other galaxies?

Why are you saying that?

 

[Old Smuggler]

With just galaxies, I would expect that curve-fitting gives reasonable results if the amount of CDM is somehow related to the amount of visible matter.

For spiral galaxies, the observations indicate a very close relationship between the dark halo and the visible matter in the disk; this is called "the disk-halo conspiracy". That is, the dark and the visible matter distributions apparently conspire such that the rotation curve turns out to behave as described by MOND and being asymptotically flat. Moreover, the asymptotic rotational speed fulfils the baryonic Tully-Fisher relation, valid over a range of 5 decades in spiral galaxy mass.

This close connection between the visible and dark matter is difficult to understand since the DM is collisionless and should only interact gravitationally with the visible matter. That is, especially in the outer regions where the DM dominates completely, the visible matter should have little influence on rotation curves. But this is not what is being observed; rather, small variations in the density of visible matter as a rule have counterpart features in the rotation curve. So for spiral galaxies, it seems that the tail is wagging the dog!

The problem is then that if the DM halo plus the visible disk were the full story, one would expect to see a much wider variety of rotation curve shapes. But one only sees those which agrees with MOND. (Someone once synthesized a bunch of artificial rotation curves just to see if DM could cope. The result was that DM could fit those rotation curves, but MOND could not. DM is more flexible than MOND!)

 

[Jonathan Scott]

Are there measurements on smaller scales? I thought that smaller objects had accelerations that were too large to see MOND-like phenomena.

One of the least satisfactory things about the MOND formula is that it includes an artificial cut-off at higher accelerations, which appears to be in order to avoid a potentially measurable (but extremely tiny) conflict with GR and/or Newtonian theory at the laboratory and solar system scales.

This cut-off is particularly unsatisfactory because in practice every particle of a star or even a solar system is typically experiencing local acceleration far in excess of the MOND threshold, yet somehow MOND is supposed to apply to the overall acceleration of the system, requiring the motion of the whole to be different from the sum of the motion of the parts. This isn't totally impossible, but I've so far not seen any satisfactory explanation.

(However, given the smallness of the deviation even within the solar system, and the lack of a specific theory about the exact local nature for calculation purposes, it still seems possible to me that the cut-off might not apply and that the MOND acceleration simply adds to the Newtonian / GR acceleration, giving consistency, but is smaller than predicted by the naive version of the theory).

Other aspects of MOND in its basic form are also unsatisfactory (for example it doesn't conserve momentum because the force of A on B is not the same as the force of B on A) but these have been addressed via more sophisticated variants such as TeVeS.

Another empirical "theory" related to this is Moffat's STVG / MOG, which also claims to handle galaxy clusters and other complex cases, but which has several more tunable parameters.

 

[Tanelorn]

Vanadium, I just thought that neither theory would be able to explain the chaotic movement of stars after two galaxies pass by or collide.


Since the hoag object is very regular in shape:
http://en.wikipedia.org/wiki/Hoag's_Object

I wondered does MOND and/of CDM accurately predict the rotational velocity of this ring of stars?

 

[Vanadium 50]

CDM can't predict anything, since you don't know how much dark matter you have. MOND can; I have no idea if the prediction matches or not.

 

[Chalnoth]

Vanadium, I just thought that neither theory would be able to explain the chaotic movement of stars after two galaxies pass by or collide.Since the hoag object is very regular in shape:
http://en.wikipedia.org/wiki/Hoag's_Object

I wondered does MOND and/of CDM accurately predict the rotational velocity of this ring of stars?

The problem is that when it comes to examining objects in the universe, we have to understand the history of the objects to understand what a specific theory predicts what structure it will take.

This is why most of the evidence which contrasts MOND and CDM isn't concerned with why galaxies (and other objects) take on a particular structure, but instead with just how the gravitational attraction in the current structure behaves. For example, star rotation curves don't mention anything about why a galaxy obtained a particular density profile: they are merely concerned with how gravity keeps it in (approximately) that shape.

When it comes to formation, well, galaxy formation in general is an unsolved problem, not because we don't understand gravity, but because it's fantastically difficult to take into account the full effect of the behavior of normal matter (e.g. what impact do supernovae have on galaxy shapes? What about the supermassive black hole at the center?). This isn't to say we know nothing about how galaxies form, but rather that there are lots of big, unsolved questions here.

So when we want to compare gravity theories, the thing to do is focus on observations where these other uncertainties, e.g. regarding galaxy formation and structure, simply do not play a role. This is one reason why the bullet cluster observation is so neat: it's a very clean observation of a pair of galaxy clusters that recently passed through one another, as can be clearly seen by the bow shock of the hot x-ray cluster gas. With about 10 times as much matter in this cluster gas than exists in the normal matter in the galaxies, a modified gravity theory would tend to predict that most of the mass should have been surrounding this hot cluster gas instead of the galaxies. Instead, the galaxies had most of the mass, which indicates that the dark matter (which, like the galaxies, was not slowed by the collision) is what contains most of the mass of these galaxies.

Some alternative gravity theory advocates claim that their theory can explain this without dark matter, but then they require a new species of heavy neutrino (i.e. another sort of dark matter). At that point, the whole enterprise becomes rather ridiculous.

 

[Chalnoth]

CDM can't predict anything, since you don't know how much dark matter you have. MOND can; I have no idea if the prediction matches or not.

That is completely false. MOND most definitely has parameters whose value is not known a priori and must be determined from observation. And the idea that CDM doesn't predict anything is positively ludicrous. Consider the CMB: the relative height difference between the even and odd harmonic peaks is a direct prediction of the effect of having most of the matter in our universe, as measured by mass, being cold dark matter. Now, the exact ratio of the even-odd peak heights was not predicted, but that there would be such a difference was.

 

[matt.o]

Let me start by saying I do not believe MOND is the correct description of reality.

That said, there are some really bad arguments used by LCDM proponents, and worse, these are often the ones that are used with the public.

The worst one is "any problem with MOND is an argument in favor of LCDM". We don't accept this line of argument from creationists, so we shouldn't accept it from ourselves. The second worst one is the Bullet Cluster. This is billed as a single-event "smoking gun" which proves LCDM. Problem #1 is that while the aftermath of the collision in the Bullet Cluster looks like one would expect from LCDM, the collision itself does not: the relative velocities are too fast.

I have to guess at which paper you are referring to here. Initial claims of the incompatibility of the Bullet cluster with LCDM inferred the velocity of the Bullet subcluster from the shock properties. The velocity of the Bullet has been reduced for two reasons: 1) The newest Chandra calibrations lower the shock temperature and, therefore, the Mach number and measured velocity for the shock (although the density jump at the shock may have been used to determine the Mach number). 2) The velocity measured from the shock is not the same as the velocity of the Bullet subcluster (see e.g., Springel and Farrar 2007). The speed of the bullet subcluster is more like 2300km/s (c.f. ~4700 km/s from the shock). This is compatible with LCDM.

If you are referring to work which compares initial velocities taken from the idealised simulation of the Bullet cluster and compares them to those seen in cosmological simulations, then this method assumes that the initial conditions in such idealised simulations reflect what happens in the universe. Also, these initial conditions are tweaked so that the end result of the simulations mattch the observed properties of the Bullet. This means that, if the initial conditions are to reflect the real universe, the physics of the collision must be well known and modeled. I think these two assumptions fail and, therefore, so any test comparing initial velocities from idealiased simulations to LCDM simulations is flawed.

Problem #2 is Abell 520, another pair of colliding galaxies where the inferred dark/normal matter distributions are reversed. The same people who argue that the Bullet Cluster proves LCDM, when asked about Abell 520 say "Well, you can't tell anything from a single event, and maybe this is an optical illusion, and blah blah blah".

Correction: Colliding *clusters* of galaxies.

The offsets in A520 are as expected. The issues was the dark core, which has since disappeared (see Clowe et al. 2012).