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Ranku
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How do we distinguish between the effect of dark matter and MOND with respect to flat rotation curves in galaxies? How would the shape of the rotation curve differ between the two?
If I understand it correctly, they don't vary much within a single galaxy. Where MOND fail is with multiple galaxies. For example, in the Bullet Cluster.Ranku said:How do we distinguish between the effect of dark matter and MOND with respect to flat rotation curves in galaxies? How would the shape of the rotation curve differ between the two?
The key difference is that dark matter is like a Rule of Lesbos -- you can bend and shape it to fit anything (and therefore it explains nothing).Ranku said:How do we distinguish between the effect of dark matter and MOND with respect to flat rotation curves in galaxies? How would the shape of the rotation curve differ between the two?
Suppose, for argument's sake, matter itself were to contain more mass than is observationally accounted for, would its gravitational effect be similar to dark matter or MOND?strangerep said:The key difference is that dark matter is like a Rule of Lesbos -- you can bend and shape it to fit anything (and therefore it explains nothing).
With MOND, in contrast, one builds a mass model of a galaxy (using observations that are independent of principles involving Newtonian gravity), and solves the Poisson equation using that mass model as source. Then one applies the MONDian (universal) radial acceleration relation to predict a rotation curve. Such MOND-predicted rotation curves do very well against the actual data.
Btw, statements like "the Bullet Cluster falsifies MOND" are very much overblown, although the current situation remains fluid.
For a recent, but lengthy, summary of MOND-vs-DM phenomenology, see Stacy McGaugh's recent blog post: Checking in on Troubles with Dark Matter
This is a pointless speculation.Ranku said:Suppose, for argument's sake, matter itself were to contain more mass than is observationally accounted for, would its gravitational effect be similar to dark matter or MOND?
Ok, let me re-phrase: Suppose there is more matter than is observationally accounted for, with distribution typical of matter, will its gravitational effect be similar to dark matter or MOND?Ranku said:Suppose, for argument's sake, matter itself were to contain more mass than is observationally accounted for, would its gravitational effect be similar to dark matter or MOND?
The more is exactly what dark matter is (if you mean observationally accounted for as electromagnetically interacting).Ranku said:Suppose there is more matter than is observationally accounted for
You mean, if we just take the visible matter density and increase it by a uniform scale factor? Then no. If you are not adjusting the law of gravity but just adding mass then this is just what dark matter does. So your extra mass needs a distribution similar to the dark matter distribution, not the visible matter distribution.Ranku said:Suppose there is more matter than is observationally accounted for, with distribution typical of matter, will its gravitational effect be similar to dark matter or MOND?
What is ironic about this characterization is that actually, a Lesbian rule is an extremely useful invention, and analogous applications in science are central to how it functions. (Consider Euclidean geometry, the classic example of a "straight rule" that turned out to not work in some applications.) The problem with a normal straight rule is that it is fixed in advance, before you encounter the application you need help with. We certainly find great use in this, but it only works when we can anticipate the problems we will face. For example, a fixed straight rule is great if you know you are going to build a rectangular structure, and you are fine with that constraint. Where a Lesbian rule comes in is when you need greater flexibility than rectangular structures, but you want something that is consistent from one stone to the next. So it's actually fine to use a Lesbian rule, so long as once you have crafted it, you are able to use it over and over. The problem comes in if you find you have to keep bending it anew, that is what would compromise its value. I would say we are still in the testing phase of dark matter, to see if the same Lesbian rule can be used over and over (for example, if the dark matter fraction in the universe is a fixed value across all ages), just not in a preordained way, or if we eventually give up on that approach.strangerep said:The key difference is that dark matter is like a Rule of Lesbos -- you can bend and shape it to fit anything (and therefore it explains nothing).
strangerep said:With MOND, in contrast, one builds a mass model of a galaxy (using observations that are independent of principles involving Newtonian gravity), and solves the Poisson equation using that mass model as source. Then one applies the MONDian (universal) radial acceleration relation to predict a rotation curve. Such MOND-predicted rotation curves do very well against the actual data.
Yes, I agree with your characterization.Ken G said:So it's actually fine to use a Lesbian rule, so long as once you have crafted it, you are able to use it over and over. The problem comes in if you find you have to keep bending it anew, that is what would compromise its value.
I was under the impression that the necessary form of DM halos are rather different for different galaxies, whereas a single MOND interpolation function needs to work widely (at least, within experimental error).Ken G said:I would say we are still in the testing phase of dark matter, to see if the same Lesbian rule can be used over and over (for example, if the dark matter fraction in the universe is a fixed value across all ages), just not in a preordained way, or if we eventually give up on that approach.
Does MOND do better in galaxies which otherwise requires less proportion of dark matter-to-matter?phinds said:There are also galaxies that simply cannot be explained at all by MOND, but only by dark matter. Here's an example of one where the rotation curve cannot be explained by MOND:
99-percent-dark-matter/
Yes, if dark matter is the cause, then different galaxy types have different amounts of dark matter acting in different ways. This would be analogous to how we need different metallicities in different galaxy types to understand how they behave. Not necessarily a bad thing, if it is indeed what is happening, but what's required is to have some other way of understanding why those differences exist. Like with metallicity, it conjures the fact that the action of stars raises metallicity over time, so we must correlate the different metallicities with different star formation histories. The added complexity becomes an opportunity to learn something else, not just an additional free parameter, but that's only useful if we can indeed learn that "something else."strangerep said:I was under the impression that the necessary form of DM halos are rather different for different galaxies, whereas a single MOND interpolation function needs to work widely (at least, within experimental error).
Be careful. This is not a degree of freedom for DM. For whatever reason, DM arranges itself (probably via the formation of galaxies) to look like MOND, or if you prefer, Tully-Fisher. For whatever unknown reason, we see variations in the dark to luminous matter ratio, but always one that follows a particular relation.strangerep said:that the necessary form of DM halos are rather different for different galaxies
Peace, just like they have it in the Middle East!Ken G said:peace
Could you elaborate?Vanadium 50 said:we see variations in the dark to luminous matter ratio, but always one that follows a particular relation.
What do you mean by this sentence, exactly? How do the variations in ratio also follow a particular relation?Vanadium 50 said:we see variations in the dark to luminous matter ratio, but always one that follows a particular relation.
Comparing dark matter and MOND is comparing apples and oranges.Ranku said:How do we distinguish between the effect of dark matter and MOND with respect to flat rotation curves in galaxies? How would the shape of the rotation curve differ between the two?
I'm not understanding how MOND can fail but another gravity based explanation could succeed in the domain you consider, because I thought "MOND" literally meant "modification to Newtonian dynamics," so anything that proposes a modification to Newton's universal law of gravity would count as MOND. It sounds like you have something more specific in mind when you refer to MOND, so what is the difference between MOND and a proposed gravity based explanation that replaces the need for copious amounts of dark matter?ohwilleke said:But there are also multiple proposed gravity based explanations for dark matter phenomena that work in domains of applicability where MOND fails.
MOND is a specific theory devised by Prof. Mordehai Milgrom in 1983. It proposes that gravity behaves in a way consistent with general relativity below a threshold gravitational acceleration magnitude a0 and is stronger according to a simple formula that creates flat rotation curves for weaker gravitational accelerations, subject to an "external field effect". See generally, this Scholarpedia article by Professor Milgrom about MOND.Ken G said:That's quite a treatise on a complicated situation, thank you. I did have one specific question though, about this part:
I'm not understanding how MOND can fail but another gravity based explanation could succeed in the domain you consider, because I thought "MOND" literally meant "modification to Newtonian dynamics," so anything that proposes a modification to Newton's universal law of gravity would count as MOND. It sounds like you have something more specific in mind when you refer to MOND, so what is the difference between MOND and a proposed gravity based explanation that replaces the need for copious amounts of dark matter?
None-the-less, the generally held belief is that it does. I'm sure I have read numerous times here on PF that "MOND" is a collection of theories.ohwilleke said:The term does not have the broader meaning that you mistakenly ascribed to it.
There are a collection of variations on MOND theories. But those various generalizations of Milgrom's 1983 theory are themselves distinct from non-MOND modified gravity theories.phinds said:None-the-less, the generally held belief is that it does. I'm sure I have read numerous times here on PF that "MOND" is a collection of theories.
While there are a number of MOND theories, that acronym is not generally used to refer to the whole universe of gravity based explanations of dark matter phenomena or modifications of gravity. It is one specific approach centered around a transition at the physical constant a0.Vanadium 50 said:"MOND" as generally used corresponds to a whole family of theories. Either side of F=ma can be modified. You might get more force at low accelerations, or you might get more acceleration for the same force (which might or might not be restricted to gravity). We could call these models MOND. MONG and MONI. But we don't.
Similarly, MOND and Moffat's theories are distinguished from each other (even though both explain DM-phenomena with gravitational modifications) in Yongda Zhu, et al., "How Close Dark Matter Halos and MOND Are to Each Other: Three-Dimensional Tests Based on Gaia DR2" arXiv:2211.13153 (November 23, 2022) (accepted for publication in MNRAS).Milgromian dynamics (MOND) can successfully fit the rotation curves with the same acceleration scale a0 measured at z≃0.
I would say that the balance has already tipped. Lambda-CDM which is the paradigm is dying a death of several dozens serious cuts, if not the thousand of the Chinese proverb.Ken G said:Still, now that I understand you are taking a strict meaning of MOND, I can better understand how you are summarizing the landscape. In part from what I've seen, and in part from your summary, it seems that we are as far away from a new theory of gravity to add to relativistic and quantum dynamics as we are from a new set of dark matter particles to add to the particle zoo. One can see why there is general favoritism to simply finding the "dark matter particle", since then we just have the attributes and interactions of that particle to understand, much like the discovery of the neutrino, and we can otherwise maintain the existing dynamical machinery. If it's MOND, then the particles stay the same, but the complete dynamical machinery must change! That's a lot of overhead. I have faith in the process of science, that this monumental task will eventually be undertaken successfully, but I haven't much faith it will happen any time soon, so I doubt I will see it. If it takes a century, none of us will!
On the other hand, one of the things that does seem to offer promise to be resolved in the foreseeable future is "precision cosmology." Some claim it's already here, but I am not among them, not as long as their remains tension in the determination of the Hubble parameter. But if I'm an optimist, and I think the next round of cosmological observations will line everything up nicely as things appear to be turning out, then we are going to be looking at dark matter and dark energy as the key elements of cosmological dynamics. As long as that continues to be true, then dark matter is pretty much here to stay, regardless of any successes of MOND. As I said above, it may turn out that MOND is only used as a convenience, a way to parametrize the effects of dark matter in galaxies because it is easier to use than some whole new paradigm for equipping galaxies with dark matter and its interactions. Or it might turn out that dark matter is only used as a convenience in cosmology, a way to parametrize some unknown dynamical effect on the expansion of the universe. An uneasy peace, perhaps, and maybe only a period of "cheating" as alluded to above, but that does seem to be where we are, and I'm not sure I see that situation changing any time soon. But who knows?
We certainly don't have a final answer yet.Ken G said:That's quite an interesting take, you are either way ahead of the curve on this, or buying off on speculative results that are not as well vetted as you claim. I don't know which, but either is still interesting, and more promising than the perspective I expressed above!