A galaxy (elliptic, or spiral) is made out of billions of stars, like our sun.
These stars rotate around the center of the galaxy (very slowly, think millions of years for 1 rotation).
A rotation curve measures the velocity of stars as a function of distance from the center of the galaxy.
Newtonian physics (or Einstein's GR) says that the rotation curve should decay with distance, ie. with greater distance the stars' velocity should go down --- assuming the matter in the galaxy is the visible matter that we see, ie. the stars (which shine light).
The problem is, there is a rich set of observational data, from many different experiments, telescopes, and methodologies that show that the rotational curve is in fact flat, it does not decay.
There are 2 big competing theories to explain this discrepancy:
1. Assume that there is a lot of unseen, non-shining, ie. Dark Matter (DM) in the galaxies (also ours). If you put the appropriate amount of dark matter in there, with the right distribution, you can reproduce the observed rotational curve. There are also other places is astrophysics/cosmology where having dark matter (specifically Cold Dark Matter, CDM, where cold just means "slowly moving") is useful. The biggest example is to explain the history of the Universe and the observed Hubble-constant. In fact the standard model of cosmology is called λCDM, CDM for Cold Dark Matter (λ for the cosmological constant, currently modeled as Dark Energy, not relevant for this discussion).
2. Assume that Newton was wrong and gravity is not exactly 1/r^2 --- this is called MOND, Modified Newtonian Dynamics. This way you can also reproduce the observed rotation curves. This is much less popular, because: (i) physicsts don't want to give up the beautiful and geometric simplicity of 1/r^2 (ii) Dark Matter is also useful for solving other discrepancies in astrophysics/cosmology.
What this article is saying is that, even in the first Dark Matter model, per the model DM distributions inside galaxies that also work with all the other places where DM is used to explain something (eg. in cosmology), at some distance from the center, the dark matter bubble has an edge and stops --- and then the velocities should finally break down. However, these latest observations are showing that the velocities remain constant even beyond the modeled/assumed DM bubbles. This is an additional ε argument in favor of MOND, and science proceeds.
I'm not a physicist but 1/r^2 strikes me as conceptually very important, because it's the relative contribution of any fixed area of spherical surface to the total area of that surface. So the total strength of gravitational field emanating from a particular object, at a given distance from that object, is a constant.
It's somewhat weird to think of the total gravitational "force field" _increasing_ in magnitude with distance. Decreasing, sure. Increasing? That makes no sense. Certainly not at a large enough function of distance for the rotational curve to be _flat_. That's got to be some kind of wonky power term over distance which implies potential energy from the field goes up with distance as well.
As above, I'm not a physicist, but a linear rotational curve breaks every intuition I've ever gleaned from physics about the nature of what's really going on with relativity, particle mediation of forces, or even the concept of a field.
Maybe it means spacetime curvature is way higher than we think.
Well the general solution is to invent a field with force mediating particles that have the exact same properties as dark matter but insist it's not dark matter.
Maybe not so weird if gravity isn't the curvature of spacetime but a symptom of there being either more or less of it, and mass creates spacetime.
Replace the highly curved spacetime region close to a blackhole with the idea that huge amount of spacetime is being created by the mass of the blackhole, so there is more spacetime near the blackhole.
The more spacetime being created and 'flowing outwards' away from the mass, the faster the apparent 'velocity' of an object through that region of spacetime ner the blackhole (and have this work out that the spacial component handles the physical motion and time slows down to compensate - just like it does in highly curved spacetime), and consequently the slower it moves relative to an external observer.
Areas further from mass see much more 'dilute' spacetime (whatever the heck that means) and travel with relative slower spacial velocity but faster in time, so it appears to be travelling faster up. This would be doubly obvious at the scale of galaxies.
I think this ridiculousness would rely on the relativity of simultaneity in rather a large way!
The other interesting thing is, if mass does create spacetime then pockets of mass like galaxies should move away from each other faster and faster as they make more of it in between themselves.
(NOTE - this is just a silly thought experiment, don't take it seriously)
It’s not at all silly. There’s some nice visualisations[1] of GR in YouTube that look like space is being swallowed up by matter.
A toy model I like to use in my mind is that matter absorbs spacetime. It is literally sucked in!
A possible extension of this model is that the tension introduced in the vacuum causes it to stretch out. That could potentially explain the non-r^2 terms in galactic rotation curves.
I don't think this sounds silly at all. Virtually everywhere we look in cosmology the last decade or so, you get a sensation of things unseen. Like we're looking at one of those optical illusions that change shape when you cover your left eye, or like the McGurk Effect, when your audio perception shifts completely because you can see someone's lips.
Another element that's being discussed is, like with mass and spacetime, that the spatial dimensions themselves are emergent phenomenon arising from bulk entanglement. Sean Carrol has talked about it a fair amount, and it's been surfing around maybe harder than it would ordinarily, because it provides some edge cases that are, at least conceivably, testable without solar-system-sized accelerators or a DeLorean to the beginning of the cosmos. It's an evocative thought. In one interpretation of this, Double Slit restricts many of the spatial dimensions, resulting in a particle that might seem to be in different places, but which is, in some respects, the same particle. Another interesting notion is that singularities, in some dimensions, might be the same place.
Combined with your notion, it almost re-frames mass as - forgive me for getting poetic here - a measure of fate. How much does this resist doing that?
For a more detailed description of the Milky Way's rotation curve, this is a brief segment from David Butler's How Far Away Is It video series (which I highly recommend):
> If you put the appropriate amount of dark matter in there, with the right distribution, you can reproduce the observed rotational curve.
What is the "right distribution"? If it's not roughly uniform, it's unlikely to result in a uniform flatness of rotation curves, across the distances in the galaxy. This seems almost impossible when accounting for clustering within the galaxy. I would believe such a uniform distribution of DM would be possible, if Dark Matter is something that exists/acts differently than matter. For example, if there were space-time bumps that form. Small bubbled/hilled spacetime is created in reaction to masses traversing it? ie the classic ball on a sheet, except it behaves more like a liquid than a sheet.
This feels like a blow to Dark Matter theories, regardless.
> (i) physicsts don't want to give up the beautiful and geometric simplicity of 1/r^2
Not a physicist here, so maybe this is a naive question: but is this really something that they care about? Why does a formula describing some physical principle have to beautiful and simple? Aren't we supposed to observe reality and then come up with the math? Rather than start with a "known true" equation and add factors and parameters it as more and more observations call the equation into question? Who's in charge of the direction physics proceeds? The observing scientists or the mathematicians?
It's not about the formula. The formula comes from the geometry.
1/r^2 falls out as the formula, starting from the geometry of "flux" and "field lines", along with "conservation of flux", in 3d space.
That's the idea that the force acts like something that's radiated in all directions, that doesn't fade with distance, instead it just spreads out so it seems weaker at individual points. The amount of spreading out, if it's uniform in 3d, turns out to be exactly 1/r^2.
You get the same 1/r^2 if you measure the flow of water in a 3d volume with a point source of water in the centre, or electric current in a 3d block of metal with a point source of electric current in the centre. (In both cases, presumably through a thin pipe or cable to the centre).
In 2d space, you get a different formula from the geometry, 1/r. If you see a force, or flow, reducing by 1/r^2 in a system you thought was 2d, you might ask "is there a third dimension involved here which I haven't accounted for?"
In 1d, the force or flow doesn't reduce with distance. For example, current in an electrical wire is the same all along the wire.
And if you see 1/r^3 in 3d, you might speculate about a hidden fourth dimension to explain it.
A general principle of science is that simpler explanations are more likely. Rule-of-thumb described by Occam's razor, formalized by concepts like Solomonoff induction. Rules like 1/r^2, being simple, are assigned a higher prior probability.
This makes sense because otherwise you waste a bunch of time on overfitted theories.
But that 1/r^2 is very old, with a lot of people standing at the cemetery peeking at their watch and thinking "now how much longer will this take, come on!".
Are you aware of any visuals that show what the density distribution of DM looks like to fix the expected the rotation curves for some example galaxies?
DM distribution is different for every single galaxy, and so can only be fit to the galaxy's rotation curve after observing it. In contrast, MOND successfully predicts rotation curves a priori using only the visible matter.
> physicsts don't want to give up the beautiful and geometric simplicity of 1/r^2
Eh, just like MOND proponents don't want to give up the beautiful simplicity of "If something is attracting me gravitationally I'd better see it!"
When you think about it, there's no a priori reason why a particle with mass should interact with any other force. We'd just like to assume it because it seems "simpler" that way.
Nor that there should be just one family of interconnected fields. We’ve got, what, two dozen or so?
Some affect each other, some don’t. You can create a graph from that, and you get one that’s dense in places but have some nearly disconnected regions. Why not a graph with actual disjoint subgraphs? We’d only be able to tell through gravity.
For those who aren't sure what Rotation curves are: it's the observation that the orbits of stars in galaxies around said galaxy's center do not slow down the further away from the center they are. In fact some even rotate faster the further out you get!
I would settle for achieving intuition or even credible sources'acknowledgment in popular press on just what point in space is being orbited when the satellite is millions of LY (or 8 light minutes :) ) out from the massive object (which is translating at a measurable fraction of c ) it is orbiting.
I can't search it up now, but I have seen a derivation that if you work all the way through the math, it turns out your intuition is wrong and in the case of something like a planet orbiting a star, if the star is moving at a reasonably constant speed (ignoring all the relativity details around how to define that for now) it turns out that the planet actually will effectively orbit around the "current" position of the star, even though gravity only travels at the speed of light [1]. Our naive expectation that it would orbit only where it sees the star in the sky right now fails to account for some additional correction terms that show up when you take the dynamics of the situation into account, and it happens to come out in what you may think of as a coincidence to "correcting" the point actually being orbited to what is also the current position of the star. (Technically I think it is still off, but galactic orbits are very, very slow, so the errors introduced by them are also very very small.) You may recall in physics class how the difficulty of understanding situations amped up once you move from statics to dynamics, especially if you took a real calculus-based version of it. In much the same way that even if you use Newtonian gravity, simply knowing the formula may still leave you surprised at quite a lot of what can happen in orbital mechanics.
If the star suddenly disappeared or zoomed off in another direction, it would be a light-speed delay before the planet "noticed" anything, but that generally does not happen, obviously.
I expect the derivation I saw would not be valid for orbits involving speeds close to c, but I would expect the general observation that the effective center of the orbit is in fact not the time-delayed location would still hold.
The same thing happens in electric fields. The electric field of a uniformly-moving charge points to where it is, not to where it was when the light was emitted from it that is now reaching the observer measuring the electric field.
It's also a straightforward implication of relativity. If there was offset like that between a mass and the point that gets orbited, then you could measure the objective speed of the mass relative to the aether.
If it’s translating at constant velocity, the satellite is orbiting the point in space where the object actually is. There is no “lag”, gravity points to where the center is now, not where it was 8 minutes ago.
Why does MOND keep showing up on HN? Pretty odd considering it's not particularly popular elsewhere and fails to explain a lot of observations of dark matter (bullet cluster, CMB). It's worth noting these rotation curves are not all the same curve, and we've discovered galaxies with varying quantities of implied dark matter (eg https://www.aanda.org/articles/aa/full_html/2023/07/aa46291-...) - so is MOND different for every galaxy?
Hacker News is just a part of the larger web. It's the same reason you see so many people arguing over lab leak, ketogenic diets, barefoot running. Anything whatsoever that is vaguely heterodox and hints that you might have some knowledge that either the "establishment" doesn't know or is lying about is a tremendous ego boost. If your default position epistemically is you don't really have the expertise or time to investigate everything out there, which is more or less true for everyone no matter how intelligent you are or how much expertise you have on one specific topic, then if you're going to throw darts anyway, you may as well throw them at the target that, if correct, makes you look and feel just a little bit more special than the unwashed masses being spoonfed that food pyramid.
I can't remember where at this point, but decades ago I heard this kind of thing called "insight porn" and have myself been guilty of it for much of my life. It's endemic to web communities composed of relatively smart people, at least if we take "smart" to mean something like intellectually curious and having right tail levels of raw cognitive ability, rather than meaning anything you believe is more likely to be true.
1) The vast majority of matter in the Universe is invisible. We've tried looking for it in a variety of ways for decades but we can't find it but we're sure it's there.
2) Our model of the Universe is slightly incorrect. It works in many many cases but not in interesting outlier situations. eg. at very low accelerations.
MOND doesn't explain all of the missing mass, it just reduces it to a fifth of what it was. So you need MOND and something else, which is two things, which Occam's Razor disfavors anyway.
> So you need MOND and something else, which is two things, which Occam's Razor disfavors anyway.
Misapplication of Occam's razor. LCDM also needs multiple things: non-interacting dark matter + a fine-tuned distribution of DM that cannot be a priori predicted from any observations, but only post-hoc fitted after observation. By contrast, MOND has successfully predicted rotation curves (and lots more) from the visible matter alone.
You can't naively apply Occam's razor to two theories that both fail some set of observations. However, as a scientific theory, MOND has a better track record of successful predictions.
I don't necessarily believe canonical MOND is the answer, just that it seems more reasonable to me to assume we don't have the Universe completely figured out and accurately modeled just yet vs. being emphatically sure most matter never interacts with light but only with gravity despite continued failure in finding direct evidence of it because it would satisfy our current model without the need to adjust it.
It took us until the 20th century to discover radiation, despite it having extremely direct and powerful interactions. Gravity is a shitty, weak interaction that we largely don't have a full understanding of, so why is it so unlikely we haven't properly discovered matter that only interacts weakly with gravity? How the hell else would we "discover" it?
> Why does MOND keep showing up on HN? Pretty odd considering it's not particularly popular elsewhere and fails to explain a lot of observations of dark matter (bullet cluster, CMB).
And DM fails to explain flat rotation curves out to 1000 parsecs, or how MOND was able to make so many successful a priori predictions where DM has to be curve fitted after the fact. This paper is just the latest to refute LCDM, but no doubt proponents will add yet more parameters to correct for this failure, as they have done many times in the past when observations refuted DM predictions.
Honestly, you and many others have fallen for the DM propaganda. Both DM and MOND are problematic and fail in various ways. DM is not nearly as successful a scientific theory as most think, and MOND is not nearly as problematic:
The simple answer is 1) because physicists don't want to muck with general relativity which has had many successes across sub-galactic scales, so they are trying as hard as possible to jam a particle into the hole that's been observed when extending GR/Newton to galactic scales, 2) MOND is just an "effective theory" that seems to fit observations but doesn't really have a fully fleshed out theory explaining why this change to gravity happens; this "inelegance" rubs math-oriented physicsts the wrong way.
I've mentioned elsewhere here that astrophysicsts tend to prefer DM because they like particles and mathematical elegance, and astronomers are more open to MOND because they like theories with few parameters that make successful predictions (a reductive generalization, but broadly true I think). JWST has validated many MOND predictions, so I hope people open their minds a little more now, but as I said, neither theory is fully satisfactory in the end.
Edit: the author of this post is an astronomer that has been working in this field for a long time and he posted another article that reviews MOND and LCDM with some history:
There's also outside support due to converging interests. If DM is a particle that gives particle physicists an excuse to fund numerous experiments in an attempt to detect it. There are just more physicists motivated overall to prefer particle DM.
It's also safe. There is no reputational risk because "everyone" (who matters to you professionally) already agrees with you. There isn't much immediate upside to sticking your neck out. Especially in the era of social media.
MOND remains popular because it's made so many successful a priori predictions that DM failed to do. At the risk of being too reductive: DM is popular among maths-oriented astrophysicists because as physicists they like particles and deep theories, and MOND by contrast, doesn't have a full theory justifying the adjustments to gravity and they don't like that; MOND tends to be more popular among astronomers than astrophysicists because they like effective theories that make successful predictions using few parameters, and MOND is somehow better at that (as with this paper).
It's easy to be a MOND enthusiast without a physics PhD, it's a lot harder to be an axion or WIMP enthusiast.
"Just tweak the laws of physics, bozos!" is a really easy idea for laymen to latch on to, which is why you see so much interest in it among science enthusiasts.
"The experts are wrong because they are overcomplicating things" is also just a rhetorical trick that has pretty much always worked on some segment of the population.
It's because people don't like the idea of matter that you can't interact with especially if it makes up a double digit percentage of all mass in the universe.
but... how is that compatible with the article I linked here? Some galaxies look like they have very little dark matter, a MOND with no free parameters can't explain why different galaxies have different inconsistencies in their rotation curves.
You actually have to do the math, it's not just a "no dark matter hurr durr" thing.
MOND predicts "no dark matter" if the acceleration regime is high. Most galaxies that havd "very little dark matter" are in the high acceleration regime.
Most of the others seem to be susceptible to observational error (e.g. ultra diffuses). Galaxy rotation curves measurements are highly sensitive to orientation of the rotational axis and distance to us (you're gonna have a hard time measuring it if the galaxy is face on)
Worth noting that recently (when couple of months is considered recent), she mentioned that she had falling out with MOND (due to some new study coming to ger attention).
"MOND predictions keep being corroborated, yet the community persists in ignoring its implications, even in terms of dark matter. It’s gotta be telling us something."
No, it doesn't. MOND falls apart in every single theory they put forth. The fact that this happens without fail should lead one to understand the answer probably lies elsewhere than MOND.
It's not like MOND (or even dark matter) are even single theories with a fixed set of predictions. They are groups of competing ever evolving theories. I don't understand the need to believe one over the other as a predictive tool without some conclusive evidence. If you're not the astronomer who makes that measurement (or explains it), it just feels like galactic sports betting.
MOND successfully predicts many things we observe that DM fails to, or that requires special tuning for DM to fit observations (which is not prediction). DM actually has quite a poor predictive record, even if it can be made to fit observation.
MOND as a theory is probably not correct. It's not relativistic for a start, although I believe there are other modified gravity theories that are.
I'm getting interested now, the comments have alleged several predictions MOND made. On the other hand I'm pretty sure the evidence for dark matter is pretty strong as well (I believe there are several ways to calculate the amount of dark matter, all of which agree).
Can you give an example of MOND falling apart? One that requires so much fine-tuning that it cannot be adequately explained, or one requiring a violation of one of the more fundamental laws of physics?
> I'm getting interested now, the comments have alleged several predictions MOND made. On the other hand I'm pretty sure the evidence for dark matter is pretty strong as well (I believe there are several ways to calculate the amount of dark matter, all of which agree).
> Can you give an example of MOND falling apart? One that requires so much fine-tuning that it cannot be adequately explained, or one requiring a violation of one of the more fundamental laws of physics?
Pick any MOND theory you like, it all fails when it hits relativity, which is a theory with an embarrassment of riches of evidence in its favor, and can't be reconciled with relativity either. So while it explains galactic rotational speeds, it then fails to explain lots of other things, so it's a huge step backwards. It's the equivalent of saying Newtonian physics is wrong because it can't explain Mercury's precession, so let's go back to epicycles. To favor MOND we give up tremendously more than we gain.
MOND CAN explain things, if I have been interpreted to say it never predicts anything, that was never my intent. The problem is that MOND can't explain much else, so rather than making our theories simpler (which usually means we're in the right direction) it complicates things.
To make relativity work for the things MOND looks at, we only add 1 thing, WIMPs, particles we theorize but haven't seen. We've predicted lots of particle and found them, so this isn't a problem. chances we have missed a particle that turns out to be highly non-interactive? High. It took us ages to really solve the missing neutrino problem by discovering them, and we predicted the Higgs boson with high accuracy too. So this is a road we've been down before.
To make MOND work, we throw out a lot of theory, and we have nothing to replace it with under MOND. Chances all those other theories are wrong even though they work great? Low.
Do tell me if I'm completely wrong but isn't part of the problem there that you're simply expecting too much from MOND?
I mean a theory that is a modification of classical dynamics is not going to work well in a relativistic regime. That just means that somehow the classical limit ends up being slightly different from what we thought it was for reasons we don't know yet. I've always viewed MOND as more of an empirical law that we don't have a good explanation for yet. To me at least it seems obvious it will be deficient when you extrapolate too far and it can't replace more fundamental models.
That MOND can't handle relativity is not a direct reason to dismiss it. It would be a problem if relativity can't handle MOND (i.e. relativity cannot result in a MOND like theory) but I can't tell if that is the case. We could also dismiss MOND if it didn't explain anything, but as you said that's not quite the case either.
So yeah, dark matter seems more reasonable, but it's hard to completely ignore a theory seems to have more predictive power (even if we know it is flawed).
> Do tell me if I'm completely wrong but isn't part of the problem there that you're simply expecting too much from MOND?
No, and the reason is because MOND is a fundamentally flawed path. We're fairly sure unified theories are the way the universe works, MOND sticks out of that like a sore thumb. If MOND wants to explain one thing, then it has to fit everything else that effects or it's wrong. Really, for a theory to overturn a preferred one, it has to be better in some way. MOND isn't, it's explains few things than dark matter, and less well. At no point have I ever heard anyone state that MOND has a better predictive power than any other accepted theory, so I'm not sure what you're referring to there. In fact, that's the primary issue with MOND theories is that they ALL fail to meet dark matter with parity, none exceed it's predictive qualities.
For one thing MOND doesn't actually appear to eliminate the need for dark matter[1]. It gets rid of ~80% of the missing mass requirement, but not all of it. Whereas pure DM can just eliminate MOND.
Then you've got the "Bullet Cluster"[2] - where two colliding galaxies have had their observable and dark matter masses apparently separated. MOND can't explain this one without a lot of tweaking, but it's pretty trivial for DM: electromagnetically interacting matter is "sticky" where as gravity only matter isn't. The Bullet Cluster shows a galaxy shaped blob of gravitational lensing exactly where you'd expect it to be if a bunch of non-interacting matter had flown through each other, whereas the electromagnetic matter has interacted and re-shaped.
NGC 1052-DF2[3] and NGC 1052-DF4 are both ultra-diffuse galaxies which have no, or very little dark matter. That is, they appear to have normal galactic rotation curves fully explained by their observed visible mass. This works totally fine for DM existing (it's a problem for lambda-CDM though because it's not clear how they could've formed without dark matter, but I mean - we also don't yet know how black holes actually manage to ever merge either yet we do observe them too). This one always seems like a problem to me: MOND proposes a new universal principle of matter, then suddenly we have some matter where it's not doing that.
The theoretical problems[4] are somewhat beyond me, but they get well into issues with violating relativity and that's a big one: relativity is stupidly, reliably accurate under every single test we put it through, to absurd levels of precision. Build a better instrument, you can just dial in your precision and get the answer out ahead of time before you launch the satellite which is testing it. Also without careful adjustment you get violations of conservation of momentum (conversely, if MOND is real this would be handy because maybe it means we can reactionless spacedrives).
It's worth noting that none of this is implicitly fatal. lambda-CDM could be wrong, a MOND variant could be right. But a list of convenient things MOND explains easily doesn't escape the need to also include the things it can't - and appeals to the idea that DM is being "tweaked" to match observations unnaturally ignores the fact that MOND has to have the same thing done to it to fix within cosmology.
> Then you've got the "Bullet Cluster"[2] - where two colliding galaxies have had their observable and dark matter masses apparently separated. MOND can't explain this one without a lot of tweaking
The bullet cluster is so over-played as a refutation of MOND. "A lot of tweaking" basically reduces to adding sterile neutrinos, as one possible solution. All galactic clusters have issues in both MOND and LCDM, the bullet cluster was nothing new when it was discovered, it was just visually dramatic because they could image the gravitational lensing.
> but it's pretty trivial for DM: electromagnetically interacting matter is "sticky" where as gravity only matter isn't.
Actually LCDM can't explain the bullet cluster either:
As always, MOND and LCDM appear to just trade off one set of issues for other equally problematic set of issues. Neither is favoured very strongly by the sum of evidence. Physicists have just gotten in the habit of ignoring all of the problems with LCDM and consider even trivial problems with MOND to be fatal.
> relativity is stupidly, reliably accurate under every single test we put it through, to absurd levels of precision
Those precise tests do not extend to galactic scales, which is exactly where the problems appear. It would be nice if our existing theory worked across all scales, but that doesn't mean it must.
> "The bullet cluster is so over-played as a refutation of MOND. "A lot of tweaking" basically reduces to adding sterile neutrinos, as one possible solution."
If MOND requires "dark matter" to explain the bullet cluster, then what is the appeal of MOND?
Read the article this thread is about. Particle DM is not sufficient to explain all observations without extreme contortions that make MOND seem more reasonable. Also, MOND predictions made decades ago keep being validated. Why does this keep happening if there's nothing to MOND?
Finally, as I said, particle DM can't fully explain the Bullet Cluster either. The evidence is screaming in our faces that we need better thinking here.
It is one brand new paper. I am skeptical. Rotation curves are perfectly flat out to whatever arbitrary distance that they happen to be able to measure? I am very skeptical.
> "particle DM can't fully explain the Bullet Cluster either."
According to one scientist, who happens to be the same scientist claiming that particle DM cannot explain rotation curves. I will not check every claim, but the bullet cluster collision speed "problem" is readily explained in the reference in the wikipedia article: https://arxiv.org/abs/1410.7438
The broader point is that every time MOND has claimed to refute dark matter so far, the refutation has been refuted, so I will wait to see the outcome of this new claim.
I wasn't referring to the bullet cluster specifically, but this obsession with the bullet cluster is typical of the confirmation bias in this field: hyperfocus on what confirms bias and ignore the countervailing evidence. The past 30+ years have seen many "corrections" to get LCDM to fit observations it did not predict [1]. Clusters in general pose challenges to both MOND and LCDM for different reasons [2,3], but LCDM's typically get ignored and MOND's treated as a fatal blow. As I said, neither theory is fully satisfactory, but it's clear that research on these questions is fairly one-sided.
I personally hope that the solution to the handful or two of things that the LCDM model does not (currently) explain will be more interesting than a trivial tweak (which seems to have no physical motivation) to the gravity equation that MOND is.
Wikipedia's bullet cluster article is written in a very misleading way. The bullet cluster is not a hard problem for MOND since regular baryonic gas easily explains the lensing. Working out the bullet cluster with LambdaCDM actually took quite a bit longer because there were problems in the initial data set that took astronomers 10 years to work out. In short, the bullet cluster doesn't "disprove" MOND anymore than it "proves" LambdaCDM.
NGC 1052-DF2 is as big a problem for LambdaCDM as it is for MOND. LambdaCDM requires dark matter for galaxy formation. Which means either LambdaCDM is wrong about how galaxies form, or there was some event (of which we currently lack evidence) that removed it. As for MOND, it's possible that actually doing the math may show that this galaxy is not particularly weird. There's aren't many folks fluent in MOND, so work on these kinds of issues tends to lag their discovery.
As for MOND and cosmology, yeah, MOND sucks at cosmology. The fact that it can so easily explain so many galactic dynamics (far better than LambdaCDM) is really weird though. MOND isn't a great theory, but it's a really interesting model because it really highlights how poorly LambdaCDM predicts (not explains!) the galaxies we see today.
This is an apple's to oranges comparison. Lambda-CDM is a theory of all cosmology, including galaxy formation. The dark-matterless galaxies are a problem for Lambda CDM because they don't easily fall out of numerical simulation during formation. Supermassive black hole mergers also don't fall out of numerical simulation easily, but we still observe them.
They're not a problem for the concept of dark matter in general, since they have good agreement with the idea that the phenomenon creating dark matter appears to be a massive particle of some sort capable of being spatially dislocated from visible mass. So a theory proposing a pervasive but ill-described massive, mostly non-interacting particle may just be under-explored or the ramifications of the full scope of possible dark matter configurations (as pure mass) not explored.
It is a problem for MOND, which proposes that all matter is generating this effect which looks like dark matter (and then has an extremely poor ability to explain the rest of cosmology or work within the framework of general relativity).
The latest is, afaik, is called RelMOND, short for relativistic MOND. It describes the cosmos very similarly to the standard model, but with MOND as well. So slightly better matching observations than lambda CDM, but still imperfect. They plan to incorporate electromagnetism in future models.
A really interesting result. On it's own, it doesn't destroy dark matter as a theory, but it's putting quite a large dent in it.
Obviously will need additional work and review, it's only one paper. Maybe there are mistakes or factors not fully considered.
There have been a number of other papers recently on measuring wide binaries. Different papers claimed different results on these.
Still, it's certainly something that merits a lot more attention. We may be looking at needing some new theory of gravity (maybe not MOND, but something other than dark matter).
> A really interesting result. On it's own, it doesn't destroy dark matter as a theory
It refutes LCDM though. No DM halos could be responsible for this behaviour this far out. And this isn't the first time LCDM has been refuted by evidence before they tweaked it with yet more parameters/epicycles to make it fit.
It's been clear for awhile now that neither particle dark matter nor MOND are adequate explanations for observations. MOND clearly matches some data better and with fewer parameters (like this), and DM others, like cluster-scale lensing. New thinking is needed, and hopefully this paper will surprise people into taking MOND-like approaches a little more seriously.
Presuming the behavior is real and not an artifact of the model. This is a statistical technique, needing specially selected targets in order to be observed reliably - and also assuming those targets themselves are typical.
There's plenty of observations which can accidentally vanish because of subtle problems with assumptions, so declaring a total refutation is beyond premature.
Like to wit, if MOND is real then you've really got to explain how sometimes it also selectively just bails out on some galaxies apparently[1].
I'm tired of people trotting out "epicycles" to attack theories they don't like: you're gonna be adding a lot of those to get a MOND which can explain all the data as well (which is to say, it's a trite insult and not useful argument).
Iirc, the udg makes sense if the distance to it is off by something like 25% and the orientation of rotation is off by a bit. Those parameters are very hard to measure in UDGs.
> I'm tired of people trotting out "epicycles" to attack theories they don't like
That's disingenuous. LCDM has a long history of failing to successfully predict later observations and adding parameters to fit the data, where MOND has made many successful a priori predictions without any added parameters since the 1980s. This is not just a matter of not liking something, successful predictions vs. post-hoc curve fitting strikes at the very core of what it means to be a good scientific theory. See:
He is a bit more nuanced than that, I've been reading the blog for quite some time.
He has pointed out areas where MOND falls short in other posts, but it is true that he thinks that LCDM has some serious flaws, and that MOND like theories have a better predictive record.
It's just that from a more complete point-of-view, there are many other areas than rotation curves that depend on dark matter and has been modelled to really require the specific properties of DM that the lambda-CDM model has, and it's just a bit worrisome every time a blog post talks exclusively about rotation curves. Sounds good that he writes about more of the issues elsewhere! The real picture might require a new blend of both DM and MOND of course..
However, what if it holds, but if inertia is quantized, then you get less gravitational effect at 90 degrees to the path of motion at astronomic distances as it recedes into the quantum noise.
Which fits observations with inventing dark matter, or tweaking gravity.
I just watched Sabine Hossenfelder's video (Gravity without Mass, https://youtu.be/Q0fwRMvNkoA) where she talks about hollow spheres. Inspired by this video I have this hypothesis:
There's quantum fluctuation. Particles appear out of nowhere and disappear again. Hossenfelder talked about negative mass, so allow me to do this as well: What if a pair of two particles, one of negative and one of positive mass can very rarely appear?
They disappear immediately again but for a short moment we have acceleration (more about that in Hossenfelder's video). Could this be enough to explain "dark matter"?
As told in certain science fiction, I like to imagine that dark matter/energy is the "pollution" or other side effects caused by some alien civilizations [over]use of faster-than-light travel.
Which is actually a terrifying existential threat to think about: The more an earlier civilization uses FTL, the more space will expand, and eventually it will become all but impossible for younger civilizations to traverse space without becoming dependent on the elder races.
A galaxy (elliptic, or spiral) is made out of billions of stars, like our sun.
These stars rotate around the center of the galaxy (very slowly, think millions of years for 1 rotation).
A rotation curve measures the velocity of stars as a function of distance from the center of the galaxy.
Newtonian physics (or Einstein's GR) says that the rotation curve should decay with distance, ie. with greater distance the stars' velocity should go down --- assuming the matter in the galaxy is the visible matter that we see, ie. the stars (which shine light).
The problem is, there is a rich set of observational data, from many different experiments, telescopes, and methodologies that show that the rotational curve is in fact flat, it does not decay.
There are 2 big competing theories to explain this discrepancy:
1. Assume that there is a lot of unseen, non-shining, ie. Dark Matter (DM) in the galaxies (also ours). If you put the appropriate amount of dark matter in there, with the right distribution, you can reproduce the observed rotational curve. There are also other places is astrophysics/cosmology where having dark matter (specifically Cold Dark Matter, CDM, where cold just means "slowly moving") is useful. The biggest example is to explain the history of the Universe and the observed Hubble-constant. In fact the standard model of cosmology is called λCDM, CDM for Cold Dark Matter (λ for the cosmological constant, currently modeled as Dark Energy, not relevant for this discussion).
2. Assume that Newton was wrong and gravity is not exactly 1/r^2 --- this is called MOND, Modified Newtonian Dynamics. This way you can also reproduce the observed rotation curves. This is much less popular, because: (i) physicsts don't want to give up the beautiful and geometric simplicity of 1/r^2 (ii) Dark Matter is also useful for solving other discrepancies in astrophysics/cosmology.
What this article is saying is that, even in the first Dark Matter model, per the model DM distributions inside galaxies that also work with all the other places where DM is used to explain something (eg. in cosmology), at some distance from the center, the dark matter bubble has an edge and stops --- and then the velocities should finally break down. However, these latest observations are showing that the velocities remain constant even beyond the modeled/assumed DM bubbles. This is an additional ε argument in favor of MOND, and science proceeds.