In my opinion, this article is misleading at best. "...scans of ancient galaxies gathered by the JWST seem to contradict the commonly accepted predictions of the most widely accepted Cold Dark Matter theory, Lambda-CDM." --> LCDM doesn't predict what galaxies should look like, it simply predicts how much mass is in collapsed structures and that dark matter haloes grow hierarchically. In contrast, with JWST we see light and need to infer what the underlying properties of the system are. It was shown very early on that the theoretical upper limit (i.e. taking all of the gas that is available in collapsed structures and turning it into stars) predicts a luminosity function (i.e. number of galaxies per unit luminosity) that is orders of above what JWST has observed (e.g. https://ui.adsabs.harvard.edu/abs/2023MNRAS.521..497M/abstra...). This means that there is plenty of space within the context of LCDM to have bright and seemingly large and massive galaxies early on. Based on current JWST data at these early epochs, there are really no convincing arguments for or against LCDM because it's highly sensitive to the galaxy formation model that's adopted.
> there are really no convincing arguments for or against LCDM because it's highly sensitive to the galaxy formation model that's adopted.
To be fair, that is absolutely not the way ΛCDM would have been described to someone in the pre-Webb days. It was a well-regarded theory and the hope was (a-la the Higgs detection) that new data would just better constrain the edges and get us on to the next phase of the problem.
But instead it's a wreck, and we didn't see what we were expecting at all, and so now we're retreating to "Well, ΛCDM wasn't exactly proven wrong, was it?!"
That doesn't mean it's wrong either, and it for sure doesn't mean MOND is right. But equally for sure this is a Kuhnian paradigm shift moment and I think it's important for the community to be willing to step back and entertain broader ideas.
> with JWST we see light and need to infer what the underlying properties of the system are
Every theory of dark matter is based exclusively on light-emitting objects. There is no "contrast" between JWST's methods and those of others. Casting aspersions on JWST because it can only see light is like casting aspersions on Galileo because he could only build telescopes. If we could teleport to the things we study and get more information that way, it would be nice, but we live in reality and must bend to its rules.
> highly sensitive to the galaxy formation model that's adopted
I should only need to remind the reader of the classic idiom "cart before the horse" to remind them that this line of reasoning is invalid.
My hangup with MOND is still general relativity. We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold. Any model of gravity based on an inverse law is simply wrong.
Another comment linked to https://tritonstation.com/new-blog-page/, which is an excellent read. It makes the case that GR has never been tested at low accelerations, that is might be wrong. But we know for a fact MOND is wrong at high accelerations. Unless your theory can cover both, I don't see how it can be pitched as an improvement to GR.
Edit: this sounds a bit hostile. to be clear, I think modified gravity is absolutely worth researching. but it isn't a silver bullet
TeVeS is definitely interesting, but it still has problems like you said. AFAICT gravitational wave observations are particularly bad for TeVeS theories. TeVeS isn't dead, but if dark matter theories are criticized for being patched up post-hoc, that standard should also apply to modified gravity.
For the fields to be considered particles, they have to be freely propagating in space. TeVeS adds a vector field, a scalar field, and some lagrange fields that are part of their coupling. The degrees of freedom aren't consistent with one or more particles.
>We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold
[citation needed]
The consensus is that gravity - outside of extreme mass/energy environments - works just as Newton described it to many many decimal places.
Emphasized part added because people in the replies thought that I literally think that General Relativity is somehow wrong. Don't be dense. All I'm saying is that gravity at galactic scales works as Newton described it. General Relativity has extremely tiny effect at those scales.
You're simply wrong. There's no other way to put it. The GPS system would have been simply impossible to deploy without the general theory of relativity. There's no extreme energy or mass involved, just precision requirements that are influenced by the minuscule differences in time experienced by the surface of the earth and orbiting satellites.
Also Newton's laws famously could not account for Mercury's orbit. Mercury is just an ordinary planet orbiting an ordinary star. Nothing extreme is involved. He knew his laws were incomplete. But they were so dead-on in basically every other scenario that could be physically observed at the time that he figured there was some small tweak missing (or maybe another planetary body that hadn't been spotted yet).
Easy there champ. Noone is shitting on general relativity.
All I'm saying is that the effect of general relativity at galactic scales is so minuscule, that galactic dynamics is - for all intents and purposes - governed by the Newtonian limit of gravity.
If you propose that gravity doesn't behave like the Newtonian limit at those scales, then you're contradicting general relativity as well, since the far-field limit of the Schwartzschild metric is literally Newton's inverse square law.
In layman terms, modified Newtonian gravity, that the article talks about, is an attempt to explain why galaxies don't rotate the way they should according to Newton (and Einstein, because at those distances the two are the same!!!).
I had the impression that "shitting on general relativity" was exactly what MOND was about. That is, it starts from the position that Einstein is wrong, and searches for ways to support that.
Citation needed? That's ridiculous. The empirical evidence is well over century old at this point. Start with the anomalous precession of Mercury's perihelion. That already can't be accounted for by Newtonian gravity.
I don't think they're saying the relativistic effects don't exist, just that they're still largely unimportant compared to Newtonian effects.
For precession of perihelion of Mercury we mostly noticed because any error is cumulative over time and we could integrate over an arbitrarily wide timebase. The relativistic effects are <10^-8 of the total, around 1/10th of the change imparted by Newtonian gravity of planets much, much further away. The BepiColombo orbiter should allow us to correct for the relativistic effects of other planets' pull on Mercury, but it's expected to be a change of <10^-12.
So I guess "many, many decimal places" is in the ballpark of 6-12.
Nobody said that general relativity is "switched on" around black holes.
But ok, let me put it this way: outside of extreme energy/mass environments, gravity is described by Newton's law of gravitation with very high precision. If you look very hard, you may notice differences on the order of 10e-MANY. But for all intents and purposes, gravity is Newtonian in 99.99999% of the universe.
My first thought was that we only know Cavendish's constant to a little over 4 significant figures, so how could this be right? The relativistic effects at Earth's surface would change this by only ~10^-8, so I think the challenge in refining the Cavendish gravitic constant lie elsewhere.
So spacetime (interactions between mass, space, and time) are required for any sort of precision explanation. If "extreme" means planet size masses, I guess, but I generally consider our solar system pretty normal. However we cannot explain the planetary motion of mercury without relativity, so define your extreme.
But sure, newton is good enough to handle most ground based scenarios where we only care about forces at low precision.
If we are asking whether MOND is useful, then the answer is probably yes. You might use it for simulations of galaxy formation where Newtonian gravity is considered a reasonable approximation today. But MOND is not a correct model of the universe. There is no place in the universe that Newtonian gravity applies, only places where the error is an acceptable trade-off for simpler calculation.
By the same logic, there's no place in the universe that general relativity applies either, since it breaks down at the quantum level. There's no place in the universe where any theory other than the one true grand unified theory applies, because everything else is just an approximation. At which point we're just arguing about semantics, and I don't see a reason for continuing it on my part.
There are vastly different scales where the approximation is correct for newton vs general relativity. Perhaps you can define the scales that you are calling relevant so we understand what you mean.
The scale of galaxies? Which the original article is about? I feel like I need to spell out everything, but ok:
The article is about modified Newtonian dynamics (MOND), which is a theory that modifies Newtonian gravitation to fix some observed differences in galaxies' motion, without invoking dark matter. The original commenter then proclaims "haha, MOND cannot be right, because we know that Newtonian gravity is incorrect". Yeah, no shit Sherlock; it is "incorrect" because it is just a limiting case of general relativity. But that's completely besides the whole point of MOND, which tries to "fix" gravity at galactic scales, which is a Newtonian regime even with general relativity. MOND is trying to tweak the Newtonian formula at those extreme distances, and if it works, then maybe it can be worked out to be a limiting case of a "modified general relativity", just as Newtonian gravity is a limiting case of GR. Got it?
These extremes exist, and GR predictions are better than Newton’s in those cases. Closest to home is mercury’s perihelion drift. We have observed black hole mergers, gravitational lensing, and GR is also an essential component in understanding the universe’s expansion(that we know from redshift and the CMB). Likely MOND will address these, but Newtonian mechanics will not get you there.
When you say "outside of" - that's the thing where it doesn't hold.
It's interesting and not even wrong to say "these rules work in these contexts" but as far as I can tell we're looking for the scenario invariant rules.
To be fair, there are relativistic generalizations of MOND, in the sense of relativistic theories that simplify to MOND dynamics in the low energy case. My understanding (this not being my field) is that they're sort of kludgey and non-calculable and that no one takes them very seriously. All the "real work" on MOND is just done using the classical stuff.
And yeah, that seems like pretty terrible cheating. It's one thing to hang a big theory on a single conjecture, but you still need to be trying to prove the conjecture.
« Stunning evidence »
… then later on:
« Instead, the readings _seem_ to support a basis for MOND, which _would_ force astronomers and cosmologists to reconsider this alternative and long-controversial theory of gravity. »
What’s conditional evidence? I may be missing the overall picture, but I view such writing as non precise at its best.
Well, it’s evidence that a) must be verified on a mathematical and empirical level, and b) (arguably) fits better with a currently unpopular theory than the dominant one. There’s so many unknowns in physics that opponents can easily reply “well your theory doesn’t explain XYZ yet, so we likely just need to tweak our theory”.
In other words, reasonable minds do disagree. AFAIU as an amateur.
Why why why do people share articles with sensational headlines like this? Its no wonder science journalism gets a bad rap. This kind of thing really undermines all the people who are actually trying to communicate science properly.
Are any of the MOND theories consistent with this new data also consistent with recent gravitational wave observations? My understanding is that gravitational wave detectors have recently ruled out most plausible MOND theories. The linked paper doesn't seem to discuss this.
Waiting for Angela Collier to make a video on this, I'm sure many people will forward her this article. MOND is actually a niche in cosmology despite its PR.
I follow the lead author, Stacy McGaugh, via his blog where he posts discussions and musings about the latest research into the dark matter vs MOND debate: https://tritonstation.com/new-blog-page/
His arguments are very convincing and relatively clear. I am not an astrophysicist but I have two degrees in physics and have always found the dark matter theory to be lacking -- in absence of any evidence of causation whatsoever, dark matter can only be described trivially as "where we would put matter if we could to make our theory of gravity make sense," which is totally backwards from a basic scientific perspective.
Predictions based on modern MOND postulates are shown to be more and more accurate as our observational instruments continue to improve in sensitivity.
> which is totally backwards from a basic scientific perspective
This is not right, because if we have a situation where our theories and observations don't cohere, it's not given whether the theory requires modification or we're missing something in our observations (or both). A classical illustration is the orbit of Uranus being observed in the nineteenth century to be contrary to the predictions of Newtonian theory. Calculations were made assuming the truth of the Newtonian theory and that we were missing something in our observations - the position of Neptune was predicted and it was subsequently discovered.
On the other hand, the orbit of Mercury diverged from the prediction of Newton's theory. Again, a previously unobserved planet closer to the sun was postulated as being responsible, but in this case it really did require a modification to the theory of gravity: general relativity, which accurately predicted the 43 arcseconds per century of perihelion precession by which Mercury's orbit diverges from Newtonian predicitions.
GR has obviously made many other predictions, such as the gravitational bending of light, black holes, and gravitational waves, which have been vindicated.
So there's obviously a problem of the theory and observations not cohering, but whether the solution is a modification of the theory or a new form of matter is not clear in advance, and the latter is not unreasonable and certainly it's not unscientific to make as a hypothesis, to see where it leads.
The difficulty is in coming up with a theoretical framework that retains all the successful predictions of GR while also accounting for the galactic rotation curves.
One difference between dark matter and Neptune is that the existence of Neptune is falsifiable. The formulation of dark matter inherently is not. Falsifiable hypotheses is the cornerstone of science.
Maybe if you're being very broad in definitions then some class of proposals describable as "dark matter" might be unfalsifiable, but to be taken seriously as a scientific proposal I think it should be specific, concrete, and indeed testable, and there are a few of these within the "dark matter" class.
Again, we're in the perhaps unsatisfying position of having observations which don't cohere with our current theoretical understanding. What's the solution? It's not easy...
Is the existence of a planet so easily falsifiable? It hasn't been so long since the Planet Nine hypothesis started going around, and while we've observationally ruled out a big chunk of the original parameter space, there's still lots of room for a big dark dwarf planet to be floating around out there. It doesn't seem so different from how we've gradually been ruling out the parameter space for dark-matter observations.
Surely the idea of it being a new kind of matter that interacts gravitationally but not electromagnetically yields some testable result? Does it actually yield nothing testable with today’s experimental methods?
There is a lot of indirect evidence for dark matter. All the direct tests for dark matter particles we have performed have found nothing so far - but since we have no idea what it might be, there's a lot of possibilities to test.
I mean, dark matter may be discoverable, we just don't know how if it exists. There was time between the irregularities that were noticed in the orbit and the discovery of a new planet.
Well put, thanks for sharing! Never saw it phrased in such a clear narrative. As a novice, it seems like there's one big difference between those anecdotes and the current situation, though: sample size. Sure, if we were observing Andromeda spinning too slowly I'd be open to our instruments not capturing some massive objects/clouds, but we're actively observing, what, ~1E5-6 galaxies? In the case of a missing planet there were accidents of history/solar system makeup that led to our otherwise solid frameworks missing a key piece of information. But that clearly couldn't happen millions of times; whatever explains the inconsistencies we're seeing has to be a fundamental misunderstanding.
Once we've arrived at this point, we can compare the two theoretical re-workings on their own terms: one is that we're glossing over some important detail of how gravitational relations in spacetime work, and the other is that we're failing to observe some new class of matter. I mean, right? There's no way this conundrum will be solved by "whoops turns out there was more plain ol' dust than we thought" at this point, right?
In those terms, I feel parsimony clearly favors one possibility over the other. Every hypothesis is worth exploring (I mean, QM and GR are dumb as hell, yet nonetheless turned out to be correct), but when funding is on the line it's also not out of line to favor one explanation explicitly. That's already happening anyway, just in the other direction.
But also I'm just some kid who's awed and grateful to be living in times of such profound mystery and discovery. Could be totally off base -- I barely passed physics I!
What we have learned so far is that our theories and models are only correct up to our ability to precisely observe and measure.
In that sense, Newtonian physics is still very much correct under a very wide set of circumstances, and as such amazingly useful.
GR improves on that (adds precision) on what would be extreme cases for NP, but it is likely as correct as Newtonian laws are: up to a point.
All this to say that "correct" is not the right term to use: many of the theories are simultaneously "correct" with sufficient constraints and a particular error range. What matters more is if they are useful in predicting behaviour, and that's where I like using "correct" instead (as above).
> where we would put matter if we could to make our theory of gravity make sense
Dark matter behaves in a fundamentally different way from baryonic matter. We can constrain the total amount of matter in the universe (both dark and baryonic) from the observed abundances of baryogenesis. But dark matter has a different effect on the relative amplitudes of peaks in the CMB.
As far as I can tell, MOND has never really had any success outside of modeling galaxy rotation curves.
The skepticism I've seen towards dark matter vs. MOND has always been strange to me. Dark matter doesn't really require much in the way of new physics --- there's just a new particle to add to the standard model. But most MOND theories violate Lorentz invariance which is a vastly more radical departure from standard physics. (And in my mind, the more sophisticated MOND theories that maintain Lorentz invariance like TeVeS are really a theory of dark matter dressed up in the language of MOND.)
These successful predictions are all generally variants on modeling galactic dynamics, though. The trouble is that galaxies and galaxy clusters are very messy places, so it's hard to make sure you've incorporated all the relevant physics.
By contrast something like baryon acoustic oscillations are very simple to model, so you can be quite confident that you've incorporated all the relevant processes. And in that regime LCDM performs beautifully and MOND completely fails. So it's reasonable to suspect that in more complicated environments the problem is that we're not modeling the systems correctly rather than that there's new physics going on.
MOND doesn't cover the existence of CBM, distribution of galaxies, non-metallic abundance - things all covered by LCDM.
What MOND has going for it is that galactic rotation curves are readily consumed by popsci readers and the story of the "little guy" vs the scientific establishment is an easily available frame story popsci authors can sell clicks for.
The proportion of lay people who think MOND could be true greatly outnumbers the proportion of MOND researchers and doesn't reflect the veracity of the theory.
MOND is not a cosmological theory unlike LCDM, and it isn't relativistic. So we should not expect it to cover the range of things that LCDM tries to.
It's just a tweak to Newtonian gravity, which surprisingly matches observation very well, and has accurately predicted quite a few things in the regime it operates in, before they were observed.
The fact it works so well in the areas it does apply to is the reason that science hasn't given up on it yet (regardless of what pop science or lay people think).
I don’t think that’s quite fair. That approach is exactly how we find planets. Here’s an unexpected variance in the motion of a planet or star. It could be explained by a planet over there. Oh look, there’s a planet over there.
Hypothesizing that a planet might be over there is a testable hypothesis.
Have we found a way to verify the presence of dark matter yet? Or is it still an untestable hypothesis sprinkled around distant galaxies so their acceleration curves look right?
That's not quite true. General relativity predicts gravitational lensing, not dark matter. Lensing has been used as an experimental probe for the presence of dark matter.
MOND is an alternative theory of gravity competing with GR. People usually forget that while MOND started to present a different explanation for Dark Matter, it is a theory of gravity. Dark Matter is not a theory of gravity and is compatible with GR.
I’m particularly amused by the hypothesis that spacetime can be bent without the presence of matter. We can’t detect dark matter because there’s no such thing, it’s just a brute topological fact.
Right, which is why it quickly led to the detection of dark matter...hmm.
I think a better analogy would be "that approach is exactly how we explain failing to find planets like Vulcan; we hypothesize that they are made of as-yet-unknown stuff that you can't see, touch, hear, smell, or in fact detect at all. But we know they're there because our calculations say they are."
It's actually a better example than you think. This exact theory led to long and protracted searches for the planet Vulcan, which would explain Mercury's strange behavior.
I usually understand "dark matter" to be shorthand for the discrepancy between theory and observation. The explanation might indeed be matter that is dark, or it might be solved by entirely unexpected observations and/or changes to theory.
Not really. You might think this after watching Angela Coulliers video, but when you read something like "25% of the universe's energy content is made of dark matter", they do not mean changes to some theory. They literally mean non-baryonic matter.
Nope. It can mean change to some theory, without a need for matter. It is the difference between relativistic gravity and the corresponding observed mass.
In my opinion, this article is misleading at best. "...scans of ancient galaxies gathered by the JWST seem to contradict the commonly accepted predictions of the most widely accepted Cold Dark Matter theory, Lambda-CDM." --> LCDM doesn't predict what galaxies should look like, it simply predicts how much mass is in collapsed structures and that dark matter haloes grow hierarchically. In contrast, with JWST we see light and need to infer what the underlying properties of the system are. It was shown very early on that the theoretical upper limit (i.e. taking all of the gas that is available in collapsed structures and turning it into stars) predicts a luminosity function (i.e. number of galaxies per unit luminosity) that is orders of above what JWST has observed (e.g. https://ui.adsabs.harvard.edu/abs/2023MNRAS.521..497M/abstra...). This means that there is plenty of space within the context of LCDM to have bright and seemingly large and massive galaxies early on. Based on current JWST data at these early epochs, there are really no convincing arguments for or against LCDM because it's highly sensitive to the galaxy formation model that's adopted.
> there are really no convincing arguments for or against LCDM because it's highly sensitive to the galaxy formation model that's adopted.
To be fair, that is absolutely not the way ΛCDM would have been described to someone in the pre-Webb days. It was a well-regarded theory and the hope was (a-la the Higgs detection) that new data would just better constrain the edges and get us on to the next phase of the problem.
But instead it's a wreck, and we didn't see what we were expecting at all, and so now we're retreating to "Well, ΛCDM wasn't exactly proven wrong, was it?!"
That doesn't mean it's wrong either, and it for sure doesn't mean MOND is right. But equally for sure this is a Kuhnian paradigm shift moment and I think it's important for the community to be willing to step back and entertain broader ideas.
> with JWST we see light and need to infer what the underlying properties of the system are
Every theory of dark matter is based exclusively on light-emitting objects. There is no "contrast" between JWST's methods and those of others. Casting aspersions on JWST because it can only see light is like casting aspersions on Galileo because he could only build telescopes. If we could teleport to the things we study and get more information that way, it would be nice, but we live in reality and must bend to its rules.
> highly sensitive to the galaxy formation model that's adopted
I should only need to remind the reader of the classic idiom "cart before the horse" to remind them that this line of reasoning is invalid.
My hangup with MOND is still general relativity. We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold. Any model of gravity based on an inverse law is simply wrong.
Another comment linked to https://tritonstation.com/new-blog-page/, which is an excellent read. It makes the case that GR has never been tested at low accelerations, that is might be wrong. But we know for a fact MOND is wrong at high accelerations. Unless your theory can cover both, I don't see how it can be pitched as an improvement to GR.
Edit: this sounds a bit hostile. to be clear, I think modified gravity is absolutely worth researching. but it isn't a silver bullet
MOND isn't pitched as an improvement to GR. It was always a Newtonian theory - it's in its name!
There are relativistic versions of MOND, for example, TeVeS [1], but they all still have some problems.
[1] https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%...
TeVeS is definitely interesting, but it still has problems like you said. AFAICT gravitational wave observations are particularly bad for TeVeS theories. TeVeS isn't dead, but if dark matter theories are criticized for being patched up post-hoc, that standard should also apply to modified gravity.
The weirdest thing about TeVeS IMO is that it adds additional fields that warp spacetime, so how is it not a dark matter theory?
For the fields to be considered particles, they have to be freely propagating in space. TeVeS adds a vector field, a scalar field, and some lagrange fields that are part of their coupling. The degrees of freedom aren't consistent with one or more particles.
>We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold
[citation needed]
The consensus is that gravity - outside of extreme mass/energy environments - works just as Newton described it to many many decimal places.
Emphasized part added because people in the replies thought that I literally think that General Relativity is somehow wrong. Don't be dense. All I'm saying is that gravity at galactic scales works as Newton described it. General Relativity has extremely tiny effect at those scales.
You're simply wrong. There's no other way to put it. The GPS system would have been simply impossible to deploy without the general theory of relativity. There's no extreme energy or mass involved, just precision requirements that are influenced by the minuscule differences in time experienced by the surface of the earth and orbiting satellites.
Also Newton's laws famously could not account for Mercury's orbit. Mercury is just an ordinary planet orbiting an ordinary star. Nothing extreme is involved. He knew his laws were incomplete. But they were so dead-on in basically every other scenario that could be physically observed at the time that he figured there was some small tweak missing (or maybe another planetary body that hadn't been spotted yet).
Easy there champ. Noone is shitting on general relativity.
All I'm saying is that the effect of general relativity at galactic scales is so minuscule, that galactic dynamics is - for all intents and purposes - governed by the Newtonian limit of gravity.
If you propose that gravity doesn't behave like the Newtonian limit at those scales, then you're contradicting general relativity as well, since the far-field limit of the Schwartzschild metric is literally Newton's inverse square law.
In layman terms, modified Newtonian gravity, that the article talks about, is an attempt to explain why galaxies don't rotate the way they should according to Newton (and Einstein, because at those distances the two are the same!!!).
I had the impression that "shitting on general relativity" was exactly what MOND was about. That is, it starts from the position that Einstein is wrong, and searches for ways to support that.
Citation needed? That's ridiculous. The empirical evidence is well over century old at this point. Start with the anomalous precession of Mercury's perihelion. That already can't be accounted for by Newtonian gravity.
I don't think they're saying the relativistic effects don't exist, just that they're still largely unimportant compared to Newtonian effects.
For precession of perihelion of Mercury we mostly noticed because any error is cumulative over time and we could integrate over an arbitrarily wide timebase. The relativistic effects are <10^-8 of the total, around 1/10th of the change imparted by Newtonian gravity of planets much, much further away. The BepiColombo orbiter should allow us to correct for the relativistic effects of other planets' pull on Mercury, but it's expected to be a change of <10^-12.
So I guess "many, many decimal places" is in the ballpark of 6-12.
https://en.wikipedia.org/wiki/Tests_of_general_relativity#Pe... is the example I learned in school. You don't need to be around a black hole for GR to suddenly switch on.
Newtonian gravity is an approximation. A perfectly acceptable one in many contexts, but still measurably incorrect.
Nobody said that general relativity is "switched on" around black holes.
But ok, let me put it this way: outside of extreme energy/mass environments, gravity is described by Newton's law of gravitation with very high precision. If you look very hard, you may notice differences on the order of 10e-MANY. But for all intents and purposes, gravity is Newtonian in 99.99999% of the universe.
My first thought was that we only know Cavendish's constant to a little over 4 significant figures, so how could this be right? The relativistic effects at Earth's surface would change this by only ~10^-8, so I think the challenge in refining the Cavendish gravitic constant lie elsewhere.
So spacetime (interactions between mass, space, and time) are required for any sort of precision explanation. If "extreme" means planet size masses, I guess, but I generally consider our solar system pretty normal. However we cannot explain the planetary motion of mercury without relativity, so define your extreme.
But sure, newton is good enough to handle most ground based scenarios where we only care about forces at low precision.
Not for all intents and purposes.
If we are asking whether MOND is useful, then the answer is probably yes. You might use it for simulations of galaxy formation where Newtonian gravity is considered a reasonable approximation today. But MOND is not a correct model of the universe. There is no place in the universe that Newtonian gravity applies, only places where the error is an acceptable trade-off for simpler calculation.
By the same logic, there's no place in the universe that general relativity applies either, since it breaks down at the quantum level. There's no place in the universe where any theory other than the one true grand unified theory applies, because everything else is just an approximation. At which point we're just arguing about semantics, and I don't see a reason for continuing it on my part.
There are vastly different scales where the approximation is correct for newton vs general relativity. Perhaps you can define the scales that you are calling relevant so we understand what you mean.
The scale of galaxies? Which the original article is about? I feel like I need to spell out everything, but ok:
The article is about modified Newtonian dynamics (MOND), which is a theory that modifies Newtonian gravitation to fix some observed differences in galaxies' motion, without invoking dark matter. The original commenter then proclaims "haha, MOND cannot be right, because we know that Newtonian gravity is incorrect". Yeah, no shit Sherlock; it is "incorrect" because it is just a limiting case of general relativity. But that's completely besides the whole point of MOND, which tries to "fix" gravity at galactic scales, which is a Newtonian regime even with general relativity. MOND is trying to tweak the Newtonian formula at those extreme distances, and if it works, then maybe it can be worked out to be a limiting case of a "modified general relativity", just as Newtonian gravity is a limiting case of GR. Got it?
The inaccuracy of the Newtonian theory of gravity is large enough that it was already noticed by astronomers in the mid-1800s.
that's like saying the visible mass of the universe is 99% hydrogen and helium, so we don't need to learn about chemistry.
So you're saying we should model galaxies down to the level of individual protons? Lol.
Galactic dynamics is governed by gravity, which is Newtonian at those scales.
No I did not say that.
These extremes exist, and GR predictions are better than Newton’s in those cases. Closest to home is mercury’s perihelion drift. We have observed black hole mergers, gravitational lensing, and GR is also an essential component in understanding the universe’s expansion(that we know from redshift and the CMB). Likely MOND will address these, but Newtonian mechanics will not get you there.
We can see gravitational redshift on Harvard's campus thanks to gamma ray Mossbauer spectroscopy.
When you say "outside of" - that's the thing where it doesn't hold. It's interesting and not even wrong to say "these rules work in these contexts" but as far as I can tell we're looking for the scenario invariant rules.
To be fair, there are relativistic generalizations of MOND, in the sense of relativistic theories that simplify to MOND dynamics in the low energy case. My understanding (this not being my field) is that they're sort of kludgey and non-calculable and that no one takes them very seriously. All the "real work" on MOND is just done using the classical stuff.
And yeah, that seems like pretty terrible cheating. It's one thing to hang a big theory on a single conjecture, but you still need to be trying to prove the conjecture.
« Stunning evidence » … then later on: « Instead, the readings _seem_ to support a basis for MOND, which _would_ force astronomers and cosmologists to reconsider this alternative and long-controversial theory of gravity. » What’s conditional evidence? I may be missing the overall picture, but I view such writing as non precise at its best.
It's just typical pop sci journalism, with a click baity headline. Read the paper instead.
Thanks, I will. https://iopscience.iop.org/article/10.3847/1538-4357/ad834d
Not entirely typical. MOND proponents seem to be trying more and more sell their approach to the public.
It annoys me but I suppose every theory has to do that now, "the mouse trap must go to market now" and all.
Well, it’s evidence that a) must be verified on a mathematical and empirical level, and b) (arguably) fits better with a currently unpopular theory than the dominant one. There’s so many unknowns in physics that opponents can easily reply “well your theory doesn’t explain XYZ yet, so we likely just need to tweak our theory”.
In other words, reasonable minds do disagree. AFAIU as an amateur.
There is no consensus yet, there is no repeatable metric
It is perfectly valid to say “hey look over there for further review”
Why why why do people share articles with sensational headlines like this? Its no wonder science journalism gets a bad rap. This kind of thing really undermines all the people who are actually trying to communicate science properly.
Without this article and HN discussion I’d never have known about MOND, which is (at the very least) a fun theory.
What’s MOND really mean? Here’s the Wikipedia entry https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
Are any of the MOND theories consistent with this new data also consistent with recent gravitational wave observations? My understanding is that gravitational wave detectors have recently ruled out most plausible MOND theories. The linked paper doesn't seem to discuss this.
Waiting for Angela Collier to make a video on this, I'm sure many people will forward her this article. MOND is actually a niche in cosmology despite its PR.
I follow the lead author, Stacy McGaugh, via his blog where he posts discussions and musings about the latest research into the dark matter vs MOND debate: https://tritonstation.com/new-blog-page/
His arguments are very convincing and relatively clear. I am not an astrophysicist but I have two degrees in physics and have always found the dark matter theory to be lacking -- in absence of any evidence of causation whatsoever, dark matter can only be described trivially as "where we would put matter if we could to make our theory of gravity make sense," which is totally backwards from a basic scientific perspective.
Predictions based on modern MOND postulates are shown to be more and more accurate as our observational instruments continue to improve in sensitivity.
> which is totally backwards from a basic scientific perspective
This is not right, because if we have a situation where our theories and observations don't cohere, it's not given whether the theory requires modification or we're missing something in our observations (or both). A classical illustration is the orbit of Uranus being observed in the nineteenth century to be contrary to the predictions of Newtonian theory. Calculations were made assuming the truth of the Newtonian theory and that we were missing something in our observations - the position of Neptune was predicted and it was subsequently discovered.
On the other hand, the orbit of Mercury diverged from the prediction of Newton's theory. Again, a previously unobserved planet closer to the sun was postulated as being responsible, but in this case it really did require a modification to the theory of gravity: general relativity, which accurately predicted the 43 arcseconds per century of perihelion precession by which Mercury's orbit diverges from Newtonian predicitions.
GR has obviously made many other predictions, such as the gravitational bending of light, black holes, and gravitational waves, which have been vindicated.
So there's obviously a problem of the theory and observations not cohering, but whether the solution is a modification of the theory or a new form of matter is not clear in advance, and the latter is not unreasonable and certainly it's not unscientific to make as a hypothesis, to see where it leads.
The difficulty is in coming up with a theoretical framework that retains all the successful predictions of GR while also accounting for the galactic rotation curves.
One difference between dark matter and Neptune is that the existence of Neptune is falsifiable. The formulation of dark matter inherently is not. Falsifiable hypotheses is the cornerstone of science.
I'm not sure it's inherently unfalsifiable. There are some specific proposals for dark matter that could be ruled out by experiments, such as right-handed neutrinos: https://en.wikipedia.org/wiki/Sterile_neutrino#Sterile_neutr...
Maybe if you're being very broad in definitions then some class of proposals describable as "dark matter" might be unfalsifiable, but to be taken seriously as a scientific proposal I think it should be specific, concrete, and indeed testable, and there are a few of these within the "dark matter" class.
Again, we're in the perhaps unsatisfying position of having observations which don't cohere with our current theoretical understanding. What's the solution? It's not easy...
Is the existence of a planet so easily falsifiable? It hasn't been so long since the Planet Nine hypothesis started going around, and while we've observationally ruled out a big chunk of the original parameter space, there's still lots of room for a big dark dwarf planet to be floating around out there. It doesn't seem so different from how we've gradually been ruling out the parameter space for dark-matter observations.
Surely the idea of it being a new kind of matter that interacts gravitationally but not electromagnetically yields some testable result? Does it actually yield nothing testable with today’s experimental methods?
There is a lot of indirect evidence for dark matter. All the direct tests for dark matter particles we have performed have found nothing so far - but since we have no idea what it might be, there's a lot of possibilities to test.
I mean, dark matter may be discoverable, we just don't know how if it exists. There was time between the irregularities that were noticed in the orbit and the discovery of a new planet.
Well put, thanks for sharing! Never saw it phrased in such a clear narrative. As a novice, it seems like there's one big difference between those anecdotes and the current situation, though: sample size. Sure, if we were observing Andromeda spinning too slowly I'd be open to our instruments not capturing some massive objects/clouds, but we're actively observing, what, ~1E5-6 galaxies? In the case of a missing planet there were accidents of history/solar system makeup that led to our otherwise solid frameworks missing a key piece of information. But that clearly couldn't happen millions of times; whatever explains the inconsistencies we're seeing has to be a fundamental misunderstanding.
Once we've arrived at this point, we can compare the two theoretical re-workings on their own terms: one is that we're glossing over some important detail of how gravitational relations in spacetime work, and the other is that we're failing to observe some new class of matter. I mean, right? There's no way this conundrum will be solved by "whoops turns out there was more plain ol' dust than we thought" at this point, right?
In those terms, I feel parsimony clearly favors one possibility over the other. Every hypothesis is worth exploring (I mean, QM and GR are dumb as hell, yet nonetheless turned out to be correct), but when funding is on the line it's also not out of line to favor one explanation explicitly. That's already happening anyway, just in the other direction.
But also I'm just some kid who's awed and grateful to be living in times of such profound mystery and discovery. Could be totally off base -- I barely passed physics I!
> ...turned out to be correct
What we have learned so far is that our theories and models are only correct up to our ability to precisely observe and measure.
In that sense, Newtonian physics is still very much correct under a very wide set of circumstances, and as such amazingly useful.
GR improves on that (adds precision) on what would be extreme cases for NP, but it is likely as correct as Newtonian laws are: up to a point.
All this to say that "correct" is not the right term to use: many of the theories are simultaneously "correct" with sufficient constraints and a particular error range. What matters more is if they are useful in predicting behaviour, and that's where I like using "correct" instead (as above).
> where we would put matter if we could to make our theory of gravity make sense
Dark matter behaves in a fundamentally different way from baryonic matter. We can constrain the total amount of matter in the universe (both dark and baryonic) from the observed abundances of baryogenesis. But dark matter has a different effect on the relative amplitudes of peaks in the CMB.
As far as I can tell, MOND has never really had any success outside of modeling galaxy rotation curves.
The skepticism I've seen towards dark matter vs. MOND has always been strange to me. Dark matter doesn't really require much in the way of new physics --- there's just a new particle to add to the standard model. But most MOND theories violate Lorentz invariance which is a vastly more radical departure from standard physics. (And in my mind, the more sophisticated MOND theories that maintain Lorentz invariance like TeVeS are really a theory of dark matter dressed up in the language of MOND.)
There are more successful predictions than just rotation curves. For example, see:
http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html
These successful predictions are all generally variants on modeling galactic dynamics, though. The trouble is that galaxies and galaxy clusters are very messy places, so it's hard to make sure you've incorporated all the relevant physics.
By contrast something like baryon acoustic oscillations are very simple to model, so you can be quite confident that you've incorporated all the relevant processes. And in that regime LCDM performs beautifully and MOND completely fails. So it's reasonable to suspect that in more complicated environments the problem is that we're not modeling the systems correctly rather than that there's new physics going on.
There are other predictions MOND makes. For example, it predicts higher collision velocities than LCDM, for example, see:
https://ieeexplore.ieee.org/document/8193356
And, of course, it predicted that the early universe would have bigger and more structured galaxies (which is what the posted article is about).
Dark matter has a slew of problems of its own; it's not the case that LCDM is problem free, despite good success in some areas.
MOND doesn't cover the existence of CBM, distribution of galaxies, non-metallic abundance - things all covered by LCDM.
What MOND has going for it is that galactic rotation curves are readily consumed by popsci readers and the story of the "little guy" vs the scientific establishment is an easily available frame story popsci authors can sell clicks for.
The proportion of lay people who think MOND could be true greatly outnumbers the proportion of MOND researchers and doesn't reflect the veracity of the theory.
MOND is not a cosmological theory unlike LCDM, and it isn't relativistic. So we should not expect it to cover the range of things that LCDM tries to.
It's just a tweak to Newtonian gravity, which surprisingly matches observation very well, and has accurately predicted quite a few things in the regime it operates in, before they were observed.
The fact it works so well in the areas it does apply to is the reason that science hasn't given up on it yet (regardless of what pop science or lay people think).
Very interesting. Do you know an article that ELI25 this?
For a more non-technical overview, Sean Carroll had a nice episode on his podcast where he talked about the evidence for dark matter among other things: https://www.preposterousuniverse.com/podcast/2023/07/31/245-...
For something more technical, this article just came out as an overview of the evidence for dark matter: https://arxiv.org/abs/2411.05062
The mond theories that add a factor that behaves like dark matter do a rather good job of matching observational data.
I don’t think that’s quite fair. That approach is exactly how we find planets. Here’s an unexpected variance in the motion of a planet or star. It could be explained by a planet over there. Oh look, there’s a planet over there.
Hypothesizing that a planet might be over there is a testable hypothesis.
Have we found a way to verify the presence of dark matter yet? Or is it still an untestable hypothesis sprinkled around distant galaxies so their acceleration curves look right?
Dark matter predicted lensing effect which were successfully tested. Same for the baryonic acoustic oscillations in the CMB.
That's not quite true. General relativity predicts gravitational lensing, not dark matter. Lensing has been used as an experimental probe for the presence of dark matter.
MOND is an alternative theory of gravity competing with GR. People usually forget that while MOND started to present a different explanation for Dark Matter, it is a theory of gravity. Dark Matter is not a theory of gravity and is compatible with GR.
Dark matter isn’t much of a theory in the first place.
I’m particularly amused by the hypothesis that spacetime can be bent without the presence of matter. We can’t detect dark matter because there’s no such thing, it’s just a brute topological fact.
Right, which is why it quickly led to the detection of dark matter...hmm.
I think a better analogy would be "that approach is exactly how we explain failing to find planets like Vulcan; we hypothesize that they are made of as-yet-unknown stuff that you can't see, touch, hear, smell, or in fact detect at all. But we know they're there because our calculations say they are."
It's actually a better example than you think. This exact theory led to long and protracted searches for the planet Vulcan, which would explain Mercury's strange behavior.
Planets are visible when you look for them.
Dark matter - so far - isn't.
What do you mean by “visible when you look for them”? Like, with light?
Does gravitational lensing count as “visible” to you?
I usually understand "dark matter" to be shorthand for the discrepancy between theory and observation. The explanation might indeed be matter that is dark, or it might be solved by entirely unexpected observations and/or changes to theory.
Not really. You might think this after watching Angela Coulliers video, but when you read something like "25% of the universe's energy content is made of dark matter", they do not mean changes to some theory. They literally mean non-baryonic matter.
Energy content not only comes from matter but also from fields.
Nope. It can mean change to some theory, without a need for matter. It is the difference between relativistic gravity and the corresponding observed mass.