James Webb Space Telescope finds evidence for alternate theory of gravity

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.

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.

Planets are visible when you look for them.

Dark matter - so far - isn't.

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."

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?

> 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.

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.

> 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.)

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.

There are more successful predictions than just rotation curves. For example, see:

http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html

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!

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.

> ...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).

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.

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.

Dark matter predicted lensing effect which were successfully tested. Same for the baryonic acoustic oscillations in the CMB.

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.

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.

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?

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.

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 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.

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.

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.

Very interesting. Do you know an article that ELI25 this?

The mond theories that add a factor that behaves like dark matter do a rather good job of matching observational data.

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.

Dark matter isn’t much of a theory in the first place.

Energy content not only comes from matter but also from fields.

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

What do you mean by “visible when you look for them”? Like, with light?

Does gravitational lensing count as “visible” to you?

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'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...

Thanks. I'm also no expert - I'm just learning general relativity - but that's also my rough understanding: either there needs to be a modification of the theory, or there's a new form of matter. It might seem more parsimonious to modify the theory, but then how do you do that in a way that retains all the successful predictions of GR while explaining the recalcitrant observations? That's the hard part.

It seems at the moment that the minimal and most elegant adjustment to the worldview required is to postulate the new form of matter. But I think it's safe to say it's a genuine problem in our knowledge: we don't know how to solve it

Have you ever encountered the phrase "grasping at straws"? The pursuit of explaining dark matter has gone through many waves of "we just need to invent detectors for this particle that has never been observed" and is littered with the wreckage.

Planets that reflect light are easy to detect.

"Evidence" in heavy scare quotes, considering, again, the tautological nature of the claims around the existence of dark matter. "Something must be here that we are missing" is, frankly, a bullshit hypothesis that need not be entertained unless researchers can actually prove there is some worthiness to the claim. Anything stronger than "maybe our theory is wrong" would suffice!

It is tendentious to point out only the difficulties in finding affirmative evidence for dark matter when MOND is doing no better in that regard. If, by that standard, dark matter is bullshit, then, mutatis mutandis, so is every other hypothesis that has been presented so far - but the observations that prompted them in the first place are not going away. It is inconsistent to call just one of them bullshit, and pointless to call them all that.

By that extremely simplistic logic, so is literally any other theory of gravity. This is not an argument, this is a flailing and empty justification.

Well of course. What I clearly meant was that DM predicts lensing effects in a magnitude that cannot be explained with ordinary matter. See bullet cluster or weak lensing observations.

Most of the history of physics involves making detectors for things that weren't previously observed... Consider: either most researchers in the field are stupid, for still pursuing an idea which you've apparently ruled out by simple reasoning, or your simple reasoning is fallacious

Almost: most of the history of physics is based on detectors being sensitive to things people didn't even know existed. Fits and starts based on happy accidents. The teleology of scientific progress is a myth. The most famous example is of course the Galilean moons.

Proposing detectors for particles that no one is even sure can exist is like setting up traps for Bigfoot...