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?
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.
What’s MOND really mean? Here’s the Wikipedia entry https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
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.
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?
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:
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.
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.
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."
Planets are visible when you look for them.
Dark matter - so far - isn't.
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.