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When I see dark matter in the news I'm always reminded of the story of Vulcan.
In the 1800s, detailed observations of the planet Mercury showed that its orbit was slightly different than Newtonian mechanics predicted-- a difference of about 43 arcseconds per century. The study was rigorous enough to rule out any observation errors.
Le Verrier, the astronomer who made these observations, wondered how to explain the difference. A decade earlier, he had noticed a similar irregularity in the orbit of Uranus, which led to the discovery of Neptune, whose gravity caused these perturbations. So Le Verrier reasoned that something similar must be going on for Mercury, and he posited the existence of Vulcan, a tiny planet close to the Sun.
Many attempts were made for decades to observe Vulcan. It was even included on some maps of the Solar System at the time (https://www.loc.gov/resource/g3180.ct003790). But it was never conclusively observed.
When Einstein published his theory of relativity in 1915, the mystery of Mercury's orbit was finally explained-- Newtonian mechanics were simply incomplete, and the irregularity of Mercury's orbit was due to relativistic effects.
Could it be that something similar is happening today? Observations of gravity on galactic scales doesn't quite align with what relativity would predict, so we use dark matter to fill the gaps. We've tried for decades to detect dark matter, with no dice. Is our theory of gravity simply incomplete?
MOND may not be the solution, but I'm still skeptical about dark matter.
There are multiple independent observations pointing toward dark matter:
1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lenses and infrared. 3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg).
MOND only explains 1.
Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.
> Only catch is that it hasnt been directly observed.
Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.
Associating dark matter with epicycles is unfair, but it's still a risk (CDM, WDM, SIDM, and probably more by next year). In that light, of course it has more observations (by virtue of creating branches to explain incompatible observations) - but counterpoint is that it has made some predictions.
My stance is that anyone pointing at it in either light probably isn't taking everything into account. It's an incredibly immature theory space - are we going to get 20 more branches of it (making it modern epicycles), or are we going to see one of the current branches pay off?
I mean there are modified gravity candidates other than MOND. I think those are worth more consideration. MOND seems pretty well disproven at this point, yet somehow it's still what people focus on in terms of modified gravity! Where are the tests of other modified gravity theories?
Indeed, in terms of the negatives of MOND, I'll go further than you -- MOND barely explains galaxy rotation curves at all. When you look at the actual rotation curves compared to MOND's predictions, well, yes MOND does better than Newton w/o dark matter, but it's still pretty heavily fudged. If you have to fudge it that much, it seems to me there's no point. So I'd say that really MOND doesn't explain any of the three.
It can't be observed, but it affects all these things.
This reminds me of arguments on the alt.athiesm newsgroup back in the day.
MOND explains 1.
2 is "who can say" because nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND).
3 is subject to questions like "is the CMB really what we think it is" -- if it's early thermalized dust, then that ALSO resolves hubble tension, e.g.
MOND explains several things LCDM cannot:
- why most elliptical galaxies seem to "not have dark matter" (effectively a prediction)
- external field effect (predicted and confirmed)
- renzo's rule
- DM halos that are way too big
- early galaxies (this was a prediction)
HM. people have been downvoting. Anyone care to post a substantive rebuttal?
Of course it can be that this is the case. There are predictions from the dark matter hypothesis, though, that have been testable. For example, there're some of the asymmetries in the cosmic microwave background that are explained presently by the dark matter hypothesis.
Something that's worth bearing in mind is that "dark matter" doesn't actually mean "totally new never before seen thing" it means "we don't know what this matter is". So, for example, a candidate that wouldn't be super novel but could fit the bill is microscopic black holes. In that sense, the hypothesis is more mundane than it might seem.
As far as I know, dark matter hasn't made any testable predictions which were subsequently proved correct. It's an infinitely tunable hypothesis that can be retro fitted to new observations, since it has many tweakable parameters.
Could you point me to a prediction made by dark matter theory which was subsequently confirmed by observation, rather than a dark matter theory which explains it post facto?
100%. It's important to realize our understanding of "dark matter" is fuzzy because we only understand it through data anomalies. Dark matter is a classic catch-all concept that we use as a crutch while we try to understand the underlying system better.
Similar to how we used to believe in "aether" to explain how light could travel through empty space. It is important to understand how these crutches help and hinder understanding.
It's kind of weird in this case, though. All the math acts like there's something invisible and heavy everywhere that we find clumps of visible matter. When we look at the motion of galaxies, they behave as if they're much more massive than the count of stars and such in the would have you believe, and in ways that otherwise jibe with our understanding of physics if only that galaxy were heavier.
If you have one galaxy that's acting heavier than you can eyeball, measured by things like light bending around it, then maybe you have some weird phenomenon. When every galaxy calculates out to be about 6x fatter than you'd expect, something else is going on.
The constituent particles of nature are under no requirement to be apparent or achievable to you.
The strongest evidence that something like MOND isn't the answer is that in some galaxy collisions, the visible matter and the dark matter appear to separate: the collision disrupts the visible matter and the dark matter appears to pass right through, uninterrupted, and we see galaxy remnants that look like they don't have dark matter. If MOND or some other modification of gravity were the answer we'd never see this kind of sorting.
See, for example, https://www.caltech.edu/about/news/dark-matter-flies-ahead-o...
You might be interested in that. We have observations of galaxies where dark matter likely is not present.
Astrophysicists have gone all through this. Absent direct observation or at least a good explanation of what it actually is, I don't think anyone really likes dark matter. The main objection is that it feels a little too flexible: you can explain a lot of things by adding mass in different places. But it's not infinitely flexible: you don't need arbitrary and utterly weird distributions of dark matter. And so it's generally emerged as the best explanation of observations so far, and not for lack of trying with tweaks to theories of gravity themselves.
yes - absent experimental confirmation, the effects attributed to dark matter may in fact be due to an unrelated as-yet-undiscovered phenomenon.
If you follow Sabine Hossenfelder's channel, she has a MONDOmeter. With MOND (modified Newtonian gravity) on one side and dark matter on the other side.
As new papers come out the needle goes back and forth, and I guess that she will make a new video if she hasn't already, with the needle moving one step towards dark matter.
I find it interesting how it doesn't seem to settle. Dark matter is still the favorite, but there is a lot of back and forth between "MOND is dead" and "we found new stuff we couldn't explain with dark matter, but it matches MOND predictions".
MOND does amazingly well at galactic rotation curves, less well at anything else. If you think it started with Vera Rubin in 1966 MOND seems natural, but if you know that it started with Fritz Zwicky in 1933 than dark matter is easier to believe.
MOND only really does well on galactic rotation curves because it has free parameters that are tuned to "predict" the correct answer for galactic rotation curves.
There are galaxies that appear to be free of dark matter and rotate accordingly. How does MOND account for that?
My understanding is that these observations are a fatal blow to any serious MOND models.
We are likely going to find out that both are unfixably faulty.
It'll take either the next Einstein or some groundbreaking experimental observation to get there in my opinion.
If it was possible to incrementally fix these theories, the army of postdocs working on these would have already done so in the last decade or so.
But at least the experimental results disproving these incremental fixes should be exactly the kind of thing the next Einstein should need for coming up with an entirely new way of looking at things
you think the deepest mysteries of reality and the universe should just reveal themselves because we have a couple thousand smart people working on it for... 10 years?
MOND is kinda like a dead horse now, that people like to keep flogging
I think it's possible for an alternative gravitational law to work, but not MOND
MOND is stronger at longer distances than Newtonian Gravity. To me that does not pass the sniff test. It could be a step in understanding a more exact law but to me it feels weird
my understanding is that there are a few MOND champions who are still holding on to the idea while everyone else has moved on.
so MOND is the new String Theory...
Once I joked that a lot of things in the universe make sense if you view it as a "simulation with optimizations like lazy loading".
MOND is dead is a true statement if we say MOND is dead as a general theory of gravity. It does not mean is does not have its success with explaining galactic rotation curves but failing at mostly everything else.
Until you definitively rule out all the dark matter candidates or get a direct detection the controversy will remain
Dark Matter : supposedly makes up a big amount of the mass of the universe, but cant be seen, does not emit, absorb, or reflect light. Also it can 'pass through' other normal matter, and other dark matter.
It's basically magic aka not actually real, just something in vogue to pretend is real at the present moment.
I mean, there are modified gravity candidates other than MOND. I think people should perhaps give more consideration to those and not focus on MOND as their idea of modified gravity. I feel like the evidence is pretty well against MOND at this point. But other ideas of modified gravity, that aren't MOND-like, may still be worth considering. Framing it as "dark matter vs MOND" implicitly excludes these and I think that's a mistake.
It's funny how for MOND we cant accept that it has some unknowns yet but we are more than willing to accept the FULL UNKNOWN Dark Matter. it's easy. put "Dark" in front of something and you don't have to explain it at all, no matter that something else explains at least 60-70% instead of 0.
I thought newtonian gravity was already proven to be inaccurate with Einstein's Special Relativity (or General Relativity?) giving better results on cosmic scales (basically analogous to an approximation vs an exact formula)?
General Relativity reduces to Newtonian gravity as the curvature goes to zero, that is when you're very far away from objects relative to their masses, for slow non-relativistic objects like stars and galaxies.
Galaxies are typically so far away from another they're almost like point sources to each other, hence Newtonian gravity explains their motion very well.
However, inside galaxies things do not behave as expected, as stars in almost all the galaxies we've measured does not move like Newtonian (nor GR) behaves based on the matter in the galaxy we see. One alternative to the mainstream theories of dark matter is to modify Newtonian gravity, called MOND.
This work tested if MOND fit the motion of galaxies in galaxy clusters. They found it did not.
MOND already does not explain other phenomena that dark matter can so it's not terribly surprising. Here[1] is a nice accessible talk going through all the evidence for dark matter.
But it is technically a possibility that there's two things are going on, something MOND-like as well as dark matter, so worth checking.
Special Relativity is an extension of Galilean/Newtonian mechanics (motion of projectiles and other objects) to the case where the object is travelling at speeds that are a fraction of the speed of light. It deals with non-accelerating frames of reference. Satelites need to use this to correct for time dilation effects, but tracking the trajectory of an arrow/etc. or a car/etc. travelling from one location to another then classical mechanics is sufficient.
General Relativity is an extension of Newtonian gravity. It is also an extension of Special Relativity to cover accelerating frames of reference. Satelites need to use this, as does tracking the orbit of Mercury. However, for the orbits of other planets and the moon, using Newtonian gravity is sufficient for a reasonable degree of accuracy, and is used for tracking things like equinoxes/solstices, full moons, etc..
At these scales (entire galaxies, very weak forces), it doesn't make a significant difference.
There are ways of adapting MOND to match general relativity, should it turn to be correct at explaining what it is supposed to explain (like the movement of galaxies).
General Relativity. It explained the anomaly in the precession of Mercury's perihelion, and the bending of starlight by the Sun (double the value predicted by Newton's law).
The test here is for the inverse square law of gravity. The rival theory in this case isn't GR, but MOND: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
Free variant of this research paper:
We measure the matter distribution by its affect on light (strong/weak lensing). We also measure the matter distribution by the amount of light coming from it. The results are not the same. The simplest explanation is that there is matter which does not produce or reflect light via e/m, i.e. it is dark. Dark Matter.
We know of particles which behave the same way. Neutrinos for example.
> We know it holds incredibly well terrestrially and within individual galaxies
I somehow thought that this was not really holding true and that Mr. Einstein had something to say.
I thought that without Einstein's "corrections" stuff like GPS wouldn't be useful at all.
Maybe I misunderstood last 100+ years in gravitational physics.
It's bizarre that Newton's law passes this test, when the linearization of general relativity predicts a gravitational equivalent of a magnetic analog for gravity.
Gravity Probe B.
There exists an even simpler derivation for a magnetic term, but its in my file drawer... (simpler in that it just relies on observations from special relativity)
If it had failed, would we be hearing about it in Science before the researchers continued testing it until it passed? (i.e. is this a blindspot in our collective epistemology?)
If it had failed it would've been a top science news. Every scientist dreams about the day when they're mentioned as "(...)'s experiment puts Newton's law of gravity in jeopardy."
Woow
just another nag screen ...