By the way, apart from the fact that dark matter predictions have been contradicted by increasingly sensitive experiments over and over and over, there is another inconvenient piece of observational evidence: An analysis of stellar velocities at the edge of the Milky Way, published last year [1] shows that they are "Keplerian", i.e. following classical behavior without the influence of any purported dark matter. So either the Milky Way is anomalously dark matter deficient (how very convenient :), or maybe the discrepancy that dark matter was invented to explain will simply disappear when more accurate astronomical data including the fainter stars becomes available.
That link also claims that the results you're listing are not likely to hold up, including comments from an author of the paper used for the article. So if one of the paper;s own authors expects these claims to disappear once more data is gathered, then maybe it's too early to claim the end of dark matter.
After all, DM has been measured in many different ways. One surprising finding doesn't negate hundreds of surprising findings without significantly more evidence.
> shows that they are "Keplerian", i.e. following classical behavior without the influence of any purported dark matter.
That is NOT at all what the paper says [1]. if you actually read the paper, and don't interject your beliefs and reword it, in every instance they use the phrase "Keplerian decline," which does not AT ALL mean classical, which would break relativity and be a massive surprise. The phrase "Keplerian decline" means the measured items are less than expected for the previous model. The movement, even with this decline, are far beyond what Kepler's laws would imply (and even Kepler's laws are demonstrably wrong in our solar system - see the precession of Mercury for example).
That is begging the question. Claiming that DM has been measured many ways presupposes that the different ways that "it has been measured" are have the same cause, which itself is the dark matter hypothesis.
Until we get a multimodal observation of DM itself we can't claim that DM has been measured many ways. We are very much still in the "guessing that these things are DM" phase.
> we can't claim that DM has been measured many ways
Yes we can.
It's shown up in 1) precise measurements of galaxy rotation curves (relevant to this paper, which only addresses this method), 2) velocity spread of bound stars, 3) x-ray emission from hot gas, 4) gravitational lensing, 5) cosmic background radiation measurements in CMBFAST and others, 6) provides solutions to issues in stellar strucutre formation, 7) supernovae behavior, 8) baryon oscillations support DM via empirical evolution compared computed with and without, 9) redshift observations support DM also. There are more.
So yes, we could claim there are 39 different causes, but that the effects of DM would give all these results, historical (and Occam's Razor, a still useful part of physics) means the most likely explanation is the simplest - 1 cause until proven otherwise.
If you dig, you can find papers covering all this with the math and experimental and computational error bounds to see how well all of these (and more) line up.
Next you'll tell me there are 5 types of electrons, despite all experimental evidence being consistent with one type of electron.
The 1st is good, but also not predicted a priori by dark matter. In it's defense, it doesn't need fine tuning to work (it works out with ~the estimated amount of DM)
For all the minor ones there's a lot of fine tuning necessary to obtain the results. So it's not surprising that an LCDM model (which can select an arbitrary distribution of DM) can fit some sundry minor observations. Again, you're cherry picking the minor ones.
LCDM has a hard time with: external field effect, renzo's rule, Tully fisher relation (requires tons of fine tuning), early galaxies, why dense elliptical and lenticular galaxies as a rule have no DM, etc. those are all phenomena explained and predicted (except for Tully fisher) by e.g. MOND.
LCDM doesn't explain how the mitochondria function, or why my dog prefers to shit in the yard instead of in the house, so it is clearly a failed model for not having explanatory power on those observations
We've also seen instances where visible "clumping" in a galactic disc correspond to differences in the rotation curves, which dark matter can't really explain. But this is still a topic of much debate.
However, there's lots of good evidence for dark matter besides rotation curves of galaxies. For instance, models of galactic formation work a lot better with it than without it (without dark matter, as galaxies coalesce, they get hot and the pressure keeps the gas apart and makes star formation really hard).
We also see the Bullet Cluster, where two galaxies collided/passed through each other. The gas and dust has been slowed down from collisions, but the dark matter has passed right through. We know this from the gravitational lensing. The lensing happens around the mass of the galaxy, but with the bullet cluster, the lens is off to the side where there is no normal matter, because the dark matter kept going when the regular matter slowed down.
In other words, we have some really good evidence for dark matter, but there's a few things going on here and there we can't explain.
I remember reading/hearing somewhere that the bullet cluster actually fits MOND better than dark matter. I think it was a video by Sabine Hossenfelder but I can't find it right now and I'm not qualified to say whether it made sense or not.
Bullet cluster is inconsistent with LCDM on the grounds that given the apparent dark matter:matter ratio a collision between clusters of that size is something like unlikely to the tune of one in a trillion? IIRC universes. (Aka the collision should not have happened in the first place).
The thing about lensing is that we don't have a solution in (GR would need to be tweaked if MOND is true -- the math is much harder!!) so we can't really say what the lensing would look like in any given MOND-like theory yet. Seems weird to declare that MOND can't explain lensing. It's should be more qualified: "we don't think MOND can explain the lensing"
It's not clear that GR would need to be tweaked to match MOND. The GR solutions we currently use in LCDM are based on the FLRW metric, but that metric could just be the wrong fit for our universe.
I wasn't familiar with the Bullet Cluster[0], but as you described it made it very interesting. I'm no astronomer, but from all of the examples of gravitational lensing[1] I'm familiar with have a very distinct look that I'm just not seeing with the Bullet Cluster. Where is the lensing effect occurring that is leading to this theory?
Gravitational lensing doesn't have to be apparent to the eye as those extreme examples on the wiki page are. The second image on the bullet page shows matter in pink and measured lensing in blue.
I thought gravitomagnetism was extremely weak? Wouldn't normal magnetism and its interaction with environmental plasma (which IIRC is also neglected on the grounds of being a rounding error) be more significant?
> apart from the fact that dark matter predictions have been contradicted by increasingly sensitive experiments over and over and over
My impression is that many physicists would disagree with this characterization entirely, and that they're eagerly working to constrain what it is or isn't. Ruling out big classes of phenomena that could be responsible for the things we observe isn't "contradicting" the predictions in the sense implied.
Particle physicist here. I've worked on direct detection DM experiments in the past, and personally know some folks who work on the LZ experiment. That direct detection experiments, such as LZ, have not detected a signal does not contradict any predictions.
Indeed, relevant to what an experiment like LZ might see, there really isn't much in the way of "predictions" which can be "contradicted." What we have at this point are mechanisms to calculate the interaction rate _given at least one free parameter_. If we were to detect a non-zero rate, then we would "know" the free parameter of a single-parameter theory underlying that calculation. If we were to continue to detect a non-zero rate, then we would try to do so using different materials, and look at the time dependence of the rate (or, really, the dependence of the rate on the Earth's direction of travel in our local galaxy). That would help us choose between different theories, pin down the free parameters, and confirm that what we're seeing is consistent with "heavy stuff just sitting out in the universe."
But, from a particle physics perspective, right now there are no predictions to contradict - just an opportunity to detect something.
I'd normally agree with you, but in the particular case of dark matter particles something still smells fishy. The theories that the predicted cross-sections are based on are just too flexible and numerous. I'm not sure how much we gain from ruling out yet more. What if there are in fact no weakly interacting particles? At what point do we decide that enough has been ruled out to start looking elsewhere? Like plasma dynamics or something (please don't shoot me if this is too silly to even contemplate).
Reprioritization of direct experimental searches for dark matter is already happening. WIMPs are by no means being abandoned, but because we are closing in on the neutrino fog background (which is mentioned in another comment), it's been recognized that to myopically cling to the same kind of experiment which dominated the 2000s and 2010s is not a strategic move (both from the "we expect to see something" and the "responsible use of tax dollars" perspectives).
For example: axions, an alternative DM candidate mentioned in another comment, have seen a significant growth in attention in recent years, and the usual detector technology for axion searches is currently being refined and scaled up, from benchtop-scale, dedicated experiments to lab-scale, wide searches.
At the same time, different groups which have developed past WIMP detectors are merging to collaborate on the larger, next-generation detectors. And there is R&D and prototyping happening to create detectors which, although looking for WIMPs, are sensitive in entirely different mass ranges than those of yesteryear.
That's just how particle physics research works. You build a detector that is designed to detect things with specific properties. You run the detector. Did you find the thing? If so, great, if not, well, that's the way the cookie crumbles. Either way you write a paper and you build the next detector.
The Higgs boson has numerous experiments exclude numerous mass ranges excluded before it was finally found.
> At what point do we decide that enough has been ruled out to start looking elsewhere?
"We" don't make that decision. The various institutions who pay for these things decide, one by one, that they're going to fund some thing that sounds more promising instead.
It does kinda suck that there's something there in the universe that is perniciously difficult to see--in fact, that's how it's defined--but that is so important in the way the universe works that we can't simply ignore it. But this is the universe we're given, so this is the universe we'll run experiments on.
> maybe the discrepancy that dark matter was invented to explain will simply disappear when more accurate astronomical data including the fainter stars becomes available
FWIW, that would be a much more surprising result than anything to do with dark matter. We can see "faint stars" just fine in the near field. What you're positing here is that somehow "faint" stars in the farther universe behave in notably different ways than they do in the Milky Way, which is exactly the kind of theory you're arguing against.
No, a few confusing results isn't going to throw dark matter out. It's too strong a signal, and too hard to explain via classical means (I mean, it's not like astronomers haven't tried!).
> maybe the discrepancy that dark matter was invented to explain will simply disappear when more accurate astronomical data including the fainter stars becomes available.
That's unlikely. Measuring the milky way galaxy's rotation curve is bitchingly hard because we're inside it. Curves of other galaxies are much, much easier to measure and for example Andromeda is likely to be correct.
Moreover galaxies with exceptionally large satellite galaxies are known to have keplerian decline, so this could have been predicted.
The more interesting recent results is that for some galaxies the rotation curve is flat very very far out (those measurements might not be as accurate ofc) which would imply much larger dark matter halos which are inconsistent with e.g. cmb ringing
Even better recent evidence shows that rotation curves are flat up to a million light years, which no feasible particle dark matter halo could reproduce:
This is an old article (2023 yea I know that's recent but it's still out of date) and the crisis in cosmology (AKA dark matter) came back harder than ever with subsequent studies: https://www.youtube.com/watch?v=T1JuCPhONlg
As written in that article, there are questions to be answered before this observation can be considered a serious challenge to dark matter. Such as: why don't we observe the same phenomenon in other galaxies? Why are things orbiting the Milky Way farther out (such as the Magellanic clouds) not affected either?
> maybe the discrepancy that dark matter was invented to explain will simply disappear when more accurate astronomical data including the fainter stars becomes available.
Clearly you mean galactic rotation curves. However, your statement seems to ignore the numerous, independent probes of dark matter which we have seen since then, most of which are much stronger evidence, most notably the shape of the CMB angular powerspectrum.
[1] https://www.scientificamerican.com/article/the-milky-way-may...