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Galaxies like this truly challenge MOND theories of modified gravity. If there is no such thing as dark matter, why doesn't this galaxy behave as the others do
by mabbo 5y ago
Galaxies like this truly challenge MOND theories of modified gravity.
If there is no such thing as dark matter, why doesn't this galaxy behave as the others do? It has the same amount of normal matter as it's neighbours, yet acts totally differently. Dark Matter as a theory has an answer: this galaxy has none. MOND has a much harder time explaining it.
- sahil50 5y agoAnswer on the 5th figure on this page. https://tritonstation.com/2022/01/10/the-curious-case-of-agc-114905-an-isolated-galaxy-devoid-of-dark-matter/ https://tritonstation.com/2022/01/10/the-curious-case-of-agc...
- awb 5y agohttps://tritonstation.files.wordpress.com/2022/01/btfr_off.png?w=1024 https://tritonstation.files.wordpress.com/2022/01/btfr_off.p... > Baryonic Tully-Fisher relation showing the ultradiffuse galaxies discussed by Mancera Piña et al. (2019) as gray circles. These are all outliers from the relation; AGC 114905 is highlighted in orange. Placing much meaning in the outliers is a classic case of missing the forest for the trees. The outliers are trees. The Tully-Fisher relation is the forest.
- contravariant 5y agoCalling them outliers isn't much of an explanation for something that shouldn't be able to exist.
- dnautics 5y agoThe tully-fisher relation is an empirical relationship IIRC used to calculate distances to galaxies. You don't know that the tully-fisher relation applies in exactly the same way to a class of galaxies that are characteristically different from those galaxies which provide the dataset for calibrating the distance (IIRC, this is done by using cepheid variable stars). By necessity, UDGs are an extrapolation of the tully-fisher relationship. Suppose the tully-fisher is off by two digit percent due to UDGs being strange -- then it's possible that our calculations about the galaxies "dark matter content" could be completely wrong.
- 8note 5y agoAfaik dark matter content is calculated by the rotation speed of the galaxy. I'm not clear why the distance errors wouldn't average themselves out over the rotation
- The_rationalist 5y agoOnly considering rotation speed for MOND seems like a simplist, poor approximation. One needs to take into account how wide/large is the galaxy. Its 3D shape. And a map of its local main density/sparsity points with estimated masses. A failure to estimate accurately any of those metrics seems like the issue with this galaxy, not MOND itself.
- sahil50 5y agoGreat pull quote. For people new to the Tully-Fisher relation, the essence of it is that at galactic scale, gravity is not like v^2/r = GM/r^2. It's more like v^2/r ~ sqrt(GM/r^2), so v^4 ~ M
- leetrout 5y agoMOND Modified newtonian dynamics https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.m.wikipedia.org/wiki/Modified_Newtonian_dynamics
- suifbwish 5y agoThank you. I prefer to use full terminology for my fields abreviations to prevent others needing to look them up just to know what I meant. I feel pretentious if I don’t.
- 8note 5y agoI only get annoyed if the first search results for an acronym are inconclusive. MOND is quite distinct
- SubiculumCode 5y ago[Are Black Holes and Dark Matter the Same? Scientists Upend Textbook Explanations (scitechdaily.com)] https://news.ycombinator.com/item?id=29799644 https://news.ycombinator.com/item?id=29799644 If so, would it be strange to find a galaxy with less blackhole mass?
- irjustin 5y agoWhen you dive into it, there are problems with this theory. PBS SpaceTime did pretty good coverage of it[0]. Each non-dark matter theory (i.e. MOND, primordial blackholes effort) usually has some pretty large glaring issue that's unsolvable. I'm all game to refute dark-matter but still waiting for the contender that fits the observations[1]. [0] https://www.youtube.com/watch?v=qy8MdewY_TY https://www.youtube.com/watch?v=qy8MdewY_TY [1] https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...
- awb 5y agoOr, maybe dark matter and black holes are the same thing? https://www.sciencedaily.com/releases/2021/12/211220120813.htm https://www.sciencedaily.com/releases/2021/12/211220120813.h... Maybe these disperse galaxies have little to no black holes?
- XorNot 5y agoThe big problem with that hypothesis is that you have to explain why we see so little matter falling into these blackholes and they don't seem to: dark matter you can basically pass right through and have your trajectory disrupted a little, whereas a black hole once you hit it becomes a zone of infinite attraction - i.e. a black hole passing near a star is pretty likely to setup in orbit around it because any matter that gets close enough inevitably gets sucked in. Dark matter - having finite mass and no singularity - doesn't have this problem.
- Enginerrrd 5y agoIt doesn't matter, you are assuming a dark matter theory to begin with here. You are just assuming the dark matter is made up of black holes. This is an established idea of dark matter that has been given serious attention at times, though it's never been a leading candidate that I know of.
- ncmncm 5y agoDark matter made out of black holes would be upsetting to cosmologists, because they need for the universe to be only 4% "hadronic" matter. If the black holes are made of hadrons, then there is too much of that. But maybe the stock of primordial black holes started as Dark Matter particles. They then have the problem of explaining why so much of the Dark Matter started out in or close to a black hole. Maybe each Dark Matter particle is so dense it is a black hole all by itself? You have to be careful talking about what a black hole is made of. Stuff that falls in only has a description before it falls in. After it falls in, it can't be thought of as made out of anything, anymore. So if black holes were part of the initial universe, there is no saying which flavor of matter they represent. After things have settled down, these black holes could proceed to swallow any amount of regular matter, and any amount of "other stuff", if there is any. The assumption is generally that the "other stuff" has difficulty cooling off enough to be sucked wholesale into black holes, unlike regular matter that is happy to radiate away a big fraction of its energy as photons, leaving the rest able to collapse and form planets and stars, and thence perhaps be gathered into the nearest black hole. Meanwhile, the Dark Matter can mill around a black hole's vicinity indefinitely, maybe swooping close and getting ejected, but otherwise hardly bothered unless it blunders straight in.
- puffoflogic 5y agoI can't help but think of this very similar argument from HHGG (Douglas Adams): > Now it is such a bizarrely improbable coincidence that anything so mindbogglingly useful could have evolved purely by chance that some thinkers have chosen it to see it as a final and clinching proof of the non-existence of God. > The argument goes something like this: "I refuse to prove that I exist," says God, "for proof denies faith, and without faith I am nothing." > "But," says Man, "the Babel fish is a dead giveaway isn't it? It could not have evolved by chance. It proves you exist, and therefore, by your own arguments, you don't. QED." > "Oh dear," says God, "I hadn't thought of that," and promptly vanishes in a puff of logic. > "Oh, that was easy," says Man, and for an encore goes on to prove that black is white and gets killed on the next zebra crossing
- laumars 5y agoNearly every page in that book contains a genius quote. Such a good book
- ncmncm 5y agoIn other words, any evidence reported can only ever be interpreted to support Dark Matter. Seems to have too much mass? Dark Matter. Seems not to have too much mass? Dark Matter! A theory compatible with all conceivable evidence is meaningless. But maybe you didn't mean to say what you said.
- 8note 5y agoLots of the same resulting gravity for the same amount of observed matter suggests no dark matter. The more variation we see, the more likely that we're missing something
- kadoban 5y agoFeel free to propose an alternative explanation for this datapoint that's consistent with the rest as well.
- nostrebored 5y agoThe beauty of skepticism is that you don’t have to. This is an unreasonable burden of proof.
- benreesman 5y agoI mean, it’s better than: we can’t make String Theory predict anything…still nothing…science was wrong the whole time!
- vba616 5y agoThe available evidence includes many galaxies. "Seems to have too much mass" and "seems not to have too much mass" are both things that a correct theory has to explain. It's not sufficient to explain one at a time with kettle logic. Are you sure that any theory which allows for different things happening in different locations is inherently and unavoidably meaningless? Or is there something else that bothers you? I would like to understand why people such as yourself viscerally dismiss "dark matter" as if it were inconsistent with all science and rational thinking. Can you describe what's obviously special about it compared to all of the other things we can't see and take for granted? I assume we all accept the idea of matter that is invisible because it is cold and doesn't emit light...there's nothing particularly weird about asteroids and planets adrift far from stars that are "dark" due to lack of light, right? And electromagnetic radiation that we can't see is everyday stuff right? Like radio waves, and ultraviolet, and infrared, and x-rays? There's nothing weird about feeling the heat from something and not seeing the rays that carry it, correct? Everybody knows that nuclear radiation can kill you while being completely invisible, right? Gamma rays and neutrons don't look like anything. It's not weird that gases are invisible, like air. We all accept that air is around us and without contaminants is transparent. Gravity itself is invisible, and we take for granted that it exists. Gravitational wave detectors correlate sometimes with visual events, meaning the invisible gravitational waves have definitely traveled over unimaginable distances, passing through everything in the way without being affected. Neutrino detectors are old hat at this point. Neutrinos are incredibly unlikely to interact with what they pass through, so the detectors have gigantic amounts of material underground with detectors to see mindbogglingly rare flashes when a neutrino hits something. I think neutrinos may still be a candidate for dark matter. Sound is normally invisible, even though it is the motion of atoms around us which are real. Heat is invisible within a normal range. If a frying pan is 350 degrees it doesn't look different from 250 degrees. Electricity in a wire is invisible. Magnetic fields are invisible. Even lasers made of light itself are invisible if you look from the side and there's no dust in the air. Are any of these besides "dark matter" unbelievable?
- marcosdumay 5y agoWell, unless people get a viable theory explaining why the thing that is 80% of the Universe is missing on an entire galaxy, it challenges dark matter theories too. They are just some not very well known bodies that may challenge all of our assumptions, or may fall in line with our theories once we get to know more about them. What is well, the best you can expect from new observations.
- ericbarrett 5y agoIn fact it's evidence for dark matter, as we can now point to areas of space where we see it and areas where we don't; and baryons and leptons in those spaces behave as expected in the presence or absence of something with mass that doesn't otherwise interact, which is all that "dark matter" is as a broad theory.
- Koshkin 5y agoA galaxy is small compared to the Universe, so why would it be difficult to imagine that some of the galaxies were formed in regions devoid of dark matter?
- marcosdumay 5y agoBecause cosmology has a lot of evidence that the clusters of mater we have around were created by gravity alone, and gravity acts the same way on normal matter and dark matter. And if you expect a difference on particle genesis at the beginning of the Universe, it would be a major kind of anisotropy that is against much more than just the cosmology theories. Why is it so hard to accept that we may have no good theory for explaining a galaxy without dark matter?
- Koshkin 5y agoDark matter is not required for the formation of a galaxy, so amongst trillions of galaxies there may be some that have none inside them?
- vba616 5y ago
- tynpeddler 5y agoLast I read, such galaxies aren't a problem for MOND. MOND doesn't say that all matter in the universe is detectable, it just says that due to modified gravity, the apparent missing mass we observe can be explained by dust or some other cold baryons. MOND has explained similar galaxies in the past by postulating that there's very little dust.
- 8note 5y agoIf mond has dark matter anyways, why bother with mond?
- vardump 5y agoI think (s)he meant some kind of theory that's basically MOND, but with still some dark matter included. Dark matter and MOND might not exclude each other.
- throwawayffffas 5y agoBecause that could be the actual physical reality, gravity or inertia might not fully work how we think it does, and there might also be some stuff we can't detect. The objective should be to make correct predictions and understand how things actually work not for one side to win the argument.
- tynpeddler 5y agoThe term "Dark Matter" refers to an unknown kind of matter that interacts gravitationally but not electromagnetically. My comment is referring to cool baryonic matter, ie the same kind of matter than you and I are made of. In other words, MOND explains galaxy dynamics by modifying gravity and postulating the existence of small amounts of conventional matter that are below the detection threshold of our telescopes. Dark matter theories explain galaxy dynamics by postulating the existence of huge amounts of an exotic matter (alongside some conventional dust) that we can't see and haven't been able to detect in any experiments.
- faangiq 5y agoClearly not a legit answer.
- jug 5y agoMOND theories seem by far most popular on Internet forums nowadays! There are just so many "But what if they are wrong and dark matter doesn't even exist, if they can't explain it..." that it's quite overwhelming! It is not as simple as this. In fact, modifying gravity to fit the set of observations here is so hard that most scientists have just given up on it by now. I mean, it looks like dark matter comes and goes, clumps together... But I think this is ultimately a good thing. I'm pretty sure we're on the right path here with several decent explanations to dark matter, like sterile neutrinos or axions. Some candidates are testable today and bright people are working on it right now. Dark matter feels like one of those things that may suddenly see a path towards being solved within years, but that it might also take much longer time.
- dnautics 5y ago> In fact, modifying gravity to fit the set of observations here is so hard that most scientists have just given up on it by now. Wait what? I was under the impression that more scientists are turning to MOND than ever. I have a hot take prediction that in 5-10 years MOND will be the preferred explanation over LCDM. Sterile neutrinos were ruled out late last year. Axions were ruled out a decade? ago... Though I guess "revised" axions are still on the table; IIRC last year another huge swath of axion masses were ruled out.
- choeger 5y agoStill, dark matter as a theory, or at least as the name of a theory, has the big problem to predict something that is conveniently invisible. A proponent of MOND could probably declare the existence of some form of dark exotic matter in galaxies that behave "Newtonian". My point is: As long as a dark matter theory doesn't make predictions that can be experimented upon, we will always have these kind of interpretation games. Side question: What would actually distribute dark matter throughout the universe in the first place? If it is only affected by gravity and there is much more dark matter than visible matter, shouldn't it be forming a huge invisible ball somewhere?
- gliptic 5y agoSo they would propose dark matter+MOND? I will just throw out MOND then because it's superfluous.
- choeger 5y agoIt's an interesting philosophical question whether a more complex theory and a small amount of invisible matter or a simpler theory and a large amount of invisible matter is the simpler combination. In any case, I think that any modified theory of gravitation has to fight an uphill battle because it seems to be easy to come up with a configuration of dark matter that explains any observation. That's my biggest grievance with DM, btw.: It doesn't make any predictions that could be used to falsify it, at least not in a sense that I could intuitively understand it. Effectively, it appears to act as a completely free parameter, distributed however we need it at the moment.
- andreareina 5y agoSomething that is only affected by gravity won't tend to collect in one place because whatever gravity would attract it there also gives it momentum to get out. You need collisions to get things to lose momentum to heat and clump together. Normal matter has collisions via interaction with the electromagnetic field, dark matter doesn't. Neutrinos also don't clump together, but are disqualified as dark matter candidates due to there not being enough of them; something about the relative abundance of different fundamental particles (e.g. electrons and neutrons) being important to get the universe we observer today. ETA: modified gravity doesn't give anything we can experiment on either; the difference of TeVeS[1] vs GR are not detectable at non-cosmological scales. [1] https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93...
- Maursault 5y agoPersonally I've always felt both the Dark Matter hypothesis and MOND are dead wrong, and that everyone is in the muck messing around with these as though one or the other are the only possible explanations. Dark Matter has not remotely been proven, not even in the remotest sense, and yet it is a fixture in the paradigm of modern cosmology. It's as messed up as when the luminiferous ether was held similarly as incontrovertible science... also without a shred of proof. When someone actually detects Dark Matter is when I will give that hypothesis any attention, while MOND is equally unpersuasive. Solving the "missing matter" problem is going to happen, and it's either going to be an adjustment of the data (or more precise or better measurements), or it'll be that we just didn't have the whole picture, and once we do, it's going to be a "duh" moment (which I suspect is going to be that there was never any missing matter, and everything and the kitchen sink thrown in that Dark Matter is invented to explain will be explained without the need for Dark Matter).
- mhh__ 5y agoNo one has ruled out dark matter either. If you are fumbling around in new physics you have to start somewhere, expressive power isn't a bad place to start.
- Maursault 5y agoIf we wait a bit, Occam will be along to help us here with what is more likely, exotic matter we can't detect, or a simpler solution we have not yet conceived. When we knew that light was a wave, and we knew that waves only travel through a medium, we were certain about the existence of luminiferous aether, which also misled hoards of young physicists that would have been better off, making Physics as a study better off, without every new entrant into the field being misled.