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Astrophysicist here. This is the same shtick from from a known anti-DM guy. A few points. * DM particle haven't yet been found. * The current best theory of

Astrophysicist here. This is the same shtick from from a known anti-DM guy. A few points. * DM particle haven't yet been found. * The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations. * There is a broad variety of Dark matter models that are consistent with simulations. * It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO. * There are a few cases where you can find tensions with the existing DM based paradigm. I.e. the is test pointed in the article involving galactic bar in the paper "Fast galaxy bars continue to challenge standard cosmology". Note the toned down title. In this paper they just compared one specific feature of galactic disks in the simulations to the data and show that it doesn't match. I'd argue there are many reasons that could be the case that doesnt' involve killing DM. The same applies to other tensions. * A final point. Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background.

replygirl 4y ago
dark matter - the new epicycle?
- croes 4y ago
  Only if we find a better explanation.
  - kloch 4y ago
    This is a constant bug/feature of human thought. When we try to see beyond the limits of our technology we find it irresistible promote explanations that can't be proved yet. Often these placeholder theories turn out to be complete nonsense (epicycles). Often they are incredibly convincing (Dark Matter), and sometimes they even turn out to be mostly correct - so this process is generally a good thing. Where we run into trouble is by assigning too much credence (or too little skepticism) for that which has not been proven yet. This can be counter-productive as it may stifle research into other areas (in this case gravity). When we take the "best current theory" and deride skeptics of it to the point that it discourages other areas of investigation, that's a bug, not a feature. Our current understanding of gravity is woefully inadequate, as shown by our inability to reproducibly measure the gravitational constant as well as we should be able to with current technology. The flyby anomaly is another example where DM does not help. I hope that the current popularity of DM does not stifle research into gravity.
    - Georgelemental 4y ago
      Epicycles had low predictive power and was needlessly complicated compared to later theories, but it wasn't "nonsense"; the predictions the theory made about the relative positions of the Earth and planets were pretty accurate.
      - deleted 4y ago
        [deleted]
      - SAI_Peregrinus 4y ago
        And thanks to our knowledge of Fourier Series we could come up with an even more accurate epicyclic model today! It's only when you try to explain tides or Lagrange points that a geocentric epicyclic model falls apart.
- mort96 4y ago
  Or dark matter - the new germ theory of disease. Both introduce a new, hard-to-observe-with-current-technology entity which explains observations better than any other theory we've came up with. MOND doesn't explain structure formations or lower rotational speed galaxies; miasma didn't explain non-airborne illness. It's incredibly hard to know until we've either observed DM particles or made good alternative hypothesises which also fit the data.
  - glitchc 4y ago
    Dark matter sounds suspiciously like aether to non-experts. I’m not saying it is, but do recall that the concept of aether filling the universe was an immensely popular one prior to the turn of the last century, so not too long ago. We don’t believe it any more, it’s hogwash of course.
    - Ekaros 4y ago
      As non-expert I have always wondered about Dark Matter and particle physics. So just how many MeV do we need? And how much would that cost?
      - SAI_Peregrinus 4y ago
        Lots, and more than we can afford, assuming particle colliders could even produce it. It's popular to look for types of Dark Matter that interact very weakly with forces other than gravity (WIMPs). That's a lot like the joke of the man searching for his keys under a street light even though he dropped them away from the light, because the light is better there. It's perfectly possible that Dark Matter ONLY interacts gravitationally, and so will never be detected by any of the current approaches. Detecting the gravitational force of an elementary particle is ridiculously difficult, well beyond the sensitivity of any current detectors. Even if we could, there would be a huge noise floor from all the normal matter around!
        davrosthedalek 4y ago
        I thought that a purely gravitationally interacting dark matter would not produce the right clumping, i.e. there must be an additional force -- either one of the known ones, or a new one.
- cwp 4y ago
  Not a physicist, but it feels more like the orbit of Mercury. Newtonian physics didn't quite predict it correctly, but it was still the best theory available until relativity came along. That seems to be the thrust of the GP comment: "you got a better idea?"
  - throwawaymaths 4y ago
    Forget GR. I got a better idea for the orbit of mercury. A ball of dark matter pulling the planet around. Does this seem silly?
- sega_sai 4y ago
  I have personally no problems with epicycles. Those were effectively the Fourier decompositions of the elliptical orbits while viewed from the Earth. At some point we found a simpler and a more predictive theory and that naturally produces the right shapes (kepler laws + classical gravitational dynamics) and we choose that instead. I think many people would be happy (myself included) if there is theory that wouldn't require dark matter and dark energy. But at the moment there is none and to me (and many astrophysicsts) the universe with dark matter is the best description we've found so far of what we observe.
  - replygirl 4y ago
    from the angle of "explaining observations" a fundamentally incorrect theory can work well. but it breaks down as soon as you interact with the object of the theory in new ways. in epicycles' case that means traveling to a satellite that doesn't orbit the earth. usually the theory is disproven before it kills people, but you know, possibility's there
    - DubiousPusher 4y ago
      I think the OP's point is that we should understand our current models of natural phenomenon to be just that, models. Outside of their predictive capabilities we should remain humble about how much "truth" they contain and be skeptical of their ability to tell us what's "really going on". Kepler and Newton's models produce better predictions over a long period than Ptolemaic models, which is great. But it's a bit of delusion to think that necessarily means they are analogous to the cosmological mechanisms that produce orbital mechanics.
      - replygirl 4y ago
        > I think the OP's point is that we should understand our current models of natural phenomenon to be just that, models this was my point with the initial epicycle comparison
    - sega_sai 4y ago
      I don't believe in fundamentally correct theories. All theories have limitations. They work until they do not. Is general relativity fundamentally correct ? I wouldn't call it that (others may disagree), because it doesn't explain quantum effects or very early universe. So to me theories are just tools that explain what we see and predict the future.
- bsder 4y ago
  Epicycles aren't wrong. In fact, the Copernican model of the solar system is less correct than the Ptolemaic epicycle model that was being used because Copernicus still viewed orbits as circular. More to the point, since epicycles are just a Fourier decomposition, they're actually perfectly correct. The problem, though, is that they are a nightmare to compute with unless you have a modern computer. Once Kepler decided to use an ellipse to model solar orbits; however, things were vastly easier to compute. At that point, the epicycle model got swiftly pushed aside.
  - throwawaymaths 4y ago
    Epicycles aren't wrong, they're not even wrong. You could model an orbit that looks like homer Simpson's face with epicycles, but the ancients weren't sophisticated enough to understand that. The problem is that the technique of epicycles this has diminishing explanatory power, as you add more free variables (how many epicycles you are "allowed" to have. The reason why LCDM gets called epicycles is that there is charitably one free parameter per galaxy (dm: bm ratio -- but actually more, because it's an arbitrary distribution)... So the fact that it has high explanatory power is unsurprising. And what's should be dismaying is the number of things it can't explain, given just how many free parameters you're allowed to have in a DM model.
mixmastamyk 4y ago
Start with an ad hominem, continue with this: > Without such theory, claiming DM doesn't exist is simply stupid IMO. Expressing flaws in an idea does not require a better one in any way. The search continues.
- candiodari 4y ago
  Expressing flaws doesn't require a better theory, no. By all means, do that. Expecting others to give up their theories that work well for 99% of the problem, or even only work well for the 10% they care about, just because there's a problem with it DOES require something better AND a reason to switch. For example, architects (technically stability engineers. Architects don't really design buildings except in the artistic sense) are still simulating Newtonian dynamics on plates that fly through empty space held up by nothing. You want them to change? No problem, but you'll have to make something better and point out why they'd want to use something better ... but, truthfully, it works pretty well.
  - mixmastamyk 4y ago
    The piece says, thing X doesn't exist, here's how it was already figured out, etc. We should find something better. I don't think it says we aren't allowed to keep it around when it is sometimes useful as a thought device. It also reduces the religious aspect, demoting DM to simply a tool used in specific situations, "we don't have anything better at the moment."
xnoreq 4y ago
May I get your thoughts on this idea of mine: We all know the marbles on a rubber sheet experiment. Masses produce "dents" in spacetime that "pull" on other objects. Couldn't it be that that these dents are balanced out by bulges between the masses, that would additionally "push" smaller masses (like solar systems) towards larger masses (like the centers of galaxies)? In an experiment it would look like this: the rubber sheet seals the top of a container filled with water. Now if you push down at some point, the increasing water pressure will push up all around that point. If you now added a marble onto that bulge, it would roll down a steeper angle than it would in the normal experiment ... just like if there was extra mass ("dark matter").
- Ygg2 4y ago
  Another fun possiblity I remember from Universe in a nutshell, is that Dark matter is gravitation leaking from other branes. In your analogy rubber sheet analogy, it's about stacking another rubber layer on top of current one. It has no prediction powers , but it would explain things like why gravity is so weak and why we can't detect it.
  - ggm 4y ago
    Analogies are great for explaining versions of complex theories, but they rarely prove or disprove the theory because they aren't a 1:1 mapping to the theory, they're analogous. Similar. But similar is not "is the same"
- rrauenza 4y ago
  Sounds like a waterbed… and there are google hits of other people with the same question! You are not alone. https://science.slashdot.org/story/20/11/22/1714220/to-explain-away-dark-matter-gravity-would-have-to-be-really-weird https://science.slashdot.org/story/20/11/22/1714220/to-expla...
- ninkendo 4y ago
  Sabine Hoffensteder had apparently the same intuition as you and set out to develop a theory of dark matter as being anti-gravitating matter. It didn’t work out… https://backreaction.blogspot.com/2021/11/does-anti-gravity-explain-dark-energy.html?m=1 https://backreaction.blogspot.com/2021/11/does-anti-gravity-... > Because I had this idea that anti-gravitating matter could surround normal galaxies and push in on them. Which would create an additional force that looks much like dark matter. Normally the excess force we observe is believed to be caused by more positive mass inside and around the galaxies. But aren’t those situations very similar? More positive mass inside, or negative mass outside pushing in? And if you remember, the important thing about dark energy is that it has negative pressure. Certainly if you have negative energy you can also get negative pressure somehow. > So using anti-gravitating matter to explain dark matter and dark energy sounds good at first sight. But at second sight neither of those ideas work. The idea that galaxies would be surrounded by anti-gravitating matter doesn’t work because such an arrangement would be dramatically unstable. Remember the anti-gravitating stuff wants to clump just like normal matter. It wouldn’t enclose galaxies of normal matter, it would just form its own galaxies. So getting anti-gravity to explain dark matter doesn’t work even for galaxies, and that’s leaving aside all the other evidence for dark matter. She goes into a lot more detail in the video (the blog post is just a transcript), but hearing her describe the thought process is interesting because she spent a lot of time on this theory (I’m assuming after she got her phd), so you’re in good company for coming to this initial conclusion at least :)
_9hey 4y ago
My general relativity professor claimed the need to postulate the existence of dark matter disappears when using basis vectors for derivations. To be honest, as the only undergraduate in the class, it was mostly way over my head. Hopefully I'm not misrepresenting his views, I think the paper is DOI:10.1140/epjp/i2011-11032-x
- JadeNB 4y ago
  Clickable: Goedecke - Global embedding via coordinate basis vectors; http://doi.org/10.1140/epjp/i2011-11032-x http://doi.org/10.1140/epjp/i2011-11032-x. I am skeptical of the idea that the need for dark matter can be "conventioned away", but, while I am a mathematician, I'm not even glancingly familiar with the physics.
  - hither_shores 4y ago
    I don't have access to the paper right now, but the abstract is off to a very bad start. > any curved Riemannian space must be a subspace of a larger flat host space This is not even wrong. Yes, you can always embed a Riemannian manifold in a Euclidean space. In fact, there are infinitely many ways to do so. But this is extra structure that you can choose to impose, if you want to - it's not intrinsic. You might as well say that every string must be a substring of a larger "host" string. > the 4D Riemann-Christoffel curvature tensor is identically equal to a geometrical tensor associated with the complementary subspace of the host space Impossibly vague. Yes, sometimes things are equal to other things. What tensor, and associated how? > Einstein’s field equations are automatically geometrized, with the stress-energy tensor expressed in terms of the contracted complementary tensor This is a tautology. The Einstein field equations relate the Ricci tensor to the metric and the stress energy tensor, and the Ricci tensor is a contraction of the Riemann curvature tensor. This remains true even if you rename the Riemann curvature tensor "the complementary tensor". But the deeper problem is this: the entire point of general relativity is that physics does not, in fact, care what coordinates we use. And this is built into the mathematical structure of the theory - there can't be any such thing as the "the coordinate basis vector approach to tensor calculus", because tensors (properly speaking: tensor fields) are, by definition, coordinate-independent objects.
nickpinkston 4y ago
Has anyone in the MoND world looked into the James Webb observations that seem to contradict our current theories of galaxy formation in the early universe? ie the observations seem to show more mature structures than predicted, we're finding. I've been wondering if some MoND / non-DM kind of theory may better explain these new JWST observations. - I'm not anything close to an astrophysicist, so give me some rope :-)
- Covzire 4y ago
  I believe 99.99% of new discoveries in astrophysics create more problems for cosmic models than they had before. Pluto shocked the scientific world in 2015 when we got up close photos of what was supposed to be a cold frozen stone, being so small and so far from the sun, but it's still geologically active, something no model could account for at the time, nor since afaik.
- arcastroe 4y ago
  This made me dig up an old comment I left 8 months ago regarding JWST predictions https://news.ycombinator.com/item?id=29488993 https://news.ycombinator.com/item?id=29488993 > the observations seem to show more mature structures than predicted Thank you for closing the loop for me! (would you happen to have a source?)
  - nickpinkston 4y ago
    Nice recall! Thanks for posting this.
- aeneasmackenzie 4y ago
  The MOND guys are aware: https://tritonstation.com/2022/07/21/jwst-twitter-bender/ https://tritonstation.com/2022/07/21/jwst-twitter-bender/ Specifically this paper[0] is mentioned as predicting it > Remarkably, the data roughly follow the green line, which is an L* galaxy magically put in place at the inconceivably high redshift of z=10. Galaxies seem to have gotten big impossibly early. This is why you see us astronomers flipping our lids at the JWST results. Can’t happen. > Except that it can, and was predicted[0] to do so by Bob Sanders a quarter century ago: “Objects of galaxy mass are the first virialized objects to form (by z=10) and larger structure develops rapidly.” 0: https://arxiv.org/abs/astro-ph/9710335 https://arxiv.org/abs/astro-ph/9710335
worik 4y ago
> * DM particle haven't yet been found. We do not know that dark matter exists > * The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations. The stories we like to tell require dark matter. Otherwise our stories are wrong > * There is a broad variety of Dark matter models that are consistent with simulations. We have a lot of stories using dark matter. We really like it > * It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO. Our stories are just stories but it is all we have > * There are a few cases where you can find tensions with the existing DM based paradigm. Our stories are not internally consistent. > * A final point. Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background. We really like stories with dark matter I love Astrophysics! Pure math with constraints. Do not confuse it with reality
- giulianob 4y ago
  Wat... "Like" implies preference. You're using the word "story" to imply someone pulled it out of their imagination. What they said was "we currently have no plausible theories that doesn't require dark matter". These theories have to be consistent with other theories and observations.
  - xqcgrek2 4y ago
    plausibility is a bayesian prior Different people have different priors. For some dark matter is more unlikely.
    - ggm 4y ago
      If they don't agree with the hole in the other theories which use DM to reconcile observations, thats an issue worth exploring. If they just don't like the DM label on those holes, its a fine point. What do you want to call the emerging shape in theory which is a hole, in otherwise very fine, testable propositions?
pdonis 4y ago
> This is the same shtick from from a known anti-DM guy. Whether it's a "shtick" or not, his basic point seems like one that is worthy of some kind of substantive counter argument, if one exists. His basic point is that small galaxies orbiting larger galaxies should experience Chandrasekhar dynamical friction if dark matter exists, but no such thing is observed. Therefore dark matter cannot exist. Has any astrophysicist who supports the dark matter hypothesis published a counter argument to this? Edit: A published paper of Kroupa's that gives much more detail about his point is here: https://arxiv.org/pdf/1406.4860.pdf https://arxiv.org/pdf/1406.4860.pdf
- rob_c 4y ago
  > Therefore dark matter cannot exist. How about therefore thing we've yet to observe if it exists fails to behave as we naively expect. This is a grand claim from one piece of work and feels more like a straw man grab at further research funding rather than finding the heffalon at the LHC https://arxiv.org/abs/1303.7367 https://arxiv.org/abs/1303.7367 Or in other words Occam's razor says you have a stripey horse in northern Europe rather than a zebra.
  - pdonis 4y ago
    > How about therefore thing we've yet to observe if it exists fails to behave as we naively expect. As I understand it, the "behavior" of dark matter in Kroupa's analysis is the behavior that is claimed for it by dark matter proponents. So if dark matter doesn't behave the way dark matter proponents say it does, the burden would be on the dark matter proponents to modify their models to say how it does behave.
    - rob_c 4y ago
      Trust me this is a straw man picking at a hole of a yet incomplete theory to claim the whole deck of cards is invalid. AFAIK none of these alternatives offer a serious explanation of gravitational lensing for instance (yet, I'm happy for someone to make a theoretical breakthrough that explains away the need for an unknown new particle, but that just opens more questions around CP violation and so on)
mynegation 4y ago
So what would be a DM theory explanation of a lack of Chandrasekhar friction?
cratermoon 4y ago
This Pavel Kroupa guy is popping up a lot across the futurist and credulous science fandom forums. To cut to the chase, his theory only applies to galactic dynamics and it doesn't even do a very good job at it. There are many other aspects that MOND doesn't address. Kroupa doesn't even bring interesting critiques of DM to the table, just claims about his theories and then concluding DM doesn't exist.
- naasking 4y ago
  > To cut to the chase, his theory only applies to galactic dynamics and it doesn't even do a very good job at it. There are many other aspects that MOND doesn't address. There are many more that DM doesn't address, or only addressed by extending it with parameters to fine-tune the result, a process which can also be done with MOND. I hate to keep posting this, but it's a good thorough review of the evidence for/against both DM and MOND, and DM does not fair as well as you imply: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm
  - cratermoon 4y ago
    I'd love to see some papers from more, shall we say, impactful peer-reviewed journals.
naikrovek 4y ago
I feel like scientists today have forgotten a fundamental truth about science and understanding: We can never _know_ when we are right about our theories. We can only ever _know_ when we are wrong about them. New evidence can arrive at any time, and that evidence will either bolster our theories, or prove one or more theories incorrect. New evidence is always arriving. I mean do we really believe that we know everything about this? Somehow, we A) know we have large knowledge gaps, AND B) believe that we understand things we have unanswered questions about. We have a LOT of confidence about dark matter and exactly zero direct evidence. It seems much more likely to me (I am not a scientist) that we are wrong about things that we assume to be hard facts and those error(s) have artificially produced the need for dark matter in order for things to make sense.
- AaronFriel 4y ago
  > We can only ever _know_ when we are wrong about them. I think this is what the OP is saying though: There are many ideas about what dark matter is or could be, but astrophysicists model the early universe using only baryonic matter, the results _are wrong_. As a theory, it's quite simple: in its absence, the physics doesn't work. If you add matter that behaves in a particular way, simulations align to observation. What is dark matter? shrug I don't think we can call any answer to that question a theory yet, we don't have any way to falsify it. We have many hypotheses, but those aren't theories.
  - naasking 4y ago
    > As a theory, it's quite simple: in its absence, the physics doesn't work. If you add matter that behaves in a particular way, simulations align to observation. They actually don't, numerous extra parameters have to be added for DM to account for observations. DM was invented to fix one problem with observation, and 15 more have cropped up which don't fit.
    - AaronFriel 4y ago
      What are those extra parameters?
spfzero 4y ago
I don't understand this: "Without such theory, claiming DM doesn't exist is simply stupid IMO." You are saying that without another theory to explain something, the current theory must be true because ... How does this logic work exactly? In the middle ages, there wasn't a theory that explained headaches, so dismissing the idea that we should cut hole in people's skulls to let out the evil spirits is "stupid"? Dismissing the idea that lighting is caused by Zeus being angry is "stupid" because there is no other theory yet?
- throwawaymaths 4y ago
  I mean MOND has made "surprising predictions that have come true". Here's one that seems very relevant: https://tritonstation.com/2022/01/03/what-jwst-will-see/ https://tritonstation.com/2022/01/03/what-jwst-will-see/
  - scotty79 4y ago
    Hasn't MOND been disproven by observed existance of galaxy without dark matter?
    - throwawaymaths 4y ago
      No? Those galaxies are ultradiffuse galaxies and if their distance estimates are off by about 25%, which is reasonable -- we don't have good models for distances to such cosmic entities -- then the calculation for their rotations bring them in line with expected.
- Tagbert 4y ago
  Theories are not necessarily absolutely true. They are models that describe and help us understand problems. In most (all?) cases they are approximations. We know that there are problems with them but they are the best tool we have to understand something. Until another tool comes along that works better, the current tool will sufficient. We know that General Relativity is incomplete. It cannot describe the work at high energies or very small scales. It is useful as far as it goes but it is not a perfect match. Quantum Mechanics does describe the world of the small and the hot, but we cannot apply it to the larger world. QD, too, is incomplete. We look for a new theory that can combine the features of both but it evades us, so far. Dark Matter is a similar theory. It describes a world. It seems to describe this work better than other theories like MOND. Looking for actual dark matter will let us see if DM is a good enough theory or not. If another theory can be found that is a better match, we can switch to that. In the meantime, every time we continue to try to verify/disprove DM.
deleted 4y ago
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briantakita 4y ago
What if the "dark matter" is actually ions not observable from our telescopes? Plasma Physics hold ions permeating the Universe as a premise & has physical experiments reproducable in the lab to back up their claims...And can use classical EM equations explain how the stars & galaxies work instead of some novel Physics which needs constructs such as "Dark Matter" to balance the equations...
- hither_shores 4y ago
  > What if the "dark matter" is actually ions not observable from our telescopes? We have radio telescopes that can see all the way down to a few tens of MHz. This is far colder than the CMB, and certainly much too cold for the intergalactic medium. > Plasma Physics hold ions permeating the Universe as a premise & has physical experiments reproducable in the lab to back up their claims Yes, plasmas exist. There are lots of them in space. This is completely uncontroversial. > And can use classical EM equations explain how the stars & galaxies work Absolutely not. Galaxies are firmly in the nonclassical regime.
  - briantakita 4y ago
    > We have radio telescopes that can see all the way down to a few tens of MHz. This is far colder than the CMB, and certainly much too cold for the intergalactic medium. PV = nRT What if the P is very low & V is very big...like it is in space? What would the plasma in dark discharge radiate? > Absolutely not. Galaxies are firmly in the nonclassical regime. According to some models. Do these models have reproducible experimental evidence? If not, I don't see how you can credibly be so sure of yourself... Sure, exotic & speculative math is way more fun than reconciling with physical reality. Without reproducible experimental evidence, the model is non-falsifiable (& non-provable). In contrast, the Plasma model does have physically reproducible experiments that are able to create galactic phenomena in the lab. Live physical experimentation...not computer simulations using math that has never been verified by physical experiment.
    - hither_shores 4y ago
naasking 4y ago
> * The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations. Dark matter models are fine-tuned to reproduce certain observations. These don't really count as evidence in favour of DM. > * It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO. MOND has more points in favour for it as a preferred theory than DM models. See [1]. > Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background. I'm not sure why I should find your claim that DM is "essential" as persuasive given MOND has received orders of magnitude less attention and less development than DM models. I would certainly not bet any money on MOND being incapable of reproducing these observations. [1] From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm
- alecst 4y ago
  I worked in this field for a minute (XENON1T) and I heard a few talks about MOND. We were all pretty open minded but it failed to stick because it didn’t convince us, for a number of reasons. Curious about your background here.
- elcomet 4y ago
  > Dark matter models are fine-tuned to reproduce certain observations That's the definition of physics. Models are fine-tuned to match the observation and be predictive.
  - 3um35w1l 4y ago
    sure, but you didn't get the point. just because a model can predict certain results doesn't mean its correct. early astronomers were able to very precisely predict the positions (w/ looping motions and all) of stellar bodies in the night sky despite using a geocentric model. if you make it complex enough, you can engineer quite a few models that can predict natural phenomenon pretty well.
onlyrealcuzzo 4y ago
Do people actually believe in "Dark Matter" - or do they just believe in there being something that doesn't make sense and this is what explains it?
- kadoban 4y ago
  What's the difference?
  - somenameforme 4y ago
    The former will tend towards a defensive attachment to an idea which leads to, as Planck put it, "Science [advancing] one funeral at a time." The openness of the latter leads to less certainty, but greater ability for progress. Consider the heliocentric/geocentric argument some centuries passed. Contrary to pop-science portrayals this wasn't an issue of Man against Church. The heliocentric view initially had numerous flaws and invalid assumptions. But because scientists of the time genuinely believed in the geocentric universe, they obsessed on these flaws and trying to use those to discredit the view, instead of impartially considering the idea and seeing if it could be patched and whether it would ultimately make more sense. The "right" answer, with negligible refinement, can often look much worse than the wrong answer which has had decades, if not centuries of mass refinement. And that can drive those with a "belief" to the contrary, to want to attack it. The analogs outside of science are as endless as those within it.
- tsimionescu 4y ago
  People actually believe that there is an yet-undiscovered substance that has mass but interacts very weakly (or not at all) with light, and that there are vast quantities of this substance in the universe. They call this "dark matter".
taylorius 4y ago
QI (quantized inertia) is a theory created by Mike McCulloch in the UK. It predicts galaxy rotations, including wide binaries extremely well, amongst other things. It is well worth a look.
- sethrin 4y ago
  This is a crackpot theory created by a known crackpot.
  - taylorius 4y ago
    Why? Because you say so? With a dismissal like that you ought to elaborate with some evidence. Especially since you seem to be attacking the man, along with his theory.
    - sethrin 4y ago
      He's associated with the EmDrive fraud, which should be sufficient for anyone to dismiss his ideas. https://www.forbes.com/sites/briankoberlein/2017/02/15/quantized-inertia-dark-matter-the-emdrive-and-how-to-do-science-wrong/amp/ https://www.forbes.com/sites/briankoberlein/2017/02/15/quant... https://arxiv.org/pdf/1908.01589.pdf https://arxiv.org/pdf/1908.01589.pdf I'm not here to debate with physics cranks; further replies will not be read.
scotty79 4y ago
Does this guy also reject DM models based on MACHOs? I think they expect particles to be thrown out and result with slowdown but dark objects as massive as stars wouldn't be thrown at such a huge rate to observe slowdown, am I right?
- alrlroipsp 4y ago
  There's only one way to find out. It involves reading the article. I mean, what else could you bring to the discussion by only guessing what is being discussed?
  - scotty79 4y ago
    I read it and I ask the question here because I didn't find the answer for it in the article. The article just talks about particles and dismisses them. I see nothing about DM composed of massive objects. What exactly does your comment bring to the discussion?
_0w8t 4y ago
We cannot simulate evolution of galaxies using equations of General Relativity. So at best the simulations are done using Newton mechanics with minimal corrections like accounting for finite speed of light. And often it is not even done using Newton mechanics per see, but numerical models use artificial constrains like a minimal distance between stars to avoid dealing with numerical instabilities. Yet there is no proof that such simplifications or constrains are valid on the scale of a typical galaxy. Moreover, when people managed to use full equations of General Relativity, then it was possible to account the effects previously thought to require the dark matter hypothesis. So until it is mathematically proven that the current simulations are valid approximations, I will remain very skeptical about the dark matter.
- rich_sasha 4y ago
  Like the GP, I'm not in love with DM hypothesis and wouldn't mind something else. I also used to think the same as you. But DM explains a lot of disparate effects, in ways that MOND does not. The best-well known effect is the "missing mass" in rotating galaxies. Well recently we found galaxies that look like they have no dark matter in them - i.e. they behave as you would expect from classical gravity/centripetal force arguments [1]. So while you can tweak General Relativity to match some of the effects in typical galaxies, you would then expect the results to hold in all galaxies. The "outliers" are easily explained by Dark Matter - they just don't have it for whatever reason. The simulations of Universe formation, sure have their limitations, but look much worse without DM. Galaxy clusters look like they have unexplained mass in them. Crucially, this isn't only from observing their rotational speeds (so like galaxy rotations) but also from looking at the strength of gravitational lensing they induce. This is as compared to fluid-dynamics-y calculations of gas temperature, pressure etc. Bullet cluster is another interesting case [2]. It is the result of a collision of two galaxies. In such cases, you expect (a) the stars to "fly through" each other, as they are basically point masses and only interact gravitationally, but (b) the gasses and dust to collide. In this case, most of the mass is in fact gas and dust, so it should be distributed in the middle of the cluster (having just had an inelastic collision). But in fact observed mass is where the stars are, further out of what looks like center of mass. [1] https://www.nature.com/articles/d41586-022-01410-x https://www.nature.com/articles/d41586-022-01410-x [2] https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster
jbakhos 4y ago
I would like to toss my own idea into this discussion. This idea incorporates a mechanism that would explain why gravity might slightly deviate from Newtonian and/or GR in some circumstances. This paper also adapts GR in such a way that it is consistent with galactic rotation rates, the anisotropies of the CMB, and cosmological expansion -- while showing that the simple operation of gravity is the cause of each of these phenomena. Cyclic Gravity and Cosmology (CGC) predicts that there are discrete specific sizes allowable for macro-objects. The instability of Bennu and the fact that it behaves more like loosely held scree rather than a compact mass -- is an example of a mass that is not exactly at one of the discrete allowable sizes. Please also refer to the link I included wherein I uploaded a video simulation of the formation of a solar system using this type of force law. (This is in section 18 of the paper) I would greatly appreciate any comments on this idea. Copies may be downloaded here: https://vixra.org/pdf/2203.0032v3.pdf https://vixra.org/pdf/2203.0032v3.pdf
garyclarke27 4y ago
It’s not shtick, it makes perfect sense to me - unlike your post which has no substance. Group-think does not equal truth. Just because there is no compelling alternative theory does not make the Dark Matter Theory correct. It’s probably nonsense, like many theories in Physics such as the Big Bang. Our level of intelligence and understanding, at the moment, is simply too small to comprehend the mysteries of the Universe that we live in.

replygirl 4y ago

dark matter - the new epicycle?

croes 4y ago
Only if we find a better explanation.
- kloch 4y ago
  This is a constant bug/feature of human thought. When we try to see beyond the limits of our technology we find it irresistible promote explanations that can't be proved yet. Often these placeholder theories turn out to be complete nonsense (epicycles). Often they are incredibly convincing (Dark Matter), and sometimes they even turn out to be mostly correct - so this process is generally a good thing. Where we run into trouble is by assigning too much credence (or too little skepticism) for that which has not been proven yet. This can be counter-productive as it may stifle research into other areas (in this case gravity). When we take the "best current theory" and deride skeptics of it to the point that it discourages other areas of investigation, that's a bug, not a feature. Our current understanding of gravity is woefully inadequate, as shown by our inability to reproducibly measure the gravitational constant as well as we should be able to with current technology. The flyby anomaly is another example where DM does not help. I hope that the current popularity of DM does not stifle research into gravity.
  - Georgelemental 4y ago
    Epicycles had low predictive power and was needlessly complicated compared to later theories, but it wasn't "nonsense"; the predictions the theory made about the relative positions of the Earth and planets were pretty accurate.
    - deleted 4y ago
      [deleted]
    - SAI_Peregrinus 4y ago
      And thanks to our knowledge of Fourier Series we could come up with an even more accurate epicyclic model today! It's only when you try to explain tides or Lagrange points that a geocentric epicyclic model falls apart.
mort96 4y ago
Or dark matter - the new germ theory of disease. Both introduce a new, hard-to-observe-with-current-technology entity which explains observations better than any other theory we've came up with. MOND doesn't explain structure formations or lower rotational speed galaxies; miasma didn't explain non-airborne illness. It's incredibly hard to know until we've either observed DM particles or made good alternative hypothesises which also fit the data.
- glitchc 4y ago
  Dark matter sounds suspiciously like aether to non-experts. I’m not saying it is, but do recall that the concept of aether filling the universe was an immensely popular one prior to the turn of the last century, so not too long ago. We don’t believe it any more, it’s hogwash of course.
  - Ekaros 4y ago
    As non-expert I have always wondered about Dark Matter and particle physics. So just how many MeV do we need? And how much would that cost?
    - SAI_Peregrinus 4y ago
      Lots, and more than we can afford, assuming particle colliders could even produce it. It's popular to look for types of Dark Matter that interact very weakly with forces other than gravity (WIMPs). That's a lot like the joke of the man searching for his keys under a street light even though he dropped them away from the light, because the light is better there. It's perfectly possible that Dark Matter ONLY interacts gravitationally, and so will never be detected by any of the current approaches. Detecting the gravitational force of an elementary particle is ridiculously difficult, well beyond the sensitivity of any current detectors. Even if we could, there would be a huge noise floor from all the normal matter around!
      - davrosthedalek 4y ago
        I thought that a purely gravitationally interacting dark matter would not produce the right clumping, i.e. there must be an additional force -- either one of the known ones, or a new one.
cwp 4y ago
Not a physicist, but it feels more like the orbit of Mercury. Newtonian physics didn't quite predict it correctly, but it was still the best theory available until relativity came along. That seems to be the thrust of the GP comment: "you got a better idea?"
- throwawaymaths 4y ago
  Forget GR. I got a better idea for the orbit of mercury. A ball of dark matter pulling the planet around. Does this seem silly?
sega_sai 4y ago
I have personally no problems with epicycles. Those were effectively the Fourier decompositions of the elliptical orbits while viewed from the Earth. At some point we found a simpler and a more predictive theory and that naturally produces the right shapes (kepler laws + classical gravitational dynamics) and we choose that instead. I think many people would be happy (myself included) if there is theory that wouldn't require dark matter and dark energy. But at the moment there is none and to me (and many astrophysicsts) the universe with dark matter is the best description we've found so far of what we observe.
- replygirl 4y ago
  from the angle of "explaining observations" a fundamentally incorrect theory can work well. but it breaks down as soon as you interact with the object of the theory in new ways. in epicycles' case that means traveling to a satellite that doesn't orbit the earth. usually the theory is disproven before it kills people, but you know, possibility's there
  - DubiousPusher 4y ago
    I think the OP's point is that we should understand our current models of natural phenomenon to be just that, models. Outside of their predictive capabilities we should remain humble about how much "truth" they contain and be skeptical of their ability to tell us what's "really going on". Kepler and Newton's models produce better predictions over a long period than Ptolemaic models, which is great. But it's a bit of delusion to think that necessarily means they are analogous to the cosmological mechanisms that produce orbital mechanics.
    - replygirl 4y ago
      > I think the OP's point is that we should understand our current models of natural phenomenon to be just that, models this was my point with the initial epicycle comparison
  - sega_sai 4y ago
    I don't believe in fundamentally correct theories. All theories have limitations. They work until they do not. Is general relativity fundamentally correct ? I wouldn't call it that (others may disagree), because it doesn't explain quantum effects or very early universe. So to me theories are just tools that explain what we see and predict the future.
bsder 4y ago
Epicycles aren't wrong. In fact, the Copernican model of the solar system is less correct than the Ptolemaic epicycle model that was being used because Copernicus still viewed orbits as circular. More to the point, since epicycles are just a Fourier decomposition, they're actually perfectly correct. The problem, though, is that they are a nightmare to compute with unless you have a modern computer. Once Kepler decided to use an ellipse to model solar orbits; however, things were vastly easier to compute. At that point, the epicycle model got swiftly pushed aside.
- throwawaymaths 4y ago
  Epicycles aren't wrong, they're not even wrong. You could model an orbit that looks like homer Simpson's face with epicycles, but the ancients weren't sophisticated enough to understand that. The problem is that the technique of epicycles this has diminishing explanatory power, as you add more free variables (how many epicycles you are "allowed" to have. The reason why LCDM gets called epicycles is that there is charitably one free parameter per galaxy (dm: bm ratio -- but actually more, because it's an arbitrary distribution)... So the fact that it has high explanatory power is unsurprising. And what's should be dismaying is the number of things it can't explain, given just how many free parameters you're allowed to have in a DM model.

mixmastamyk 4y ago

Start with an ad hominem, continue with this: > Without such theory, claiming DM doesn't exist is simply stupid IMO. Expressing flaws in an idea does not require a better one in any way. The search continues.

candiodari 4y ago
Expressing flaws doesn't require a better theory, no. By all means, do that. Expecting others to give up their theories that work well for 99% of the problem, or even only work well for the 10% they care about, just because there's a problem with it DOES require something better AND a reason to switch. For example, architects (technically stability engineers. Architects don't really design buildings except in the artistic sense) are still simulating Newtonian dynamics on plates that fly through empty space held up by nothing. You want them to change? No problem, but you'll have to make something better and point out why they'd want to use something better ... but, truthfully, it works pretty well.
- mixmastamyk 4y ago
  The piece says, thing X doesn't exist, here's how it was already figured out, etc. We should find something better. I don't think it says we aren't allowed to keep it around when it is sometimes useful as a thought device. It also reduces the religious aspect, demoting DM to simply a tool used in specific situations, "we don't have anything better at the moment."

xnoreq 4y ago

May I get your thoughts on this idea of mine: We all know the marbles on a rubber sheet experiment. Masses produce "dents" in spacetime that "pull" on other objects. Couldn't it be that that these dents are balanced out by bulges between the masses, that would additionally "push" smaller masses (like solar systems) towards larger masses (like the centers of galaxies)? In an experiment it would look like this: the rubber sheet seals the top of a container filled with water. Now if you push down at some point, the increasing water pressure will push up all around that point. If you now added a marble onto that bulge, it would roll down a steeper angle than it would in the normal experiment ... just like if there was extra mass ("dark matter").

Ygg2 4y ago
Another fun possiblity I remember from Universe in a nutshell, is that Dark matter is gravitation leaking from other branes. In your analogy rubber sheet analogy, it's about stacking another rubber layer on top of current one. It has no prediction powers , but it would explain things like why gravity is so weak and why we can't detect it.
- ggm 4y ago
  Analogies are great for explaining versions of complex theories, but they rarely prove or disprove the theory because they aren't a 1:1 mapping to the theory, they're analogous. Similar. But similar is not "is the same"
rrauenza 4y ago
Sounds like a waterbed… and there are google hits of other people with the same question! You are not alone. https://science.slashdot.org/story/20/11/22/1714220/to-explain-away-dark-matter-gravity-would-have-to-be-really-weird https://science.slashdot.org/story/20/11/22/1714220/to-expla...
ninkendo 4y ago
Sabine Hoffensteder had apparently the same intuition as you and set out to develop a theory of dark matter as being anti-gravitating matter. It didn’t work out… https://backreaction.blogspot.com/2021/11/does-anti-gravity-explain-dark-energy.html?m=1 https://backreaction.blogspot.com/2021/11/does-anti-gravity-... > Because I had this idea that anti-gravitating matter could surround normal galaxies and push in on them. Which would create an additional force that looks much like dark matter. Normally the excess force we observe is believed to be caused by more positive mass inside and around the galaxies. But aren’t those situations very similar? More positive mass inside, or negative mass outside pushing in? And if you remember, the important thing about dark energy is that it has negative pressure. Certainly if you have negative energy you can also get negative pressure somehow. > So using anti-gravitating matter to explain dark matter and dark energy sounds good at first sight. But at second sight neither of those ideas work. The idea that galaxies would be surrounded by anti-gravitating matter doesn’t work because such an arrangement would be dramatically unstable. Remember the anti-gravitating stuff wants to clump just like normal matter. It wouldn’t enclose galaxies of normal matter, it would just form its own galaxies. So getting anti-gravity to explain dark matter doesn’t work even for galaxies, and that’s leaving aside all the other evidence for dark matter. She goes into a lot more detail in the video (the blog post is just a transcript), but hearing her describe the thought process is interesting because she spent a lot of time on this theory (I’m assuming after she got her phd), so you’re in good company for coming to this initial conclusion at least :)

_9hey 4y ago

My general relativity professor claimed the need to postulate the existence of dark matter disappears when using basis vectors for derivations. To be honest, as the only undergraduate in the class, it was mostly way over my head. Hopefully I'm not misrepresenting his views, I think the paper is DOI:10.1140/epjp/i2011-11032-x

JadeNB 4y ago
Clickable: Goedecke - Global embedding via coordinate basis vectors; http://doi.org/10.1140/epjp/i2011-11032-x http://doi.org/10.1140/epjp/i2011-11032-x. I am skeptical of the idea that the need for dark matter can be "conventioned away", but, while I am a mathematician, I'm not even glancingly familiar with the physics.
- hither_shores 4y ago
  I don't have access to the paper right now, but the abstract is off to a very bad start. > any curved Riemannian space must be a subspace of a larger flat host space This is not even wrong. Yes, you can always embed a Riemannian manifold in a Euclidean space. In fact, there are infinitely many ways to do so. But this is extra structure that you can choose to impose, if you want to - it's not intrinsic. You might as well say that every string must be a substring of a larger "host" string. > the 4D Riemann-Christoffel curvature tensor is identically equal to a geometrical tensor associated with the complementary subspace of the host space Impossibly vague. Yes, sometimes things are equal to other things. What tensor, and associated how? > Einstein’s field equations are automatically geometrized, with the stress-energy tensor expressed in terms of the contracted complementary tensor This is a tautology. The Einstein field equations relate the Ricci tensor to the metric and the stress energy tensor, and the Ricci tensor is a contraction of the Riemann curvature tensor. This remains true even if you rename the Riemann curvature tensor "the complementary tensor". But the deeper problem is this: the entire point of general relativity is that physics does not, in fact, care what coordinates we use. And this is built into the mathematical structure of the theory - there can't be any such thing as the "the coordinate basis vector approach to tensor calculus", because tensors (properly speaking: tensor fields) are, by definition, coordinate-independent objects.

nickpinkston 4y ago

Has anyone in the MoND world looked into the James Webb observations that seem to contradict our current theories of galaxy formation in the early universe? ie the observations seem to show more mature structures than predicted, we're finding. I've been wondering if some MoND / non-DM kind of theory may better explain these new JWST observations. - I'm not anything close to an astrophysicist, so give me some rope :-)

Covzire 4y ago
I believe 99.99% of new discoveries in astrophysics create more problems for cosmic models than they had before. Pluto shocked the scientific world in 2015 when we got up close photos of what was supposed to be a cold frozen stone, being so small and so far from the sun, but it's still geologically active, something no model could account for at the time, nor since afaik.
arcastroe 4y ago
This made me dig up an old comment I left 8 months ago regarding JWST predictions https://news.ycombinator.com/item?id=29488993 https://news.ycombinator.com/item?id=29488993 > the observations seem to show more mature structures than predicted Thank you for closing the loop for me! (would you happen to have a source?)
- nickpinkston 4y ago
  Nice recall! Thanks for posting this.
aeneasmackenzie 4y ago
The MOND guys are aware: https://tritonstation.com/2022/07/21/jwst-twitter-bender/ https://tritonstation.com/2022/07/21/jwst-twitter-bender/ Specifically this paper[0] is mentioned as predicting it > Remarkably, the data roughly follow the green line, which is an L* galaxy magically put in place at the inconceivably high redshift of z=10. Galaxies seem to have gotten big impossibly early. This is why you see us astronomers flipping our lids at the JWST results. Can’t happen. > Except that it can, and was predicted[0] to do so by Bob Sanders a quarter century ago: “Objects of galaxy mass are the first virialized objects to form (by z=10) and larger structure develops rapidly.” 0: https://arxiv.org/abs/astro-ph/9710335 https://arxiv.org/abs/astro-ph/9710335

worik 4y ago

> * DM particle haven't yet been found. We do not know that dark matter exists > * The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations. The stories we like to tell require dark matter. Otherwise our stories are wrong > * There is a broad variety of Dark matter models that are consistent with simulations. We have a lot of stories using dark matter. We really like it > * It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO. Our stories are just stories but it is all we have > * There are a few cases where you can find tensions with the existing DM based paradigm. Our stories are not internally consistent. > * A final point. Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background. We really like stories with dark matter I love Astrophysics! Pure math with constraints. Do not confuse it with reality

giulianob 4y ago
Wat... "Like" implies preference. You're using the word "story" to imply someone pulled it out of their imagination. What they said was "we currently have no plausible theories that doesn't require dark matter". These theories have to be consistent with other theories and observations.
- xqcgrek2 4y ago
  plausibility is a bayesian prior Different people have different priors. For some dark matter is more unlikely.
  - ggm 4y ago
    If they don't agree with the hole in the other theories which use DM to reconcile observations, thats an issue worth exploring. If they just don't like the DM label on those holes, its a fine point. What do you want to call the emerging shape in theory which is a hole, in otherwise very fine, testable propositions?

pdonis 4y ago

> This is the same shtick from from a known anti-DM guy. Whether it's a "shtick" or not, his basic point seems like one that is worthy of some kind of substantive counter argument, if one exists. His basic point is that small galaxies orbiting larger galaxies should experience Chandrasekhar dynamical friction if dark matter exists, but no such thing is observed. Therefore dark matter cannot exist. Has any astrophysicist who supports the dark matter hypothesis published a counter argument to this? Edit: A published paper of Kroupa's that gives much more detail about his point is here: https://arxiv.org/pdf/1406.4860.pdf https://arxiv.org/pdf/1406.4860.pdf

rob_c 4y ago
> Therefore dark matter cannot exist. How about therefore thing we've yet to observe if it exists fails to behave as we naively expect. This is a grand claim from one piece of work and feels more like a straw man grab at further research funding rather than finding the heffalon at the LHC https://arxiv.org/abs/1303.7367 https://arxiv.org/abs/1303.7367 Or in other words Occam's razor says you have a stripey horse in northern Europe rather than a zebra.
- pdonis 4y ago
  > How about therefore thing we've yet to observe if it exists fails to behave as we naively expect. As I understand it, the "behavior" of dark matter in Kroupa's analysis is the behavior that is claimed for it by dark matter proponents. So if dark matter doesn't behave the way dark matter proponents say it does, the burden would be on the dark matter proponents to modify their models to say how it does behave.
  - rob_c 4y ago
    Trust me this is a straw man picking at a hole of a yet incomplete theory to claim the whole deck of cards is invalid. AFAIK none of these alternatives offer a serious explanation of gravitational lensing for instance (yet, I'm happy for someone to make a theoretical breakthrough that explains away the need for an unknown new particle, but that just opens more questions around CP violation and so on)

mynegation 4y ago

So what would be a DM theory explanation of a lack of Chandrasekhar friction?

cratermoon 4y ago

This Pavel Kroupa guy is popping up a lot across the futurist and credulous science fandom forums. To cut to the chase, his theory only applies to galactic dynamics and it doesn't even do a very good job at it. There are many other aspects that MOND doesn't address. Kroupa doesn't even bring interesting critiques of DM to the table, just claims about his theories and then concluding DM doesn't exist.

naasking 4y ago
> To cut to the chase, his theory only applies to galactic dynamics and it doesn't even do a very good job at it. There are many other aspects that MOND doesn't address. There are many more that DM doesn't address, or only addressed by extending it with parameters to fine-tune the result, a process which can also be done with MOND. I hate to keep posting this, but it's a good thorough review of the evidence for/against both DM and MOND, and DM does not fair as well as you imply: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm
- cratermoon 4y ago
  I'd love to see some papers from more, shall we say, impactful peer-reviewed journals.

naikrovek 4y ago

I feel like scientists today have forgotten a fundamental truth about science and understanding: We can never _know_ when we are right about our theories. We can only ever _know_ when we are wrong about them. New evidence can arrive at any time, and that evidence will either bolster our theories, or prove one or more theories incorrect. New evidence is always arriving. I mean do we really believe that we know everything about this? Somehow, we A) know we have large knowledge gaps, AND B) believe that we understand things we have unanswered questions about. We have a LOT of confidence about dark matter and exactly zero direct evidence. It seems much more likely to me (I am not a scientist) that we are wrong about things that we assume to be hard facts and those error(s) have artificially produced the need for dark matter in order for things to make sense.

AaronFriel 4y ago
> We can only ever _know_ when we are wrong about them. I think this is what the OP is saying though: There are many ideas about what dark matter is or could be, but astrophysicists model the early universe using only baryonic matter, the results _are wrong_. As a theory, it's quite simple: in its absence, the physics doesn't work. If you add matter that behaves in a particular way, simulations align to observation. What is dark matter? shrug I don't think we can call any answer to that question a theory yet, we don't have any way to falsify it. We have many hypotheses, but those aren't theories.
- naasking 4y ago
  > As a theory, it's quite simple: in its absence, the physics doesn't work. If you add matter that behaves in a particular way, simulations align to observation. They actually don't, numerous extra parameters have to be added for DM to account for observations. DM was invented to fix one problem with observation, and 15 more have cropped up which don't fit.
  - AaronFriel 4y ago
    What are those extra parameters?

spfzero 4y ago

I don't understand this: "Without such theory, claiming DM doesn't exist is simply stupid IMO." You are saying that without another theory to explain something, the current theory must be true because ... How does this logic work exactly? In the middle ages, there wasn't a theory that explained headaches, so dismissing the idea that we should cut hole in people's skulls to let out the evil spirits is "stupid"? Dismissing the idea that lighting is caused by Zeus being angry is "stupid" because there is no other theory yet?

throwawaymaths 4y ago
I mean MOND has made "surprising predictions that have come true". Here's one that seems very relevant: https://tritonstation.com/2022/01/03/what-jwst-will-see/ https://tritonstation.com/2022/01/03/what-jwst-will-see/
- scotty79 4y ago
  Hasn't MOND been disproven by observed existance of galaxy without dark matter?
  - throwawaymaths 4y ago
    No? Those galaxies are ultradiffuse galaxies and if their distance estimates are off by about 25%, which is reasonable -- we don't have good models for distances to such cosmic entities -- then the calculation for their rotations bring them in line with expected.
Tagbert 4y ago
Theories are not necessarily absolutely true. They are models that describe and help us understand problems. In most (all?) cases they are approximations. We know that there are problems with them but they are the best tool we have to understand something. Until another tool comes along that works better, the current tool will sufficient. We know that General Relativity is incomplete. It cannot describe the work at high energies or very small scales. It is useful as far as it goes but it is not a perfect match. Quantum Mechanics does describe the world of the small and the hot, but we cannot apply it to the larger world. QD, too, is incomplete. We look for a new theory that can combine the features of both but it evades us, so far. Dark Matter is a similar theory. It describes a world. It seems to describe this work better than other theories like MOND. Looking for actual dark matter will let us see if DM is a good enough theory or not. If another theory can be found that is a better match, we can switch to that. In the meantime, every time we continue to try to verify/disprove DM.

deleted 4y ago

[deleted]

briantakita 4y ago

What if the "dark matter" is actually ions not observable from our telescopes? Plasma Physics hold ions permeating the Universe as a premise & has physical experiments reproducable in the lab to back up their claims...And can use classical EM equations explain how the stars & galaxies work instead of some novel Physics which needs constructs such as "Dark Matter" to balance the equations...

hither_shores 4y ago
> What if the "dark matter" is actually ions not observable from our telescopes? We have radio telescopes that can see all the way down to a few tens of MHz. This is far colder than the CMB, and certainly much too cold for the intergalactic medium. > Plasma Physics hold ions permeating the Universe as a premise & has physical experiments reproducable in the lab to back up their claims Yes, plasmas exist. There are lots of them in space. This is completely uncontroversial. > And can use classical EM equations explain how the stars & galaxies work Absolutely not. Galaxies are firmly in the nonclassical regime.
- briantakita 4y ago
  > We have radio telescopes that can see all the way down to a few tens of MHz. This is far colder than the CMB, and certainly much too cold for the intergalactic medium. PV = nRT What if the P is very low & V is very big...like it is in space? What would the plasma in dark discharge radiate? > Absolutely not. Galaxies are firmly in the nonclassical regime. According to some models. Do these models have reproducible experimental evidence? If not, I don't see how you can credibly be so sure of yourself... Sure, exotic & speculative math is way more fun than reconciling with physical reality. Without reproducible experimental evidence, the model is non-falsifiable (& non-provable). In contrast, the Plasma model does have physically reproducible experiments that are able to create galactic phenomena in the lab. Live physical experimentation...not computer simulations using math that has never been verified by physical experiment.
  - hither_shores 4y ago

naasking 4y ago

> * The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations. Dark matter models are fine-tuned to reproduce certain observations. These don't really count as evidence in favour of DM. > * It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO. MOND has more points in favour for it as a preferred theory than DM models. See [1]. > Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background. I'm not sure why I should find your claim that DM is "essential" as persuasive given MOND has received orders of magnitude less attention and less development than DM models. I would certainly not bet any money on MOND being incapable of reproducing these observations. [1] From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm

alecst 4y ago
I worked in this field for a minute (XENON1T) and I heard a few talks about MOND. We were all pretty open minded but it failed to stick because it didn’t convince us, for a number of reasons. Curious about your background here.
elcomet 4y ago
> Dark matter models are fine-tuned to reproduce certain observations That's the definition of physics. Models are fine-tuned to match the observation and be predictive.
- 3um35w1l 4y ago
  sure, but you didn't get the point. just because a model can predict certain results doesn't mean its correct. early astronomers were able to very precisely predict the positions (w/ looping motions and all) of stellar bodies in the night sky despite using a geocentric model. if you make it complex enough, you can engineer quite a few models that can predict natural phenomenon pretty well.

onlyrealcuzzo 4y ago

Do people actually believe in "Dark Matter" - or do they just believe in there being something that doesn't make sense and this is what explains it?

kadoban 4y ago
What's the difference?
- somenameforme 4y ago
  The former will tend towards a defensive attachment to an idea which leads to, as Planck put it, "Science [advancing] one funeral at a time." The openness of the latter leads to less certainty, but greater ability for progress. Consider the heliocentric/geocentric argument some centuries passed. Contrary to pop-science portrayals this wasn't an issue of Man against Church. The heliocentric view initially had numerous flaws and invalid assumptions. But because scientists of the time genuinely believed in the geocentric universe, they obsessed on these flaws and trying to use those to discredit the view, instead of impartially considering the idea and seeing if it could be patched and whether it would ultimately make more sense. The "right" answer, with negligible refinement, can often look much worse than the wrong answer which has had decades, if not centuries of mass refinement. And that can drive those with a "belief" to the contrary, to want to attack it. The analogs outside of science are as endless as those within it.
tsimionescu 4y ago
People actually believe that there is an yet-undiscovered substance that has mass but interacts very weakly (or not at all) with light, and that there are vast quantities of this substance in the universe. They call this "dark matter".

taylorius 4y ago

QI (quantized inertia) is a theory created by Mike McCulloch in the UK. It predicts galaxy rotations, including wide binaries extremely well, amongst other things. It is well worth a look.

sethrin 4y ago
This is a crackpot theory created by a known crackpot.
- taylorius 4y ago
  Why? Because you say so? With a dismissal like that you ought to elaborate with some evidence. Especially since you seem to be attacking the man, along with his theory.
  - sethrin 4y ago
    He's associated with the EmDrive fraud, which should be sufficient for anyone to dismiss his ideas. https://www.forbes.com/sites/briankoberlein/2017/02/15/quantized-inertia-dark-matter-the-emdrive-and-how-to-do-science-wrong/amp/ https://www.forbes.com/sites/briankoberlein/2017/02/15/quant... https://arxiv.org/pdf/1908.01589.pdf https://arxiv.org/pdf/1908.01589.pdf I'm not here to debate with physics cranks; further replies will not be read.

scotty79 4y ago

Does this guy also reject DM models based on MACHOs? I think they expect particles to be thrown out and result with slowdown but dark objects as massive as stars wouldn't be thrown at such a huge rate to observe slowdown, am I right?

alrlroipsp 4y ago
There's only one way to find out. It involves reading the article. I mean, what else could you bring to the discussion by only guessing what is being discussed?
- scotty79 4y ago
  I read it and I ask the question here because I didn't find the answer for it in the article. The article just talks about particles and dismisses them. I see nothing about DM composed of massive objects. What exactly does your comment bring to the discussion?

_0w8t 4y ago

We cannot simulate evolution of galaxies using equations of General Relativity. So at best the simulations are done using Newton mechanics with minimal corrections like accounting for finite speed of light. And often it is not even done using Newton mechanics per see, but numerical models use artificial constrains like a minimal distance between stars to avoid dealing with numerical instabilities. Yet there is no proof that such simplifications or constrains are valid on the scale of a typical galaxy. Moreover, when people managed to use full equations of General Relativity, then it was possible to account the effects previously thought to require the dark matter hypothesis. So until it is mathematically proven that the current simulations are valid approximations, I will remain very skeptical about the dark matter.

rich_sasha 4y ago
Like the GP, I'm not in love with DM hypothesis and wouldn't mind something else. I also used to think the same as you. But DM explains a lot of disparate effects, in ways that MOND does not. The best-well known effect is the "missing mass" in rotating galaxies. Well recently we found galaxies that look like they have no dark matter in them - i.e. they behave as you would expect from classical gravity/centripetal force arguments [1]. So while you can tweak General Relativity to match some of the effects in typical galaxies, you would then expect the results to hold in all galaxies. The "outliers" are easily explained by Dark Matter - they just don't have it for whatever reason. The simulations of Universe formation, sure have their limitations, but look much worse without DM. Galaxy clusters look like they have unexplained mass in them. Crucially, this isn't only from observing their rotational speeds (so like galaxy rotations) but also from looking at the strength of gravitational lensing they induce. This is as compared to fluid-dynamics-y calculations of gas temperature, pressure etc. Bullet cluster is another interesting case [2]. It is the result of a collision of two galaxies. In such cases, you expect (a) the stars to "fly through" each other, as they are basically point masses and only interact gravitationally, but (b) the gasses and dust to collide. In this case, most of the mass is in fact gas and dust, so it should be distributed in the middle of the cluster (having just had an inelastic collision). But in fact observed mass is where the stars are, further out of what looks like center of mass. [1] https://www.nature.com/articles/d41586-022-01410-x https://www.nature.com/articles/d41586-022-01410-x [2] https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster

jbakhos 4y ago

I would like to toss my own idea into this discussion. This idea incorporates a mechanism that would explain why gravity might slightly deviate from Newtonian and/or GR in some circumstances. This paper also adapts GR in such a way that it is consistent with galactic rotation rates, the anisotropies of the CMB, and cosmological expansion -- while showing that the simple operation of gravity is the cause of each of these phenomena. Cyclic Gravity and Cosmology (CGC) predicts that there are discrete specific sizes allowable for macro-objects. The instability of Bennu and the fact that it behaves more like loosely held scree rather than a compact mass -- is an example of a mass that is not exactly at one of the discrete allowable sizes. Please also refer to the link I included wherein I uploaded a video simulation of the formation of a solar system using this type of force law. (This is in section 18 of the paper) I would greatly appreciate any comments on this idea. Copies may be downloaded here: https://vixra.org/pdf/2203.0032v3.pdf https://vixra.org/pdf/2203.0032v3.pdf

garyclarke27 4y ago

It’s not shtick, it makes perfect sense to me - unlike your post which has no substance. Group-think does not equal truth. Just because there is no compelling alternative theory does not make the Dark Matter Theory correct. It’s probably nonsense, like many theories in Physics such as the Big Bang. Our level of intelligence and understanding, at the moment, is simply too small to comprehend the mysteries of the Universe that we live in.