5 ms·

fyi, "dark matter" is not a real thing. Modified gravity (MOND) accurately models galactic rotation for hundreds of galaxies with sqrt(GM/R^2 * c*H / 2pi), with

• sahil50 5y ago
  http://astroweb.case.edu/ssm/mond/fit_compare.html http://astroweb.case.edu/ssm/mond/fit_compare.html
  • renewiltord 5y ago
    If that is the case, then how do the sparse galaxies described in the article work? In the article, the galaxies described behave aligned with unmodified newtonian dynamics. So if we consider the inverse linear relationship in MOND, then for some reason, that relationship does not occur for these galaxies. That seems like evidence against MOND. EDIT: Nvm, parent answered similar question here https://news.ycombinator.com/item?id=29914380 https://news.ycombinator.com/item?id=29914380 Following links it appears that proponents of MOND believe that running the calculations will show that MOND predicts this.
    • sahil50 5y ago
      Yep 5th figure at that link. Also McGaugh's words here, https://www.scientificamerican.com/article/dark-matter-may-be-missing-from-this-newfound-galaxy-astronomers-say/ https://www.scientificamerican.com/article/dark-matter-may-b....
• SkittyDog 5y ago
  You state things as absolute fact, while there IS disagreement amongst legitimate experts in these fields. Please, furnish us with YOUR credentials that allow you to have such unique insights into this field. Otherwise, you just sound like a crank. I don't know if you're right or wrong, but your unsourced certainty is definitely setting off my bullshit meter.
  • sahil50 5y ago
    I have a degree in Computer Science from Yale. I built https://zedtime.live https://zedtime.live and I'm building https://spase.io https://spase.io, and I've been in direct communication with cold dark matter (CDM) and modified gravity (MOND) cosmologists for the last 20 weeks. I basically ask the questions that I'm gonna guess everyone has on their mind, and I'm just moving the conversation along faster.
    • nwiswell 5y ago
      > I've been in direct communication with cold dark matter (CDM) and modified gravity (MOND) cosmologists for the last 20 weeks. It's not really any of my business, but in your shoes I would be making sure to disclaim these statements regarding MOND as secondhand opinion rather than presenting them as established scientific fact.
      • sahil50 5y ago
        Appreciate the feedback. These statements are a synthesis of ideas from people who've already synthesized all the complexity of the CDM vs MOND debate, but now I'm talking at the level of HackerNews so people do something!
        • mlyle 5y ago
          I'd suggest being more humble. - It's a debate, not a settled fact. - When your opening salvo describes the other side as "stupid and ludicrous" you should not be surprised that some people respond with hostility in kind. - Having studied something somewhat intensely for 20 weeks doesn't put your personal opinion on the subject on a higher standing than people who have studied this at a high academic level for a lifetime. You're free to make arguments, but not to proclaim absolute truths. - For the particular article we're discussing: A galaxy that seems to have no rotational speed anomaly is a blow for both CDM & MOND theories, but it would seem to be a bigger blow for MOND (e.g. CDM anomaly could just be a weird distribution of dark matter).
• PaulHoule 5y ago
  Dark matter shows up in different ways at different scales. It is not just about galactic rotation curves and MOND has a hard time beyond it. It hit me one day that the galactic rotation curves had me thinking about it backwards. That had led me to think a galaxy has dark matter in it while more likely the dark matter has a galaxy in it.
  • notdemo88 5y ago
• jwuphysics 5y ago
  Maybe I'm just an "academic in charge" but I don't understand how MOND being able to account for galaxy rotation curves means that dark matter is not real. There is missing matter in galaxy clusters. Matter is missing if we look at galaxy-galacy and cluster-galaxy strong and weak gravitational lensing. This matter appears to be non-interacting (see Bullet Cluster, El Gordo, etc). There is missing matter from the angular power spectrum of the cosmic microwave background if we only account for baryons. There is missing matter if we are to forward model gravitational collapse of large scale structure at early times. MOND doesn't explain any of these things.
  • sahil50 5y ago
    Hey, glad to meet you. Maybe we can have a discussion. Glad we agree on modified gravity (MOND) being able to accurately account for galaxy rotation. - galaxy clusters - lensing - Bullet Cluster, El Gordo all have answers in the work of modified gravity (MOND) cosmologists. Stacy McGaugh does a really good job going through each one. The essential thing, and I mean really essential, is that modified gravity (MOND) uses fewer free parameters than cold dark matter (CDM), by a long shot.
    • perhonen 5y ago
      but... this is not true? McGaugh himself [1] mentions that there are mass discrepancies even using MOND analysis, and that this favors the dark matter paradigm [2]. [1] https://twitter.com/dudedarkmatter/status/1094526391620722688 https://twitter.com/dudedarkmatter/status/109452639162072268... [2] https://twitter.com/DudeDarkmatter/status/1094527929994932224 https://twitter.com/DudeDarkmatter/status/109452792999493222... certainly, these are still open questions in cosmology
      • sahil50 5y ago
        The tweets above are good questions! This is how to do science! Meanwhile it's good to restate the obvious. Modified gravity (MOND), where acceleration at galactic scales isn't just GM/R^2 but modeled as sqrt(GM/R^2 * cH/(2pi)), works for 100s of galaxies, and is more precise than cold dark matter (CDM). Modified gravity only has one free parameter. cH/(2pi) = c^2 / 87 bly.
• XorNot 5y ago
  > without the stupid and ludicrous idea of invisible, noninteracting blobs of dark matter around every galaxy. This is a very strange thing to declare definitely can't be happening, in a universe where subatomic particle interactions do not have definite results or even particularly comprehensible abstract models of how they work to human minds. Especially when the alternative is "gravity has some minimum force over distance which is exactly small enough we can't detect it on Earth".
  • R0b0t1 5y ago
    That last point is a very good way to describe MOND. It seems quite compelling, tbh. A minimum force is in line with a maximum speed and minimum area. I'm aware the jury is out on it.
• mhio 5y ago
  Was there a particular piece (or pieces) of research that stood out as the thing that convinced you MOND was the complete model?
  • sahil50 5y ago
    http://astroweb.case.edu/ssm/mond/fit_compare.html http://astroweb.case.edu/ssm/mond/fit_compare.html cold dark matter (CDM) is a superimposed amorphous blob of invisible non-interacting matter, hence its inability to model the kinks in the real rotation curve modified gravity (MOND) models the kinks in the rotation curve
• gwerbret 5y ago
  While other more knowledgeable folks than I have pointed out some very important things about your highly-declarative statement about dark matter, I'll raise one minor point about this research finding, which you can glean by reading the abstract of the actual paper [0]. The galaxy in question, AGC 114905, is described as lacking dark matter _because_ its rotational characteristics are readily explained by its baryonic matter content alone. MOND, therefore (and your entire argument), does not apply. You may have mistaken this for a regular galaxy with "missing" matter that also happens to lack dark matter, which would then suggest the need for an alternate explanation. EDIT: The abstract also says, among other things: "We also find that the rotation curve of AGC 114905 deviates strongly from the predictions of Modified Newtonian dynamics." [0]: https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stab3491/6461100 https://academic.oup.com/mnras/advance-article/doi/10.1093/m...
  • sahil50 5y ago
    fyi, I started my comment with fyi. The point being that the premise of the article is overblown. McGaugh says the same here https://www.scientificamerican.com/article/dark-matter-may-be-missing-from-this-newfound-galaxy-astronomers-say/ https://www.scientificamerican.com/article/dark-matter-may-b....
    • mlyle 5y ago
      FYI means "for your information". We use it when we are telling people about settled facts. It's also three letters that based on context could be seen as condescending on their own. When we are discussing not-settled facts, there's better ways to frame our discussion. For instance: "I believe, that CDM is an antiquated theory and MOND is a better explanation, because _______. I've had discussions with ______ and in particular ______" It's less effort than your opening flame, it's less abrasive, and it opens productive discourse. Your opening statement is more like that of a political partisan than someone seeking discussion.
      • sahil50 5y ago
        Intelligent and good people have followed the advice above, and the result has been more error and harm. Therefore, learning from their experience, the one thing that works is accurately calling the status quo's invisible blobs of noninteracting kludge as what it is.
• shusaku 5y ago
  From the article > In their work, Fraternali and his colleagues tested a leading contender among dark matter alternatives, called MOND, for Modified Newtonian Dynamics, which involves tweaking Isaac Newton’s law of gravity. First proposed by Israeli physicist Mordehai Milgrom in the 1980s, MOND hypothesizes that standard gravitational physics, which accurately explains the motions of objects with high gravitational accelerations, like planets in our solar system, might not apply the same way to slowly orbiting stars at the edge of a galaxy’s disc. So the discrepancy between the expected velocities of the stars in galaxies and how fast they appear to be moving may not indicate missing mass, but rather a math error, if the MOND gravitational law is right. But while the MOND model fares well with more normal galaxies, it too couldn’t explain the rotation of Fraternali’s team’s fluffy galaxy. It fared just as poorly as dark matter models do. Maybe try reading before calling others stupid
  • sahil50 5y ago
    "stupid" is in reference to the invisible blobs of noninteracting kludge. "lost" is what I would say the people pumping it are.
• zaarn 5y ago
  MOND, as the article points out, doesn't explain this galaxy, however. So it's a point in favor of CDM, if anything, that we find a galaxy behaving according to standard newtonian dynamics, not the modified version. Doesn't really matter how many data points(galaxies) MOND can accurately predict, if it can't predict all parts of the dataset, what is the point? (I'm not saying CDM is better, it can't predict all data points either, but it seems to be better at explaining a wide range of things once you step outside galactic rotation rates).
• mhh__ 5y ago
  And how does MOND work with relativity then? Despite popular belief physicists are actually quite clever and have evaluated theories such as this.
  • leadingthenet 5y ago
    I'm not a physicist and barely understand some of the things I'm reading in this thread, but your question reminded me that I did hear about a relativistic MOND a few months back (called TeVeS): 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...