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“Dark matter” always seemed like a cop out to this layperson. You can’t just give up and attribute problems with your theory to a mysterious substance that can’

• croes 3y ago
  You mean like Planet Nine? If Dark Matter exists it can be measured by it's effects on its surroundings
• airstrike 3y ago
  As another layperson with a background in finance, it always felt like the cosmological equivalent of making a Balance Sheet balance by plugging the difference in a model
  • ixaxaar 3y ago
    "Goodwill"?
  • Sharlin 3y ago
    Well not at all. If anything, that's the MOND theories. The proper almost exact analogue to dark matter is an accountant noticing something's wrong with the figures, trying to find an honest mistake somewhere, and failing to do so, starting to reasonably suspect that there's embezzlement going on and the missing money has not simply disappeared into the ether.
    • airstrike 3y ago
      Except accounting is just fun with numbers, so by definition you know if something is off, whereas with cosmological models you can't ever know if the math is missing something or if the model itself is inaccurate I'm saying (as others seem to also be saying) that it feels akin to saying "trust me, the model is right, we just haven't been able to find the missing $1 million but I'm sure it exists somewhere" It's not a particularly innovative or insightful criticism of current models, but that's what it feels like from the outside looking in. Chalk it up to ignorance, if you'd like, but I still feel like asking "are you sure you can't fix the model instead of plugging it with some unobservable feature?"
      • tekla 3y ago
        Its ignorance. There are very good reasons why Astronomers haven't chucked out Dark Matter. And yes, they have thought of the 5 minute HN remarks and already dealt with them.
      • lonjil 3y ago
        But we do know where it is, to quite good precision. We know how much or how little dark matter is present in different galaxies, which matches quite well with the amount of dark matter that must've been present in the early universe to explain some of the anisotropy of the cosmic microwave background.
      • fatbird 3y ago
        If we're going to correct our metaphor, it's like saying "when we add up the balances of all accounts, we're missing a million dollars, but when we add up all the interest earned on those accounts, total interest only makes sense if that million dollars is somewhere in there. Therefore, there's a million dollars we can detect overall but can't locate specifically."
        • airstrike 3y ago
          I like that, thank you for humoring me and my ignorance It feels like TFA is arguing "the way we calculate interest in the current model is wrong, here's a new formula". The jury is still out on whether that new formula breaks other parts of the model, I suppose
          • fatbird 3y ago
            Yes, exactly. And carrying it just a little further, the missing million idea has been around a while and actually accounts for quite a few observed discrepancies. A new interest formula plausibly explains one, but then has to tackle the others as well or better before it'll be taken as seriously.
      • Sharlin 3y ago
        Someone investigating a potential financial crime doesn't have all the information. They only have a model of where the money is coming from and going to. It's not like it's always easy to prove that there's definitely something illegal going on if the perpetrators have been carefully hiding their tracks with shell companies and money laundering and so on. So the investigator must form hypotheses and then try to find evidence for or against said hypotheses. Another analogy: we find a person, shot, in his home. There's a bullet inside the body, calibre and rifling marks point to a particular type of firearm, powder burn marks indicate a point-blank shot, all the CSI stuff. But the murder weapon is nowhere to be found. Do we assume that the weapon – and the perp – does indeed exist, or should we "tweak the model" to include a way for bullets to spontaneously materialize out of thin air and kill people? > Chalk it up to ignorance, if you'd like, but I still feel like asking "are you sure you can't fix the model instead of plugging it with some unobservable feature?" I don't understand how people think there's a distinction between "introducing a particle" and "fixing the model". Doing the former is doing the latter! Anyway, yes I do chalk it up to ignorance, because that's still a strawman, assuming some kind of a Scientific Orthodoxy that has decided that Dark Matter It Is, Claiming Otherwise Is Heresy. It's tiring to repeat this in every thread about DM, but – everybody and their dog has been testing the dark matter hypothesis for several decades now, obviously, because finding definitive evidence against it could be worth a Nobel, and so would figuring out a way to detect dark matter particles in laboratorio. And thus far they've found zero contradictory evidence, but instead in several other subdisciplines of astronomy, observations have been made that just happen to fit exactly the same sort of dark matter model that was originally only devised to explain the galactic rotation curves, without any fiddling. All the new evidence has simply tightened some parameters of the model that used to be less well known. Meanwhile, no competing hypothesis has been able to explain all the different observations that the the single unified dark matter model does explain. Different non-dark-matter models may be finetuned to explain some pieces of evidence, but that makes them unable to match other evidence. Nobody is claiming that there couldn't be a nice elegant dark-matter-less model that explains everything neatly. Most astrophycisists and cosmologists would be elated if one were found, and people do keep on looking. But as long as no such thing exists, we must assume that something that walks like a duck and quacks like a duck is, indeed, a duck.
• Sharlin 3y ago
  Well, but it has been. Just indirectly. But we have only measured tons of things indirectly, so that's nothing special. Like what Earth's core is made of, or the fact that exoplanets exist, or that atomic nuclei are made of things called protons and neutrons, which in turns are made of things called quarks. Us humans are really biased towards giving a special importance to EM radiation, due to how important vision is to us, but that's just us. Nature does not care. "Laypersons" just never seem to know anything about dark matter, including the shitload of evidence from several different types of observations that there's something there that can't be explained by just tweaking some equations. Of course that never stops them from having an opinion based on a total strawman.
  • nullindividual 3y ago
    FYI we've directly observed exoplanets. https://en.wikipedia.org/wiki/List_of_directly_imaged_exoplanets https://en.wikipedia.org/wiki/List_of_directly_imaged_exopla...
• Jevon23 3y ago
  There’s about as much evidence for the existence of dark matter as we can possibly get, short of actually observing it. In particular we’ve observed some galaxies that seem to have little to no dark matter. Gravitationally, they behave the way that you would “naively” predict without the need to plug in a correction term that represents dark matter. This indicates that there is something physically different about these galaxies (like an unobserved substance), and it points away from MOND-style theories.
  • lottin 3y ago
    You're saying that the evidence for the existence of dark matter is that your theory doesn't agree with the observation. This is like saying that you have no evidence at all.
    • Sharlin 3y ago
      Nope. It's saying that the theory didn't agree with the observation, so we came up with a hypothesis, and – guess what – we haven't found any evidence against that hypothesis, and a lot of evidence for that hypothesis even in places we didn't think we'd find some! That's 100% proper scientific method. Please don't argue against stuff you only have a strawman understanding of. Even if you're on HN and think you're smart and all.
      • lottin 3y ago
        What evidence against the existence of dark matter should we expect to find if it didn't exist?
        • Sharlin 3y ago
          That's easy. We look at the predictions a DM model makes, and then see if the predictions match observations. People seem to think that just because DM does not interact electromagnetically, the DM hypothesis cannot make testable predictions. Models are quantitative entities – this is physics we're talking about. They almost always have quantitative parameters whose values initially have large uncertainties. We make observations in order to tighten the error bars of those parameters – to rule out parameter values inconsistent with observations. Now, if we found that two observations are inconsistent with each other – each implies a parameterization incompatible with the other – and could rule out experimental error via repeat experiments etc, then that would be evidence against the entire model. In dark matter's case, we might realize, for example, that we cannot predict both galactic rotation curves and CMB anisotropies with a single DM model. That would be evidence against DM. (But that's not the case, both seem to be perfectly consistent with a single, well-constrained model.)
• cloudwalk9 3y ago
  As another layperson, I understand it as the physics equivalent of solving for x, but far less trivial. We figure it must exist because it's a missing puzzle piece in the mathematics that accurately predicts everything else. This is reinforced by observations of indirect effects that aren't accounted for in the current math.
• tekla 3y ago
  Oh man, let me tell you how we learned about the existence of some of the planets and things like neutron stars.
• lonjil 3y ago
  Just the opposite. Sure, DM was proposed when some observations didn't match predictions, but do you think scientists liked that? Do you think they just accepted it? They did not. No one liked DM, and many people tried to disprove it. How does one disprove DM? You simply assume that it exists, and then make a prediction using that assumption. If the prediction is wrong, that's evidence against DM. What happened is that every time someone made a DM prediction, it turned out to be correct. Two early cases of this were that if invisible matter exists, there should be gravitational lensing around seemingly empty space, which was then found, and that if if something like dark matter existed before recombination, then a certain pattern should be found in the cosmic microwave background, and a few years after the prediction, sensitive enough equipment was deployed that could see enough detail in the CMB, and the prediction turned out correct. Today there's something like a dozen such predictions that have come true, ranging in scale from kiloparsecs to gigaparsecs, from things happening today to things that happened in the early universe. When a theory makes accurate predictions across many problems and areas, that is good evidence for the theory.
• gmuslera 3y ago
  It worked with planets pretty well.