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I follow the lead author, Stacy McGaugh, via his blog where he posts discussions and musings about the latest research into the dark matter vs MOND debate: http

• simonh 2y ago
  I don’t think that’s quite fair. That approach is exactly how we find planets. Here’s an unexpected variance in the motion of a planet or star. It could be explained by a planet over there. Oh look, there’s a planet over there.
  • russdill 2y ago
    It's actually a better example than you think. This exact theory led to long and protracted searches for the planet Vulcan, which would explain Mercury's strange behavior.
  • TheOtherHobbes 2y ago
    Planets are visible when you look for them. Dark matter - so far - isn't.
    • drdeca 2y ago
      What do you mean by “visible when you look for them”? Like, with light? Does gravitational lensing count as “visible” to you?
  • MarkusQ 2y ago
    Right, which is why it quickly led to the detection of dark matter...hmm. I think a better analogy would be "that approach is exactly how we explain failing to find planets like Vulcan; we hypothesize that they are made of as-yet-unknown stuff that you can't see, touch, hear, smell, or in fact detect at all. But we know they're there because our calculations say they are."
  • solid_fuel 2y ago
    Hypothesizing that a planet might be over there is a testable hypothesis. Have we found a way to verify the presence of dark matter yet? Or is it still an untestable hypothesis sprinkled around distant galaxies so their acceleration curves look right?
    • User23 2y ago
      I’m particularly amused by the hypothesis that spacetime can be bent without the presence of matter. We can’t detect dark matter because there’s no such thing, it’s just a brute topological fact.
    • mr_mitm 2y ago
      Dark matter predicted lensing effect which were successfully tested. Same for the baryonic acoustic oscillations in the CMB.
      • MattPalmer1086 2y ago
        That's not quite true. General relativity predicts gravitational lensing, not dark matter. Lensing has been used as an experimental probe for the presence of dark matter.
        • elashri 2y ago
          MOND is an alternative theory of gravity competing with GR. People usually forget that while MOND started to present a different explanation for Dark Matter, it is a theory of gravity. Dark Matter is not a theory of gravity and is compatible with GR.
          • zeroonetwothree 2y ago
            Dark matter isn’t much of a theory in the first place.
        • mr_mitm 2y ago
          Well of course. What I clearly meant was that DM predicts lensing effects in a magnitude that cannot be explained with ordinary matter. See bullet cluster or weak lensing observations.
• griffzhowl 2y ago
  > which is totally backwards from a basic scientific perspective This is not right, because if we have a situation where our theories and observations don't cohere, it's not given whether the theory requires modification or we're missing something in our observations (or both). A classical illustration is the orbit of Uranus being observed in the nineteenth century to be contrary to the predictions of Newtonian theory. Calculations were made assuming the truth of the Newtonian theory and that we were missing something in our observations - the position of Neptune was predicted and it was subsequently discovered. On the other hand, the orbit of Mercury diverged from the prediction of Newton's theory. Again, a previously unobserved planet closer to the sun was postulated as being responsible, but in this case it really did require a modification to the theory of gravity: general relativity, which accurately predicted the 43 arcseconds per century of perihelion precession by which Mercury's orbit diverges from Newtonian predicitions. GR has obviously made many other predictions, such as the gravitational bending of light, black holes, and gravitational waves, which have been vindicated. So there's obviously a problem of the theory and observations not cohering, but whether the solution is a modification of the theory or a new form of matter is not clear in advance, and the latter is not unreasonable and certainly it's not unscientific to make as a hypothesis, to see where it leads. The difficulty is in coming up with a theoretical framework that retains all the successful predictions of GR while also accounting for the galactic rotation curves.
  • bbor 2y ago
    Well put, thanks for sharing! Never saw it phrased in such a clear narrative. As a novice, it seems like there's one big difference between those anecdotes and the current situation, though: sample size. Sure, if we were observing Andromeda spinning too slowly I'd be open to our instruments not capturing some massive objects/clouds, but we're actively observing, what, ~1E5-6 galaxies? In the case of a missing planet there were accidents of history/solar system makeup that led to our otherwise solid frameworks missing a key piece of information. But that clearly couldn't happen millions of times; whatever explains the inconsistencies we're seeing has to be a fundamental misunderstanding. Once we've arrived at this point, we can compare the two theoretical re-workings on their own terms: one is that we're glossing over some important detail of how gravitational relations in spacetime work, and the other is that we're failing to observe some new class of matter. I mean, right? There's no way this conundrum will be solved by "whoops turns out there was more plain ol' dust than we thought" at this point, right? In those terms, I feel parsimony clearly favors one possibility over the other. Every hypothesis is worth exploring (I mean, QM and GR are dumb as hell, yet nonetheless turned out to be correct), but when funding is on the line it's also not out of line to favor one explanation explicitly. That's already happening anyway, just in the other direction. But also I'm just some kid who's awed and grateful to be living in times of such profound mystery and discovery. Could be totally off base -- I barely passed physics I!
• antognini 2y ago
  > where we would put matter if we could to make our theory of gravity make sense Dark matter behaves in a fundamentally different way from baryonic matter. We can constrain the total amount of matter in the universe (both dark and baryonic) from the observed abundances of baryogenesis. But dark matter has a different effect on the relative amplitudes of peaks in the CMB. As far as I can tell, MOND has never really had any success outside of modeling galaxy rotation curves. The skepticism I've seen towards dark matter vs. MOND has always been strange to me. Dark matter doesn't really require much in the way of new physics --- there's just a new particle to add to the standard model. But most MOND theories violate Lorentz invariance which is a vastly more radical departure from standard physics. (And in my mind, the more sophisticated MOND theories that maintain Lorentz invariance like TeVeS are really a theory of dark matter dressed up in the language of MOND.)
  • MattPalmer1086 2y ago
    There are more successful predictions than just rotation curves. For example, see: http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html
    • antognini 2y ago
      These successful predictions are all generally variants on modeling galactic dynamics, though. The trouble is that galaxies and galaxy clusters are very messy places, so it's hard to make sure you've incorporated all the relevant physics. By contrast something like baryon acoustic oscillations are very simple to model, so you can be quite confident that you've incorporated all the relevant processes. And in that regime LCDM performs beautifully and MOND completely fails. So it's reasonable to suspect that in more complicated environments the problem is that we're not modeling the systems correctly rather than that there's new physics going on.
      • gus_massa 2y ago
        Very interesting. Do you know an article that ELI25 this?
• halgir 2y ago
  I usually understand "dark matter" to be shorthand for the discrepancy between theory and observation. The explanation might indeed be matter that is dark, or it might be solved by entirely unexpected observations and/or changes to theory.
  • mr_mitm 2y ago
    Not really. You might think this after watching Angela Coulliers video, but when you read something like "25% of the universe's energy content is made of dark matter", they do not mean changes to some theory. They literally mean non-baryonic matter.
    • OutOfHere 2y ago
      Nope. It can mean change to some theory, without a need for matter. It is the difference between relativistic gravity and the corresponding observed mass.
    • zeroonetwothree 2y ago
      Energy content not only comes from matter but also from fields.