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I'm not positive that's true. I think the most challenging thing for MOND is that basically no one will touch it unless they already have tenure because it's a

by kayaeb 6y ago

I'm not positive that's true. I think the most challenging thing for MOND is that basically no one will touch it unless they already have tenure because it's a death sentence. I saw Pavel Kroupa (big name in Milgromian gravity) present in Heidelberg (big concentration of Astro), at the time Volker Springel (author of widely used LCDM simulation code "GADGET") was there and Illustris simulation sets (LCDM major project) had just been rolled out. And Pavel basically got heckled (in a very erudite and respectable way, but constant interruptions from the LCDM majority audience). But Pavel had one slide, I can't find it now, but it was like 72 different problems that LCDM had not solved (ok, 72 is an exaggeration; you can troll his website for mentions of a lot of them https://astro.uni-bonn.de/~pavel/kroupa_SciLogs.html https://astro.uni-bonn.de/~pavel/kroupa_SciLogs.html). And like, Pavel's a big boy, he has some of the most cited papers in all of astronomy, he's got tenure, and he's set, so he can take it and not care. But grad students / postdocs I imagine would constantly have their work politely ignored and get shunted into underfunded groups. I'm just trying to say that LCDM has things it can't explain, and MOND has things it can't explain, but the amount of resources in each theory is seriously lopsided so LCDM can frequently "tweak" itself to solve problems that MOND just doesn't have time or resources to to the same (for example disk formation in LCDM models used to be impossible until they had the supercomputing resources for the resolution required, and they found that the feedback coefficient could a) not promote disk growth, b) promote disk growth, and c) destroy disk growth, depending on how much they cranked it up. That's NOT a triumph of LCDM making an amazing replication of the observation, that's some grad student in a lab with enough CPU to tweak a meta-parameter until it looks good. Also the CMB is extremely tightly constrained... and multiple huge tightly-constraining studies, WMAP, PLANCK, Gaia, are more than 3-sigma outside of each other's results, so... perhaps too tightly constrained. Edit: Found it, here is the great astronomical bloodbath of 2014: LCDM (Springel and Rix) vs MOND (Kroupa). Great watch. https://www.youtube.com/watch?v=UPVGDXNSBZM https://www.youtube.com/watch?v=UPVGDXNSBZM

edna314 6y ago
I'm not entirely sure which one of the two, Dark Matter or MOND, is conceptually the bigger disaster. Dark matter pretty much sounds like the invisible ether that was suspected to carry EM waves. A physical theory that depends on the scale at which you look at space seems similarly awkward to me.
- teraflop 6y ago
  The main difference is that the aether was only ever a theoretical construct, whereas we have lots of indirect observational evidence that "dark matter" is a phenomenon that really exists. We don't know what it consists of, but it certainly seems like there is more of it in some places than others, and it has mass and momentum (see e.g. the Bullet Cluster that was already mentioned in this thread), so "matter" seems as good a name for it as any.
  - edna314 6y ago
    Ok, I would be curious about evidence that dark matter has momentum, because my favorite theory at the moment is that spacetime itself has certain topology on large scales, which isn’t tied to any masses. If one could show that dark matter has momentum, I would reconsider, but I don’t think that is what Bullet Cluster shows. Edit: even if it turns out that dark matter is the best theory, it would still be a conceptual disaster. But so is QM. Nature seems not to care what we find conceptually appealing.
    - DiogenesKynikos 6y ago
      Quantum mechanics is incredibly elegant and appealing as a theory. It's just counter-intuitive.
      - edna314 6y ago
        Is this why there are a bazillion of theories of the foundations of QM? I get that the mathematics is incredibly elegant, but concepts behind the mathematics are not really “understandable” to quote Feynman.
        DiogenesKynikos 6y ago
        The Many-Worlds Interpretation is the simplest one, because it makes no assumptions beyond, "The basic rules of Quantum Mechanics are correct." It's also very counter-intuitive, but very elegant.
hasjekyll 6y ago
It's also worth pointing out that LCDM is a cosmological framework. MOND (typically) has no cosmology. And then even if MOND is correct - you likely still need something like DM to explain clusters (particularly the bullet cluster). It has also been included in the paper posted by OP as scalar field at early times, which then washes out at late times. It is true though, MOND is hard to touch as an early career researcher!
DiogenesKynikos 6y ago
My experience from MOND talks is that a proponent of MOND gets up, explains their theory, and then starts explaining how their theory explains the rotation curve of galaxies. They're then confronted with a bunch of really basic questions from the audience: how does your theory explain the spatial spectrum of anisotropies in the Cosmic Microwave Background? Is your theory consistent with Big Bang Nucleosynthesis? Can your theory explain Weak Lensing measurements? How does your theory deal with the Bullet Cluster? The answer is then generally, "I'm not sure, but I'm working on it." That causes all the astrophysicists in the room to lose interest. Standard cosmology explains all of these basic observations with a minimal set of assumptions. If your theory can't or doesn't explain the most basic set of observations, and there's another theory that does, why should I care about your theory?
- auntienomen 6y ago
  This reminds me of the old joke: When an academic says "That's a good question!", you should hear "I don't know the answer". Likewise, "I'm working on it." means "No.".
- alvarelle 6y ago
  Imagine what proponents to heliocentrism such as Galileo had to face explaining their theory and how it explain a simplified elliptic orbit for the planets rather than the strange curly orbits known by geocentrists. They'd be confronted with basic question from the audience: Why would things fall down if it was not the center? (That was before Newton) Wouldn't we see paralax in the stars (stars are much further away than what was believed at the time). Wouldn't we feel it if earth turns so fast? "Maybe that explains the tides" said Galileo (but it doesn't) The previous model was also explaining the observations quite well at the time. Why should we care for another theory? (God works in mysterious ways.) I'm not saying that MOND is correct. ("they also laughed at Bozo the Clown".) Just that the fact that there are some unexplained missing piece does not mean one should reject it so quickly.
  - DiogenesKynikos 6y ago
    Comparing oneself to Galileo is generally viewed as a sign of crackpotery, especially if it's used as a response to legitimate criticism. > Just that the fact that there are some unexplained missing piece does not mean one should reject it so quickly. It's not just one missing piece. It's a whole series of basic properties of the observed universe. Most MOND theories are tailored to match one particular observation, but fail to match everything else. Until there's a MOND theory that matches a basic set of observations (like CMB anisotropies and the large-scale structure of the Universe, the ratios of abundances of the light elements, weak lensing measurements, etc.), MOND is simply uninteresting to most astrophysicists.

by kayaeb 6y ago

edna314 6y ago

I'm not entirely sure which one of the two, Dark Matter or MOND, is conceptually the bigger disaster. Dark matter pretty much sounds like the invisible ether that was suspected to carry EM waves. A physical theory that depends on the scale at which you look at space seems similarly awkward to me.

teraflop 6y ago
The main difference is that the aether was only ever a theoretical construct, whereas we have lots of indirect observational evidence that "dark matter" is a phenomenon that really exists. We don't know what it consists of, but it certainly seems like there is more of it in some places than others, and it has mass and momentum (see e.g. the Bullet Cluster that was already mentioned in this thread), so "matter" seems as good a name for it as any.
- edna314 6y ago
  Ok, I would be curious about evidence that dark matter has momentum, because my favorite theory at the moment is that spacetime itself has certain topology on large scales, which isn’t tied to any masses. If one could show that dark matter has momentum, I would reconsider, but I don’t think that is what Bullet Cluster shows. Edit: even if it turns out that dark matter is the best theory, it would still be a conceptual disaster. But so is QM. Nature seems not to care what we find conceptually appealing.
  - DiogenesKynikos 6y ago
    Quantum mechanics is incredibly elegant and appealing as a theory. It's just counter-intuitive.
    - edna314 6y ago
      Is this why there are a bazillion of theories of the foundations of QM? I get that the mathematics is incredibly elegant, but concepts behind the mathematics are not really “understandable” to quote Feynman.
      - DiogenesKynikos 6y ago
        The Many-Worlds Interpretation is the simplest one, because it makes no assumptions beyond, "The basic rules of Quantum Mechanics are correct." It's also very counter-intuitive, but very elegant.

hasjekyll 6y ago

It's also worth pointing out that LCDM is a cosmological framework. MOND (typically) has no cosmology. And then even if MOND is correct - you likely still need something like DM to explain clusters (particularly the bullet cluster). It has also been included in the paper posted by OP as scalar field at early times, which then washes out at late times. It is true though, MOND is hard to touch as an early career researcher!

DiogenesKynikos 6y ago

My experience from MOND talks is that a proponent of MOND gets up, explains their theory, and then starts explaining how their theory explains the rotation curve of galaxies. They're then confronted with a bunch of really basic questions from the audience: how does your theory explain the spatial spectrum of anisotropies in the Cosmic Microwave Background? Is your theory consistent with Big Bang Nucleosynthesis? Can your theory explain Weak Lensing measurements? How does your theory deal with the Bullet Cluster? The answer is then generally, "I'm not sure, but I'm working on it." That causes all the astrophysicists in the room to lose interest. Standard cosmology explains all of these basic observations with a minimal set of assumptions. If your theory can't or doesn't explain the most basic set of observations, and there's another theory that does, why should I care about your theory?

auntienomen 6y ago
This reminds me of the old joke: When an academic says "That's a good question!", you should hear "I don't know the answer". Likewise, "I'm working on it." means "No.".
alvarelle 6y ago
Imagine what proponents to heliocentrism such as Galileo had to face explaining their theory and how it explain a simplified elliptic orbit for the planets rather than the strange curly orbits known by geocentrists. They'd be confronted with basic question from the audience: Why would things fall down if it was not the center? (That was before Newton) Wouldn't we see paralax in the stars (stars are much further away than what was believed at the time). Wouldn't we feel it if earth turns so fast? "Maybe that explains the tides" said Galileo (but it doesn't) The previous model was also explaining the observations quite well at the time. Why should we care for another theory? (God works in mysterious ways.) I'm not saying that MOND is correct. ("they also laughed at Bozo the Clown".) Just that the fact that there are some unexplained missing piece does not mean one should reject it so quickly.
- DiogenesKynikos 6y ago
  Comparing oneself to Galileo is generally viewed as a sign of crackpotery, especially if it's used as a response to legitimate criticism. > Just that the fact that there are some unexplained missing piece does not mean one should reject it so quickly. It's not just one missing piece. It's a whole series of basic properties of the observed universe. Most MOND theories are tailored to match one particular observation, but fail to match everything else. Until there's a MOND theory that matches a basic set of observations (like CMB anisotropies and the large-scale structure of the Universe, the ratios of abundances of the light elements, weak lensing measurements, etc.), MOND is simply uninteresting to most astrophysicists.