4 ms·

The Quanta article doesn't appear to address the Bullet Cluster, nor the dark-matter-rich dwarf galaxies. Looking forward to having a gander at the paper itsel

by ISL 6y ago

The Quanta article doesn't appear to address the Bullet Cluster, nor the dark-matter-rich dwarf galaxies. Looking forward to having a gander at the paper itself. Edit for context: the challenging thing for MOND is that ΛCDM explains a number of precision observables and phenomena at once. MOND and TeVeS theories can fit individual facts in isolation, but not the ensemble simultaneously. ΛCDM fits those facts with only a single extra concept. It sounds like this iteration claims to address the seminal observation one and one of the most-tightly constrained observables simultaneously. There are several others. Will have to read the paper before saying more.

BluSyn 6y ago
Linked in the article: https://arxiv.org/abs/2007.00082 https://arxiv.org/abs/2007.00082
- sradman 6y ago
  For context, Wikipedia article on MOND [1]: > Modified Newtonian dynamics (MOND) is a hypothesis that proposes a modification of Newton's laws to account for observed properties of galaxies. It is an alternative to the hypothesis of dark matter in terms of explaining why galaxies do not appear to obey the currently understood laws of physics. [1] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
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.

by ISL 6y ago

BluSyn 6y ago

Linked in the article: https://arxiv.org/abs/2007.00082 https://arxiv.org/abs/2007.00082

sradman 6y ago
For context, Wikipedia article on MOND [1]: > Modified Newtonian dynamics (MOND) is a hypothesis that proposes a modification of Newton's laws to account for observed properties of galaxies. It is an alternative to the hypothesis of dark matter in terms of explaining why galaxies do not appear to obey the currently understood laws of physics. [1] https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

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.