7 ms·

I've been doing a deep-dive in the past few weeks of papers and data sets regarding to rotation curves, mass densities etc. (SPARC, papers describing the rotati

I've been doing a deep-dive in the past few weeks of papers and data sets regarding to rotation curves, mass densities etc. (SPARC, papers describing the rotation curves of various dwarf galaxies etc). The impression I get is that most of the authors are not critical of a CDM dark matter halo explaining rotation curve data. The papers I'm reading span from ~1990 to present.

uoaei 2y ago
Have you seen any work from Stacy McGaugh? https://tritonstation.com/new-blog-page/ https://tritonstation.com/new-blog-page/
- the__alchemist 2y ago
  No; checking it out. Ty!
  - throwawaymaths 2y ago
    not saying MOND is correct, but the list of predictions it made without fine-tuning are stunning: - keplerian rotation curves ("no dark matter") in elliptical and lenticular galaxies - EFE (a group tried to find evidence against EFE and found it instead) - early galaxies in the universe - renzo's rule - keplerian descent in the milky way (due to the effects of the magellanic clouds) There's also explaining the tully-fisher relation, which is a post-hoc rationalization, so not really a prediction, but the model wasn't fine tuned to obtain the exact numerical solution.
    - the__alchemist 2y ago
      The typical CDM response to this is "What about the Bullet Cluster?" (Gravitational lensing not correlated with visible mass). What is your thought on that? I think a convincing answer there would go a long way towards convincing people. I believe Milgrom's speculation is undetected matter.
      - throwawaymaths 2y ago
        I don't know enough about the statisical image processing to measure weak gravitational lensing to have a cogent answer, but also it's totally ok to have one or two things that don't quite match up. Some of the "there's no DM" or "there's almost entirely DM" diffuse galaxies are likely due to measurement error in the distance to the diffuse galaxy, for example.
        DiggyJohnson 2y ago
        Thanks for your original post as well. Petitioning anyone else to understand GGPs list and attempt an explanation of GP’s observation of the Bullet Cluster. Cheers all. I think it’s interesting how many of us are seriously interested laymen on this topic. I think it’s fun and fascinating.
    - Keysh 2y ago
      It's kind of amusing that you present "keplerian rotation curves" as a "prediction" of MOND, given that the whole point of MOND is a kludge to produce non-Keplerian rotation curves. That is, by definition MOND cannot produce Keplerian rotation curves. This is why the (small) number of dwarf galaxies (not "elliptical and lenticular galaxies"!) which apparently lack dark matter -- and which do not follow the Tully-Fisher relation-- are serious problems for MOND.
      - throwawaymaths 2y ago
        > by definition MOND cannot produce Keplerian rotation curves. thats a common misunderstanding of MOND and means you did not stop to understand the definition. look at the formula carefully; in high gravitational regimes it looks newtonian (obviously, e.g. solar system). MOND, by definition predicts keplerian curves when the galaxies are rotating quickly (high gravitation). You will find that ellpitical and lenticular galaxies are almost always rotating fast. it also can through EFE but i dont fully understand the math enough to explain that.
        Keysh 2y ago
        "Keplerian" is a term of art based on the rotation curve in Newtonian gravity when all the mass of a system is concentrated at its center. For the Solar System, the rotation curve outside the radius of the Sun is pretty much pure Keplerian: the velocity decreases proportional to the square root of the radius. For galaxies, which are extended objects, the rotation curve is not Keplerian when you are well inside the galaxy: it first rises to larger radii, then levels off. But since the baryonic matter (stars, gas, dust) in galaxy is rather centrally concentrated, the rotation curve should start looking more and more Keplerian as you get further and further into the outskirts and outside the galaxy. But that is not what we see. Instead, we see the rotation curves staying roughly constant with radius ("flat"); we call this "non-Keplerian". This is true for almost all galaxies, including ellipticals and lenticulars (this is a recent study of three lenticular galaxies: https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-20/aa38184-20.html https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-.... Note the rotation curves in the bottom panels of Figure 3: they do not at any point start decaying, let alone decaying as fast as R^(-1/2).) Figure 5 of that paper (https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-20/F5.html https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-...) shows the observed rotation curves; it also shows the predicted curves if just the stars and standard Newtonian gravity were operating, with the dotted red lines. Note how these lines first rise to a local peak at small radii, and then decline to larger radii: this is a (quasi-)Keplerian decline. It fails to match the actual rotation curve at large radii. The conventional response is to postulate some additional form of matter, distributed in a much more extended fashion than the baryonic matter (this produces the dashed black lines in Figure 5 of that paper). The MOND response is to modify gravity (or: to modify the acceleration due to gravity) such that it doesn't show anything like a Keplerian falloff at large radii, even at radii where the gravitating matter (assumed to be just the visible baryonic matter) is well inside. In the case of the Solar System, the Keplerian decline starts right outside the Sun, where the acceleration is strong enough to be above the MOND threshold. But if you went far enough out and could measure the circular orbital speed, MOND would start to deviate from Keplerian. In the case of galaxies, the outer radii where the rotation curves appear "flat" are where the acceleration due to gravity is low enough for MOND to matter, and so the predicted MOND curves will not be Keplerian. (I should perhaps point out that I'm a professional astronomer whose been studying galaxies, including lenticulars and ellipticals, for almost 30 years, so attempts to mansplain my field to me won't really impress me.)
Retric 2y ago
That seems really time consuming. One question I’ve had for a while but didn’t seem worth the look is if there any consideration for local gravitational interaction between stars exchanging momentum between them and thus flattening the rotation curve. I’m assuming that’s one of the first things looked at but couldn’t find a paper on the subject. Remember any references on the topic?
- Keysh 2y ago
  The problem isn't so much the flatness of the rotation curve, but its continued high value: as you go farther and farther out in distance, it should drop rapidly because most of the visible matter is concentrated toward the center of the galaxy, but it doesn't. This implies that there is more matter, less centrally concentrated than the visible matter. Note that most "rotation curves" are actually measured from gas, not stars, and also that strong gravitational interactions between individual stars are extremely rare except in very dense star clusters and galactic nuclei, due to the increasingly large distances between stars as you go out from galactic centers. The time required for individual stellar interactions in the main or outer parts of galaxies to significantly affect their motions is much larger than the age of the universe (see, e.g., https://en.wikipedia.org/wiki/Stellar_dynamics https://en.wikipedia.org/wiki/Stellar_dynamics). Finally, this wouldn't address other evidence for dark matter, like the halos of hot (millions or tens of millions of K) intergalactic gas in galaxy clusters. The pressure of the gas should have driven the gas to expand way billions of years ago, if you assume that only the gravity of the individual galaxies and the gas itself is restraining it.
  - deleted 2y ago
    [deleted]
  - Retric 2y ago
    Edited. > strong gravitational interactions between individual stars are extremely rare except in very dense star clusters We’re talking 225 million years for the sun to orbit the galaxy, rare events become commonplace on those timescales. Anyway, I’m sure someone has actually done this kind of simulation I’m just curious about what the result is and how they did it.

uoaei 2y ago

Have you seen any work from Stacy McGaugh? https://tritonstation.com/new-blog-page/ https://tritonstation.com/new-blog-page/

the__alchemist 2y ago
No; checking it out. Ty!
- throwawaymaths 2y ago
  not saying MOND is correct, but the list of predictions it made without fine-tuning are stunning: - keplerian rotation curves ("no dark matter") in elliptical and lenticular galaxies - EFE (a group tried to find evidence against EFE and found it instead) - early galaxies in the universe - renzo's rule - keplerian descent in the milky way (due to the effects of the magellanic clouds) There's also explaining the tully-fisher relation, which is a post-hoc rationalization, so not really a prediction, but the model wasn't fine tuned to obtain the exact numerical solution.
  - the__alchemist 2y ago
    The typical CDM response to this is "What about the Bullet Cluster?" (Gravitational lensing not correlated with visible mass). What is your thought on that? I think a convincing answer there would go a long way towards convincing people. I believe Milgrom's speculation is undetected matter.
    - throwawaymaths 2y ago
      I don't know enough about the statisical image processing to measure weak gravitational lensing to have a cogent answer, but also it's totally ok to have one or two things that don't quite match up. Some of the "there's no DM" or "there's almost entirely DM" diffuse galaxies are likely due to measurement error in the distance to the diffuse galaxy, for example.
      - DiggyJohnson 2y ago
        Thanks for your original post as well. Petitioning anyone else to understand GGPs list and attempt an explanation of GP’s observation of the Bullet Cluster. Cheers all. I think it’s interesting how many of us are seriously interested laymen on this topic. I think it’s fun and fascinating.
  - Keysh 2y ago
    It's kind of amusing that you present "keplerian rotation curves" as a "prediction" of MOND, given that the whole point of MOND is a kludge to produce non-Keplerian rotation curves. That is, by definition MOND cannot produce Keplerian rotation curves. This is why the (small) number of dwarf galaxies (not "elliptical and lenticular galaxies"!) which apparently lack dark matter -- and which do not follow the Tully-Fisher relation-- are serious problems for MOND.
    - throwawaymaths 2y ago
      > by definition MOND cannot produce Keplerian rotation curves. thats a common misunderstanding of MOND and means you did not stop to understand the definition. look at the formula carefully; in high gravitational regimes it looks newtonian (obviously, e.g. solar system). MOND, by definition predicts keplerian curves when the galaxies are rotating quickly (high gravitation). You will find that ellpitical and lenticular galaxies are almost always rotating fast. it also can through EFE but i dont fully understand the math enough to explain that.
      - Keysh 2y ago
        "Keplerian" is a term of art based on the rotation curve in Newtonian gravity when all the mass of a system is concentrated at its center. For the Solar System, the rotation curve outside the radius of the Sun is pretty much pure Keplerian: the velocity decreases proportional to the square root of the radius. For galaxies, which are extended objects, the rotation curve is not Keplerian when you are well inside the galaxy: it first rises to larger radii, then levels off. But since the baryonic matter (stars, gas, dust) in galaxy is rather centrally concentrated, the rotation curve should start looking more and more Keplerian as you get further and further into the outskirts and outside the galaxy. But that is not what we see. Instead, we see the rotation curves staying roughly constant with radius ("flat"); we call this "non-Keplerian". This is true for almost all galaxies, including ellipticals and lenticulars (this is a recent study of three lenticular galaxies: https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-20/aa38184-20.html https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-.... Note the rotation curves in the bottom panels of Figure 3: they do not at any point start decaying, let alone decaying as fast as R^(-1/2).) Figure 5 of that paper (https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-20/F5.html https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-...) shows the observed rotation curves; it also shows the predicted curves if just the stars and standard Newtonian gravity were operating, with the dotted red lines. Note how these lines first rise to a local peak at small radii, and then decline to larger radii: this is a (quasi-)Keplerian decline. It fails to match the actual rotation curve at large radii. The conventional response is to postulate some additional form of matter, distributed in a much more extended fashion than the baryonic matter (this produces the dashed black lines in Figure 5 of that paper). The MOND response is to modify gravity (or: to modify the acceleration due to gravity) such that it doesn't show anything like a Keplerian falloff at large radii, even at radii where the gravitating matter (assumed to be just the visible baryonic matter) is well inside. In the case of the Solar System, the Keplerian decline starts right outside the Sun, where the acceleration is strong enough to be above the MOND threshold. But if you went far enough out and could measure the circular orbital speed, MOND would start to deviate from Keplerian. In the case of galaxies, the outer radii where the rotation curves appear "flat" are where the acceleration due to gravity is low enough for MOND to matter, and so the predicted MOND curves will not be Keplerian. (I should perhaps point out that I'm a professional astronomer whose been studying galaxies, including lenticulars and ellipticals, for almost 30 years, so attempts to mansplain my field to me won't really impress me.)

Retric 2y ago

That seems really time consuming. One question I’ve had for a while but didn’t seem worth the look is if there any consideration for local gravitational interaction between stars exchanging momentum between them and thus flattening the rotation curve. I’m assuming that’s one of the first things looked at but couldn’t find a paper on the subject. Remember any references on the topic?

Keysh 2y ago
The problem isn't so much the flatness of the rotation curve, but its continued high value: as you go farther and farther out in distance, it should drop rapidly because most of the visible matter is concentrated toward the center of the galaxy, but it doesn't. This implies that there is more matter, less centrally concentrated than the visible matter. Note that most "rotation curves" are actually measured from gas, not stars, and also that strong gravitational interactions between individual stars are extremely rare except in very dense star clusters and galactic nuclei, due to the increasingly large distances between stars as you go out from galactic centers. The time required for individual stellar interactions in the main or outer parts of galaxies to significantly affect their motions is much larger than the age of the universe (see, e.g., https://en.wikipedia.org/wiki/Stellar_dynamics https://en.wikipedia.org/wiki/Stellar_dynamics). Finally, this wouldn't address other evidence for dark matter, like the halos of hot (millions or tens of millions of K) intergalactic gas in galaxy clusters. The pressure of the gas should have driven the gas to expand way billions of years ago, if you assume that only the gravity of the individual galaxies and the gas itself is restraining it.
- deleted 2y ago
  [deleted]
- Retric 2y ago
  Edited. > strong gravitational interactions between individual stars are extremely rare except in very dense star clusters We’re talking 225 million years for the sun to orbit the galaxy, rare events become commonplace on those timescales. Anyway, I’m sure someone has actually done this kind of simulation I’m just curious about what the result is and how they did it.