5 ms·

Does this have any implications on dark matter? I think most of the evidence for dark matter is from outside the galaxy, so not really. But might we not have m

Does this have any implications on dark matter? I think most of the evidence for dark matter is from outside the galaxy, so not really. But might we not have made similar over estimations of baryonic matter at the edges of other galaxies? If they were true, does it imply there must be even more dark matter than we thought?

layer8 3y ago
The implication is that there may be significantly less dark matter in the Milky Way than previously estimated. From [0]: “Using this newly derived rotation curve, a new mass was deduced for our Milky Way. And surprise... it is much lighter. It is now estimated to be only two hundred billion times that of the Sun (~2.06 x 10^11), so about four to five times lower than the previous estimates. The amount of ordinary matter did not change, thus, there must be a lot less dark matter in the Milky Way than previously thought. The new expectation is that the ordinary matter like stars and gas in the Milky Way now makes up about 1/3 of the Milky Way mass and the other 2/3 are accounted to dark matter.” [0] https://www.cosmos.esa.int/web/gaia/iow_20230927 https://www.cosmos.esa.int/web/gaia/iow_20230927
- benreesman 3y ago
  I’m not a physicist or cosmologist, just a fan. Is it possible that another turn of the crank or two takes dark matter to zero? Dark matter has always had that string theory vibe: yeah, maybe it sounds ridiculous but it’s the only game in town man.
  - 8bitsrule 3y ago
    Apart from the Vera Rubin game, of course.
  - eru 3y ago
    > Is it possible that another turn of the crank or two takes dark matter to zero? Not sure about the Milky Way. But we already found galaxies out there that don't seem to have any dark matter; ie where their rotation curves match what you would predict from the ordinary matter alone. Interestingly enough that is evidence in favour of dark matter: it's easy to conceive of mechanisms that can separate dark matter from visible matter with some low probability in a galaxy merger or near-merger. (Similarly for galaxies having more than the average amount of dark matter.) But alternatives like Modified Newtonian Dynamics (MOND) would have a much harder time accommodating those differences.
    - username332211 3y ago
      Without knowing anything about MOND, this never made sense to me. If MOND can explain away all cases where m > 0[*], it'd be a bit strange it can't explain m = 0. If MOND can only accommodate a certain range, wouldn't that also present a problem for dark matter? If all previously known galaxies had dark matter in that range, why would every galaxy have roughly the same amount of dark matter except the one that doesn't have any? [*] Where I guess m would be the ratio of dark to ordinary matter.
      - eru 3y ago
        MOND can most easily explain a certain fixed value of what you call m. It has a much harder time explaining lots of different m at the same time. > If all previously known galaxies had dark matter in that range, why would every galaxy have roughly the same amount of dark matter except the one that doesn't have any? They don't all have the same amount of dark matter. There's a distribution. You were right, that a weird distribution would invite investigation.
    - throwawaymaths 3y ago
      No. MOND predicts a large external field effect in the milky way due to satellite galaxies (like the large magellanic clouds). This would manifest itself in as "a reduced DM halo", and an keplerian decline (see figure in TFA). LCDM typically has a difficult time with this because in order to have a flat curve, the dark matter must be distributed in a halo that is MUCH bigger than the galaxy, and that leaves no explanation for the decline. Not having run the numbers, at first glance it looks like these new data are STRONG support for MOND. MOND also generally predicts that denser fast rotating galaxies should appear to have ~no dark matter (in these cases centripetal acceleration is > a0 constant, so it's in the Newtonian regime) , which is highly consistent with observations of galaxies. Naively (I have not run simulations) thinking about it it seems puzzling that the denser galaxies would have no DM -- since DM is supposed to be the nucleus for galactic formation in the early universe. Conversely, the galaxies that are highly diffuse (UDGs) seem to have extra dark matter, which is very consistent with MOND (since being diffuse, gravitational acceleration a < a0). Again, this makes little sense with LCDM as you would expect a pocket of extra DM to attract mass and nucleate the formation of a very dense galaxy. > separate dark matter from visible matter with some low probability in a galaxy merger Generally these observations have been done with weak lensing, and unfortunately there is not really yet a good model that reconciles GR with MOND. Apparently the math is hard. It is entirely possible that when you combine the two the calculated spacetime curvature solves to what you would expect from the observed weak lensing effects.
  - ck2 3y ago
    Very smart people have been working on the problem for a very long time https://www.youtube.com/watch?v=RchRrngfjQY https://www.youtube.com/watch?v=RchRrngfjQY I'll look forward to what Matt has to say about this new paper eventually.
  - shiroiuma 3y ago
    The dark matter hypothesis has always seemed like BS to me, like the simple product of a math error. I think the observational results are caused by something else that's not understood or seen yet. What's funny to me is how people can be so confident about astrophysics theories, when we're just a primitive race that hasn't even ventured beyond our own moon and fighting brutal wars over whose imaginary god is correct.
    - TheBigSalad 3y ago
      I think almost everyone realizes dark matter is a placeholder for the unknown. It doesn't mean we shouldn't explore the possibility of special dark matter particles that don't interact with regular matter. In fact, there's not much else to do.
      - throwawaymaths 3y ago
        There are two historical dark matter theories in early cosmology. Neptune and Vulcan. One of them turned out to be real. It would do to remember that the other one did not.
      - benreesman 3y ago
        I think this is the most honest-sounding reply to a layperson. Our data doesn’t fit GR with the boundary conditions that make sense more locally. Positing a bunch of non-interacting mass/energy is closer to observations than doing nothing. Calling it a placeholder seems honest, reasonable, and productive. Getting pushy about WIMPs and stuff (which is deeply intertwined with the refusal to give up on supersymmetry because, careers) in a world where Michio Kaku is on TV talking about quantum computing and AI in a way that’s less scientific than Matt Gaetz yelling about UFOs on C-SPAN to distract from the child-trafficking charges is icky.
    - Pxtl 3y ago
      There's a good cosmology YouTuber who basically explains: dark matter is not a theory, it's a set of observations. Galaxies behave like there's a halo of invisible mass surrounding them, one that varies in density per-galaxy. The halos even perturb as expected in galaxy collisions. But nobody's got a coherent theory on what that halo is. https://m.youtube.com/watch?v=PbmJkMhmrVI&pp=ygUbZGFyayBtYXR0ZXIgaXMgbm90IGEgdGhlb3J5 https://m.youtube.com/watch?v=PbmJkMhmrVI&pp=ygUbZGFyayBtYXR...
  - Beldin 3y ago
    Actually, dark matter isn't the only game in town. The whole need for dark matter arose from the fact that our theories (on gravity) didn't match some of our observations - while matching other observations phenomenally well. Abstractly speaking, when theory doesn't match observations, either the theory is lacking, or the observations are. Dark matter is the hypothesis that the observations are lacking. The alternative is to come up with modifications of Einsteinian gravity that preserve accordance with observations where it's extremely good (e.g. perihelion shift of Mercury), and modify only the results for the parts where the match with observations is lacking. To the best of my knowledge, this category is collectively called MOND. There are some good reasons to think that there exists a type of matter that we cannot observe - not that we didn't happen to see it like some small black hole in the middle of nowhere, but that our current instruments literally cannot see it, only its effects. There are also some reasons to think otherwise. The main one for that is that dark matter, by its nature, should be something. So not only should dark matter theories make predictions that can be proven, evidence for dark matter should eventually answer the question "what is dark matter?" That is a bit of a problem: right now, it might as well be unicorn farts.
    - gpderetta 3y ago
      IANAP, but I thought is that MOND is a modification of classical gravity and that GR-compatible extensions to MOND end up introducing dark-matter like terms.
  - layer8 3y ago
    No, because galaxy rotation curves are not the only evidence of dark matter, by far. There is a lot of independent strong evidence, see https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid....
    - edgyquant 3y ago
      It would be great for you to link somewhere other than Wikipedia
  - Pxtl 3y ago
    Dark Matter is the opposite of String Theory imho. String Theory was about desperately seeking a Grand Unifying Theory that everybody wants and mashing math together until you figured out something that matched the evidence but was so overcomplicated it was completely untestable. Dark Matter is about having evidence that something is terribly wrong with the math, and the evidence pointing to something goddamned absurd that everybody hates, and reluctantly finding more and more evidence that this thing everybody hates might be true. The main thing they have in common is that they're both incredibly ugly solutions to problems in physics, but one is invented to reconcile a an inconsistency in theory, and the other is invented to reconcile an inconsistency in evidence. And honestly, that's why I'm comfy thinking dark matter is probably real: I'm a pessimist. The ugly solution that everybody finds kinda gross is probably the truth.
  - hyperman1 3y ago
    We've been in this situation before a few times. E.g. when geology was new, the earth was much too old, compared to what the heath radiating from it allowed. Chemical reactions were simply not up to the job over such long time spans. We needed wild new physics, nuclear reactions, before things started to make sense. Everybody involved knew something was very wrong. A lot of ink was used trying to explain the error. But finally, the error was settled as a sidekick result from a completely different branch of science.

layer8 3y ago

The implication is that there may be significantly less dark matter in the Milky Way than previously estimated. From [0]: “Using this newly derived rotation curve, a new mass was deduced for our Milky Way. And surprise... it is much lighter. It is now estimated to be only two hundred billion times that of the Sun (~2.06 x 10^11), so about four to five times lower than the previous estimates. The amount of ordinary matter did not change, thus, there must be a lot less dark matter in the Milky Way than previously thought. The new expectation is that the ordinary matter like stars and gas in the Milky Way now makes up about 1/3 of the Milky Way mass and the other 2/3 are accounted to dark matter.” [0] https://www.cosmos.esa.int/web/gaia/iow_20230927 https://www.cosmos.esa.int/web/gaia/iow_20230927

benreesman 3y ago
I’m not a physicist or cosmologist, just a fan. Is it possible that another turn of the crank or two takes dark matter to zero? Dark matter has always had that string theory vibe: yeah, maybe it sounds ridiculous but it’s the only game in town man.
- 8bitsrule 3y ago
  Apart from the Vera Rubin game, of course.
- eru 3y ago
  > Is it possible that another turn of the crank or two takes dark matter to zero? Not sure about the Milky Way. But we already found galaxies out there that don't seem to have any dark matter; ie where their rotation curves match what you would predict from the ordinary matter alone. Interestingly enough that is evidence in favour of dark matter: it's easy to conceive of mechanisms that can separate dark matter from visible matter with some low probability in a galaxy merger or near-merger. (Similarly for galaxies having more than the average amount of dark matter.) But alternatives like Modified Newtonian Dynamics (MOND) would have a much harder time accommodating those differences.
  - username332211 3y ago
    Without knowing anything about MOND, this never made sense to me. If MOND can explain away all cases where m > 0[*], it'd be a bit strange it can't explain m = 0. If MOND can only accommodate a certain range, wouldn't that also present a problem for dark matter? If all previously known galaxies had dark matter in that range, why would every galaxy have roughly the same amount of dark matter except the one that doesn't have any? [*] Where I guess m would be the ratio of dark to ordinary matter.
    - eru 3y ago
      MOND can most easily explain a certain fixed value of what you call m. It has a much harder time explaining lots of different m at the same time. > If all previously known galaxies had dark matter in that range, why would every galaxy have roughly the same amount of dark matter except the one that doesn't have any? They don't all have the same amount of dark matter. There's a distribution. You were right, that a weird distribution would invite investigation.
  - throwawaymaths 3y ago
    No. MOND predicts a large external field effect in the milky way due to satellite galaxies (like the large magellanic clouds). This would manifest itself in as "a reduced DM halo", and an keplerian decline (see figure in TFA). LCDM typically has a difficult time with this because in order to have a flat curve, the dark matter must be distributed in a halo that is MUCH bigger than the galaxy, and that leaves no explanation for the decline. Not having run the numbers, at first glance it looks like these new data are STRONG support for MOND. MOND also generally predicts that denser fast rotating galaxies should appear to have ~no dark matter (in these cases centripetal acceleration is > a0 constant, so it's in the Newtonian regime) , which is highly consistent with observations of galaxies. Naively (I have not run simulations) thinking about it it seems puzzling that the denser galaxies would have no DM -- since DM is supposed to be the nucleus for galactic formation in the early universe. Conversely, the galaxies that are highly diffuse (UDGs) seem to have extra dark matter, which is very consistent with MOND (since being diffuse, gravitational acceleration a < a0). Again, this makes little sense with LCDM as you would expect a pocket of extra DM to attract mass and nucleate the formation of a very dense galaxy. > separate dark matter from visible matter with some low probability in a galaxy merger Generally these observations have been done with weak lensing, and unfortunately there is not really yet a good model that reconciles GR with MOND. Apparently the math is hard. It is entirely possible that when you combine the two the calculated spacetime curvature solves to what you would expect from the observed weak lensing effects.
- ck2 3y ago
  Very smart people have been working on the problem for a very long time https://www.youtube.com/watch?v=RchRrngfjQY https://www.youtube.com/watch?v=RchRrngfjQY I'll look forward to what Matt has to say about this new paper eventually.
- shiroiuma 3y ago
  The dark matter hypothesis has always seemed like BS to me, like the simple product of a math error. I think the observational results are caused by something else that's not understood or seen yet. What's funny to me is how people can be so confident about astrophysics theories, when we're just a primitive race that hasn't even ventured beyond our own moon and fighting brutal wars over whose imaginary god is correct.
  - TheBigSalad 3y ago
    I think almost everyone realizes dark matter is a placeholder for the unknown. It doesn't mean we shouldn't explore the possibility of special dark matter particles that don't interact with regular matter. In fact, there's not much else to do.
    - throwawaymaths 3y ago
      There are two historical dark matter theories in early cosmology. Neptune and Vulcan. One of them turned out to be real. It would do to remember that the other one did not.
    - benreesman 3y ago
      I think this is the most honest-sounding reply to a layperson. Our data doesn’t fit GR with the boundary conditions that make sense more locally. Positing a bunch of non-interacting mass/energy is closer to observations than doing nothing. Calling it a placeholder seems honest, reasonable, and productive. Getting pushy about WIMPs and stuff (which is deeply intertwined with the refusal to give up on supersymmetry because, careers) in a world where Michio Kaku is on TV talking about quantum computing and AI in a way that’s less scientific than Matt Gaetz yelling about UFOs on C-SPAN to distract from the child-trafficking charges is icky.
  - Pxtl 3y ago
    There's a good cosmology YouTuber who basically explains: dark matter is not a theory, it's a set of observations. Galaxies behave like there's a halo of invisible mass surrounding them, one that varies in density per-galaxy. The halos even perturb as expected in galaxy collisions. But nobody's got a coherent theory on what that halo is. https://m.youtube.com/watch?v=PbmJkMhmrVI&pp=ygUbZGFyayBtYXR0ZXIgaXMgbm90IGEgdGhlb3J5 https://m.youtube.com/watch?v=PbmJkMhmrVI&pp=ygUbZGFyayBtYXR...
- Beldin 3y ago
  Actually, dark matter isn't the only game in town. The whole need for dark matter arose from the fact that our theories (on gravity) didn't match some of our observations - while matching other observations phenomenally well. Abstractly speaking, when theory doesn't match observations, either the theory is lacking, or the observations are. Dark matter is the hypothesis that the observations are lacking. The alternative is to come up with modifications of Einsteinian gravity that preserve accordance with observations where it's extremely good (e.g. perihelion shift of Mercury), and modify only the results for the parts where the match with observations is lacking. To the best of my knowledge, this category is collectively called MOND. There are some good reasons to think that there exists a type of matter that we cannot observe - not that we didn't happen to see it like some small black hole in the middle of nowhere, but that our current instruments literally cannot see it, only its effects. There are also some reasons to think otherwise. The main one for that is that dark matter, by its nature, should be something. So not only should dark matter theories make predictions that can be proven, evidence for dark matter should eventually answer the question "what is dark matter?" That is a bit of a problem: right now, it might as well be unicorn farts.
  - gpderetta 3y ago
    IANAP, but I thought is that MOND is a modification of classical gravity and that GR-compatible extensions to MOND end up introducing dark-matter like terms.
- layer8 3y ago
  No, because galaxy rotation curves are not the only evidence of dark matter, by far. There is a lot of independent strong evidence, see https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid....
  - edgyquant 3y ago
    It would be great for you to link somewhere other than Wikipedia
- Pxtl 3y ago
  Dark Matter is the opposite of String Theory imho. String Theory was about desperately seeking a Grand Unifying Theory that everybody wants and mashing math together until you figured out something that matched the evidence but was so overcomplicated it was completely untestable. Dark Matter is about having evidence that something is terribly wrong with the math, and the evidence pointing to something goddamned absurd that everybody hates, and reluctantly finding more and more evidence that this thing everybody hates might be true. The main thing they have in common is that they're both incredibly ugly solutions to problems in physics, but one is invented to reconcile a an inconsistency in theory, and the other is invented to reconcile an inconsistency in evidence. And honestly, that's why I'm comfy thinking dark matter is probably real: I'm a pessimist. The ugly solution that everybody finds kinda gross is probably the truth.
- hyperman1 3y ago
  We've been in this situation before a few times. E.g. when geology was new, the earth was much too old, compared to what the heath radiating from it allowed. Chemical reactions were simply not up to the job over such long time spans. We needed wild new physics, nuclear reactions, before things started to make sense. Everybody involved knew something was very wrong. A lot of ink was used trying to explain the error. But finally, the error was settled as a sidekick result from a completely different branch of science.