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You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy. Instead you 'just' need to add a weakly inte

You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy. Instead you 'just' need to add a weakly interacting particle with the right density (30% of critical). That particle needs to be 'slow enough', i.e. non-relativistic. After that it turns out that the evolution of the universe basically reproduces the vast array of observations. * The right primordial abundance of different elements * The amount of structure in the cosmic microwave background. * The rotation speeds/velocity dispersions in large number of galaxies * Large scale structure/The amount of clustering of galaxies. * Gravitational lensing observations. And much much more. It is certainly true that there are individual cases that look challenging for Lambda CDM paradigm (CDM stands for Cold Dark Matter), but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM. (there are currently some tensions with the Hubble constant measurements and some other cosmological parameters between different techniques, and it remains to to be seen whether those are systematic errors or point towards some modification in the theory). Comment on the article itself. I think at some point I would be fine with dark matter being essentially a computing trick, i.e. if the behaviour of the universe is well reproduced when we add gravitationally interacting but not interacting otherwise matter. It would be up to theorists to see if somehow that can be written as some sort of extra-term in Einsten's Equations and show we don't really need an actual particle. I think there is a possibility of that, but personally I still think it's more likely there is a particle responsible for dark matter.

naasking 4y ago
> You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy. No, he's correct that each galaxy does need tuning. Specifically, the amount of DM and it's specific distribution is tuned to match each observation, which is what the OP said. > but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM. I think this "strong case" for LCDM is often oversold. See this review of the evidence for LCDM compared to modified gravity: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm
- DiogenesKynikos 4y ago
  > the amount of DM and it's specific distribution is tuned to match each observation This is simply not true. No cosmological theory tries to explain why each star is in its exact observed location. They try to predict the statistics of galaxies, star formation, etc. If you run a simulation of the universe, beginning with very simple, random initial conditions, and then run physics forward for 13 billion years, you get a universe that statistically looks almost identical to our own. You get the same mix of spiral and elliptical galaxies as observed, clustered together in the same way, with the same star formation history, etc. You don't get exactly our universe, with the Milky Way exactly here and Andromeda exactly there, because you begin with random starting conditions. However, you get a universe that is statistically identical in most ways. Note that this is not the case for Milgromian theories. So far, no one has managed to create a theory without dark matter that can achieve this. Alternative theories generally get one or a few things right, at the cost of getting a whole host of other things wrong. It's not that astrophysicists a priori like dark matter. It's just that dark matter is a very simple paradigm that explains a huge number of phenomena with only a handful of free variables, and none of the alternatives work so far.
  - naasking 4y ago
    > No cosmological theory tries to explain why each star is in its exact observed location. Nice strawman. > Note that this is not the case for Milgromian theories. So far, no one has managed to create a theory without dark matter that can achieve this. LCDM also fails at this (see Dwarf galaxy problem, for one example), so this is not a point in your favour. > It's just that dark matter is a very simple paradigm that explains a huge number of phenomena with only a handful of free variables, and none of the alternatives work so far. LCDM doesn't work that great either. See the review I linked.
    - DiogenesKynikos 4y ago
      > see Dwarf galaxy problem, for one example I assume you're referring to the "missing satellites problem." This problem is already essentially solved: 1. Newer, more sensitive surveys have detected more Milky Way satellite galaxies. 2. Reionization and supernova feedback quench star formation in small dark matter halos, meaning that the smallest dark matter halos never form galaxies. > See the review I linked. I think we've talked about this elsewhere on HN before, and I pointed out that the paper you're linking to gets a lot of things wrong and is published in an obscure journal (presumably because it wouldn't survive peer review at any of the major journals).

naasking 4y ago

> You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy. No, he's correct that each galaxy does need tuning. Specifically, the amount of DM and it's specific distribution is tuned to match each observation, which is what the OP said. > but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM. I think this "strong case" for LCDM is often oversold. See this review of the evidence for LCDM compared to modified gravity: From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm https://www.mdpi.com/2073-8994/14/7/1331/htm

DiogenesKynikos 4y ago
> the amount of DM and it's specific distribution is tuned to match each observation This is simply not true. No cosmological theory tries to explain why each star is in its exact observed location. They try to predict the statistics of galaxies, star formation, etc. If you run a simulation of the universe, beginning with very simple, random initial conditions, and then run physics forward for 13 billion years, you get a universe that statistically looks almost identical to our own. You get the same mix of spiral and elliptical galaxies as observed, clustered together in the same way, with the same star formation history, etc. You don't get exactly our universe, with the Milky Way exactly here and Andromeda exactly there, because you begin with random starting conditions. However, you get a universe that is statistically identical in most ways. Note that this is not the case for Milgromian theories. So far, no one has managed to create a theory without dark matter that can achieve this. Alternative theories generally get one or a few things right, at the cost of getting a whole host of other things wrong. It's not that astrophysicists a priori like dark matter. It's just that dark matter is a very simple paradigm that explains a huge number of phenomena with only a handful of free variables, and none of the alternatives work so far.
- naasking 4y ago
  > No cosmological theory tries to explain why each star is in its exact observed location. Nice strawman. > Note that this is not the case for Milgromian theories. So far, no one has managed to create a theory without dark matter that can achieve this. LCDM also fails at this (see Dwarf galaxy problem, for one example), so this is not a point in your favour. > It's just that dark matter is a very simple paradigm that explains a huge number of phenomena with only a handful of free variables, and none of the alternatives work so far. LCDM doesn't work that great either. See the review I linked.
  - DiogenesKynikos 4y ago
    > see Dwarf galaxy problem, for one example I assume you're referring to the "missing satellites problem." This problem is already essentially solved: 1. Newer, more sensitive surveys have detected more Milky Way satellite galaxies. 2. Reionization and supernova feedback quench star formation in small dark matter halos, meaning that the smallest dark matter halos never form galaxies. > See the review I linked. I think we've talked about this elsewhere on HN before, and I pointed out that the paper you're linking to gets a lot of things wrong and is published in an obscure journal (presumably because it wouldn't survive peer review at any of the major journals).