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>but I think it is worth at least pursuing the idea that maybe DM doesn't exist and there is something about gravity that isn't fully explained by general relat

>but I think it is worth at least pursuing the idea that maybe DM doesn't exist and there is something about gravity that isn't fully explained by general relativity. I don't think anyone is disagreeing with this. However, the current observations are best explained by dark matter. The various MOND models don't fit as well. We either actually detect and confirm dark matter or someone comes up with a MOND model that fits as well or better and has testable predictions. When people deride MOND, it's not to say that it can't be an answer later but people who are pushing it now are all ignoring some gaping hole in their theory. Continuing research is fine but, confidence wise, dark matter is in the lead by a lot.

throwawaymaths 4y ago
It's easy seem like youre in the lead when you have one free parameter for every galaxy.
- hither_shores 4y ago
  Dark matter ratios are not free parameters, because galaxies did not form through independent processes. The evidence for a highly homogeneous early universe is overwhelming, and any dark matter candidate that required large-scale inhomogeneity would be rejected out of hand.
  - throwawaymaths 4y ago
    If they aren't free parameters then why do we assign arbitrary dark matter content and distribution to galaxies like the bullet cluster/ultradiffuse galaxies with "too much/almost no/no DM" to "make it work out"? If dark matter distribution weren't arbitrarily assignable, why would reputable physicists suggest that some galaxies might have a dark matter clump at the center and not a supermassive black hole? The evidence for a highly homogeneous early universe has basically been refuted, unless systematic difficulties in assigning galaxy distances (certainly a possibility) JWST is observing galaxies that are forming WAY too early for a standard gravitational model; MOND has predicted early galaxy formation for more than a decade now.
    - parineum 4y ago
      > If they aren't free parameters then why do we assign arbitrary dark matter content and distribution to galaxies like the bullet cluster/ultradiffuse galaxies with "too much/almost no/no DM" to "make it work out"? Something has to explain why galaxies aren't all exactly the same right? Whatever other theory out there will have some "free parameters" that explain that difference won't they? The observable matter differs, why shouldn't dark matter? I think you're being really cynical about the scientific process here. The process is a new observation comes along that's a bit out of line with the norm and scientists look at the parameters and see if Dark Matter is still a candidate to explain it. Do you think it's unrealistic to assume that different galaxies might have different amounts of dark matter (should it exist)? They have different amounts of observable matter. If there was some galaxy that just didn't make sense by just adding some arbitrary amount of dark matter, it'd put a real damper on the theory but that hasn't happened yet. As of today, the only thing that seems "off" about galaxies is the amount of gravity which is very simply explained by adding in some amount dark matter. I think my intuition of that is probably the same as your, that it seems kind of ridiculous to just add some invisible matter to the equation but it does actually do a really good job of explaining a few different phenomenon. I suspect the process for the acceptance of dark matter initially began as a "these galaxies are behaving _as if_ they had more matter" and a lot of people trying to explain why that might be to no avail until they kind of just accepted that maybe there just _is_ more matter that we don't see. At least that's how it's been for me. I'm still holding out some kind of hope/expectation that there's a better theory out there that actually explains dark matter/energy in a more satisfactory way.
      - throwawaymaths 4y ago
        It's not rediculous to add invisible matter. This is how we discovered Neptune, and Uranus. But it didn't work for Vulcan and Pluto was super sketchy. But to scientist's credit they gave up on dark matter Vulcan when nothing was found, over and over again, and GR explained mercury's orbit. The analogy would be if we didn't figure out GR, used dark matter to explain mercury's precession, then used dark matter to explain aberrant voyager probe kinetics, then rejected GR because "it failed to explain the weird motion of voyager" Adding invisible matter is not crazy, but over time you make your model increasingly unfalsifiable as the observations fail to meet the expectations. Ideally that would decrease confidence in the model, but for dark matter it seems to have entrenched the conviction of mainstream astro.

throwawaymaths 4y ago

It's easy seem like youre in the lead when you have one free parameter for every galaxy.

hither_shores 4y ago
Dark matter ratios are not free parameters, because galaxies did not form through independent processes. The evidence for a highly homogeneous early universe is overwhelming, and any dark matter candidate that required large-scale inhomogeneity would be rejected out of hand.
- throwawaymaths 4y ago
  If they aren't free parameters then why do we assign arbitrary dark matter content and distribution to galaxies like the bullet cluster/ultradiffuse galaxies with "too much/almost no/no DM" to "make it work out"? If dark matter distribution weren't arbitrarily assignable, why would reputable physicists suggest that some galaxies might have a dark matter clump at the center and not a supermassive black hole? The evidence for a highly homogeneous early universe has basically been refuted, unless systematic difficulties in assigning galaxy distances (certainly a possibility) JWST is observing galaxies that are forming WAY too early for a standard gravitational model; MOND has predicted early galaxy formation for more than a decade now.
  - parineum 4y ago
    > If they aren't free parameters then why do we assign arbitrary dark matter content and distribution to galaxies like the bullet cluster/ultradiffuse galaxies with "too much/almost no/no DM" to "make it work out"? Something has to explain why galaxies aren't all exactly the same right? Whatever other theory out there will have some "free parameters" that explain that difference won't they? The observable matter differs, why shouldn't dark matter? I think you're being really cynical about the scientific process here. The process is a new observation comes along that's a bit out of line with the norm and scientists look at the parameters and see if Dark Matter is still a candidate to explain it. Do you think it's unrealistic to assume that different galaxies might have different amounts of dark matter (should it exist)? They have different amounts of observable matter. If there was some galaxy that just didn't make sense by just adding some arbitrary amount of dark matter, it'd put a real damper on the theory but that hasn't happened yet. As of today, the only thing that seems "off" about galaxies is the amount of gravity which is very simply explained by adding in some amount dark matter. I think my intuition of that is probably the same as your, that it seems kind of ridiculous to just add some invisible matter to the equation but it does actually do a really good job of explaining a few different phenomenon. I suspect the process for the acceptance of dark matter initially began as a "these galaxies are behaving _as if_ they had more matter" and a lot of people trying to explain why that might be to no avail until they kind of just accepted that maybe there just _is_ more matter that we don't see. At least that's how it's been for me. I'm still holding out some kind of hope/expectation that there's a better theory out there that actually explains dark matter/energy in a more satisfactory way.
    - throwawaymaths 4y ago
      It's not rediculous to add invisible matter. This is how we discovered Neptune, and Uranus. But it didn't work for Vulcan and Pluto was super sketchy. But to scientist's credit they gave up on dark matter Vulcan when nothing was found, over and over again, and GR explained mercury's orbit. The analogy would be if we didn't figure out GR, used dark matter to explain mercury's precession, then used dark matter to explain aberrant voyager probe kinetics, then rejected GR because "it failed to explain the weird motion of voyager" Adding invisible matter is not crazy, but over time you make your model increasingly unfalsifiable as the observations fail to meet the expectations. Ideally that would decrease confidence in the model, but for dark matter it seems to have entrenched the conviction of mainstream astro.