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There is only a handful of parameters. They describe the probability distribution of finding one particular matter configuration. We are simply observing one re

There is only a handful of parameters. They describe the probability distribution of finding one particular matter configuration. We are simply observing one realization of that probability distribution, in which we should be able to calculate the percentage of galaxies with an unusual amount of dark matter for any definition of "unusual". We don't consider the position and momentum of every baryon a parameter of the standard model either, so why should we do so with DM? DM can be falsified perfectly well. If the rotation curves of all observed galaxies behaved as predicted by Newton, there would be no DM and every reasonable physicist would consider DM falsified. DM simply passes a lot of falsification experiments. If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation. Differentiating it from MoND or similar theories will then be based purely on how well the theory fits. However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable. Why shouldn't there be heavy particles that don't interact electromagnetically, like a heavy neutrino? Or particles that don't even interact weakly? Or at least, why should their abundance be low compared to baryons? Edit: Regarding falsification of DM, it's not like DM isn't challenged anywhere. Like I mentioned above, we should be able to compute the abundance of galaxies of certain sizes (dark or not), and the dwarf galaxies are coming up short. It's called the dwarf galaxy problem and poses indeed a challenge to DM, albeit not as much as DM alternatives in the mind of most cosmologists. They think it will be solved eventually as our understanding of galaxy formation improves, but who knows, maybe it's the first glimpse at something bigger. https://en.wikipedia.org/wiki/Dwarf_galaxy_problem https://en.wikipedia.org/wiki/Dwarf_galaxy_problem

treis 4y ago
>If DM interacts purely gravitationally If it's purely gravitational then that implies infinite density and infinite force for any matter that occupies the exact same space.
- treis 4y ago
  Also, if there's 5x as much dark matter in the universe why is our solar system so devoid of it? We should be able to detect the equivalent of dark matter suns or planets around us.
  - rocqua 4y ago
    All of the below is according to theory, not claiming it to be true. Because DM passes right through itself it doesn't clump like the stuff that made our planets do. Instead it remains spread out bound by gravity but not clumping. That makes it incredibly diffuse. But space is also quite big, so it can still contribute a lot of mass.
- panda-giddiness 4y ago
  On the contrary, if dark matter were to interact purely gravitationally, then there would be little friction to slow it down. This would prevent dark matter from clumping, which is in agreement with observations.
  - treis 4y ago
    I don't think so. Say you had two stationary whatevers of dark matter 1 meter apart alone in the universe. At t=0 they accelerate towards each other. At some time they cross and occupy the exact same point. R becomes 0 and the force becomes infinite. They should be stuck together.
    - gpderetta 4y ago
      That would make DM collapse in a black hole. I don't think that DM black holes are considered impossible, just not likely.
      - raattgift 4y ago
        The key weakness in your comment's parent comment was, "occupy the exact same point". One might ask why shining a flashlight during broad daylight (or say while doing work around the international space station) doesn't produce black holes. Or why there aren't black holes in the first image at <https://en.wikipedia.org/wiki/Caustic_(optics) https://en.wikipedia.org/wiki/Caustic_(optics)>. Particle DM is still quantum particles, with Compton and de Broglie wavelengths. A cold milliectronvolt axion has a wavelength on the order of a kilometre. How do you trap a pair of them in a volume comparable to the Schwarzschild radius of about 10^-50 metres? Additionally, an appreciable mass-energy of dark matter (one you can expect to be described tractably by the Raychaudhuri equations) will be a many-particle system occupying a noncompact volume. Newton's constant G in SI units is the force in Newtons between two one-kilogram masses at rest with respect to each other at one metre apart. It's small (< 7e-11). You will wait a long time -- about a full Earth day -- for two kg of anything initial 1m apart to get very much closer to each other because of (only) their mutual gravitational interaction. However, each of the pair of kg-mass blobs of DM particles is not likely to stick around intact that long because (by virtue of them being weakly interacting particles) nothing around them is strong enough to keep them from running away due to thermal motion. They are only cold in the sense they move slowly compared to hot dark matter in the form of relativistic neutrinos; they can still move about at many m/s, which is a lot faster than m/day. Newton's constant is rather small, and rather too tiny when thinking of coaxing a couple kg of CDM into a volume comparable to the Schwarzschild radius of about 10^-27 metres. One might compare this with two kg-masses of water ice. In a ball of ice various interactions trap molecules within the ball even against sublimation. The ice balls will come into contact in about a day. Thus we can have things like Saturn's rings or periodic comets, helped by the non-gravitational interactions of water ice keeping the mass-energy-momentum from flying away before gravity can do its work. Indeed, keeping galaxy and galaxy-cluster dark matter halos from flying away puts an upper bound on their temperature (if you heat up the water ice above to a couple hundred kelvins it can sublimate away in minutes). Halo DM needs to be cold in order for it to stick around. tl;dr, two sets of non-self-interacting (collisionless) dark matter just slide right through each other. cf. the famous Bullet cluster collision. You'd need at least a galaxy worth of dark matter and at least many millions of years of Chandrasekhar dynamical friction (radiating away very low amplitude gravitational waves as structures ("subhalos") form and move through each other) to make a black hole from DM alone.
rocqua 4y ago
This seems like a very nicely detailed response to OPs question. Thanks! Your response brought up some questions and remarks from myself. > DM can be falsified perfectly well. I recall hearing that DM was attempted to be falsified based on gravitational lensing. Is that correct? > If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation. Would DM as matter not have inertia, whereas treating it like an artifact of space-time would leave it without inertia? > Differentiating it from MoND or similar theories will then be based purely on how well the theory fits. A simple question here would be: do all galaxies of similar size have the same amount of dark matter. If so, then the uniformity of dark-matter would need explanation. If not, then I think something like MoND would have a hard time explaining why these similar galaxies rotate at different speeds.
naasking 4y ago
> However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable. I think this is a strawman. MOND proponents don't discount that some observations are better suited to a particle, they just assert that the particle dark matter sledgehammer is vastly overused, and that modified gravity has been unusually successful in cases where it had no right to be if gravity acts as it does under GR. For instance, this review [1] of the astrophysical evidence by a MOND proponent makes a good case for MOND + sterile neutrinos. This review also covers numerous falsifications of DM, so if you're correct that DM can be falsified, why did these falsifications fail to convince the majority? [1] 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
- sigmoid10 4y ago
  >why did these falsifications fail to convince the majority? Because they are cherry picked. In the big picture, for every area where particle DM struggles, you will find a dozen datapoints where MOND struggles. Physicists are also wary of approaches like MOND+particle DM, because it just introduces more parameters which you can arbitrarily tune to explain data. It feels like every week someone comes with a new model that fits certain edge cases better, but doesn't actually tell us anything new or could act as a reasonable guide for future work. You can imagine it like fitting a polynomial of degree N to N data points. Yes it will fit perfectly, but that doesn't mean it's a good model. This is just sophisticated curve fitting to generate papers and not good science.
  - naasking 4y ago
    > Because they are cherry picked. Sure, as are the objections to MOND. > Physicists are also wary of approaches like MOND+particle DM, because it just introduces more [...] You can imagine it like fitting a polynomial of degree N to N data points. Yes it will fit perfectly, but that doesn't mean it's a good model. This is just sophisticated curve fitting to generate papers and not good science. That's exactly what's happening with cold dark matter models. I agree the state of things is unsatisfactory for either theory, we need bolder ideas. Superfluid dark matter was promising but doesn't currently match some observations. We need more of that.

treis 4y ago

>If DM interacts purely gravitationally If it's purely gravitational then that implies infinite density and infinite force for any matter that occupies the exact same space.

treis 4y ago
Also, if there's 5x as much dark matter in the universe why is our solar system so devoid of it? We should be able to detect the equivalent of dark matter suns or planets around us.
- rocqua 4y ago
  All of the below is according to theory, not claiming it to be true. Because DM passes right through itself it doesn't clump like the stuff that made our planets do. Instead it remains spread out bound by gravity but not clumping. That makes it incredibly diffuse. But space is also quite big, so it can still contribute a lot of mass.
panda-giddiness 4y ago
On the contrary, if dark matter were to interact purely gravitationally, then there would be little friction to slow it down. This would prevent dark matter from clumping, which is in agreement with observations.
- treis 4y ago
  I don't think so. Say you had two stationary whatevers of dark matter 1 meter apart alone in the universe. At t=0 they accelerate towards each other. At some time they cross and occupy the exact same point. R becomes 0 and the force becomes infinite. They should be stuck together.
  - gpderetta 4y ago
    That would make DM collapse in a black hole. I don't think that DM black holes are considered impossible, just not likely.
    - raattgift 4y ago
      The key weakness in your comment's parent comment was, "occupy the exact same point". One might ask why shining a flashlight during broad daylight (or say while doing work around the international space station) doesn't produce black holes. Or why there aren't black holes in the first image at <https://en.wikipedia.org/wiki/Caustic_(optics) https://en.wikipedia.org/wiki/Caustic_(optics)>. Particle DM is still quantum particles, with Compton and de Broglie wavelengths. A cold milliectronvolt axion has a wavelength on the order of a kilometre. How do you trap a pair of them in a volume comparable to the Schwarzschild radius of about 10^-50 metres? Additionally, an appreciable mass-energy of dark matter (one you can expect to be described tractably by the Raychaudhuri equations) will be a many-particle system occupying a noncompact volume. Newton's constant G in SI units is the force in Newtons between two one-kilogram masses at rest with respect to each other at one metre apart. It's small (< 7e-11). You will wait a long time -- about a full Earth day -- for two kg of anything initial 1m apart to get very much closer to each other because of (only) their mutual gravitational interaction. However, each of the pair of kg-mass blobs of DM particles is not likely to stick around intact that long because (by virtue of them being weakly interacting particles) nothing around them is strong enough to keep them from running away due to thermal motion. They are only cold in the sense they move slowly compared to hot dark matter in the form of relativistic neutrinos; they can still move about at many m/s, which is a lot faster than m/day. Newton's constant is rather small, and rather too tiny when thinking of coaxing a couple kg of CDM into a volume comparable to the Schwarzschild radius of about 10^-27 metres. One might compare this with two kg-masses of water ice. In a ball of ice various interactions trap molecules within the ball even against sublimation. The ice balls will come into contact in about a day. Thus we can have things like Saturn's rings or periodic comets, helped by the non-gravitational interactions of water ice keeping the mass-energy-momentum from flying away before gravity can do its work. Indeed, keeping galaxy and galaxy-cluster dark matter halos from flying away puts an upper bound on their temperature (if you heat up the water ice above to a couple hundred kelvins it can sublimate away in minutes). Halo DM needs to be cold in order for it to stick around. tl;dr, two sets of non-self-interacting (collisionless) dark matter just slide right through each other. cf. the famous Bullet cluster collision. You'd need at least a galaxy worth of dark matter and at least many millions of years of Chandrasekhar dynamical friction (radiating away very low amplitude gravitational waves as structures ("subhalos") form and move through each other) to make a black hole from DM alone.

rocqua 4y ago

This seems like a very nicely detailed response to OPs question. Thanks! Your response brought up some questions and remarks from myself. > DM can be falsified perfectly well. I recall hearing that DM was attempted to be falsified based on gravitational lensing. Is that correct? > If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation. Would DM as matter not have inertia, whereas treating it like an artifact of space-time would leave it without inertia? > Differentiating it from MoND or similar theories will then be based purely on how well the theory fits. A simple question here would be: do all galaxies of similar size have the same amount of dark matter. If so, then the uniformity of dark-matter would need explanation. If not, then I think something like MoND would have a hard time explaining why these similar galaxies rotate at different speeds.

naasking 4y ago

> However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable. I think this is a strawman. MOND proponents don't discount that some observations are better suited to a particle, they just assert that the particle dark matter sledgehammer is vastly overused, and that modified gravity has been unusually successful in cases where it had no right to be if gravity acts as it does under GR. For instance, this review [1] of the astrophysical evidence by a MOND proponent makes a good case for MOND + sterile neutrinos. This review also covers numerous falsifications of DM, so if you're correct that DM can be falsified, why did these falsifications fail to convince the majority? [1] 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

sigmoid10 4y ago
>why did these falsifications fail to convince the majority? Because they are cherry picked. In the big picture, for every area where particle DM struggles, you will find a dozen datapoints where MOND struggles. Physicists are also wary of approaches like MOND+particle DM, because it just introduces more parameters which you can arbitrarily tune to explain data. It feels like every week someone comes with a new model that fits certain edge cases better, but doesn't actually tell us anything new or could act as a reasonable guide for future work. You can imagine it like fitting a polynomial of degree N to N data points. Yes it will fit perfectly, but that doesn't mean it's a good model. This is just sophisticated curve fitting to generate papers and not good science.
- naasking 4y ago
  > Because they are cherry picked. Sure, as are the objections to MOND. > Physicists are also wary of approaches like MOND+particle DM, because it just introduces more [...] You can imagine it like fitting a polynomial of degree N to N data points. Yes it will fit perfectly, but that doesn't mean it's a good model. This is just sophisticated curve fitting to generate papers and not good science. That's exactly what's happening with cold dark matter models. I agree the state of things is unsatisfactory for either theory, we need bolder ideas. Superfluid dark matter was promising but doesn't currently match some observations. We need more of that.