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My intuitive objection against dark matter is that it's basically an infinite amount of additional parameters to be fitted to the observations. Galaxy behaves w

My intuitive objection against dark matter is that it's basically an infinite amount of additional parameters to be fitted to the observations. Galaxy behaves weirdly? Add dark matter! It behaves as expected? It obviously has lost its dark matter! It stands to reason that for every possible observation, a particular DM configuration can be calculated. I fail to see how dark matter could ever be falsified. The best potential replacement would be a theory that's simpler, but even then dark matter would probably explain the same observations. The second question is: Is dark matter even matter if it only interacts via gravity? Wouldn't it be better named a property of space-time? That is, dark matter is a name for a recursive property of gravity?

namaria 4y ago
Unless you have worked extensively with the data and math of modern cosmology I very much doubt your 'intuition' about 'dark matter' is anywhere close to the intended meaning.
- AnIdiotOnTheNet 4y ago
  Right? After quantum mechanics and relativity there's absolutely no reason to believe that our intuition is worth squat when it comes to how physics works.
  - npc12345 4y ago
doliveira 4y ago
Dark matter is the simplest theory.
- TEP_Kim_Il_Sung 4y ago
  *hypothesis Theory implies they found out what it is.
  - mr_mitm 4y ago
    In physics, we have laws, rules, models, equations, theories, theorems, postulates, and maybe more. I can't think of one instance in which something has been "promoted" from one to the other after some data came in and couldn't even rank these in terms of certainty. We still call it Newton's law even though we know it's wrong. We still call it Born rule even though it's one of the most tested things we have. We still call it the standard model even though it's the thing that works better than everything else. We call particular Lagrangians in Quantum Field Theory "theory" even though we know it's just a toy theory which doesn't describe reality. I mean, "toy theory" or "toy model" is a thing. Physicists aren't nearly as nit-picky as internet commentators when it comes to this topic.
    - newaccount2021 4y ago
    - TEP_Kim_Il_Sung 4y ago
      Thanks for expanding on my comment. You said it, bruh!
mr_mitm 4y ago
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.
antonvs 4y ago
Dark matter is not a “recursive property of gravity” because it’s not consistent like that, as your own initial objection suggests. The concept of a particle that only interacts via gravity is straightforward. Consider neutrinos, which only interact via gravity and the weak force. Trillions of them pass through your body every second. Take away the weak force interaction and add some mass, and you have a dark matter candidate particle. > I fail to see how dark matter could ever be falsified. You’d have to explain what should be done about that. It could nevertheless be a correct theory, and we might one day verify it more directly. Naive falsificationism is an almost useless principle at the frontiers of modern physics. We’re long past the days when you could run some simple experiments and come up with a clear yea or nay. That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.
- choeger 4y ago
  > You’d have to explain what should be done about that. Well, there should be something that follows from the assumption that there are huge amounts of particles that only interact via gravity. What's the prediction that this model makes? If it doesn't make any useful prediction, it's pretty much worthless. > That’s living in the 19th century, basically - which admittedly, many people still seem to want to do. I think this attitude is inappropriate. The scientific progress made with 19th century methods, and that includes most of the stuff discovered until the 1950s, is massive. Maybe scientists should focus more on a clear nay or yea? I mean, the LHC is certainly a significant achievement, but compared to the detection of gravitational waves or even just exoplanets it doesn't seem to deliver as much bang for the buck.
  - YawningAngel 4y ago
    Indulge the counterfactual for a second. Suppose that dark matter really does exist, and we can't ascertain that fact with any certainty. What do you want Physics as a whole to do about that? I think everybody agrees a simple answer would be great, we're just unable to provide one in this context
    - 4y ago
nonrandomstring 4y ago
"Dark Matter" defies parsimony. Occam's Razor suggests rejecting the assumption that spacetime is uniform. The search for "Laws" may be a parochialism built into human epistemology. Advanced aliens may be comfortable with the idea of "space weather" and local variations. :)
rocqua 4y ago
I very much agree with you, and have raised these questions before. I got some interesting responses that half-mollified my worries. Originally dark matter was hypothesized because of galaxy rotation mismatches. This allows for mapping where 'dark matter' should be. One falsification of dark matter that was tried is gravitational lensing. If there is indeed extra curvature of space-time in these galaxies, then they should bend light-rays, Potentially showing the same galaxy twice. This was tested, and confirmed. Hypothesized dark matter, as detected by galaxy rotation, causes the expected gravitational lensing. I suppose another potential falsification is by 'moving' dark matter through a gravitational tractor (very heavy mass with rocket engines) and then detect the resultant change in gravity. You could attempt that detection by gravitational lensing, orbit velocity, or perhaps something else. Despite all this, dark matter still feels like our theory of gravity was falsified, and instead of concluding the theory was wrong, it was attempted to add random parameters to the theory. Its not like there is a better theory though (I believe MONS has trouble with explaining gravitational lensing), so perhaps this is a problem of PR much more than a problem of science. As for the second question, for dark matter to be matter, I think it suffices if it has mass and inertia. My gravity tractor detection idea tries to work with that. Edit: also see this: https://medium.com/starts-with-a-bang/7-independent-pieces-of-evidence-for-dark-matter-3692126a2283 https://medium.com/starts-with-a-bang/7-independent-pieces-o... from a different reply in this HN discussion.
scott_w 4y ago
My intuitive objection to your comment is that it ignores the basis of the article: that it’s entirely about learning more about dark matter. This is the complete antithesis to what your comment claims.
sega_sai 4y ago
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
_a_a_a_ 4y ago
Where does "infinite amount of additional parameters" come from? It's just one parameter - something's (we call dark matter) is either there or it isn't. > I fail to see how dark matter could ever be falsified I don't think science cares. Frankly if you could back up your opinion with some actual reasoned argument, it would be a better post.
mhh__ 4y ago
> infinite amount of additional parameters Not in any technical sense I can think of.

namaria 4y ago

Unless you have worked extensively with the data and math of modern cosmology I very much doubt your 'intuition' about 'dark matter' is anywhere close to the intended meaning.

AnIdiotOnTheNet 4y ago
Right? After quantum mechanics and relativity there's absolutely no reason to believe that our intuition is worth squat when it comes to how physics works.
- npc12345 4y ago

doliveira 4y ago

Dark matter is the simplest theory.

TEP_Kim_Il_Sung 4y ago
*hypothesis Theory implies they found out what it is.
- mr_mitm 4y ago
  In physics, we have laws, rules, models, equations, theories, theorems, postulates, and maybe more. I can't think of one instance in which something has been "promoted" from one to the other after some data came in and couldn't even rank these in terms of certainty. We still call it Newton's law even though we know it's wrong. We still call it Born rule even though it's one of the most tested things we have. We still call it the standard model even though it's the thing that works better than everything else. We call particular Lagrangians in Quantum Field Theory "theory" even though we know it's just a toy theory which doesn't describe reality. I mean, "toy theory" or "toy model" is a thing. Physicists aren't nearly as nit-picky as internet commentators when it comes to this topic.
  - newaccount2021 4y ago
  - TEP_Kim_Il_Sung 4y ago
    Thanks for expanding on my comment. You said it, bruh!

mr_mitm 4y ago

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.

antonvs 4y ago

Dark matter is not a “recursive property of gravity” because it’s not consistent like that, as your own initial objection suggests. The concept of a particle that only interacts via gravity is straightforward. Consider neutrinos, which only interact via gravity and the weak force. Trillions of them pass through your body every second. Take away the weak force interaction and add some mass, and you have a dark matter candidate particle. > I fail to see how dark matter could ever be falsified. You’d have to explain what should be done about that. It could nevertheless be a correct theory, and we might one day verify it more directly. Naive falsificationism is an almost useless principle at the frontiers of modern physics. We’re long past the days when you could run some simple experiments and come up with a clear yea or nay. That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.

choeger 4y ago
> You’d have to explain what should be done about that. Well, there should be something that follows from the assumption that there are huge amounts of particles that only interact via gravity. What's the prediction that this model makes? If it doesn't make any useful prediction, it's pretty much worthless. > That’s living in the 19th century, basically - which admittedly, many people still seem to want to do. I think this attitude is inappropriate. The scientific progress made with 19th century methods, and that includes most of the stuff discovered until the 1950s, is massive. Maybe scientists should focus more on a clear nay or yea? I mean, the LHC is certainly a significant achievement, but compared to the detection of gravitational waves or even just exoplanets it doesn't seem to deliver as much bang for the buck.
- YawningAngel 4y ago
  Indulge the counterfactual for a second. Suppose that dark matter really does exist, and we can't ascertain that fact with any certainty. What do you want Physics as a whole to do about that? I think everybody agrees a simple answer would be great, we're just unable to provide one in this context
  - 4y ago

nonrandomstring 4y ago

"Dark Matter" defies parsimony. Occam's Razor suggests rejecting the assumption that spacetime is uniform. The search for "Laws" may be a parochialism built into human epistemology. Advanced aliens may be comfortable with the idea of "space weather" and local variations. :)

rocqua 4y ago

I very much agree with you, and have raised these questions before. I got some interesting responses that half-mollified my worries. Originally dark matter was hypothesized because of galaxy rotation mismatches. This allows for mapping where 'dark matter' should be. One falsification of dark matter that was tried is gravitational lensing. If there is indeed extra curvature of space-time in these galaxies, then they should bend light-rays, Potentially showing the same galaxy twice. This was tested, and confirmed. Hypothesized dark matter, as detected by galaxy rotation, causes the expected gravitational lensing. I suppose another potential falsification is by 'moving' dark matter through a gravitational tractor (very heavy mass with rocket engines) and then detect the resultant change in gravity. You could attempt that detection by gravitational lensing, orbit velocity, or perhaps something else. Despite all this, dark matter still feels like our theory of gravity was falsified, and instead of concluding the theory was wrong, it was attempted to add random parameters to the theory. Its not like there is a better theory though (I believe MONS has trouble with explaining gravitational lensing), so perhaps this is a problem of PR much more than a problem of science. As for the second question, for dark matter to be matter, I think it suffices if it has mass and inertia. My gravity tractor detection idea tries to work with that. Edit: also see this: https://medium.com/starts-with-a-bang/7-independent-pieces-of-evidence-for-dark-matter-3692126a2283 https://medium.com/starts-with-a-bang/7-independent-pieces-o... from a different reply in this HN discussion.

scott_w 4y ago

My intuitive objection to your comment is that it ignores the basis of the article: that it’s entirely about learning more about dark matter. This is the complete antithesis to what your comment claims.

sega_sai 4y ago

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

_a_a_a_ 4y ago

Where does "infinite amount of additional parameters" come from? It's just one parameter - something's (we call dark matter) is either there or it isn't. > I fail to see how dark matter could ever be falsified I don't think science cares. Frankly if you could back up your opinion with some actual reasoned argument, it would be a better post.

mhh__ 4y ago

> infinite amount of additional parameters Not in any technical sense I can think of.