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> * The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations. Dark ma

> * The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations. Dark matter models are fine-tuned to reproduce certain observations. These don't really count as evidence in favour of DM. > * It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO. MOND has more points in favour for it as a preferred theory than DM models. See [1]. > Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background. I'm not sure why I should find your claim that DM is "essential" as persuasive given MOND has received orders of magnitude less attention and less development than DM models. I would certainly not bet any money on MOND being incapable of reproducing these observations. [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

alecst 4y ago
I worked in this field for a minute (XENON1T) and I heard a few talks about MOND. We were all pretty open minded but it failed to stick because it didn’t convince us, for a number of reasons. Curious about your background here.
elcomet 4y ago
> Dark matter models are fine-tuned to reproduce certain observations That's the definition of physics. Models are fine-tuned to match the observation and be predictive.
- 3um35w1l 4y ago
  sure, but you didn't get the point. just because a model can predict certain results doesn't mean its correct. early astronomers were able to very precisely predict the positions (w/ looping motions and all) of stellar bodies in the night sky despite using a geocentric model. if you make it complex enough, you can engineer quite a few models that can predict natural phenomenon pretty well.
  - kubanczyk 4y ago
    > if you make it complex enough, you can engineer quite a few models that can predict natural phenomenon pretty well. Well, yes. Next thing is to pick the easiest of these models: Occam's razor. But pondering whether such model is "correct" won't bring any further improvements.
  - elcomet 4y ago
    Yes of course, but this applies to any model. We cannot know of any model is "correct", they are just tools to describe what we observe. If they are more predictive than other models, then we should use them. Or course it's better to use a simpler model that gives the same predictions.
- abainbridge 4y ago
  I agree. I find it hard to argue against the claim that all physics is just curve fitting. We always end up with tuned parameters to make the maths work. For example the G in F = Gmm/r^2 (Newtonian mechanics). I'm not an expert, but it seems like Dark Matter theories allow the DM distribution to be tuned. Is that the case? Can you say, "oh, there must be a clump of DM over here with mass X". If so, that would give you lots of parameters to tune per situation you're modelling. It may be that IS how the universe works. The problem is that the theory is hard to falsify if it has so much wiggle room to fit every situation.
  - Avshalom 4y ago
    Same way normal matter has a clump here or a galaxy there or a super cluster over there.
    - abainbridge 4y ago
      Except we don't get to invent where they are because you can see normal matter. We tried before to invent some normal matter to explain the orbit of Mercury. https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet).
  - GoblinSlayer 4y ago
    Fine tuning usually means some kind of teleological goal, not merely uniqueness of present state.
- naasking 4y ago
  > That's the definition of physics. Models are fine-tuned to match the observation and be predictive. That's not what "fine-tuned" means. Epicycles can be fine-tuned to reproduce the motion we see from elliptical orbits. However, we should clearly prefer the theory of elliptical orbits because it reproduces observations without any extra parameters needed to fine-tune it.
  - raattgift 4y ago
    In physics, a (free) parameter is finely-tuned when the model is highly sensitive to different values at high precisions. If you get radically different results for some parameter k = 0.0000000010 and k = 0.0000000011, and observations match only the first case but clearly do not match the second (or any value much larger or smaller than either of these), then the value of parameter k is finely-tuned. One may need to add more free parameters to a theory, but those parameters need not be finely tuned. A free parameter is one that must be measured or estimated, as opposed to dependent parameters which are fixed by the theory (and possibly its free parameters). The pointwise energy-density of dark matter, or its contribution to the stress-energy tensor at a point -- a free parameter -- is presently only estimated, but that estimate is not finely-tuned. In fact, that's part of the practical problem of proving particle dark matter: the pointwise density is underdetermined by observation. As a result, we don't know whether the DM particles must all have very small masses, or whether there can be a mix of masses right up to masses comparable to that of Jupiter. We also don't know the density-gradient very precisely within galaxy clusters and individual galaxies. However, we get coarsely-similar galaxies and clusters in simulations for a wide variety of choices in these areas. So therefore, the dark-matter density (and its gradient, if any) are not finely-tuned. We must therefore search the parameter space to exclude many families of possibilities as significant contributors to dark matter energy-density. (Which has happened: many types of MACHO have been ruled out by observation; various supersymmetry candidates for WIMPs have been ruled out by experiment; most "warm" dark matter has been ruled out; and so forth). If only the distribution of dark matter was finely-tuned, we could quickly narrow down the search space : <https://en.wikipedia.org/wiki/Dark_matter#Composition https://en.wikipedia.org/wiki/Dark_matter#Composition> (the last box in the table is for the cases where one has the mathematical freedom to choose to move some or all of the non-metric-tensor additional field(s) from the curvature side of the Einstein Field Equations to the matter side (where the stress-energy tensor is found), without prejudice as to whether it is more naturally (in a conceptual sense) a function on the curvature (which is why the cosmological constant appears on that side, where it could just as easily be written down as a function on stress-energy)).
- baryphonic 4y ago
  If this is true, then were epicycle models physics? How about the luminiferous aether?
sdoering 4y ago
As my understanding goes the models used are there to see what would be needed to replicate the current best know state of the universe, given what we know about its beginnings and as said its "current" state. Running very many different scenarios the best models need something the astro "community" came to call "dark matter". As long as a opposing theory can't replicate these simulations from initial state to known "now" state without the use of some form of "whatever" it is quite interesting to call out that there is nothing like "dark matter" (whatever this something may be). But that is just my layman's take.
eterevsky 4y ago
> MOND has more points in favour for it as a preferred theory than DM models. Isn't MOND completely falsified by the discovery of galaxies without dark matter?
- sigmoid10 4y ago
  It was already falsified by well established limits on self interaction of DM from galaxy collisions. And from the already mentioned temperature fluctuations in the CMB. Every now and then some MOND supporter comes out if they found a way to tune their model to one specific observation, but particle DM remains the only approach that can fit all these observations at the same time. Ofc that doesn't guarantee that particle DM is the only explanation. After all, it could be a combination of DM+MOND. But MOND alone is just not possible, whereas DM alone might be.
  - naasking 4y ago
    By that argument, DM was already falsified by the observations pointed out in this article and the paper I linked. Your claim that DM could "fit all of these observations at the same time" is just wrong. DM has been tuned to correct its parameter-free predictions, so if you're going to compare like against like, then MOND should be permitted to be tuned to fit as well, and we can judge in the end which theory requires the least fine-tuning. As for galaxies without DM, those observations are still being debated, but even if true, that doesn't entail MOND can't explain those results.
    - eterevsky 4y ago
      I don't think it's fair to say that it's "tuned" to fit observation. It's not like several different types of DM are introduced to fit the observation. Rather the observations add some constraints to the DM properties. That's how it's supposed to be.
      - naasking 4y ago
        If DM has to have a very specific initial distribution to reproduce what we see, for example, that too is a constraint but it's also fine-tuning. Another theory that is free of such parameters should be preferred. Some types of constraints really should decrease a theory's plausibility.

alecst 4y ago

I worked in this field for a minute (XENON1T) and I heard a few talks about MOND. We were all pretty open minded but it failed to stick because it didn’t convince us, for a number of reasons. Curious about your background here.

elcomet 4y ago

> Dark matter models are fine-tuned to reproduce certain observations That's the definition of physics. Models are fine-tuned to match the observation and be predictive.

3um35w1l 4y ago
sure, but you didn't get the point. just because a model can predict certain results doesn't mean its correct. early astronomers were able to very precisely predict the positions (w/ looping motions and all) of stellar bodies in the night sky despite using a geocentric model. if you make it complex enough, you can engineer quite a few models that can predict natural phenomenon pretty well.
- kubanczyk 4y ago
  > if you make it complex enough, you can engineer quite a few models that can predict natural phenomenon pretty well. Well, yes. Next thing is to pick the easiest of these models: Occam's razor. But pondering whether such model is "correct" won't bring any further improvements.
- elcomet 4y ago
  Yes of course, but this applies to any model. We cannot know of any model is "correct", they are just tools to describe what we observe. If they are more predictive than other models, then we should use them. Or course it's better to use a simpler model that gives the same predictions.
abainbridge 4y ago
I agree. I find it hard to argue against the claim that all physics is just curve fitting. We always end up with tuned parameters to make the maths work. For example the G in F = Gmm/r^2 (Newtonian mechanics). I'm not an expert, but it seems like Dark Matter theories allow the DM distribution to be tuned. Is that the case? Can you say, "oh, there must be a clump of DM over here with mass X". If so, that would give you lots of parameters to tune per situation you're modelling. It may be that IS how the universe works. The problem is that the theory is hard to falsify if it has so much wiggle room to fit every situation.
- Avshalom 4y ago
  Same way normal matter has a clump here or a galaxy there or a super cluster over there.
  - abainbridge 4y ago
    Except we don't get to invent where they are because you can see normal matter. We tried before to invent some normal matter to explain the orbit of Mercury. https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet) https://en.wikipedia.org/wiki/Vulcan_(hypothetical_planet).
- GoblinSlayer 4y ago
  Fine tuning usually means some kind of teleological goal, not merely uniqueness of present state.
naasking 4y ago
> That's the definition of physics. Models are fine-tuned to match the observation and be predictive. That's not what "fine-tuned" means. Epicycles can be fine-tuned to reproduce the motion we see from elliptical orbits. However, we should clearly prefer the theory of elliptical orbits because it reproduces observations without any extra parameters needed to fine-tune it.
- raattgift 4y ago
  In physics, a (free) parameter is finely-tuned when the model is highly sensitive to different values at high precisions. If you get radically different results for some parameter k = 0.0000000010 and k = 0.0000000011, and observations match only the first case but clearly do not match the second (or any value much larger or smaller than either of these), then the value of parameter k is finely-tuned. One may need to add more free parameters to a theory, but those parameters need not be finely tuned. A free parameter is one that must be measured or estimated, as opposed to dependent parameters which are fixed by the theory (and possibly its free parameters). The pointwise energy-density of dark matter, or its contribution to the stress-energy tensor at a point -- a free parameter -- is presently only estimated, but that estimate is not finely-tuned. In fact, that's part of the practical problem of proving particle dark matter: the pointwise density is underdetermined by observation. As a result, we don't know whether the DM particles must all have very small masses, or whether there can be a mix of masses right up to masses comparable to that of Jupiter. We also don't know the density-gradient very precisely within galaxy clusters and individual galaxies. However, we get coarsely-similar galaxies and clusters in simulations for a wide variety of choices in these areas. So therefore, the dark-matter density (and its gradient, if any) are not finely-tuned. We must therefore search the parameter space to exclude many families of possibilities as significant contributors to dark matter energy-density. (Which has happened: many types of MACHO have been ruled out by observation; various supersymmetry candidates for WIMPs have been ruled out by experiment; most "warm" dark matter has been ruled out; and so forth). If only the distribution of dark matter was finely-tuned, we could quickly narrow down the search space : <https://en.wikipedia.org/wiki/Dark_matter#Composition https://en.wikipedia.org/wiki/Dark_matter#Composition> (the last box in the table is for the cases where one has the mathematical freedom to choose to move some or all of the non-metric-tensor additional field(s) from the curvature side of the Einstein Field Equations to the matter side (where the stress-energy tensor is found), without prejudice as to whether it is more naturally (in a conceptual sense) a function on the curvature (which is why the cosmological constant appears on that side, where it could just as easily be written down as a function on stress-energy)).
baryphonic 4y ago
If this is true, then were epicycle models physics? How about the luminiferous aether?

sdoering 4y ago

As my understanding goes the models used are there to see what would be needed to replicate the current best know state of the universe, given what we know about its beginnings and as said its "current" state. Running very many different scenarios the best models need something the astro "community" came to call "dark matter". As long as a opposing theory can't replicate these simulations from initial state to known "now" state without the use of some form of "whatever" it is quite interesting to call out that there is nothing like "dark matter" (whatever this something may be). But that is just my layman's take.

eterevsky 4y ago

> MOND has more points in favour for it as a preferred theory than DM models. Isn't MOND completely falsified by the discovery of galaxies without dark matter?

sigmoid10 4y ago
It was already falsified by well established limits on self interaction of DM from galaxy collisions. And from the already mentioned temperature fluctuations in the CMB. Every now and then some MOND supporter comes out if they found a way to tune their model to one specific observation, but particle DM remains the only approach that can fit all these observations at the same time. Ofc that doesn't guarantee that particle DM is the only explanation. After all, it could be a combination of DM+MOND. But MOND alone is just not possible, whereas DM alone might be.
- naasking 4y ago
  By that argument, DM was already falsified by the observations pointed out in this article and the paper I linked. Your claim that DM could "fit all of these observations at the same time" is just wrong. DM has been tuned to correct its parameter-free predictions, so if you're going to compare like against like, then MOND should be permitted to be tuned to fit as well, and we can judge in the end which theory requires the least fine-tuning. As for galaxies without DM, those observations are still being debated, but even if true, that doesn't entail MOND can't explain those results.
  - eterevsky 4y ago
    I don't think it's fair to say that it's "tuned" to fit observation. It's not like several different types of DM are introduced to fit the observation. Rather the observations add some constraints to the DM properties. That's how it's supposed to be.
    - naasking 4y ago
      If DM has to have a very specific initial distribution to reproduce what we see, for example, that too is a constraint but it's also fine-tuning. Another theory that is free of such parameters should be preferred. Some types of constraints really should decrease a theory's plausibility.