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It's a bit of the same from Sabine. Dark matter hypothesis allowed us to simulate a vast range of processes in the Universe, from large scale cosmological simul

It's a bit of the same from Sabine. Dark matter hypothesis allowed us to simulate a vast range of processes in the Universe, from large scale cosmological simulations, predicting clustering of galaxies, cosmic microwave background to smaller scale simulations of galaxy formation and individual galaxies. The hypothesis have been extremely successful and predictive. It is also true, that some of the systems we've been discovering are a bit harder to explain within our Cold Dark matter framework, requiring changing some things about how galaxies form (i.e. making star formation less efficient in small galaxies, introducing feedback effects from accreting supermassive black hols and things like that). I (and many astrophysicists) do not see that as a problem or evidence that dark matter needs to be abandoned, but instead see that as a refinement to the theory. Regarding modified gravity theories, it's true that you can mimic some of the dark matter behaviour by the modification of gravity laws. Also if you can add some kind of global scalar field you can similarly mimic dark matter behaviour. The problem however is that the modified gravity theories (at this point) do not allow the same range of simulations that cold dark matter(CDM) simulations can do (for various reasons, such as many of those theories do not have a general relativity formulation). Because these theories can't make as many/as varied simulations as CDM ones, they are not as tested as regular CDM and much less predictive. Therefore based on that fact alone IMO these theories are less useful. It is also true that if we continue for many years searching for the dark matter particle and will still be unable to find it, maybe we will need to refocus our attention on alternative gravity theories, but the problem is that so far the part of parameter space where we looked for the dark matter particle is still pretty small and there is no strong reason to think that dark matter should have been there.

posix_me_less 5y ago
> The hypothesis have been extremely successful and predictive. Epicycles were successful and predictive. But they were later found to be the wrong way to understand the celestial motions. The problem is these calculations with dark matter are just fitting observations with zillion parameters. It is busywork, yes sometimes it is useful, but it is not discovering new laws, just chugging along without having to challenge our concept of how star/galaxy motions really work.
- sega_sai 5y ago
  Epicycles were a good theory that approximated a ellipses by combinations of circles (essentially through a Fourier transform). When a simpler theory came that was easier to compute, was more predictive and came from a first principles calculations it was adopted instead. That's not the case with the modified gravity (at least right now). And please don't BS about zillion of parameters. That's not the case. The dark matter only simulations don't have that many parameters (other than the basic cosmological ones) . When you start putting baryons in, yes that's where the parameters start appearing (probably of the order of a 100), but it's hardly surprising give the need to describe the complexity of star formation, gas dynamics, black hole accretion etc. The modified gravity theories don't need as many parameters because nobody has ever run any modified gravity simulation that is even close in complexity to what Cold Dark Matter simulations do.
  - posix_me_less 5y ago
    I agree MOND results aren't that great now. But Kopernik's results weren't that great either, compared to standard theory. They gave less precise results initially. It took time for the astronomers to accept them. What kind of initial condition about dark matter distribution are you assumming in your simulations?
    - sega_sai 5y ago
      Copernicus model was not that much of a change from a model complexity point of view, as it just removed some common epicycles by putting Sun in the center. The key changes were Kepler's results and Newton's, where one had provided drastically simpler model, and another had a theoretical/(first principles) derivation. Regarding initial conditions, I'll point you towards wikipedia. But basically the assumptions is that it's collisionless fluid with very small Gaussian density perturbations and scale invariant power spectrum. In total the specification of that model requires ~ 10 parameters.
      - posix_me_less 5y ago
        So I infer you are talking about large scale simulations of many galaxies/a region much greater than single galaxy, right? If so, do these kinds of simulations with 10 parameter model predict galactical rotation curves in agreement with observation? Or do we need more detailed model of dark matter distribution for the rotation curves?
nosianu 5y ago
> I (and many astrophysicists) do not see that as a problem or evidence that dark matter needs to be abandoned As I read it, and I think it was pretty straightforward, neither does she? She wants to keep it where it seems to be an appropriate model.
slowmovintarget 5y ago
Modified gravity would not cover the main case for dark matter, though, which is not galaxy formation or patterns of motion we observe. Rather the main case is the ratio of protons and neutrons to photons produced during Big Bang nucleosythesis. We know the ratio of the number of protons to number of photons. We know the number of photons in the universe (most of them are in the CMB), and we therefore know the number of protons. That means we know the amount of "normal" matter in the universe, and it is far less than the total amount of matter in the universe (that survived from baryogenesis). This requires there to be "dark matter" in the universe. The other effects like galaxy formation and stellar motion in galaxies are just things that might be caused by dark matter (though that's come into doubt, as Sabine points out). The real reason to believe in dark matter is Big Bang nucleosynthesis. Sean Carroll discusses it here: https://youtu.be/tZQadPmTd84?t=4642 https://youtu.be/tZQadPmTd84?t=4642
- posix_me_less 5y ago
  > We know the number of photons in the universe (most of them are in the CMB), and we therefore know the number of protons. If the strongest argument for DM was fixing a particular model of nucleosynthesis, most people who care about this hypothesis would not care for it. In my opinion, extrapolating our standard theories back in time to "big bang" and drawing consequences such as "DM had to be created so that our model of nucleosynthesis can be preserved" is not a credible science. But no, DM was introduced to explain observations of celestial motions. And it works. Too well, it is too accommodating to any observations to be a great insight of what is going on with celestial motions.
Sniffnoy 5y ago
But Hossenfelder doesn't propose abandoning dark matter; this comment seems like a mischaracterization of her position as stated in the original post. Rather Hossenfelder suggests that both dark matter and modified gravity may be necessary, with each being relevant in different situations due to some sort of phase change (?), noting in the comments Khoury's idea of superfluid dark matter (although I'm unclear as to whether that includes modified gravity?) as one example of such a theory. Edit: Oops, misattributed & misdescribed superfluid dark matter
- eutectic 5y ago
  Seems like an excuse for introducing even more free parameters.
- adw 5y ago
  I'm a long time out of the game (PhD in solid state physics in the early 2000s), but the key phrase here is "phase transition". She's making an argument along the lines of: * MOND is a field-like hypothesis which explains some set of currently-unexplained astronomical observations; * Dark matter is a particle-like hypothesis which explains a (different) set of astronomical observations similarly * because of wave-particle duality the field explanation and particle explanation may both correspond to the same thing in two different regimes (one where "wave-like" behavior is the most convenient explanation, one where "particle-like" behavior is, and most likely with the transition between these regimes corresponding to observations at different length-scales) * this can be analogized to a phase transition in solid state physics * the people who understand phase transitions aren't working on astrophysics (and the communities largely do not overlap) therefore it might be that people are attacking the problem from the wrong perspective with the wrong mental models and mathematical tools. I have no real idea if the argument is sound or not, but it's centainly true that the communities don't overlap much and Sabine Hossenfelder has been one of the most active prominent scientists in her field in terms of outreach to other parts of physics so her perspective is likely to be informed.
  - raattgift 5y ago
    > the people who understand phase transitions aren't working on astrophysics I'm fairly sure there are plenty of equations of state with (first-order) phase transitions in the minds and literature of astrophysicists working on neutron stars. https://duckduckgo.com/?q=phase+transition+neutron+star https://duckduckgo.com/?q=phase+transition+neutron+star > wave-particle duality I don't see how that has anything to do with it. The superfluid dark matter idea is that dark matter behaves as the usual verrrrry-weakly (even only gravitationally) interacting (both with ordinary matter and with itself) perfect fluid in the Friedmann equations at the largest scales. However, as one takes the dark matter density higher, these rare or weak self-interactions increase in number or strength. At densities found deep enough within galaxy clusters and within individual galaxies the self-interaction produces a phase change that increases the intensity of interactions with ordinary matter at distances much greater than that of the diameters of atomic nuclei (but perhaps not infinite distances, as with electromagnetism). This is somewhat backwards from the neutron star studies: they start with microscopic physical mechanisms (quark deconfinement, pion condensation, and other exotica) and extract a change in bulk behaviour. By contrast, this family of dark matter idea starts with bulk behaviour and hopes to find microscopic physical mechanisms eventually. Which really is not very different from the standard cosmology's cold dark matter: bulk behaviour, but we haven't found the particles that generate it. But, turning to Ken Wilson (who is extremely important in generalized phase transitions [1]): density of discrete elements has an obvious length scale (the mean distance between the elements) and the phase-change idea is that above a certain length scale standard cosmology's cold dark matter is a fine effective theory; below that length scale, that effective theory needs adaptation. In these theories there's a singular point in the dark matter energy_density-viscosity curve, and on one side of that point the perfect fluid approximation is accurate for all practical purposes, and on the other additional parameters become relevant. Or turning it around, one recovers the standard perfect fluid cold dark matter from a model with additional parameters as one takes those parameters to zero, and thus the standard perfect fluid is emergent. And in turn as one studies the microscopic details of the superfluid, it is almost certain to emerge from something else, probably but not necessarily a quantum field theory. - -- [1] https://www.nobelprize.org/prizes/physics/1982/press-release/ https://www.nobelprize.org/prizes/physics/1982/press-release... [quick overview of Wilson's prize-winning work] and https://www.preposterousuniverse.com/blog/2013/06/20/how-quantum-field-theory-becomes-effective/ https://www.preposterousuniverse.com/blog/2013/06/20/how-qua... [renormalization] and http://www.preposterousuniverse.com/blog/2010/11/25/thanksgiving-5/ http://www.preposterousuniverse.com/blog/2010/11/25/thanksgi... [effective field theory more generally]

posix_me_less 5y ago

> The hypothesis have been extremely successful and predictive. Epicycles were successful and predictive. But they were later found to be the wrong way to understand the celestial motions. The problem is these calculations with dark matter are just fitting observations with zillion parameters. It is busywork, yes sometimes it is useful, but it is not discovering new laws, just chugging along without having to challenge our concept of how star/galaxy motions really work.

sega_sai 5y ago
Epicycles were a good theory that approximated a ellipses by combinations of circles (essentially through a Fourier transform). When a simpler theory came that was easier to compute, was more predictive and came from a first principles calculations it was adopted instead. That's not the case with the modified gravity (at least right now). And please don't BS about zillion of parameters. That's not the case. The dark matter only simulations don't have that many parameters (other than the basic cosmological ones) . When you start putting baryons in, yes that's where the parameters start appearing (probably of the order of a 100), but it's hardly surprising give the need to describe the complexity of star formation, gas dynamics, black hole accretion etc. The modified gravity theories don't need as many parameters because nobody has ever run any modified gravity simulation that is even close in complexity to what Cold Dark Matter simulations do.
- posix_me_less 5y ago
  I agree MOND results aren't that great now. But Kopernik's results weren't that great either, compared to standard theory. They gave less precise results initially. It took time for the astronomers to accept them. What kind of initial condition about dark matter distribution are you assumming in your simulations?
  - sega_sai 5y ago
    Copernicus model was not that much of a change from a model complexity point of view, as it just removed some common epicycles by putting Sun in the center. The key changes were Kepler's results and Newton's, where one had provided drastically simpler model, and another had a theoretical/(first principles) derivation. Regarding initial conditions, I'll point you towards wikipedia. But basically the assumptions is that it's collisionless fluid with very small Gaussian density perturbations and scale invariant power spectrum. In total the specification of that model requires ~ 10 parameters.
    - posix_me_less 5y ago
      So I infer you are talking about large scale simulations of many galaxies/a region much greater than single galaxy, right? If so, do these kinds of simulations with 10 parameter model predict galactical rotation curves in agreement with observation? Or do we need more detailed model of dark matter distribution for the rotation curves?

nosianu 5y ago

> I (and many astrophysicists) do not see that as a problem or evidence that dark matter needs to be abandoned As I read it, and I think it was pretty straightforward, neither does she? She wants to keep it where it seems to be an appropriate model.

slowmovintarget 5y ago

Modified gravity would not cover the main case for dark matter, though, which is not galaxy formation or patterns of motion we observe. Rather the main case is the ratio of protons and neutrons to photons produced during Big Bang nucleosythesis. We know the ratio of the number of protons to number of photons. We know the number of photons in the universe (most of them are in the CMB), and we therefore know the number of protons. That means we know the amount of "normal" matter in the universe, and it is far less than the total amount of matter in the universe (that survived from baryogenesis). This requires there to be "dark matter" in the universe. The other effects like galaxy formation and stellar motion in galaxies are just things that might be caused by dark matter (though that's come into doubt, as Sabine points out). The real reason to believe in dark matter is Big Bang nucleosynthesis. Sean Carroll discusses it here: https://youtu.be/tZQadPmTd84?t=4642 https://youtu.be/tZQadPmTd84?t=4642

posix_me_less 5y ago
> We know the number of photons in the universe (most of them are in the CMB), and we therefore know the number of protons. If the strongest argument for DM was fixing a particular model of nucleosynthesis, most people who care about this hypothesis would not care for it. In my opinion, extrapolating our standard theories back in time to "big bang" and drawing consequences such as "DM had to be created so that our model of nucleosynthesis can be preserved" is not a credible science. But no, DM was introduced to explain observations of celestial motions. And it works. Too well, it is too accommodating to any observations to be a great insight of what is going on with celestial motions.

Sniffnoy 5y ago

But Hossenfelder doesn't propose abandoning dark matter; this comment seems like a mischaracterization of her position as stated in the original post. Rather Hossenfelder suggests that both dark matter and modified gravity may be necessary, with each being relevant in different situations due to some sort of phase change (?), noting in the comments Khoury's idea of superfluid dark matter (although I'm unclear as to whether that includes modified gravity?) as one example of such a theory. Edit: Oops, misattributed & misdescribed superfluid dark matter

eutectic 5y ago
Seems like an excuse for introducing even more free parameters.
adw 5y ago
I'm a long time out of the game (PhD in solid state physics in the early 2000s), but the key phrase here is "phase transition". She's making an argument along the lines of: * MOND is a field-like hypothesis which explains some set of currently-unexplained astronomical observations; * Dark matter is a particle-like hypothesis which explains a (different) set of astronomical observations similarly * because of wave-particle duality the field explanation and particle explanation may both correspond to the same thing in two different regimes (one where "wave-like" behavior is the most convenient explanation, one where "particle-like" behavior is, and most likely with the transition between these regimes corresponding to observations at different length-scales) * this can be analogized to a phase transition in solid state physics * the people who understand phase transitions aren't working on astrophysics (and the communities largely do not overlap) therefore it might be that people are attacking the problem from the wrong perspective with the wrong mental models and mathematical tools. I have no real idea if the argument is sound or not, but it's centainly true that the communities don't overlap much and Sabine Hossenfelder has been one of the most active prominent scientists in her field in terms of outreach to other parts of physics so her perspective is likely to be informed.
- raattgift 5y ago
  > the people who understand phase transitions aren't working on astrophysics I'm fairly sure there are plenty of equations of state with (first-order) phase transitions in the minds and literature of astrophysicists working on neutron stars. https://duckduckgo.com/?q=phase+transition+neutron+star https://duckduckgo.com/?q=phase+transition+neutron+star > wave-particle duality I don't see how that has anything to do with it. The superfluid dark matter idea is that dark matter behaves as the usual verrrrry-weakly (even only gravitationally) interacting (both with ordinary matter and with itself) perfect fluid in the Friedmann equations at the largest scales. However, as one takes the dark matter density higher, these rare or weak self-interactions increase in number or strength. At densities found deep enough within galaxy clusters and within individual galaxies the self-interaction produces a phase change that increases the intensity of interactions with ordinary matter at distances much greater than that of the diameters of atomic nuclei (but perhaps not infinite distances, as with electromagnetism). This is somewhat backwards from the neutron star studies: they start with microscopic physical mechanisms (quark deconfinement, pion condensation, and other exotica) and extract a change in bulk behaviour. By contrast, this family of dark matter idea starts with bulk behaviour and hopes to find microscopic physical mechanisms eventually. Which really is not very different from the standard cosmology's cold dark matter: bulk behaviour, but we haven't found the particles that generate it. But, turning to Ken Wilson (who is extremely important in generalized phase transitions [1]): density of discrete elements has an obvious length scale (the mean distance between the elements) and the phase-change idea is that above a certain length scale standard cosmology's cold dark matter is a fine effective theory; below that length scale, that effective theory needs adaptation. In these theories there's a singular point in the dark matter energy_density-viscosity curve, and on one side of that point the perfect fluid approximation is accurate for all practical purposes, and on the other additional parameters become relevant. Or turning it around, one recovers the standard perfect fluid cold dark matter from a model with additional parameters as one takes those parameters to zero, and thus the standard perfect fluid is emergent. And in turn as one studies the microscopic details of the superfluid, it is almost certain to emerge from something else, probably but not necessarily a quantum field theory. - -- [1] https://www.nobelprize.org/prizes/physics/1982/press-release/ https://www.nobelprize.org/prizes/physics/1982/press-release... [quick overview of Wilson's prize-winning work] and https://www.preposterousuniverse.com/blog/2013/06/20/how-quantum-field-theory-becomes-effective/ https://www.preposterousuniverse.com/blog/2013/06/20/how-qua... [renormalization] and http://www.preposterousuniverse.com/blog/2010/11/25/thanksgiving-5/ http://www.preposterousuniverse.com/blog/2010/11/25/thanksgi... [effective field theory more generally]