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In classic internet fashion I only did a quick skim/search of the article and preprint of the paper (linked in this article). However, neither make mention of t

by podiki 3y ago

In classic internet fashion I only did a quick skim/search of the article and preprint of the paper (linked in this article). However, neither make mention of the bullet cluster [0] or "particle dark matter" and seem to rely more on galaxy rotation curves. While the bullet cluster may not be as definitive as we would like, it is at the very least very strongly suggestive of particle dark matter or something similar. As far as I remember modified gravitational theories without particle dark matter don't offer a good explanation of the bullet cluster (in short, evidence of a non- or weakly interacting but gravitational matter). I won't argue with schemes like MOND or this proposal that can explain rotation curves, but the bullet cluster at the very least is a more interesting test these days. Without even a mention of some alternative explanation here (from my quick look), it is a non-starter for a dark matter-less theory. And a pretty glaring omission (again, maybe I missed it) for what is a well-known observation. Still, alternatives for quantum gravity, and anything that might say something about dark energy, are exciting. Falsifiable ones especially. But if you didn't address the bullet cluster then... [0] https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster

naasking 3y ago
Milgrom's perspective on the Bullet Cluster: overblown and completely expected. http://astroweb.case.edu/ssm/mond/moti_bullet.html http://astroweb.case.edu/ssm/mond/moti_bullet.html
- podiki 3y ago
  I won't get into an argument about MOND. Don't take my word for it, take the vast majority (all but a few?) of anyone working in the field. And don't take Milgrom's word for it (or the one reference given) either, for that matter, as clearly he has a horse in the game. Sure you can continue to make MOND work to some degree, but is it really predictive if you keep adding things to make it work? What is the underlying theory? (The proposal in this linked article is interesting in that regard.) Okay, so maybe I did continue the argument a little :) I love for people working on alternative less mainstream ideas. But MOND was never very alive to begin with, let alone decades later. Let's move on.
  - zmgsabst 3y ago
    > Sure you can continue to make MOND work to some degree, but is it really predictive if you keep adding things to make it work? Do you hold dark matter theories to this standard?
    - podiki 3y ago
      Absolutely. I'm out of the field these days, but e.g supersymmetric WIMP dark matter was the most popular but got more and more clugely with low energy supersymmetry seeming less likely (or not as useful in solving the problems it meant to solve). Everyone would come up with some tuned ad hoc model for whatever dark matter "signal" was in fashion (before being ruled out); none of those were compelling. Fun to play with maybe, but didn't really tell us anything.
  - naasking 3y ago
    > What is the underlying theory? Who cares? This has literally never been a requirement of science and I don't know why people bring it up for MOND. What was Isaac's Newton's underlying theory when he proposed his law of gravitation? No explanation for why two masses attract, therefore we should reject an effective description? That's nonsense. A theory is scientific if it describes what we observe, period. If you have a deeper explanation right off the bat, that's a bonus, otherwise that's the goal of further research. The other poster adequately addressed the implicit hypocrisy applied to MOND vs. LCDM, so I'll just leave this reference for a proper analysis of who has been making predictions vs. tweaking their theory to fit observations: From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity, https://arxiv.org/abs/2110.06936 https://arxiv.org/abs/2110.06936
    - podiki 3y ago
      Fine, rather than "underlying theory" use some other words, basically does any new proposal give us predictions, solve some unexplained phenomenon, do something simpler and more compelling (sure, a matter of taste, but so is all of this to some degree), in short, does it tell us something new. Newton certainly did as he gave a law to describe and predict successfully the motions of the planets, for instance. I don't think we are saying anything different really. There wasn't a predicitve framework before (as far as I know), but then there was, so that is certainly progress. But the bar gets ever higher as we know more for what a theory should do. Just writing down an equation that can fit some data and can make predictions is great but is not the end of the story. We could just have some arbitrary functions that fit the data we have and call it a day.
podiki 3y ago
After a little more of a skim: the paper seems mostly to be about reproducing an equation like that of MOND (modified Newtonian dynamics). Which, fine, some people are fans of MOND (not most, and for good reason from the theoretical which this might address, to the practical like previous comment), but there's a whole lot more than just the dynamics of rotating galaxies to consider. Without even going into that (bullet cluster, lensing, n-body simulations, galaxy formation, etc.) it doesn't (yet?) replace anything.
- raattgift 3y ago
  Sure, but the origin of this preprint is in the stochastic-classical/uncertain-quantum "postquantum-classical" theory the Oppenheim group is working on. Their theory is premised on a weakening of the gravitational interaction at short distances (it's "asymptotically free" and thus renormalizable) compared to perturbative quantum gravity (which is non-renormalizable by power counting). However, they discovered that the gravitational interaction in their theory can be too strong. More technically, in their diffusion limit they extract a different scalar potential than from GR's weak field limit, with the former growing (statistically) stronger with increasing radius. This in practice means even with a highly similar source, their theory predicts significantly different geodesics compared to General Relativity. Rather than abandon the theory or complicate it such that it becomes compatible with fully classical GR, they solved a Schwarzschild-de Sitter (static spherical symmetry and an expansion term which "washes out" the extra-strength gravitational potential before it gets too strong) spacetime with a galactic mass (described simply and with some restrictions) and found that the geodesics are within a standard deviation or so of stable circular MONDian orbits. That result would relieve some of the immediate concern that their theory is unphysical, and drive them towards further study of its large-length-scale behaviour. (In fact their result is better than some earlier attempts to make a generally covariant MOND by adding auxiliary gravitational fields to GR (see the overview by Famaey & McGaugh 2012). Postquantum-classical gravity was on a relativistic footing from the start, although exactly how diffeomorphism invariance and manifest covariance of formulations works in postquantum-classical gravity is really interesting, see <https://arxiv.org/abs/2402.17844 https://arxiv.org/abs/2402.17844> if you want details) It would be interesting to see how their theory does does -- it would have to be studied numerically -- with any sort of CDM halo. Various stellar binaries are also an obvious place to look, since for sufficiently wide ones, you can't "wash out" the extra strength of the scalar potential with an expansion term. (Wide binaries are really rough on MOND too, for comparable reasons). > it doesn't (yet?) replace anything The authors know that. From just after Eqn (23) in the preprint <https://arxiv.org/abs/2402.19459 https://arxiv.org/abs/2402.19459>: "While this study demonstrates that galactic rotation curves can undergo modification due to stochastic fluctuations, a phenomenon attributed to dark matter, it is important to acknowledge the existence of separate, independent evidence supporting ΛCDM. In particular, in the CMB power spectrum, in gravitational lensing, in the necessity of dark matter for structure formation, and in a varied collection of other methods used to estimate the mass in galaxies"
deleted 3y ago
[deleted]

by podiki 3y ago

naasking 3y ago

Milgrom's perspective on the Bullet Cluster: overblown and completely expected. http://astroweb.case.edu/ssm/mond/moti_bullet.html http://astroweb.case.edu/ssm/mond/moti_bullet.html

podiki 3y ago
I won't get into an argument about MOND. Don't take my word for it, take the vast majority (all but a few?) of anyone working in the field. And don't take Milgrom's word for it (or the one reference given) either, for that matter, as clearly he has a horse in the game. Sure you can continue to make MOND work to some degree, but is it really predictive if you keep adding things to make it work? What is the underlying theory? (The proposal in this linked article is interesting in that regard.) Okay, so maybe I did continue the argument a little :) I love for people working on alternative less mainstream ideas. But MOND was never very alive to begin with, let alone decades later. Let's move on.
- zmgsabst 3y ago
  > Sure you can continue to make MOND work to some degree, but is it really predictive if you keep adding things to make it work? Do you hold dark matter theories to this standard?
  - podiki 3y ago
    Absolutely. I'm out of the field these days, but e.g supersymmetric WIMP dark matter was the most popular but got more and more clugely with low energy supersymmetry seeming less likely (or not as useful in solving the problems it meant to solve). Everyone would come up with some tuned ad hoc model for whatever dark matter "signal" was in fashion (before being ruled out); none of those were compelling. Fun to play with maybe, but didn't really tell us anything.
- naasking 3y ago
  > What is the underlying theory? Who cares? This has literally never been a requirement of science and I don't know why people bring it up for MOND. What was Isaac's Newton's underlying theory when he proposed his law of gravitation? No explanation for why two masses attract, therefore we should reject an effective description? That's nonsense. A theory is scientific if it describes what we observe, period. If you have a deeper explanation right off the bat, that's a bonus, otherwise that's the goal of further research. The other poster adequately addressed the implicit hypocrisy applied to MOND vs. LCDM, so I'll just leave this reference for a proper analysis of who has been making predictions vs. tweaking their theory to fit observations: From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity, https://arxiv.org/abs/2110.06936 https://arxiv.org/abs/2110.06936
  - podiki 3y ago
    Fine, rather than "underlying theory" use some other words, basically does any new proposal give us predictions, solve some unexplained phenomenon, do something simpler and more compelling (sure, a matter of taste, but so is all of this to some degree), in short, does it tell us something new. Newton certainly did as he gave a law to describe and predict successfully the motions of the planets, for instance. I don't think we are saying anything different really. There wasn't a predicitve framework before (as far as I know), but then there was, so that is certainly progress. But the bar gets ever higher as we know more for what a theory should do. Just writing down an equation that can fit some data and can make predictions is great but is not the end of the story. We could just have some arbitrary functions that fit the data we have and call it a day.

podiki 3y ago

After a little more of a skim: the paper seems mostly to be about reproducing an equation like that of MOND (modified Newtonian dynamics). Which, fine, some people are fans of MOND (not most, and for good reason from the theoretical which this might address, to the practical like previous comment), but there's a whole lot more than just the dynamics of rotating galaxies to consider. Without even going into that (bullet cluster, lensing, n-body simulations, galaxy formation, etc.) it doesn't (yet?) replace anything.

raattgift 3y ago
Sure, but the origin of this preprint is in the stochastic-classical/uncertain-quantum "postquantum-classical" theory the Oppenheim group is working on. Their theory is premised on a weakening of the gravitational interaction at short distances (it's "asymptotically free" and thus renormalizable) compared to perturbative quantum gravity (which is non-renormalizable by power counting). However, they discovered that the gravitational interaction in their theory can be too strong. More technically, in their diffusion limit they extract a different scalar potential than from GR's weak field limit, with the former growing (statistically) stronger with increasing radius. This in practice means even with a highly similar source, their theory predicts significantly different geodesics compared to General Relativity. Rather than abandon the theory or complicate it such that it becomes compatible with fully classical GR, they solved a Schwarzschild-de Sitter (static spherical symmetry and an expansion term which "washes out" the extra-strength gravitational potential before it gets too strong) spacetime with a galactic mass (described simply and with some restrictions) and found that the geodesics are within a standard deviation or so of stable circular MONDian orbits. That result would relieve some of the immediate concern that their theory is unphysical, and drive them towards further study of its large-length-scale behaviour. (In fact their result is better than some earlier attempts to make a generally covariant MOND by adding auxiliary gravitational fields to GR (see the overview by Famaey & McGaugh 2012). Postquantum-classical gravity was on a relativistic footing from the start, although exactly how diffeomorphism invariance and manifest covariance of formulations works in postquantum-classical gravity is really interesting, see <https://arxiv.org/abs/2402.17844 https://arxiv.org/abs/2402.17844> if you want details) It would be interesting to see how their theory does does -- it would have to be studied numerically -- with any sort of CDM halo. Various stellar binaries are also an obvious place to look, since for sufficiently wide ones, you can't "wash out" the extra strength of the scalar potential with an expansion term. (Wide binaries are really rough on MOND too, for comparable reasons). > it doesn't (yet?) replace anything The authors know that. From just after Eqn (23) in the preprint <https://arxiv.org/abs/2402.19459 https://arxiv.org/abs/2402.19459>: "While this study demonstrates that galactic rotation curves can undergo modification due to stochastic fluctuations, a phenomenon attributed to dark matter, it is important to acknowledge the existence of separate, independent evidence supporting ΛCDM. In particular, in the CMB power spectrum, in gravitational lensing, in the necessity of dark matter for structure formation, and in a varied collection of other methods used to estimate the mass in galaxies"

deleted 3y ago

[deleted]