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Folks, the observation doesn't test Verlinde's idea, this tests a very general class of models, such as the well-known MOND scheme. > Milgrom, however, support

Folks, the observation doesn't test Verlinde's idea, this tests a very general class of models, such as the well-known MOND scheme. > Milgrom, however, supports the work. He also points out that according to his own 2013 analysis of gravitational lensing data in galaxies, MOND produces similarly impressive results as Verlinde’s gravitational model does in Brouwer’s study. (Also: http://motls.blogspot.ca/2016/11/verlindes-de-sitter-mond-is-highly.html http://motls.blogspot.ca/2016/11/verlindes-de-sitter-mond-is... ) This data cannot distinguish between Verlinde and other explanations for MOND-like behavior. We've long known that MOND seems to fit a subset of all observational data better the dark matter, in particular the Tully–Fisher relation https://en.wikipedia.org/wiki/Tully%E2%80%93Fisher_relation https://en.wikipedia.org/wiki/Tully%E2%80%93Fisher_relation However, for the rest of the data (e.g., the CMB, large-scale structure, the bullet cluster), MOND is generally considered to give a worse fit than dark matter, or to be silent (because it's not obvious how to extend MOND to a complete cosmological theory). That's why experts think the totality of evidence supports dark matter. Nothing has changed.

DennisP 10y ago
Seems like it's pretty easy to make dark matter a good fit, because you can just adjust the presumed distribution of dark matter to fit the data.
- sandworm101 10y ago
  It does seem unfair to fault one theory for making up terms but to ok the invention of dark matter and its placement wherever needed. Perhaps dark matter will be like vulcan: a big chunk of mass invented and placed so as to confirm an outdated model. The new model described a new force, mooting the need for the mass.
  - TTPrograms 10y ago
    It's faulting one theory for making up terms to fit one source of data and then those same terms failing on 6+ other experiments. Dark matter's predictions are consistent across totally different experiments with fewer sets of parameters.
- jessriedel 10y ago
  Nope, this is only an issue on galactic scales and below. On supergalactic scales, where most of our data comes from, the DM distribution is determined by theory. That includes all the stuff I mentioned: LSS, CMB, bullet cluster, etc.
  - DennisP 10y ago
    So, my impression from popular articles is that physicists don't actually know what dark matter is, and have a fair number of candidate ideas. According to the theory that makes good predictions, what is dark matter made of?
    - jessriedel 10y ago
      Dark matter (DM) is typically modeled like electrons are modeled: as a species of massive fermion, described by a quantum field. The parameters that go into the model (both DM and the electron) are the mass of the particle, plus its various couplings to the other known particle species. In this sense, the only difference in our knowledge of DM and electrons is the DM mass and the DM coupling constants. For the electron, these numbers are known to high precision. For DM, they could lie anywhere in a range over many orders of magnitude (although they are generally bounded from above, because we have not seen evidence of strong interactions). At supergalactic scales, the predictions of the DM model are not very sensitive to it's mass or couplings. On galactic scales, various nonlinear processes in normal matter (such a supernovea) and possible DM interaction effects become important. Since we don't know those things, the theory has free parameters. Folks definitely consider other DM models, e.g. bosons, but these are only considered to be favored over MOND insofar as they agree with the same observational data. In other words, the set of all theories of DM which produce the same observational predictions forms an "equivalence class"; and it's the equivalence class, not a particular choice of DM mass and couplings, that is favored over MOND. This is basically what people mean when they talk about "cold dark matter", it's the CDM in the LambdaCDM model, the standard model of cosmology. https://en.wikipedia.org/wiki/Lambda-CDM_model https://en.wikipedia.org/wiki/Lambda-CDM_model
      - 10y ago
- tombone12 10y ago
  people who just make up dark matter distributions are morons, obviously you use a model derived from the theory.
acqq 10y ago
I like Ethan Siegel's explanation of the Verlinde's paper much more than the article with the "kills off" in the title: http://www.forbes.com/sites/startswithabang/2016/11/12/ask-ethan-what-if-gravity-isnt-really-fundamental/ http://www.forbes.com/sites/startswithabang/2016/11/12/ask-e... "The hope is, with the right assumptions, a full theory of gravity, That's the big hope, and what Verlinde is working towards. (Others are also working towards it independently.) This paper is an update on how it's going. So, how's it going? "There are some successes given very specific assumptions, but there are a lot of problems. The largest problem, quite simply, is that one needs to make a multitude of seemingly arbitrary "interpretation" decisions to wind up with something other than nonsense. For example: the full motivation for this approach is based in anti-de Sitter space (or space with a negative cosmological constant), but our Universe is observed to have a positive cosmological constant (i.e., de Sitter space), and the mathematics of the two spaces have very different properties. For another, you need the entropy to obey a strict area-based law to get the Einstein equations out, but you don't get a cosmological horizon out if you do. (And our Universe has one.) And finally, if you make all the assumptions you need to in order to get the gravitational acceleration for galaxies out, you destroy all of General Relativity's successes on larger-than-galaxy scales. (Verlinde, on pp. 39-40, makes the argument that it could succeed, but the observations of colliding galaxy clusters completely undermine his line of thought.)" There's even more discussion in the Ethan's article, which is much more balanced. And another take on Verlinde's paper by Sabine Hossenfelder: http://backreaction.blogspot.co.at/2016/12/can-dark-energy-and-dark-matter-emerge.html http://backreaction.blogspot.co.at/2016/12/can-dark-energy-a... "General Relativity is a rigorously tested theory with many achievements. To do any better than general relativity is hard, and thus for any new theory of gravity the most important thing is to have a controlled limit in which General Relativity is reproduced to good precision. How this might work in Verlinde’s approach isn’t clear to me because he doesn’t even attempt to deal with the general case. He starts right away with cosmology." "I'd say he takes inspiration from models that are best understood in AdS." but "he starts with de-Sitter space which means he assumes dark energy. It doesn't make sense to say that this is a source of dark energy, it's the same thing, period."
raattgift 10y ago
Well, I wouldn't focus on the MOND part of Verlinde's recent work; the crucial things are that it's emergent gravity, that it's strings underneath, and that he proposes a long range entanglement between strings from which he argues the CDM-like generator of the metric emerges (or, if you like, from which the MOND-like kinematics emerge). From a couple of days ago : https://news.ycombinator.com/item?id=13176483 https://news.ycombinator.com/item?id=13176483
- jessriedel 10y ago
  The topic of the article is the experimental data, which does not distinguish between Verlinde's model from many other alternatives to DM. You may think the story Verlinde tells gives new a-priori reason to favor this sort of theory, but it doesn't have much to do with the experimental data being discussed.
  - raattgift 10y ago
    Huh? The topic of the newscientist article is https://arxiv.org/abs/1612.03034 https://arxiv.org/abs/1612.03034 (it links to the PDF right at the bottom!) and in the preprint's references section is "Verlinde, E. P., 2016, preprint, (arXiv:1611.02269)", which is surely the "Verlinde's theory of Emergent Gravity" that Brouwer et al. mean. The evidence in Brouwer et al. is shaky, and I am not a fan of Verlinde's paper (although it is interesting), but I don't see what it has to do with MOND. Indeed, in Brouwer et al., I can't even see a reference to MOND. Only in the newscientist article is there a scattering of the abbreviation "MOND", and the article goes so far as to (entirely unjustifiably) say, "[Verlinde's Emergent Gravity] also builds on controversial models of so-called modified gravity, such as the Modified Newtonian Dynamics (MOND) theory of Mordehai Milgrom". Instead Verlinde's work builds on Ted Jacobson's work from twenty years ago (arXiv:1505.04753) (cf. Verlinde's first paragraph in section 5) and it is no surprise that discussions directly with Jacobson are disclosed in the acknowledgments section. Sure Milgrom is also further down in the acknowledgments section and MOND is in the references in force, but consider Verlinde's, "We like to emphasize that we have not derived the theory of modified Newtonian dynamics as proposed by Milgrom. In our description there is no modification of the law of inertia, nor is our result (7.43) to be interpreted as a modified gravitational field equation." (top of Verlinde's page 39). Further down, "although we derived the same relation as modified Newtonian dynamics, the physics is very different". (emphasis mine) Indeed, if you look at my link in the comment you replied to, you'll see in the first paragraph that his unremovable background is standard GR dS. Other than in the newscientist article, I think you can't really find anywhere that conflates MOND-like approaches with Verlinde's emergent gravity. They are very different theories. The key point though is that Verlinde's is more fundamental and MOND-like and FLRW-like results can emerge from it at appropriate length scales (and \Lambda is simply built in). But that's the point of emergent theories; you can derive the emergent theory from the more fundamental one, but you can't derive the fundamental theory from the emergent one. And in Verlinde's work, the fundamental theory, for better or worse (fwiw, I think worse) is strings in dS, and what emerges is General Relativity as an effective field theory with some IR corrections. Finally, what I think is most interesting about Verlinde's paper is that string people usually avoid dS. "That's interesting and not outrageously wrong" is not saying that it is anything close to a complete theory (it does not even seem wholly self-consistent), or that the paper is especially well structured. At best you could say that since I am open-minded about the possibility that our current fundamental theories emerge from something more fundamental, but that's hardly an a priori reason to favour Verlinde's EG, or anyone else's. I don't even know where you got that from; maybe instead of "You may think" you meant "Someone might think".

DennisP 10y ago

Seems like it's pretty easy to make dark matter a good fit, because you can just adjust the presumed distribution of dark matter to fit the data.

sandworm101 10y ago
It does seem unfair to fault one theory for making up terms but to ok the invention of dark matter and its placement wherever needed. Perhaps dark matter will be like vulcan: a big chunk of mass invented and placed so as to confirm an outdated model. The new model described a new force, mooting the need for the mass.
- TTPrograms 10y ago
  It's faulting one theory for making up terms to fit one source of data and then those same terms failing on 6+ other experiments. Dark matter's predictions are consistent across totally different experiments with fewer sets of parameters.
jessriedel 10y ago
Nope, this is only an issue on galactic scales and below. On supergalactic scales, where most of our data comes from, the DM distribution is determined by theory. That includes all the stuff I mentioned: LSS, CMB, bullet cluster, etc.
- DennisP 10y ago
  So, my impression from popular articles is that physicists don't actually know what dark matter is, and have a fair number of candidate ideas. According to the theory that makes good predictions, what is dark matter made of?
  - jessriedel 10y ago
    Dark matter (DM) is typically modeled like electrons are modeled: as a species of massive fermion, described by a quantum field. The parameters that go into the model (both DM and the electron) are the mass of the particle, plus its various couplings to the other known particle species. In this sense, the only difference in our knowledge of DM and electrons is the DM mass and the DM coupling constants. For the electron, these numbers are known to high precision. For DM, they could lie anywhere in a range over many orders of magnitude (although they are generally bounded from above, because we have not seen evidence of strong interactions). At supergalactic scales, the predictions of the DM model are not very sensitive to it's mass or couplings. On galactic scales, various nonlinear processes in normal matter (such a supernovea) and possible DM interaction effects become important. Since we don't know those things, the theory has free parameters. Folks definitely consider other DM models, e.g. bosons, but these are only considered to be favored over MOND insofar as they agree with the same observational data. In other words, the set of all theories of DM which produce the same observational predictions forms an "equivalence class"; and it's the equivalence class, not a particular choice of DM mass and couplings, that is favored over MOND. This is basically what people mean when they talk about "cold dark matter", it's the CDM in the LambdaCDM model, the standard model of cosmology. https://en.wikipedia.org/wiki/Lambda-CDM_model https://en.wikipedia.org/wiki/Lambda-CDM_model
    - 10y ago
tombone12 10y ago
people who just make up dark matter distributions are morons, obviously you use a model derived from the theory.

acqq 10y ago

I like Ethan Siegel's explanation of the Verlinde's paper much more than the article with the "kills off" in the title: http://www.forbes.com/sites/startswithabang/2016/11/12/ask-ethan-what-if-gravity-isnt-really-fundamental/ http://www.forbes.com/sites/startswithabang/2016/11/12/ask-e... "The hope is, with the right assumptions, a full theory of gravity, That's the big hope, and what Verlinde is working towards. (Others are also working towards it independently.) This paper is an update on how it's going. So, how's it going? "There are some successes given very specific assumptions, but there are a lot of problems. The largest problem, quite simply, is that one needs to make a multitude of seemingly arbitrary "interpretation" decisions to wind up with something other than nonsense. For example: the full motivation for this approach is based in anti-de Sitter space (or space with a negative cosmological constant), but our Universe is observed to have a positive cosmological constant (i.e., de Sitter space), and the mathematics of the two spaces have very different properties. For another, you need the entropy to obey a strict area-based law to get the Einstein equations out, but you don't get a cosmological horizon out if you do. (And our Universe has one.) And finally, if you make all the assumptions you need to in order to get the gravitational acceleration for galaxies out, you destroy all of General Relativity's successes on larger-than-galaxy scales. (Verlinde, on pp. 39-40, makes the argument that it could succeed, but the observations of colliding galaxy clusters completely undermine his line of thought.)" There's even more discussion in the Ethan's article, which is much more balanced. And another take on Verlinde's paper by Sabine Hossenfelder: http://backreaction.blogspot.co.at/2016/12/can-dark-energy-and-dark-matter-emerge.html http://backreaction.blogspot.co.at/2016/12/can-dark-energy-a... "General Relativity is a rigorously tested theory with many achievements. To do any better than general relativity is hard, and thus for any new theory of gravity the most important thing is to have a controlled limit in which General Relativity is reproduced to good precision. How this might work in Verlinde’s approach isn’t clear to me because he doesn’t even attempt to deal with the general case. He starts right away with cosmology." "I'd say he takes inspiration from models that are best understood in AdS." but "he starts with de-Sitter space which means he assumes dark energy. It doesn't make sense to say that this is a source of dark energy, it's the same thing, period."

raattgift 10y ago

Well, I wouldn't focus on the MOND part of Verlinde's recent work; the crucial things are that it's emergent gravity, that it's strings underneath, and that he proposes a long range entanglement between strings from which he argues the CDM-like generator of the metric emerges (or, if you like, from which the MOND-like kinematics emerge). From a couple of days ago : https://news.ycombinator.com/item?id=13176483 https://news.ycombinator.com/item?id=13176483

jessriedel 10y ago
The topic of the article is the experimental data, which does not distinguish between Verlinde's model from many other alternatives to DM. You may think the story Verlinde tells gives new a-priori reason to favor this sort of theory, but it doesn't have much to do with the experimental data being discussed.
- raattgift 10y ago
  Huh? The topic of the newscientist article is https://arxiv.org/abs/1612.03034 https://arxiv.org/abs/1612.03034 (it links to the PDF right at the bottom!) and in the preprint's references section is "Verlinde, E. P., 2016, preprint, (arXiv:1611.02269)", which is surely the "Verlinde's theory of Emergent Gravity" that Brouwer et al. mean. The evidence in Brouwer et al. is shaky, and I am not a fan of Verlinde's paper (although it is interesting), but I don't see what it has to do with MOND. Indeed, in Brouwer et al., I can't even see a reference to MOND. Only in the newscientist article is there a scattering of the abbreviation "MOND", and the article goes so far as to (entirely unjustifiably) say, "[Verlinde's Emergent Gravity] also builds on controversial models of so-called modified gravity, such as the Modified Newtonian Dynamics (MOND) theory of Mordehai Milgrom". Instead Verlinde's work builds on Ted Jacobson's work from twenty years ago (arXiv:1505.04753) (cf. Verlinde's first paragraph in section 5) and it is no surprise that discussions directly with Jacobson are disclosed in the acknowledgments section. Sure Milgrom is also further down in the acknowledgments section and MOND is in the references in force, but consider Verlinde's, "We like to emphasize that we have not derived the theory of modified Newtonian dynamics as proposed by Milgrom. In our description there is no modification of the law of inertia, nor is our result (7.43) to be interpreted as a modified gravitational field equation." (top of Verlinde's page 39). Further down, "although we derived the same relation as modified Newtonian dynamics, the physics is very different". (emphasis mine) Indeed, if you look at my link in the comment you replied to, you'll see in the first paragraph that his unremovable background is standard GR dS. Other than in the newscientist article, I think you can't really find anywhere that conflates MOND-like approaches with Verlinde's emergent gravity. They are very different theories. The key point though is that Verlinde's is more fundamental and MOND-like and FLRW-like results can emerge from it at appropriate length scales (and \Lambda is simply built in). But that's the point of emergent theories; you can derive the emergent theory from the more fundamental one, but you can't derive the fundamental theory from the emergent one. And in Verlinde's work, the fundamental theory, for better or worse (fwiw, I think worse) is strings in dS, and what emerges is General Relativity as an effective field theory with some IR corrections. Finally, what I think is most interesting about Verlinde's paper is that string people usually avoid dS. "That's interesting and not outrageously wrong" is not saying that it is anything close to a complete theory (it does not even seem wholly self-consistent), or that the paper is especially well structured. At best you could say that since I am open-minded about the possibility that our current fundamental theories emerge from something more fundamental, but that's hardly an a priori reason to favour Verlinde's EG, or anyone else's. I don't even know where you got that from; maybe instead of "You may think" you meant "Someone might think".