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First test of rival to Einstein’s gravity kills off dark matter

newscientist.com

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Re: First test of rival to Einstein’s gravity kills off dark matter

#61

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/verli…

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

Re: First test of rival to Einstein’s gravity kills off dark matter

#62
post #58

Earlier quoted context omitted.

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.

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?

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

Re: First test of rival to Einstein’s gravity kills off dark matter

#63

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/verli…

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

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.

Re: First test of rival to Einstein’s gravity kills off dark matter

#64
post #59
post #17

> String theorist Lubos Motl savaged Verlinde’s ideas in a recent blog post: “I wouldn’t okay this wrong piece of work as an undergraduate term paper.” I find this rudeness and pedantry in academia infuriating. There's no justification for this level of nastiness -- not in private, public, peer review, or science as a whole. Seeking out the unknown and creative explanations thereof is the hallmark of good science; ch…

That's pretty much what academia told Motl. He hasn't been a working physicist for a long time precisely because of how unnecessarily and uncharacteristically nasty he is. Characterizing him as an example of any sort of trend in academia is both uninformed and deeply insulting.

> He hasn't been a working physicist for a long time precisely because of how unnecessarily and uncharacteristically nasty he is.

It's hard to understand why New Scientist decided to quote him then.

They certainly seem to be presenting him as someone worth paying attention to.

Re: First test of rival to Einstein’s gravity kills off dark matter

#65

Earlier quoted context omitted.

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

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.

Huh? The topic of the newscientist article is 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".

Re: First test of rival to Einstein’s gravity kills off dark matter

#66
post #45

Please change the submission title. It is downright misleading. This test did not "kill off" dark matter. To do so would imply overwhelming evidence against the concept of dark matter as a whole. Instead, this theory, which eliminates the need for dark matter in certain situations, has passed its first test. The sentence "... test ... kills off dark matter" is simply false.

Moreover, it's not even a complete "theory" in the sense that it's still not known if it can even repeat the results of the General Relativity.

https://news.ycombinator.com/item?id=13197595

Re: First test of rival to Einstein’s gravity kills off dark matter

#67
post #32
post #17

> String theorist Lubos Motl savaged Verlinde’s ideas in a recent blog post: “I wouldn’t okay this wrong piece of work as an undergraduate term paper.” I find this rudeness and pedantry in academia infuriating. There's no justification for this level of nastiness -- not in private, public, peer review, or science as a whole. Seeking out the unknown and creative explanations thereof is the hallmark of good science; ch…

As others have mentioned, Lubos Motl is no longer in academia. He seemed to have spent more time criticizing others (and not just their works) than on actual physics. His appointment at Harvard was not reviewed. I'm frankly surprised New Scientist bothered to quote him.

Eh - I meant "not renewed"

Re: First test of rival to Einstein’s gravity kills off dark matter

#68
post #64
post #59

Earlier quoted context omitted.

That's pretty much what academia told Motl. He hasn't been a working physicist for a long time precisely because of how unnecessarily and uncharacteristically nasty he is. Characterizing him as an example of any sort of trend in academia is both uninformed and deeply insulting.

> He hasn't been a working physicist for a long time precisely because of how unnecessarily and uncharacteristically nasty he is. It's hard to understand why New Scientist decided to quote him then. They certainly seem to be presenting him as someone worth paying attention to.

> It's hard to understand why New Scientist decided to quote him then.

Is it harder than to understand that they publish an article with the false title and false interpretations?

Re: First test of rival to Einstein’s gravity kills off dark matter

#69
post #66
post #45

Please change the submission title. It is downright misleading. This test did not "kill off" dark matter. To do so would imply overwhelming evidence against the concept of dark matter as a whole. Instead, this theory, which eliminates the need for dark matter in certain situations, has passed its first test. The sentence "... test ... kills off dark matter" is simply false.

Moreover, it's not even a complete "theory" in the sense that it's still not known if it can even repeat the results of the General Relativity. https://news.ycombinator.com/item?id=13197595

It's not complete for other reasons, several of which are detailed in Verlinde's paper itself; however, it wholly reproduces GR in the EFT limit by design. Indeed, Verlinde starts with standard dS as his unremovable background on which he puts strings. What in GR we would consider perturbations of dS and IR corrections of GR (at ~ galactic scales) emerge from the behaviour of the strings and how they form long-distance entanglements.

However, it's not outrageously wrong, and it is noteworthy that an accomplished string theorist has decided to tackle gravity in a universe like ours rather than leaning on AdS/CFT arguments.

Re: First test of rival to Einstein’s gravity kills off dark matter

#70
post #3

You may find the top answer to this SE question helpful (asked 5 years ago, though): http://physics.stackexchange.com/questions/6561/are-modified... . Especially I think the following comment is spot on: "Galaxy rotation curves are not very good tests for GR itself, since there are so many parameters about mass distribution in a galaxy which are simply not nearly as precisely understood as GR itself"

That says MOND is a non-theory, so now I'm even more confused since this theory "builds on" MOND.

The New Scientist article writer is who is confusing you.

Verlinde does not build on MOND. An explanation and some extracts from his paper are in an earlier comment of mine:

https://news.ycombinator.com/item?id=13198573

(specifically at the paragraph half-way along in the comment, starting with "Sure Milgrom", if you want to zoom right in.)

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