[raattgift]

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

[jessriedel]

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]

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".

[jessriedel]

> 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.

Yea, I'm well aware of all this. Brouwer et al. are jumping on the entropic gravity media bandwagon. However, as stated in the New Scientist article, and as I quoted in my original top-level comment you replied to, "Milgrom...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."

So you really seem to be missing my point. I am well aware that entropic gravity tells a different and more complicated story than MOND proponents. But within the actual physical system being observed, they make indistinguishable predictions. Therefore, this experiment by Brouwer et al. does not actually support entropic gravity over MOND.

> 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

It's not a matter of conflation, it's that in many regimes (including the OP's) they have the very similar phenomenology. See, e.g., "Entropic-gravity derivation of MOND"

https://arxiv.org/abs/1201.4160

[raattgift]

> So you really seem to be missing my point

Yes, I was. I got it now, thanks for the clarification; I think we're in closer agreement than I thought.

Essentially where I think we now differ is in the motivation of Brouwer et al. -- they don't seem interested in contrasting with other approaches as much as showing that Verlinde's EG (or at least the claim at the start of their section 4) survives contact with observational data. Additionally, I think we differ on whether that's enough to make the paper worth writing in the first place.

(Where we probably most strongly agree is that there is a lot of breathless hype from Leiden, Amsterdam and Groningen lately.)