Great! Thank you for laying this out. I obviously not in this area, but find the debate here really interesting. As a computational biologist, I can even follow some of the math, and most (if not all) of the mathematical reasoning. It is an interesting puzzle to see shake out…
@PdotdQ, how likely is this crazy idea of mine? Could dark particles be a virtual or emergent particle? If that is the case, perhaps MONDS is true, and dark matter emerges from a lower level theory.
Depending on how far one is allowed to stretch the meaning of “emergent particles”, that is certainly possible. One popular extension to Lambda-CDM is to require the dark matter to be a superfluid that could have waves that could be identified as “phonons”.
Edit:
I am reminded of another “emergent” dark matter theory: the Einstein Equation of General Relativity is nonlinear, and some have proposed that dark matter simply comes from this nonlinearity.
So, if one of these pans out, I’m in line for a Nobel right?
You’ll probably have to petition the Monarch of Sweden 
Just to be clear, that would mean we would never see a dark matter particle in a collider, right?
Only if you make sure you leave your phonon. (Sorry - you won’t get much serious sense from me on mathematical cosmology!)
Again, this depends on what one actually means by “emergent particle”.
For example, in the superfluid dark matter picture, the dark matter field (the superfluid) would be detectable in a collider. The phonons (which are excitations in the dark matter field), would probably not be detectable. In the nonlinear general relativity picture, dark matter will not be seen in a collider.
What is your take on the galaxy that didn’t have dark matter? My assumption was that it helps demonstrate nothing is wrong with our description of gravity (besides on the very small quantum scale). How does MOND fit into such a galaxy?
The galaxy “without dark matter”, NGC1052-DF2 is not an evidence against MOND.
Let me explain:
To weigh a galaxy, van Dokkum and collaborators measured the speed of bright globular-cluster-like-objects in said galaxy. The more massive the galaxy, the faster these globular clusters move. They measured the average speed of the globular clusters to be ~10.5 km/s at 90% confidence.
Of course, the amount of mass in the galaxy, and thus the speed of these globular clusters must be somewhat dependent on how bright the galaxy is. This is because stars and gas are bright and massive. The brighter the galaxy, the faster these globular clusters move.
However, in dark matter theories, I can arbitrary increase the velocities of these globular clusters by adding dark matter into the galaxy. This is because dark matter increases the mass of the galaxy without changing the amount of light emitted by the galaxy.
The claim by van Dokkum et al. is that the velocities of NGC1052-DF2’s globular clusters are so low that all of it can be explained by the bright components (stars and gas) of the galaxy. In particular, van Dokkum et al. calculated that MOND will give an average velocity of ~20 km/s, way beyond the “10.5 km/s at 90% confidence” they measured.
However:
Their MOND calculation is wrong because in their calculation they assumed that NGC1052-DF2 is an isolated galaxy. In reality, NGC1052-DF2 is a satellite of the giant elliptical galaxy NGC1052 (as clued in by its name). Correcting the MOND prediction for the existence of the giant elliptical galaxy, the average velocity of the globular clusters predicted by MOND becomes ~13.4 km/s, which is within the error bars of van Dokkum et al.'s measurement.
A different team of astronomers has re-weighted NGC1052-DF2 and found that it is inconsistent with the lack of dark matter claim of van Dokkum et al.
@PdotdQ I do not doubt you but I find your post remarkable.
Keep in mind this is a Nature paper that you are critiquing, that I am sure has high visibility in the field and went through rigorous peer review.
Is this just your observation, or does the field know this yet? Was this published somewhere else or are you riffing? Was the there a comment that noted the error? I’m pretty surprised that there is an error of that scale in this paper, though you may be right.
Can you link us to that paper? I’m honestly curious how quickly the correction came out. As I understand it, physics moves really quickly now days.
So about superfluid, look at this blog post / article that Sabine just dropped. What do you think of it?
Btw, one of the three physicists (@dga471, @pevaquark, @PdotdQ) should invite her hear some time. I think the conversation will be interesting, but you guys better be in the mix or the biologists here (i.e. me) will sound silly.
Of course, here are the sources of the claims in my posts:
The links I provided are from the pre-print server so that everyone can access them without getting paywalled. However, source for 1) is published in MNRAS and 2) in ApJL, both top notch astro journals.
Let me defend van Dokkum et al. a little bit: the error in 1) is completely understandable. Most astrophysicists do not have training in MOND, and could understandably make said mistake.
So this article that links superfluid dark matter, dark energy, and emergent gravity is interesting.
In the new paper, Verlinde argues that his variant of emergent gravity gives rise to deviations from general relativity on long distances, and these deviations correspond to dark energy and dark matter.
Personally I quite like superfluid dark matter, so I am glad that it passed this check.
A few words of caution: Superfluid dark matter is still a dark matter theory - it replaces particle dark matter with superfluid dark matter. The idea is as follows:
This way superfluid dark matter produces both the success of MOND and Lambda-CDM in one neat package.
Is it going to be possible to discriminate these theories, or are we going end up with a new morass, comparable to string theory? It seems that different formalisms can describe the same underlying reality too, which means that there might be real indeterminacy in these problems.
These theories can be discriminated, for example through the lensing study that Sabine published.
May I say something about the title of this thread though: superfluid dark matter is not dark matter as an emergent particle. Indeed, these models are created to get away from particle dark matter.
What I mentioned previously was that if you have superfluid dark matter, then presumably they can have phonons (as mundane superfluids do), and these phonons would be the “emergent particles”. These phonons won’t be (or at least might not be) the ones responsible for the MOND like force.
Thanks for the info- I was going to ask for the source but @swamidass beat me to it. I’m reading through van Dokkum’s post here where he mentions the Martin paper:
https://www.pietervandokkum.com/ngc1052-df2
All in all a nice example of science in action so far with more to come!