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Comment by jph00 | Hacker News Reader

As it happens I dug into this question in some detail a couple of weeks ago when analysing the dataset, including carefully reading the wikipedia page which the question comes from. AFAICT both D and E are kinda correct, but E isn't quite right because MOND doesn't entirely "eliminate the observed missing baryonic mass", but rather just reduces it from a factor of 10 to 2.

Is that not correct? (Of course I fully accept your expertise in this matter and this is just my curiosity, not trying to tell you you're wrong!)

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Fascinating! I dug into the Wikipedia article, which cites a Scholarpedia article; the LLM answer seems to originate from a reference to this sentence [1]:

> So, MOND reduces the discrepancy in clusters at these radii to only a factor of ∼2−3 (Sanders, 1999; Ettori, et al., 2019)

So I think you're right, and today I learned something! I also checked if Stacy McGaugh had weighed in on this particular subject, and it seemed like there is still an issue for clusters [2], although interestingly the issue isn't mentioned in his latest blog post that summarizes the strengths/weaknesses with MOND [3]. Anyway, thanks for humoring me for a bit.

[1] http://www.scholarpedia.org/article/The_MOND_paradigm_of_mod... [2] https://tritonstation.com/2021/02/05/the-fat-one-a-test-of-s... [3] https://tritonstation.com/2023/06/27/checking-in-on-troubles...

I believe neither MOND nor Condensed Dark Matter are theories exactly, so much as they are schemata for classes of theories. Both are struggling to produce a verified theory that accounts for all observations, and while the latter is much more widely regarded as likely being correct, MOND has not been conclusively falsified to everyone's satisfaction. I would guess that there are, at least in principle, MOND theories which work for galaxy clusters but have residual discrepancies when applied to galaxies.

If this is so, then a multi-choice question which conflates one particular MOND theory for MOND itself, and which depends on the specifics of that particular theory for selecting the 'correct' answer, is problematic: for one thing, it may make selecting the 'correct' answer more difficult for a student who has specific knowledge about the topic. This is just one of several problems with multi-choice questions, though, fortunately, it does not seem to have any bearing on the very interesting phenomenon you have discovered.

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