Nope. MOND is not the answer. It is never going to be the answer. There are too many things that MOND cannot explain. One of the reasons why planet 9 was hypothesized to exist is because the orbits of objects far from the sun are out of plane in weird highly elliptical orbits not explainable through modification of gravity’s effects at small force scales. Dark matter is another phenomenon MOND was meant to explain away but cant. i.e the bullet cluster is an object with less than 1% of its mass accountable by visible matter. MOND cannot make up the difference. Structures like this are only explainable if dark matter actually exists. The best MOND can hope for is to account for some but not all, of the discrepancy.
I hit up the Wikipedia article. Much like a typical episode of PBS Spacetime, it was just enough over my head to feel within reach while simultaneously hammering home that I was never meant to be an astrophysicist.
The best MOND can hope for is to account for some but not all, of the discrepancy.
Your last sentence kind of shut down the whole preceding "MOND isn't be a thing!" Part of the comment.
Sure, it doesn't seem particularly likely that MOND is a thing. But given that science always allows for old theories to be disproven, and our theories of how gravity works are already known to be incomplete or otherwise shaky, I wouldn't shut down any researchers who want to give it a go.
The issue is that MOND is being marketed as being able to fix things that we already know it isnt capable of fixing. This is the astronomy equivalent of an experimental drug tauted to cure cancer rather than a specific form of it (if it works at all.) The biggest issue I have with MOND is that the mechanism for modified gravity isn’t derived like relativity was (i.e relativity has a mechanism that naturally leads to its equations) but designed post facto to fit observations. i.e a0 was not derived from scratch, it was curve fit. A curve that does not appear to explain ultra diffuse galaxies which are apparently essentially free of dark matter. If MOND had some theoretical basis behind it beyond “stars in galaxies go brrr” and could explain the apparent lack of deviation from newtonian gravity of ultra diffuse galaxies, it might have deserved more attention.
I mentioned galactic rotation curves because thats more or less where it began. If MOND fails to fit the data at those scales, it will necessarily fail within our own solar system at the outer edges where a0 would potentially be relevant.
There are many variants of MOND. RelMOND, for example, includes features that resemble dark matter more closely.
I'm not saying "it's gotta be MOND", of course. I'm just reacting to what I perceive as an unnecessarily hostile reaction to it. We have yet to actually figure out dark matter either, after all, and there are many variants of dark matter that have been proposed. So the "it's designed post facto to fit observations" is a complaint that can be directed against it right now too. I see nothing wrong with exploring all the options, especially when it's by people who have chosen to spend their efforts doing that for themselves.
None of them are based on anything though. Theyre curve fitting models which is why I am hostile to them. Show me a mechanism that derives what a0 is from scratch.
Basing a model on how well it fits a curve rather than on a mechanism that naturally derives the curve from scratch is essentially worthless.
Without a mechanism that explains why there needs to be a MOND dominated regime in the first place, its just too susceptible to being pathological science.
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