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Comment by naasking | Hacker News Reader

Milgrom's perspective on the Bullet Cluster: overblown and completely expected.

http://astroweb.case.edu/ssm/mond/moti_bullet.html

I won't get into an argument about MOND. Don't take my word for it, take the vast majority (all but a few?) of anyone working in the field. And don't take Milgrom's word for it (or the one reference given) either, for that matter, as clearly he has a horse in the game. Sure you can continue to make MOND work to some degree, but is it really predictive if you keep adding things to make it work? What is the underlying theory? (The proposal in this linked article is interesting in that regard.)

Okay, so maybe I did continue the argument a little :) I love for people working on alternative less mainstream ideas. But MOND was never very alive to begin with, let alone decades later. Let's move on.

> Sure you can continue to make MOND work to some degree, but is it really predictive if you keep adding things to make it work?

Do you hold dark matter theories to this standard?

Absolutely. I'm out of the field these days, but e.g supersymmetric WIMP dark matter was the most popular but got more and more clugely with low energy supersymmetry seeming less likely (or not as useful in solving the problems it meant to solve). Everyone would come up with some tuned ad hoc model for whatever dark matter "signal" was in fashion (before being ruled out); none of those were compelling. Fun to play with maybe, but didn't really tell us anything.

> What is the underlying theory?

Who cares? This has literally never been a requirement of science and I don't know why people bring it up for MOND. What was Isaac's Newton's underlying theory when he proposed his law of gravitation? No explanation for why two masses attract, therefore we should reject an effective description? That's nonsense.

A theory is scientific if it describes what we observe, period. If you have a deeper explanation right off the bat, that's a bonus, otherwise that's the goal of further research.

The other poster adequately addressed the implicit hypocrisy applied to MOND vs. LCDM, so I'll just leave this reference for a proper analysis of who has been making predictions vs. tweaking their theory to fit observations:

From galactic bars to the Hubble tension: weighing up the astrophysical evidence for Milgromian gravity, https://arxiv.org/abs/2110.06936

Fine, rather than "underlying theory" use some other words, basically does any new proposal give us predictions, solve some unexplained phenomenon, do something simpler and more compelling (sure, a matter of taste, but so is all of this to some degree), in short, does it tell us something new. Newton certainly did as he gave a law to describe and predict successfully the motions of the planets, for instance. I don't think we are saying anything different really. There wasn't a predicitve framework before (as far as I know), but then there was, so that is certainly progress. But the bar gets ever higher as we know more for what a theory should do. Just writing down an equation that can fit some data and can make predictions is great but is not the end of the story. We could just have some arbitrary functions that fit the data we have and call it a day.

Yes. Sone researchers went looking for the absence of EFE to disprove MOND and wound up finding it instead. MOND has predictive power. Moreover, we keep finding that lenticular and dense elliptical galaxies have "no dark matter", which is a prediction of MOND.

One of the reasons why things "keep getting added" to MOND is that it's known to be incomplete. The equations to reconcile MOND with relativity are not known and are tougher (but not necessarily impossible) than the Newtonian equations.

So it's expected that things will have to be added. That the equations are tricky for us mere mortal humans should not be held against it; the universe has no requirement that equations must be easy to solve.

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