[Avshalom]

>> MOND describes those observations very simply.

I mean it doesn't really. Worse though, it doesn't explain the observations. It's just a model fit post hoc. Everytime they try to explain why gravity would act like that they just end up reinventing dark matter but calling it a field and hoping no one points out particle/field equivalence during peer review.

[naasking]

The "why" doesn't matter to recognize a law that seems to hold across many observations. We still don't quite know why entropy must always increase and how this is linked to time, for instance, and yet we call them the laws of thermodynamics.

What's conspicuous however, is that MOND can make many successful a priori predictions about observations that LCDM needs to fit a posteriori using a "tuned" distribution of dark matter.

So MOND does indeed describe those observations simply even if it doesn't explain why they hold. An example of a better way to handle this than outright dismissing MOND, are recent proposals for superfluid dark matter that reproduce MOND in the right regimes.

[Avshalom]

> MOND can make many successful a priori predictions about observations that LCDM needs to fit a posteriori using a "tuned" distribution of dark matter.

those are the same predictions you'd get if you just set dark matter at a single uniform universal distribution. which is again, exactly what every attempt at explaining mond just ends up doing anyway... but then there's also those things that disagree with mond rendering the need for introducing a bunch of new stuff on top of it anyway.

and that's the thing mond leads us to dark matter plus extra stuff. Occam's razor is hardly an absolute truth but we like it and we can explain things without the extra stuff.

it's also like a real fucking brain worm to call "matter follows a statistical distribution": tuning or more parameters like the sibling post does. "Matter follows a statistical distribution" is what we already see in all the matter we see.

[naasking]

> but then there's also those things that disagree with mond rendering the need for introducing a bunch of new stuff on top of it anyway.

And LCDM does this too. If you want a comprehensive survey of the tuning done by each model to fit observations, see:

https://arxiv.org/abs/2110.06936v3

It seems clear that neither approach is satisfactory as they stand, but the resistance to recognizing the significant flaws in LCDM is frankly puzzling.

[throwawaymaths]

Wrong. Those models require tuning for every observation with extremely fragile parameters to get correct with LCDM, if you assume that LCDM particles are particulate and have individual momenta; in other words given the variation in momentum and density that we see in galaxies as a whole the odds that every single galaxy have exactly the right parameters to make the basic galactic curves work out is extremely low, and it's worse once you take into account stuff EFE

[simonh]

Not every galaxy does follow the behaviour that indicates a “standard” distribution of dark matter though, some seem to have a lot less, or straightforwardly behave as expected without it. Often these are galaxies that have undergone collisions and the theory is that the dark matter was stripped away in the collision. MOND can’t account for these galaxies at all, hence MOND theories end up introducing ad hoc fields or versions of dark matter anyway.

Personally, the way I think about dark matter is that it’s a map of the variances we see between the expected behaviour and actual behaviour of galaxies. That’s it. That’s it’s a distribution of gravitating matter is one possibility. That would fit the observations, so it’s a reasonable working hypothesis, that’s all.

MOND simply doesn’t describe the distribution of the effects we see. It does in many common cases, but not all by a long way and the variance is different from case to case, so it still leaves a gap we’d need to fill with something else.

[throwawaymaths]

There's an n of 1 that is unexplainable by MOND, and that's the bullet cluster. Pretty much every other Galaxy is explainable using mond, the udgs for example are likely to be errors in distance to them, a lot of other errors due to being oriented face towards us, etc.

Iirc, LCDM has trouble explaining satellite galaxies. They're traveling through the halo, which should create drag on them. They shouldn't last long enough to exist at all.

[simonh]

For individual galaxies yes the bullet cluster is the main problem, but MOND can’t explain the motion of galaxy clusters at all.

[YeGoblynQueenne]

Out of topic, but I was following this thread out of mild interest (not at all my expertise and I don't understand the subject) and I just realised yours is the same account that posted a helpful comment on that Prolog thread two days ago.

I'm curious about your background: you seem to know both physics and Prolog? How come?

Btw, thanks for your comments in the Prolog thread. Much more level-headed reaction than mine, I have to say with embarrassment.

[Avshalom]

A BS in physics from a decade ago and BS in geology (mostly geophysics) a few years ago.

Just sorta started noodling around with prolog a while back on a whim. Fits my brain well enough and it's kinda fun. Still never really used it in anger but I've been hanging around tech/programming circles for so long i can speak (somewhat) intelligently about languages well before I can do anything with them.

[YeGoblynQueenne]

Wow, two BSs? You got patience!

I hope you stick with Prolog. It's a unique thing. If only more people understood it, and used it, in their everyday work.

[Jefff8]

Another out of topic - I keep seeing your comments and in a comment about two years ago you said your profile had a link to your research. I looked at your profile to find out a bit about the things you are working on, but since then you've edited your profile. Any chance you could add a link/links again for a day or two?

Thanks!

[Avshalom]

https://github.com/stassa/louise

[Jefff8]

Thank you for pointing that out. I have different nicks most places I frequent and I forget that others are far more consistent.

[YeGoblynQueenne]

Oh, sorry, missed that! Thanks to Avshalom for posting the link to Louise.

You'll find the two journal papers in my old profile at the very end of Louise's Readme :)

[lll-o-lll]

> Worse though, it doesn't explain the observations.

So it needs more research, but pretending that the model doesn’t fit the observations with a very simple framework, and instead throwing more and more parameters at Dark Matter to get it to fit… At some point it’s starting to look silly?

Why not consider that there is probably more to understand here and try to combine these theories as Sabine is arguing?

[throwawaymaths]

Every model is a "model fit post hoc". That's science. Though perhaps you would be surprised to know that MOND has made several predictions that have turned out to be true. Some of which still have contemporary astronomers in a tizzy (early galaxies)

[6nf]

> It's just a model fit post hoc.

So does Dark Matter