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0 pointsnaasking3y ago0 comments

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.

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Avshalom3y ago

> 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.

naaskingOP3y ago

> 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.

throwawaymaths3y ago

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

simonh3y ago

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.

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