[lll-o-lll]

Dark Matter is unintuitive for the layman (which I am), and extremely intuitive for the physicist, which I suspect is where the origins of this problem lie. This article is another in long line of you just don’t get it man, which is true. Sabine Hossenfelder has some great videos that actually explain the concepts in a way that the layman can grasp, while also pointing out a number of problems with the standard orthodoxy.

The problems don’t indicate that Dark Matter (the theory) is wrong necessarily. MOND is much better at explaining a number of observations of large scale (better meaning simpler in this case), but Dark Matter is better for other observations at smaller scale. Unfortunately it seems that the physicist community (outside of a small subset) is unwilling to research in the MOND space; it has the taint. So physicists just pile on a bunch of extra variables to make Dark Matter fit certain observations, when MOND describes those observations very simply.

Sabine argues long and hard that modern particle physicists have made no fundamental progress for 50 years due to poor scientific method, and she’s acerbic and popular with the plebs (such as me). I’m glad she’s a voice out there, but I’m sure she has put herself offside with a number (maybe most) working particle physicists.

It’s unfortunate. The phenomenon that MOND and Dark Matter seek to explain are really interesting, and the depths have clearly not been plumbed. The continual search and failure to find the Dark Matter particle is not doing physics any favours.

[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.

[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

[throwawaymaths]

Why exactly is dark matter unintuitive to the layperson? (Any more so than say, germ theory, or praying to an interventionist god)?

[phkahler]

>> Why exactly is dark matter unintuitive to the layperson?

For me it's because the explanations don't actually make sense. The classic example to me is the galactic rotation "problem". It gets resolved by suggesting a spherical cloud of dark matter surrounding a galaxy. Great, but why would it affect the galaxy normally but take on such a distribution? The solution raises more questions than the problem. Not to mention it's a whole new kind of matter we can't detect. It's almost magical thinking.

[consilient]

> Great, but why would it affect the galaxy normally but take on such a distribution?

Electromagnetic interactions radiate off energy but not angular momentum, so a spinning sphere of gas will flatten over time. Dark matter doesn't interact electromagnetically and thus sheds energy much more slowly.

[lll-o-lll]

Until you have it properly explained it just looks like a kludge. It’s an obvious extrapolation once a number of fundamental concepts have been understood, but these are concepts you won’t be exposed to outside of a physics university education.

That’s all it is I think. Sabine (and probably others) have done a great job of explaining these concepts for the layman audience.

This article, and so many like it, don’t do that. So the physicist is left shaking their head, and the layman comes away still thinking “what a giant kludge that dark matter is”.

[heavyset_go]

Intangible matter that can neither be seen nor interacted with outside of gravity is unintuitive. It can sound like "what if we're all just in a simulation, man?" but coming from the best science apparently has to offer.

[Isinlor]

That's exactly what neutrinos are in terms of human experience. Ridiculous amounts of neutrinos are flowing trough the whole planet and trough you like we are nothing.

[heavyset_go]

Neutrinos can interact with ordinary matter even if it's highly unlikely. They're intuitively really small and lightweight.

[consilient]

Neutrinos interact via gravity and the weak force, just like most dark matter candidates.

[Isinlor]

Dark matter is also believed to interact with ordinary matter in ways other than gravitational.

In realm of everyday experience neutrinos and proposed dark matter would be the same ghostly thing.

[throwawaymaths]

Honestly "Dark matter is also believed to interact with ordinary matter in ways other than gravitational." is wishful thinking. Why? We have no a priori reason to believe that matter must have multimodal interactions, and there is potentially a real ontological crisis. If dark matter only interacts gravitationally - which even people who don't "believe in dm" must accept is a possibility, them dark matter is unfalsifiable, so we have to be ok with the possibility that there is something in the universe that exists but we cannot ever satisfactorily argue that it does. LCDM advocates know that they are on philosophically shakier ground if that's the case, but aren't brave enough to admit it because they are afraid it weakens their main cause by association (to be fair it does, we should probably be exploring other explanations first)

[throwawaymaths]

I guess I take issue with the vagueness of "intuitive" because that's kinda sorta "just feels"/bias/projection about the state of "lay person" thought. As I said, germ theory is "invisible stuff that interacts with your body" and an interventionist god is intangible entity that can control your fate... And I think a lot of people believe in those.

[SaberTail]

You say MOND is better because it's simpler.

We already have examples of particles that do not interact electromagnetically but have mass, and therefore interact gravitationally. They're neutrinos. How is a new theory of gravity simpler than particles that are similar to ones we know exist?

[AnimalMuppet]

Depends on which hat you're wearing. If you're wearing an astronomical or GR hat, new particles are simple. Just add their mass, and space curves differently, and you're done. But if you're wearing a particle physics/standard model hat, new particles are a huge deal. They change all kinds of things. Some of those things that change have rather large effects on cosmology.

[lll-o-lll]

I say that MOND can describe certain observations in a simpler way, which it can. For those same observations Dark Matter is more complex because you have to add a number of parameters to make it fit.

I didn’t say it’s better than Dark Matter; it’s not either or. I have no dog in this fight. I simply regard what Sabine has said “combine these two theories you idiots!” (paraphrased), and think “she’s probably got a point”.

[throwawaymaths]

It has one parameter. "Particle we can't see" has as many parameters as it takes to model the density and flow of those particles at any given place in the universe.

[mr_mitm]

That's a common misconception (at least on HN).

I debunked this idea in the last DM thread: https://news.ycombinator.com/item?id=34365591

[throwawaymaths]

Yeah and you're still wrong. The comparison is inapt because for all other cosmological phenomena our information is multimodal and this not currently the case for LCDM. Take the argument to the extreme: suppose dark matter has no weak force interaction. It will then be literally impossible to uncouple our observations of dark matter from our inference of it's distribution, and then LCDM is strictly unfalsifiable and may be used to explain almost anything.

[nwallin]

Terminology nitpick: MOND is a Dark Matter theory. Dark Matter is whatever causes the observed phenomena that galaxy rotation rates are all wonky. If it explains galaxy rotation rates, which MOND does, it's dark matter. I'll use the term CDM ("Cold Dark Matter", meaning non-relativistic matter like dark matter) which is a catchall for Axions, WIMPs, and MACHOs when I'm talking about the prevailing theory of dark matter. (note that MACHOs have been falsified by observation, even though it was originally the leading hypothesis for a while.)

> and extremely intuitive for the physicist,

No it isn't. We haven't gotten here because it's intuitive, we've gotten here because it's the only hypothesis which is consistent with the data.

> Unfortunately it seems that the physicist community (outside of a small subset) is unwilling to research in the MOND space; it has the taint.

This is false. MOND was a leading theory up until it was falsified by the Bullet Cluster. MOND was later falsified by baryon acoustic oscillations. Then again by ultra diffuse galaxies. Most MOND theories predicted gravitational waves traveling slower than the speed of light, all of which were falsified by GW 170817, the neutron star-neutron star merger a few years ago.

MOND has little support in 'orthodox' physics and cosmology because it keeps getting falsified.

> So physicists just pile on a bunch of extra variables to make Dark Matter fit certain observations, when MOND describes those observations very simply.

MOND is viable because we add more variables to it every time the latest observation comes along that falsifies it. It's true that the original, first incarnation of MOND is simpler than CDM, but every time we learn more about the universe, CDM still stands but MOND is invalidated, and so another variable has to be added to it.

CDM frontloads all its warts and complexity. "We propose new, as yet undetected and possibly undetectable particles." Everything else is just running the numbers. MOND frontloads its simplicity. "F = m a^2 / a_0". Everything else adds more variables. You want it to be compatible with GR? You need to add new variables to Einstein's equations. You want it to have stars in that are stable? You need to add new variables. You want it to be consistent with gravitational waves traveling near or at the speed of light? New variables. You want it to be compatible with BAO observations? New variables.

You want it to be compatible with the bullet cluster and ultra diffuse galaxies? Now you need to add CDM.

We are now at the point where viable MOND theories still include CDM, just less CDM than pure CDM. Ballpark numbers are that pure CDM predicts 80% of galaxy rotation curves are caused by CDM and 20% by normal matter, leading viable MOND predicts 20% by normal unadjusted gravity on normal matter, 40% by CDM, and 40% by the modified gravity equations. If you're just looking for the simpler model, MOND ain't it.