[rappatic]

When I see dark matter in the news I'm always reminded of the story of Vulcan.

In the 1800s, detailed observations of the planet Mercury showed that its orbit was slightly different than Newtonian mechanics predicted-- a difference of about 43 arcseconds per century. The study was rigorous enough to rule out any observation errors.

Le Verrier, the astronomer who made these observations, wondered how to explain the difference. A decade earlier, he had noticed a similar irregularity in the orbit of Uranus, which led to the discovery of Neptune, whose gravity caused these perturbations. So Le Verrier reasoned that something similar must be going on for Mercury, and he posited the existence of Vulcan, a tiny planet close to the Sun.

Many attempts were made for decades to observe Vulcan. It was even included on some maps of the Solar System at the time (https://www.loc.gov/resource/g3180.ct003790). But it was never conclusively observed.

When Einstein published his theory of relativity in 1915, the mystery of Mercury's orbit was finally explained-- Newtonian mechanics were simply incomplete, and the irregularity of Mercury's orbit was due to relativistic effects.

Could it be that something similar is happening today? Observations of gravity on galactic scales doesn't quite align with what relativity would predict, so we use dark matter to fill the gaps. We've tried for decades to detect dark matter, with no dice. Is our theory of gravity simply incomplete?

MOND may not be the solution, but I'm still skeptical about dark matter.

[pessimist]

There are multiple independent observations pointing toward dark matter:

1. Galaxy rotation curves. 2. Galaxy cluster mass measurements from gravitational lenses and infrared. 3. Cosmic Microwave Background models (mass measurements from inhomogeneities that correspond to acoustic waves, for eg).

MOND only explains 1.

Dark matter accounts for all 3. Only catch is that it hasnt been directly observed.

[throwaway27448]

> Only catch is that it hasnt been directly observed.

Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.

[magicalhippo]

> Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.

For some general misunderstanding to explain what we see, ie something which is not dark matter, it means our equations of gravity must be wrong.

People have tried hard to tweak the equations to fix the observations. One such proposal is indeed MOND that the submission is about.

The problem with tweaking the equations is that we have a lot of observations of very different circumstances and scales, and it's so far proved impossible to tweak the equations in a given way so they explain all of the observations. What happens is that you can tweak them so they fit one set of observations, like MOND fitting galaxy rotation curves, but then not fit other sets of observations, like MOND with the motion of galaxies in galaxy clusters as discussed in the submission.

Meanwhile, adding dark matter seems to work much better at explaining all the observations at once. I say much better because there is still some tension, but it's way less than any modifications of gravity.

Having dark matter particle(s) is also not that strange. As far as we know there's no requirement that a particle interacts with any of the known forces beyond gravity. And while the early particle candidates seem to be getting ruled out, axion-like particles, originally proposed to solve entirely different problems, are still a good dark matter candidate.

That said, it's not settled. People are proposing alternatives. One example I know of is Alexandre Deur[1], a QCD physicist which has taken his QCD knowledge into the realm of GR and proposed that perhaps graviton-graviton self-interaction can explain a lot of the dark matter phenomena. It's not mainstream, but he's getting published in peer-reviewed journals.

[1]: https://arxiv.org/search/gr-qc?searchtype=author&query=Deur,...

[crazygringo]

> Having dark matter particle(s) is also not that strange. As far as we know there's no requirement that a particle interacts with any of the known forces beyond gravity.

This is the key thing. As we still don't have a theory of everything, which explains the fundamental reasons why every particle is the way it is, dark matter is a very straightforward and simple explanation.

The name makes it sound stranger than it is. It's just particles that have mass and are affected normally by gravity, but which are unaffected by the electromagnetic force.

It's not a particularly exotic or implausible or wild idea. In fact it's so simple it's almost boring. It's just obviously inherently difficult to test directly, since gravity is such a weak force.

[bsder]

> Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.

Dark matter is the Occam's Razor theory. It explains almost all of the observations while assuming the least.

Why do you assume we have no evidence just because we don't have direct observations? Black holes were a similar phenomenon that we had no direct evidence for a fairly long time even though we had lots of other indirect phenomenon that really couldn't be explained any other way.

[towledev]

The razor is commonly misunderstood.

William of Ockham objected to his fellow theologians inventing things out of whole cloth (like dark matter question mark). That’s the point, not that a simpler explanation is more likely to be true.

The common understanding would have us believe that creationism, being simpler, outshines evolution, or that there is no such thing as a color revolution because the simplest explanation is that the mass protests are earnestly aggrieved locals.

[bsder]

Yours is a common misunderstanding, as well.

Occam's Razor only kicks in when the hypotheses have equal predictive and explanatory power.

The MOND hypothesis does NOT have equal explanatory power with the dark matter hypothesis. As such, Occam's Razor is not relevant.

[towledev]

Maybe you can point me to the "equal predictive and explanatory power" bit in William's writings.

[cactacea]

Because stuff like the Bullet Cluster exists. Dark matter is in fact the simplest answer.

https://en.wikipedia.org/wiki/Bullet_Cluster

[glitchc]

I feel compelled to point out that (a)ether was the simplest answer until we confirmed that space is a vacuum and that light can travel in the absence of a medium.

[bsder]

Abandoning aether was NOT a "simple" answer. One you abandon aether, all manner of weirdness suddenly pops in.

Light being the same speed irrespective of observer is weird. Velocity dilating length and time is weird. Not having a preferential observation point is weird. Not needing a medium for transmission is weird. Not being able to agree on simultaneity is weird. etc.

Aether wasn't just something that a bunch of dullards clung to. You have to abandon some very long held common-sense understanding when you give it up.

[glitchc]

That's my point exactly.

[whateverboat]

aether was not the simplest explanation since it was disproved by Michelson-Morley experiment. Nothing as of now yet has disproved Dark matter as thoroughly.

[srean]

The two were dealt with very differently.

Aether: observations show lack of aether so we update the theory that makes aether unnecessary.

Dark matter: observations show lack of observable matter so we keep hunting for decades locating it. The aether analogue would have been to continue to look for dark aether. Dark matter is more like god of gaps. We can 'darkify' any theory that does not fit empirical evidence.

It's hard to tell now which leans more towards the more correct theory.

[hammock]

Angry gods used to be the simplest answer for a lot of things. Inevitably we find a better model

[SiempreViernes]

You are of course very welcome to propose this supposed better model, but please do some due diligence and learn to roughly understand the current flawed models firsts.

For example "angry gods" was never a simple theory, one needs only to read some fan fiction to understand that theology gets complicated fast. Instead "angry gods" is a simplified summary of a very complicated theory about metaphysical hierarchy, creation, agency, the meaning of divinity, and cause and effect, etc etc.

[whateverboat]

Simplest in this sense does also imply predictability for Dark matter, which cannot be said for angry gods. Angry gods is anything but a simple theory.

[Tadpole9181]

Because the discrepancy exists and shows consistency. This gap needs to be called something, you can't just pretend it's not real. But the underlying model seems fine and all alternatives are broken.

So we came up with an idea that fills the gap in the current model and allows us to continue predictive science until we find out more.

People get so absurdly touchy about Dark Matter just because it's named and "weird". It's just a placeholder value.

[antognini]

We already know that some particles interact extremely weakly with others. Neutrinos were postulated as a "dark" particle in order to preserve conservation of energy and momentum, but it took more than a decade before they were observed directly. There's no reason there couldn't be another particle with an even weaker interaction.

[chmod775]

It doesn't even matter whether it actually exists, because it works for modeling the universe and making accurate predictions about it.

That means for now it's useful.

[Tuna-Fish]

We have indirectly observed it a bunch, and our understanding of physics allow the existence of material that flatly cannot be directly observed.

[perlgeek]

A "general misunderstanding" isn't really a theory whose complexity you can compare to the Dark Matter theory.

One could also argue that on the one hand you need just one concept (dark matter) to explain three phenomena (galaxy rotation curves, gravitational lensing, CMBR), and the "general misunderstanding" might need to have three aspects to make it explain all three things, making it more complex.

Occam's razor is a good guiding principle, but in some cases, it's really hard to compare the complexity of competing theories. My favorite example of this is the Many Worlds interpretation of quantum mechanics. In some sense, it's really simple, the wave function just follows the Schrödinger equation. In another sense, it implicitly postulates the existence of many parallel worlds, which you can count as a quasi-infinite complexity penalty, or you can shrug and say it's not a big deal, the theory says the parallel worlds aren't observable, so there is nothing wrong with postulating them. There's no objective way to decide that.

[fsckboy]

>Occam's razor would point toward...

Occam's razer is not a thing with explanatory or legal or even scientific force. if Occam's razor was really a thing, quantum theory and general relativity would be ruled out as complicated and neither 100 percent explanatory.

[wvenable]

> Ok, so... why do people take it seriously as a concept?

Historically, science has predicted lots of true things about the universe that could not be observed at the time.

[magicalhippo]

> Historically, science has predicted lots of true things about the universe that could not be observed at the time.

Just some examples I could think of:

The neutrino[1], predicted in 1930 due to missing mass in beta decays, discovered in 1956.

Gravitational waves[2], predicted by Einstein in 1916, indirectly detected via pulsars in 1974 and directly by LIGO in 2015.

Higgs boson[3], predicted in 1964 to solve the problem of the Standard Model at the time requiring particles to be massless, which we knew they weren't. Hints from LEP at the turn of the millennium, detection by the LHC in 2012.

Cosmic microwave background[4], predicted in 1948 as a consequence of the expanding universe on the early radiation. Detected by accident in 1964.

Gravitational lensing[5] was first proposed in 1801 as a consequence of Newtonian corpuscular theory of light. Einstein realized in 1915 that GR predicted a larger value. Weak lensing by the sun was observed in 1919. Strong lensing by a distant galaxy of an object behind the galaxy was was predicted in 1937, and first observed in 1979.

[1]: https://en.wikipedia.org/wiki/Neutrino#Pauli's_proposal

[2]: https://en.wikipedia.org/wiki/Gravitational_wave#History

[3]: https://en.wikipedia.org/wiki/Higgs_boson#History

[4]: https://en.wikipedia.org/wiki/Cosmic_microwave_background#Hi...

[5]: https://en.wikipedia.org/wiki/Gravitational_lens#History

[sigbottle]

I don't know what or why our science education is like this, but it seems like everybody's understanding of science bottomed out at a straw man version of popper & positivism.

And to be clear, falsification and being empirical & skeptical about theoretical claims is great. What I see all too often on the Internet is just pattern matching to the words "observable" and "falsification" without a second thought, without actually looking into how science develops, and any and all narratives are historically rewritten to fit only those two categories.

Which is why it's even more impressive to be a real scientist, to actually be able to navigate the muddy waters properly where it's not just some simple adjective checklists to run through. (As a non scientist)

[wwarner]

I feel the same, but then I remind myself of how I used to feel about dark matter (I really disliked it). Having an arena where no scientific question is out of bounds is great.

[Izkata]

I blame sci-fi.

As a kid in 90s who was into this stuff, it seemed like anyone who knew about dark matter knew it was just theorized and never observed. It was included as a caveat in a lot of things I read to ensure the readers understood this.

Then in sci-fi it started showing up as a material the characters would interact with, and a lot more people were introduced to the term.

Then as a teen in the 2000s the caveat was usually gone and it started being treated as something we had proof of. People I knew started thinking it was something we'd actually found and was now fact instead of theory.

[rdtsc]

> Ok, so... why do people take it seriously as a concept? Occam's razor would point towards some general misunderstanding on which we have no evidence to reasonably speculate a cause.

One scientist says "We don't know, really, it's a head scratcher" and the others say something cool like "Dark energy", which ones will get more attention and publicity?

[cedilla]

But it really isn't a head scratcher, and it's not a wild guess, and also, dark matter is not the same as dark energy.

Think about some random person born in the 1950s. It will be impossible for you to directly prove that that person has two parents, but with all your knowledge about biology, and your knowledge that humans could not be cloned in the 1950s, it's not a head scratcher, unless our understanding of biology is completely off, it's not just speculation there were two parents. That's the comparable situation with dark matter. It's not a head scratcher. Every indication shows there is a lot of mass around that doesn't interact with light.

[zamalek]

Associating dark matter with epicycles is unfair, but it's still a risk (CDM, WDM, SIDM, and probably more by next year). In that light, of course it has more observations (by virtue of creating branches to explain incompatible observations) - but counterpoint is that it has made some predictions.

My stance is that anyone pointing at it in either light probably isn't taking everything into account. It's an incredibly immature theory space - are we going to get 20 more branches of it (making it modern epicycles), or are we going to see one of the current branches pay off?

[Sniffnoy]

I mean there are modified gravity candidates other than MOND. I think those are worth more consideration. MOND seems pretty well disproven at this point, yet somehow it's still what people focus on in terms of modified gravity! Where are the tests of other modified gravity theories?

Indeed, in terms of the negatives of MOND, I'll go further than you -- MOND barely explains galaxy rotation curves at all. When you look at the actual rotation curves compared to MOND's predictions, well, yes MOND does better than Newton w/o dark matter, but it's still pretty heavily fudged. If you have to fudge it that much, it seems to me there's no point. So I'd say that really MOND doesn't explain any of the three.

[joquarky]

It can't be observed, but it affects all these things.

This reminds me of arguments on the alt.athiesm newsgroup back in the day.

[dnautics]

MOND explains 1.

2 is "who can say" because nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND).

3 is subject to questions like "is the CMB really what we think it is" -- if it's early thermalized dust, then that ALSO resolves hubble tension, e.g.

MOND explains several things LCDM cannot:

- why most elliptical galaxies seem to "not have dark matter" (effectively a prediction)

- external field effect (predicted and confirmed)

- renzo's rule

- DM halos that are way too big

- early galaxies (this was a prediction)

HM. people have been downvoting. Anyone care to post a substantive rebuttal?

[Tuna-Fish]

2 needs a more substantive rebuttal. LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be.

There is a reason why LCDM used to be a lot more disputed before the work of Clowe, Gonzales and others on the bullet cluster, and is now generally treated as settled science by practitioners. We might still be surprised by something, the universe is more wondrous and complex than we can possibly understand, but Occam's razor massively supports LCDM now. If you want to propose any alternative, you need to start by showing how it explains bullet cluster as well or better than LCDM. (And the bullet cluster specifically is not the only place where this is visible, there are others like MACS J0025.4-1222.)

[dnautics]

> LCDM correctly predicts where the dark matter is located after a galaxy collision, such as in the bullet cluster. There is no reasonable interpretation of MOND that has the center of mass of the galaxy shifted away from where it's visible matter lies, precisely how LCDM says it should be.

It does not really make that "prediction", its a post hoc assignment of dark matter density based on weak lensing for which you can make a plausible "this is how it started" explanation.

you can counter with lcdm cant explain tons of stuff that MOND can, from tully fisher relation through barred spiral galaxies (n >> thousands) etc.

[pdonis]

> nobody has reconciled MOND with Relativity (not that it's impossible, it's just hard and annoying math, could be a lack of effort thing, could also be a real theoretical constraint that invalidates MOND).

My take on this is a bit different. The obvious way to get a relativistic theory that looks like MOND is to add a scalar field. (And possibly a vector field, as in TeVeS.) But from a relativistic, quantum field theory perspective, adding more fields is just...adding a new type of "matter". I.e., dark matter!

In other words, when you take relativity into account, "MOND" and "dark matter" don't really look like two different possibilities any more. They just look like two different ways of describing the same math in imprecise ordinary language.

[firebot]

2. It's called relativistic MOND. That exists as a category. RelMOND(for short) includes TeVeS, Einstein–Aether variants, BIMOND, and others.

3. I would wager that what we call the CMB is more of a local effect than a big bang remnant.

I lack the karma to downvote, don't blame me. :)

[dnautics]

2. not a physicist but iiuc the relativistic MONDs have been disappointing? but not enough to rule out

[firebot]

They have varying degrees of 'inelegance' or 'complexity.'

But generally they closely conform to Einstein gravity/standard model, as they incorporate relativity.

But that's still the thing. No theory can explain gravity across the spectrum of variables (from micro to macro scales, apparent lack of dark matter in some galaxies, black holes, why galaxies don't fly apart, etc.) We have a dark gravity problem regardless of which theory you like best.

[lukan]

"HM. people have been downvoting. Anyone care to post a substantive rebuttal?"

Just that asking this might get you more downvotes. I upvoted as I found your input interesting. I would suggest to edit that out (then I will delete my comment)

[dnautics]

I don't really care (i'm doing just fine fake-internet-points-wise), just if I made a mistake it would be nice to know. Otherwise worth it to know that there are people out there who find "alternative explanations by professional scientists" threatening. for some reason.

[lukan]

Well, I rather meant there are guidelines to keep this space meta debate free and I do think they make sense.

(And apart from that, HN is not free of dogmatic ideologists of all sorts who feel triggered by anything straying off from mainstream theory. That is my guess here and my advice to ignore it, but I would be also curious if there is something clearly wrong with your comment deserving downvotes.)

[gwynforthewyn]

Of course it can be that this is the case. There are predictions from the dark matter hypothesis, though, that have been testable. For example, there're some of the asymmetries in the cosmic microwave background that are explained presently by the dark matter hypothesis.

Something that's worth bearing in mind is that "dark matter" doesn't actually mean "totally new never before seen thing" it means "we don't know what this matter is". So, for example, a candidate that wouldn't be super novel but could fit the bill is microscopic black holes. In that sense, the hypothesis is more mundane than it might seem.

[MattPalmer1086]

As far as I know, dark matter hasn't made any testable predictions which were subsequently proved correct. It's an infinitely tunable hypothesis that can be retro fitted to new observations, since it has many tweakable parameters.

Could you point me to a prediction made by dark matter theory which was subsequently confirmed by observation, rather than a dark matter theory which explains it post facto?

[MattPalmer1086]

Not sure why the downvotes just for asking a question.

Dark matter (well, LCDM) has predicted many things which were proven false, e.g. that big galaxies assembled through merging with smaller ones over long periods of time. JWST shows us this is not in fact the case. There are now lots of papers showing how LCDM can explain that, but these are post facto explanations.

Happy to be proven wrong if someone can point me to a prediction subsequently confirmed by observation.

[gus_massa]

Is this the "hot" vs "cold" dark mater discussion? (Not my area. I just read that in Wikipedia.)

[MattPalmer1086]

Not really, Cold Dark Matter is part of LCDM, the main cosmological model. Whether hot or cold, I'd like to see a solid prediction made for it confirmed subsequently by observation.

[lubujackson]

100%. It's important to realize our understanding of "dark matter" is fuzzy because we only understand it through data anomalies. Dark matter is a classic catch-all concept that we use as a crutch while we try to understand the underlying system better.

Similar to how we used to believe in "aether" to explain how light could travel through empty space. It is important to understand how these crutches help and hinder understanding.

[kstrauser]

It's kind of weird in this case, though. All the math acts like there's something invisible and heavy everywhere that we find clumps of visible matter. When we look at the motion of galaxies, they behave as if they're much more massive than the count of stars and such in the would have you believe, and in ways that otherwise jibe with our understanding of physics if only that galaxy were heavier.

If you have one galaxy that's acting heavier than you can eyeball, measured by things like light bending around it, then maybe you have some weird phenomenon. When every galaxy calculates out to be about 6x fatter than you'd expect, something else is going on.

[fragmede]

Except (IANotAnAstrophysicist) it's not every galaxy. There are two that stick out as having little to no dark matter.

    * NGC 1052-DF2
    * NGC 1052-DF4

Of course this is hotly contested as it destroys the argument for MOND, but research is ongoing.

[lukeify]

The constituent particles of nature are under no requirement to be apparent or achievable to you.

[not2b]

The strongest evidence that something like MOND isn't the answer is that in some galaxy collisions, the visible matter and the dark matter appear to separate: the collision disrupts the visible matter and the dark matter appears to pass right through, uninterrupted, and we see galaxy remnants that look like they don't have dark matter. If MOND or some other modification of gravity were the answer we'd never see this kind of sorting.

See, for example, https://www.caltech.edu/about/news/dark-matter-flies-ahead-o...

[gostsamo]

You might be interested in that. We have observations of galaxies where dark matter likely is not present.

https://news.ycombinator.com/item?id=47641057

[rcxdude]

Astrophysicists have gone all through this. Absent direct observation or at least a good explanation of what it actually is, I don't think anyone really likes dark matter. The main objection is that it feels a little too flexible: you can explain a lot of things by adding mass in different places. But it's not infinitely flexible: you don't need arbitrary and utterly weird distributions of dark matter. And so it's generally emerged as the best explanation of observations so far, and not for lack of trying with tweaks to theories of gravity themselves.

[Centigonal]

yes - absent experimental confirmation, the effects attributed to dark matter may in fact be due to an unrelated as-yet-undiscovered phenomenon.