Hacker News Reader

I've long noticed that this idea - that scientists just "jumped to conclusion" that there must be some dark matter to resolve some observational inconsistencies - is very prevalent among non-specialists. It could not be further from the truth, of course - as soon as the rigorous statistical study of galaxy rotation curves was provided by Vera Rubin et. al. in the 80s, the MOND classes of alternatives to gravity started to appear.

I tried to understand why this narrative is so prevalent, and it looks like it is due to its very appealing nature - both to the listener and to the speaker: "all those egghead scientists can't see an obvious solution to a problem they are fighting with for decades". I also noticed that when you get deeper into it, then you find that the source - the "speaker" part in the whole narrative - is some kind of science popularizer. Who at the same time (surprise) is a proponent of his own flavor of modified gravity theory.

Truth is - everyone, including me, had this idea as soon as they first heard about the DM. If you actually study all the evidence and discussions around the subject, you'll see that introducing an extra "dark" particle is the most obvious solution to the whole collection of observational inconsistencies. Modifying gravity in such a way so that it is self-consistent and consistent with all the observations... I actually don't think anyone completely finished that project so far.

can it be possible that the galaxy just appears to be rotating faster due to time dilation, so from the POV of the galaxy itself it doesn't seem like there is anything out of the ordinary?

[It's not my branch, so I have to guess.] They are using special and general relativity to make a lot of corrections, like red shift and gravitational lens, so there is 0% chance that all the community forgot to add that correction.

Galactic rotational velocities are on the order of 100 km/s, which, though blazingly fast by human standards, is so slow compared to the speed of light that special relativistic effects are completely negligible.

Moreover, it's not just the absolute magnitude of the velocity that is the problem, it's also the shape of the curve. Newtonian mechanics (even with the extremely tiny relativistic corrections taken into account) predict that the rotational speed of a galaxy would decrease the further you go out from the center, and we in fact find that this speed remains more or less constant.

When Sabine Hossenfelder and Sean Caroll agree on something, I pay attention.

I do take some humility there. We have stunningly brilliant contrarians who would be the first to dismantle dark matter on the public stage, and instead they tell us, “no, it’s a real thing. We just haven’t seen it in a lab on earth much yet.”

Now if Eric Weinstein started saying dark matter is actually the consequence of a massively egocentric MOND, and everyone else is a WIMP, I might start believing that too ;)

What do Hossenfelder and Carroll agree on? It was my understanding that Hossenfelder thinks we need both DM and MoND while Carroll is firmly in the conventional DM camp.

You caught me, in that she’s 100% open to new evidence. Kinda why she is so effective.

So, over the past decade, Hossenfelder made a series of videos and talks about “dark matter is real.” And then “is dark matter real?”

Her opinion evolves. But the conclusions are generally that the data points to a measurable thing along many lines of evidence.

I find the cosmic web the most compelling, personally. Like, what else could that be besides a mystery? You can’t draw the web in some other configuration, it’s a real thing somehow.

From my understanding. Her hesitation that she expresses is more linked to an underlying intuition: the universe is completely deterministic—we just can’t see all of the rules being computed, and the initial conditions are intractable.

I also find that pretty compelling.

except its not consistent with all the observations, which is why it's not accepted consensus.

Everyone got drilled into them in school the stories of scientists and philosophers clinging to outdated models and introducing extraneous elements to extend their life in light of contradicting evidence, the piling up of epicycles to explain celestial movements by circular motions, or luminiferous aether for light to be wave in a fluid medium.

So of course when you first hear "so astronomers found out the equations don't work out with the matter we can detect, so they figured that there must be uhhh... more matter, invisible matter! That exists but you can't detect no matter what!" it sounds like those examples.

If you dig in deeper, then you realize that it's not just fudging of equations and that there being mass that's very sparse and unaccounted-for makes sense.

There's a lot more to it than galaxy rotation curves. https://en.wikipedia.org/wiki/Dark_matter is worth a read, especially the Observational Evidence section.

Also, general relativity has had considerable predictive success, so replacing it with something else that works the same in some situations but differently in others seems more complicated than "there's mass here in some form we don't understand yet", though people have of course tried to find such replacements. Physicists have been trying all the hypotheses they can think of and dark matter currently fits the evidence best, they didn't jump straight to dark matter or fixate on it without considering anything else.

> general relativity has had considerable predictive success, so replacing it with something else that works the same in some situations but differently in others seems more complicated than [...]

First: you're very correct - we shouldn't expect an easy solution.

Second: precisely the same argument applies to Newtonian gravity. It was phenomenally successful. The mismatch between observations and its model were more easily explained by lack in observations - and they were! We found Neptune because of that!

That role (of plugging holes in observations) is currently being played by dark matter. So chances for dark matter existing in some form are good from a historical perspective. On the other hand, from that same perspective, chances of gravity theory being slightly(1) updated for extreme cases are also good.

(1) "slightly" in the sense that the difference with the current theory, GR, would be nil or negligible in most cases.

>general relativity has had considerable predictive success, so replacing it with something else that works the same in some situations but differently in others seems more complicated than "there's mass here in some form we don't understand yet", though people have of course tried to find such replacements.

Do the models use GR? My understanding is that they make the assumption that Newtoninan gravity models are sufficient.

If I had a euro every time someone is skeptical of dark matter because they think it only predicts galaxy rotation curves, I’d have enough money for several beers (beer is expensive in Finland!) GRC observations were the original motivator for the DM hypothesis, but in the following decades its predictions (specifically, those of a specific cold dark matter model well-constrained by evidence) have been in entire agreement with many observations in several distinct subfields of astronomy, whereas no competing hypothesis has been able to achieve the same.

Indeed DM, or something very much like it, is a required ingredient in cosmology to get a universe even remotely similar to ours! As far as we know, we wouldn’t even have galaxies, much less the intricate foam-like structure of voids and filaments, without DM, and no competing hypothesis can predict the observed large-scale structure of the universe while also explaining galaxy rotation curves and everything else, for instance the Bullet Cluster, or the specific observed anisotropies in the cosmic microwave background.

The issue is that a modified gravitational theory means getting rid of general Relativity or at least _significantly_ modifying it. As i understand, no proposed modified gravity theory has ever been able to be more accurate than GR. Every theory that's been proposed (very many for over 100 years) to resolve galaxy rotation has then failed at some other scale or situation. Most of those theories still require dark matter to explain those discrepancies. The single most accurate and explainable model currently is general relativity with some extra dark mass hiding in extremely large structures.

As well, to do away with GR means ridding ourselves of the most accurate gravitational predictions ever made. GR explains basically _everything else_. It predicts gravitational waves, black holes, the orbit of mercury and the precession of rotating bodies, as well as many other things I'm sure. And so far every big prediction has come out to be true. In physics, predictive power is everything. Most of the modified gravity theories can't even explain everything we already see let alone make accurate predictions about other things.

It just comes down to math and data. I've had this conversation with many people and while i don't want to discourage questioning or curiosity, none of these people were very versed in the actual topics of modified gravity or modern dark matter theories (not saying this about you, but I've had this conversation _a lot_). You can Google a Wikipedia article about problems with modified gravity, MOND, etc. And read about it freely if curious.

The issue i think is that it's too complicated. The science and especially the math of these theories is graduate level physics and these theories are being proposed and discounted by doctorates who've dedicated their entire lives to learning about this. It's extremely difficult for regular people with jobs and lives to have the time to truly understand the issues here, at least until someone comes along who can explain it much better than I.

I must say, I'm also not a doctorate in gr or anything, but I've been around it a while and have seen the developments unfold over time. The short answer is that modified gravity theories have more problems than gr + dark matter, at least right now.

What about another force that shows up at larger scales that is to gravity what gravity is to the strong force ?

And what is gravity to the strong force, exactly? I'm pretty sure if you solve that one you might win a Nobel Prize.

Anyway, positing other forces is a good thought experiment, but ultimately that's more complex than LCDM, which is a pretty simple kind of thing to potentially exist relative to how much it explains.

> Immediately I see this as being a gravitational issue. Maybe gravity works in ways we do not understand at this level. But instead scientists jump directly to "there must be matter here that we can't see".

You go for the most likely explanation first. When I go to my car and my keys aren't in my pocket, I go and check the counter where I usually put them when I get home. I don't start checking behind the toilet or similar.

In this case we had a theory, general relativity, which worked very well for all the tests we'd done so far, and there was nothing in the theory to indicate it shouldn't be the same all over. Meanwhile astronomical observations are difficult and we knew, and still know, there's more to particle physics. Hence more mass is a more likely explanation.

Keep in mind that trying to replace an existing, validated theory can be very hard, since you need to not just explain the new stuff, but also pass all the checks the existing theory did. So far nobody has been able to come up with a modification of general relativity that can do this.

And it's not been for lack of trying[1].

[1]: https://en.wikipedia.org/wiki/Alternatives_to_general_relati...

I believe the rotational speed discrepancy in galaxies is simply the most intuitive example. There are many more cases where mass appears to be missing: https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... . Crucially, these occur at vastly different scales, from individual galaxies to clusters and whole observable universe.

> ...have we really jumped to matter being the culprit without taking a look at gravity behaving in unexpected ways?

This is called Modified Gravity, or MOND: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics and is well-researched. It stands as a niche but actively pursued alternative to Dark Matter. For the most part, Dark Matter fits observations better and more parsimoniously.

But of course we don't have definitive proof/disproof for either.

> I do not understand how they jumped directly to matter being the solution to the gravitational oddities

This never happened. "Dark Matter" is just a name for a phenomenon. Galaxies rotate as if they have a bunch of matter in them that can't be accounted for by looking at the light coming from them. So it's "dark" matter. Get it?

No one has a "solution" here. It's a problem, and it needs a name. What would you prefer?

Once again "naming things is hard" strikes again :p

Because it's the most economical assumption.

We know that there are various different particles and there are some that we haven't discovered yet. There's no reason why some particle wouldn't only interact via gravity and weak interaction.

Adding some "dark" mass in the simulations paints the picture that matches what we see.

Some galaxies were found without these gravitational anomalies, which all but rules out a general systemic issues with the theory of gravity.

I'll add, it paints the _entire_ picture (including smaller gravitational bodies and new found anomalies) much better than other theories

I wasn't aware of the galaxy rotation argument for DM - I have always heard it argued for using the bullet cluster observation for galaxy collision (https://en.wikipedia.org/wiki/Bullet_Cluster).

That's not to say DM is the only way to explain such data, but it is a good motivation for it.

Bullet cluster came much later and indeed the mass/visible matter discrepancy was discovered specifically because they were looking for evidence for dark matter. It’s just one piece of additional evidence for the theory that dark matter is specifically matter.

Seen this way it seems DM theories are a failure of applying Occam's Razor on a scientifically complex topic. Whereby our current tools and theories cannot explain some observed discrepancies, we aim for a more complicated explanation for them (special types of matter) rather than limitations of our current techniques.

At the same time science doesn't progress by just shrugging and saying "we can't know", so maybe it's an inherent conflict of interest at play here?

More complicated? That makes zero sense and is some sort of a strange cognitive bias against particles (or possibly against things we can’t directly see?)

When it was found that experiments didn’t support Thomson’s "raisin pudding" model of the atom, the solution was not to modify electromagnetism but to introduce a hitherto unknown particle, the proton. When it was found that the mass of a nucleus wasn’t simply the sum of the masses of the protons, the solution was not to modify theories of inertial mass and gravity, but to introduce a hitherto unknown particle, the neutron. When it was found that in a beta decay some of the mass-energy was unaccounted for, the solution was not a modification to Einstein’s equations but to introduce a hitherto unknown particle, the neutrino, whose existence was only experimentally proven quite a bit later.

All our evidence from distinct sub-disciplines of astronomy suggest that some specific well-constrained type of matter that interacts via gravity but not via EM is needed to make predictions match observations. No competing hypothesis has been proposed that is as good a match. Scientific method 101 says that if something walks like a duck, swims like a duck, and quacks like a duck, we should assume that it is indeed a duck until and unless additional evidence rules out the duck model.

I mean we're trying to determine the mass of objects many light years away.

The concept that we might not be able to perceive everything isn't a real huge leap. Far shorter a leap than "everything we know about gravity is wrong".