Why do people in the comments
always ignore that dark matter has way more evidence for it than just galaxy rotation curves (something very well explained in the article)? It's quite tiresome to read the same objections as if physicists somehow hadn't thought of them.
Probably because it's the only piece of evidence you can understand with just high school physics. Most laypeople have trouble understanding that the universe is not expanding from a single point, so how would they be able to understand what the cosmic background radiation is, much less why its angular power spectrum has peaks at particular positions?
It's a common pattern with crackpots as well, that they focus on "big" but easy to understand things like special relativity, which doesn't involve more math than you learn in high school. No crackpot ever revolutionizes condensed matter physics or statistical mechanics.
Although the bullet cluster should be a clear indicator as well, the rest of the evidence of DM appears much more subtle to a layperson.
Anyway, cosmology is a weird topic which not only HN reveals tons of arrogance over. Lots of misplaced skepticism. (Probably similar to the Covid topic.)
Yes exactly. We've all experienced reading an account on our particular domain of expertise by a journalist and realising they have absolutely no clue what they're talking about. I find it amazing how presumably otherwise intelligent people reading Hacker News don't see the issue with offering lay opinions on Cosmology of all topics.
Crackpots emerge when a conceptual model is weak. They don't care how strong a mathematical model is, which is why they ignore SM and other areas where "theory" literally means a mathematical model, and nothing else. Having a lot of crackpots proposing bad theories is a sign that your conceptual model is garbage, just a bad attempt to hand wave a bunch of math that happens to predict reality, and you should stop pretending.
There are infinitely many mathematical systems that could describe "reality" all with divergent behavior in the parts of the system we haven't observed yet. Trying to make authoritative claims by analogy to coincidentally correspondent mathematical systems is foolhardy and vain.
> Anyway, cosmology is a weird topic which not only HN reveals tons of arrogance over. Lots of misplaced skepticism. (Probably similar to the Covid topic.)
It's the curse of being clever. People believe that since they're clever and were able to be successful at what they do, it must be that all other topics are similarly easy. And with easy access (via the internet) to the most basic introductions for lay people to any topic, the Dunning Kruger effect really kicks in.
The more clever you are, the easier it is to fall into this trap. example: "Buy twitter and solve all its problems myself".
> No crackpot ever revolutionizes condensed matter physics or statistical mechanics
Well, the memory of water still has a lot of support, as the scientific explanation for homeopathy. Lots and lots of crackpots here.
I would not call this crackpot, but it took an awful amount of time for some to let go cold fusion.
As for statistics - my personal hell is when I have to read some biology or biology-like document and there are numbers that are tortured to make them sing the way they are expected to. I do not even talk about economy, psychology, sociology and other similar ologies.
Because it is the only evidence that is presented, and lay-people can easily come up with objections. These people then express their frustration or confusion why some of these objections seem completely unaddressed.
These people are effectively asking about these objections. Perhaps many of them are phrasing it differently. But, with exceptions, I think people tend not to be angry or vitriolic. At best they are frustrated. Just treat these remarks as questions.
It might be annoying to get the same questions repeatedly, but that is a fault of wider communication about dark-matter.
Everything we see in the Universe outside our own galaxy is static. We don't have enough parallax to actually perceive objects (stars) moving outside our galaxy. Ok, there's an exception, people were able to perceive the movement of one star in one of our satellite galaxies in 2012. That's about it. So, when people talk about galaxy rotation curves, that's based on a sample of one, our own galaxy. Actually, we can plot galaxy rotation curves for other galaxies, but based on indirect evidence, such as the redshift. How accurate is that? Come to think of it, we can't estimate directly the distance to any galaxy beyond and including the Andromeda galaxy. But again, we can estimate indirectly. Only that estimation is based on a stack of estimations each with its own uncertainties.
Now, if you work in the area, stop me right here and tell me you went through that stack of estimates, and check all the uncertainties, and are confident there's no potential error anywhere.
Because, as an outsider, if I do a bit of Bayesian estimation, and I have to choose between the dark matter and the possibility that everything that can be explained with dark matter is actually a result of estimation errors, I'm somewhat on the fence. Every year that the hunt for dark matter brings nothing I'm more inclined to think there's a different explanation out there. Like the frame dragging explanation discussed in [1].
I'm sorry, but misplaced confidence like yours is the tiresome part. I think you should also apply Bayesian thinking to the whole debate as well.
I'm not a cosmologist, but I have a degree in Physics. I won't be able to fill in the details, but all your objections sound pretty basic and unfounded even to my semi-amateur ears. About the cosmic distance ladder (as if parallax were the only way. I guess we don't see Earth's curvature either), about how to propagate errors (literally learnt in the 1st week of Physics. Those bars around the scatter points are literally the error bars, and we propagate them from start to finish). And even about the scientific method itself. No one can claim "there's no potential error anywhere" in anything.
And most importantly, which was my original main point, galaxy rotation curves are not our only evidence.
BTW, I would encourage you to look at how fossils are dated. To me it sounds kind of similar in spirit to how distances are measured in astronomical scales.
I'm encouraged to reply by your reference to a (undergraduate cosmology) textbook in your more recent comment in this thread. I'd suggest borrowing (or taking advantage of a free-as-in-beer Internet copy of) the most recent edition of Binney & Tremaine's _Galactic Dynamics_, a standard textbook for graduates (and a good reference for researchers) that explores in detail how galaxy mass distributions are calculated. FWIW Binney has from time to time explored and even embraced MONDian ideas, and this is captured in his publication record.
In their textbook you'll find that galaxy rotation curve studies are spectroscopic. That is they are keenly interested in the relative redshift in the 21 cm neutral atomic hydrogen lines (among others like H2 and CO molecular lines) at the limbs of edge-on disc galaxies, and adapting that to spirals and other disc galaxies that are tilted away from edge on. [Binney 2e sec 6.1]. Practically invariably the relevant lines are relatively redshifted and relatively blueshifted at the limbs, leading to the interpretation that generically there is equatorial spin in disc galaxies.
In elliptical galaxies there is practically no equatorial spin to speak of; instead the spread of relative redshift across the face of the elliptical is interpreted as blobs of hydrogen gas moving radially, that is sinking deeper into the galaxy or rising out of the galaxy's depths. It is also useful to study a wide range of absorption lines given that the clouds are backlit by a galaxy's worth of starlight (and sometimes a quasar), in a process which grinds out surface densities.
These relative redshifts do not depend on cosmological redshift (the whole galaxy, or its whole cluster, is cosmologically redshifted identically for all practical purposes, so the opposite limbs in discs are affected similarly). It is also not sensitive to an isolated galaxy's peculiar motion within a cluster. It may matter for merging galaxies.
We can also look deeper than disc limbs and ellptical surfaces. Optical interferometry is highly sensitive in this application, and provides direct evidence of the motion of the various sources of emission and absorption lines from various gas clouds, dusts, and even starlight. The Large Binocular Telescope does some work in this area. Radio interferometry is useful for looking into the bulk motions within the more central regions of galaxies and clusters; dust obscures optical signals but millimetre signals cut through.
The investigated starlight is bulk and while the interpretation depends on assumptions about the bulk stellar chemistry of an observational target (maybe metallicity varies slightly in different parts of an elliptical which might have a history of galaxy mergers) there is in no way a dependence upon any single star and its meanderings through its galaxy. We're interested in the light generated by ~billions of stars, not the positions or momenta of single stars, mostly because we just cannot resolve the latter with current technology.
> as an outsider
You seem interested in the topic. Learning how observations are made (and the history of them) is probably not inaccessible to you given your comments here. Whether that leads you into any sort of conclusions about the structure and evolution of galaxies is up to you, but I think textbooks will be better for you than whatever ultimately led you to the stackexchange link in your comment. (I did notice however that the Ciotti preprint discussed later in your link cites Binney & Tremaine multiple times, as does the Ludwig paper in the stackexchange question).
I don't particularly buy that a gravity-only interacting particle "is not physics". If that is what exists, that is what exists. And we've already detected it via dark matter observations. So it is physics. What we can't do is ever really prove it other than by exclusion, but if we racked up a thousand years of failure, we'd have to accept that it was real (hopefully by then not dogmatically and we'd let any kid who wanted to tilt at windmill to go for it).
And Woit links to a paper on dark matter being only a right handed majorana neutrino, requiring no real physics beyond the standard model (plus three right handed non-interacting neutrinos): https://arxiv.org/abs/1803.08930
And links to a comment on that in John Baez's blog:
Another big problem is that all LCDM predictions (MACHO/WIMP) were disproven but it still remains a theory. Now we need energy too high to use in universe.
It's eerily similar to Phlogiston theory that at one point was considered to have negative mass.
Reading this I realized that dark matter is starting to sound somewhat like the idea of the "ether". It's everywhere, it affects everything, but we can't see it or interact with it easily. It's somewhat different than the idea of the ether in that it is not thought to be a homogenous, pervasive field; instead it clumps around gravity. But it has some very ether-like aspects, maybe some we have not discovered yet. I always thought we abandoned the idea of an ether too soon. Why does it have to be evenly spread? Why does it have to respond to things like direction or EM waves? Maybe there are some simple experiments that could explore the idea of an ether/dark matter further. Pure speculation but that is my intuition.
Dark matter and dark energy are like what if there was no Einstein to discover relativity in early 1900s. We later continue to discover that gravity can't explain planetary motions correctly and therefore search for any explanation.
There are more laws of Physics waiting to be discovered. At least two big ones from what we currently observe.
You are cherry-picking one particular example. The neutrino and the planet Neptune were "dark matter" for decades until better detectors became available.
There is certainly new physics involved, the issue is more that for dark matter, the evidence points to matter that hardly, or perhaps not at all, interacts* with the stuff we are familiar with (standard model). It might be that the correct theory of dark matter is already found and debated, we just have no way to tell it from the false ones, as for all that we know, dark matter might be a shadow world of spaghetti monsters of which we only know its existence via their gravitation.
> the evidence points to matter that hardly, or perhaps not at all, interacts* with the stuff we are familiar with
I think this is why people get frustrated with the DM story. It's not the evidence that points to that. It's the lack of evidence that points to that. Specifically speaking, we keep hypothesizing limits on interaction in the hopes of finding it below some threshold but keep pushing those limits back. A multimodal observation would be clutch, but we keep confirming that a multimodal observation can't happen.
My intuitive objection against dark matter is that it's basically an infinite amount of additional parameters to be fitted to the observations. Galaxy behaves weirdly? Add dark matter! It behaves as expected? It obviously has lost its dark matter! It stands to reason that for every possible observation, a particular DM configuration can be calculated.
I fail to see how dark matter could ever be falsified. The best potential replacement would be a theory that's simpler, but even then dark matter would probably explain the same observations.
The second question is: Is dark matter even matter if it only interacts via gravity? Wouldn't it be better named a property of space-time? That is, dark matter is a name for a recursive property of gravity?
You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy. Instead you 'just' need to add a weakly interacting particle with the right density (30% of critical). That particle needs to be 'slow enough', i.e. non-relativistic. After that it turns out that the evolution of the universe basically reproduces the vast array of observations.
* The right primordial abundance of different elements
* The amount of structure in the cosmic microwave background.
* The rotation speeds/velocity dispersions in large number of galaxies
* Large scale structure/The amount of clustering of galaxies.
* Gravitational lensing observations.
And much much more.
It is certainly true that there are individual cases that look challenging for Lambda CDM paradigm (CDM stands for Cold Dark Matter), but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM. (there are currently some tensions with the Hubble constant measurements and some other cosmological parameters between different techniques, and it remains to to be seen whether those are systematic errors or point towards some modification in the theory).
Comment on the article itself. I think at some point I would be fine with dark matter being essentially a computing trick, i.e. if the behaviour of the universe is well reproduced when we add gravitationally interacting but not interacting otherwise matter. It would be up to theorists to see if somehow that can be written as some sort of extra-term in Einsten's Equations and show we don't really need an actual particle. I think there is a possibility of that, but personally I still think it's more likely there is a particle responsible for dark matter.
> You don't seem to quite understand the current cosmological models. You don't need special cases for every galaxy.
No, he's correct that each galaxy does need tuning. Specifically, the amount of DM and it's specific distribution is tuned to match each observation, which is what the OP said.
> but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM.
I think this "strong case" for LCDM is often oversold. See this review of the evidence for LCDM compared to modified gravity:
There is only a handful of parameters. They describe the probability distribution of finding one particular matter configuration. We are simply observing one realization of that probability distribution, in which we should be able to calculate the percentage of galaxies with an unusual amount of dark matter for any definition of "unusual". We don't consider the position and momentum of every baryon a parameter of the standard model either, so why should we do so with DM?
DM can be falsified perfectly well. If the rotation curves of all observed galaxies behaved as predicted by Newton, there would be no DM and every reasonable physicist would consider DM falsified. DM simply passes a lot of falsification experiments.
If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation.
Differentiating it from MoND or similar theories will then be based purely on how well the theory fits.
However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable. Why shouldn't there be heavy particles that don't interact electromagnetically, like a heavy neutrino? Or particles that don't even interact weakly? Or at least, why should their abundance be low compared to baryons?
Edit: Regarding falsification of DM, it's not like DM isn't challenged anywhere. Like I mentioned above, we should be able to compute the abundance of galaxies of certain sizes (dark or not), and the dwarf galaxies are coming up short. It's called the dwarf galaxy problem and poses indeed a challenge to DM, albeit not as much as DM alternatives in the mind of most cosmologists. They think it will be solved eventually as our understanding of galaxy formation improves, but who knows, maybe it's the first glimpse at something bigger. https://en.wikipedia.org/wiki/Dwarf_galaxy_problem
> However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable.
I think this is a strawman. MOND proponents don't discount that some observations are better suited to a particle, they just assert that the particle dark matter sledgehammer is vastly overused, and that modified gravity has been unusually successful in cases where it had no right to be if gravity acts as it does under GR.
For instance, this review [1] of the astrophysical evidence by a MOND proponent makes a good case for MOND + sterile neutrinos. This review also covers numerous falsifications of DM, so if you're correct that DM can be falsified, why did these falsifications fail to convince the majority?
This seems like a very nicely detailed response to OPs question. Thanks! Your response brought up some questions and remarks from myself.
> DM can be falsified perfectly well.
I recall hearing that DM was attempted to be falsified based on gravitational lensing. Is that correct?
> If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation.
Would DM as matter not have inertia, whereas treating it like an artifact of space-time would leave it without inertia?
> Differentiating it from MoND or similar theories will then be based purely on how well the theory fits.
A simple question here would be: do all galaxies of similar size have the same amount of dark matter. If so, then the uniformity of dark-matter would need explanation. If not, then I think something like MoND would have a hard time explaining why these similar galaxies rotate at different speeds.
Dark matter is not a “recursive property of gravity” because it’s not consistent like that, as your own initial objection suggests.
The concept of a particle that only interacts via gravity is straightforward. Consider neutrinos, which only interact via gravity and the weak force. Trillions of them pass through your body every second. Take away the weak force interaction and add some mass, and you have a dark matter candidate particle.
> I fail to see how dark matter could ever be falsified.
You’d have to explain what should be done about that. It could nevertheless be a correct theory, and we might one day verify it more directly.
Naive falsificationism is an almost useless principle at the frontiers of modern physics. We’re long past the days when you could run some simple experiments and come up with a clear yea or nay. That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.
> You’d have to explain what should be done about that.
Well, there should be something that follows from the assumption that there are huge amounts of particles that only interact via gravity. What's the prediction that this model makes? If it doesn't make any useful prediction, it's pretty much worthless.
> That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.
I think this attitude is inappropriate. The scientific progress made with 19th century methods, and that includes most of the stuff discovered until the 1950s, is massive. Maybe scientists should focus more on a clear nay or yea? I mean, the LHC is certainly a significant achievement, but compared to the detection of gravitational waves or even just exoplanets it doesn't seem to deliver as much bang for the buck.
> Naive falsificationism is an almost useless principle at the frontiers of modern physics. We’re long past the days when you could run some simple experiments and come up with a clear yea or nay. That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.
To a layman like myself this implies both that the things being studied then are both unprovable and not potentially of practical use or significance. I certainly am not saying that is the case, but rather am looking for more informed opinions to the contrary.
The cynical side of me feels like study of something unfalsifiable is as the study of religion. A poor investment for public funds.
As I mentioned however, I am seeking opinions here to the contrary of my gut reaction as my gut is often dumb and reactionary.
If it is not falsifiable, it is not science nor physics. It is meta-physics, ersatz-religion. Which is exactly the core of the complaint against concepts like this.
Of course same thinking can't easily be applied to things that are not always true, like social behaviour. Part of the reason the replication crisis hit so hard in the social sciences.
And of course this is not really as absolute as I wrote above. No field works like this all the time. But it is a good guideline. If falsifiability is completely ignored, then a concept really is nothing more than a religious belief and should be considered as such.
>which admittedly, many people still seem to want to do.
This occurs because most science is not the "the frontiers of modern physics". But even then, falsification in the frontiers of modern physics is still a useful tool.
Unless you have worked extensively with the data and math of modern cosmology I very much doubt your 'intuition' about 'dark matter' is anywhere close to the intended meaning.
Right? After quantum mechanics and relativity there's absolutely no reason to believe that our intuition is worth squat when it comes to how physics works.
I very much agree with you, and have raised these questions before.
I got some interesting responses that half-mollified my worries.
Originally dark matter was hypothesized because of galaxy rotation mismatches. This allows for mapping where 'dark matter' should be. One falsification of dark matter that was tried is gravitational lensing. If there is indeed extra curvature of space-time in these galaxies, then they should bend light-rays, Potentially showing the same galaxy twice.
This was tested, and confirmed. Hypothesized dark matter, as detected by galaxy rotation, causes the expected gravitational lensing.
I suppose another potential falsification is by 'moving' dark matter through a gravitational tractor (very heavy mass with rocket engines) and then detect the resultant change in gravity. You could attempt that detection by gravitational lensing, orbit velocity, or perhaps something else.
Despite all this, dark matter still feels like our theory of gravity was falsified, and instead of concluding the theory was wrong, it was attempted to add random parameters to the theory. Its not like there is a better theory though (I believe MONS has trouble with explaining gravitational lensing), so perhaps this is a problem of PR much more than a problem of science.
As for the second question, for dark matter to be matter, I think it suffices if it has mass and inertia. My gravity tractor detection idea tries to work with that.
"Dark Matter" defies parsimony. Occam's Razor suggests rejecting the
assumption that spacetime is uniform. The search for "Laws" may be a
parochialism built into human epistemology. Advanced aliens may be
comfortable with the idea of "space weather" and local variations. :)
Where does "infinite amount of additional parameters" come from? It's just one parameter - something's (we call dark matter) is either there or it isn't.
> I fail to see how dark matter could ever be falsified
I don't think science cares. Frankly if you could back up your opinion with some actual reasoned argument, it would be a better post.
My intuitive objection to your comment is that it ignores the basis of the article: that it’s entirely about learning more about dark matter. This is the complete antithesis to what your comment claims.
I’m sure physicists have already thought about this but…
Has anyone considered modeling the effect currently explained via dark matter as a new kind of force or field? Like a distinct force generated by mass and its relationship to space time?
This wouldn’t be modified gravity but a proposal for an entirely new force whose effect is what we currently explain with the “fudge factor” of dark matter.
From the rough picture I do have of dark matter this force would be one whose relationship to distance is different from the usual inverse square law that governs forces. That would be weird but maybe no weirder than particles that can never be detected or MOND.
"Dark matter" implies a new field. Or rather, a new field or force that affects gravity implies some sort of interaction exactly like a massive particle that interacts only gravitationally. Particles are just localized disturbances in fields (in quantum field theory), and new fields can have such disturbances and thus have particles. In this (trivial) sense, MOND is a type of "dark matter" theory that posits a particle with only gravitational interactions.
Of course we don't have a quantum field theory for gravity, so the above is hand-waving away a HUGE issue of what that new field would be like, how its particles would act, etc, since we don't even know what the quantum gravitational field is like and what its particles are like (assuming such a thing even makes sense).
Am I right in thinking no one has been able to detect dark matter despite decades of attempts and research?
My understanding is that based on observations of what can be seen galaxies move as if there's move stuff in them??
If you hypothesis that this extra stuff is dark matter but can't detect it, how do you know it's real?
How have you ruled out all the normal stuff that doesn't give off light or stuff we can't see at the distances we're looking over? Like planets, rocks, dust, etc. Couldn't that stuff, which to my mind there would be vastly more of than stars, make up the extra gravity?
I'm aware I might be talking complete shit because I'm completely ignorant of the detail here. I would love an understandable explanation as to why that's wrong.
> Am I right in thinking no one has been able to detect dark matter despite decades of attempts and research?
Yup [1]
> My understanding is that based on observations of what can be seen galaxies move as if there's move stuff in them??
Yup [2]
> If you hypothesis that this extra stuff is dark matter but can't detect it, how do you know it's real?
It produces certain effects in various different enviromennts or domains, which lends more support to that hypothesis. By certain different domains, I mean like Galaxy and cluster dynamics, gravitational lensing around galaxies and clusters and the precise fluctuations seen in the cosmic microwave background, these are all very different regimes but all seem to support the hypothesis of extra mass which is not interacting electromagnetically.
> How have you ruled out all the normal stuff that doesn't give off light or stuff we can't see at the distances we're looking over?
There are research efforts to look for exactly this kind of stuff; Massive compact halo object (MACHOS). However they don't see enough of it to account for the observations we see in galaxies and galaxy clusters. [3]
Sort of, but gravitational lensing has been seen in places where there is no visible matter. If dark matter doesn't exist, then you'd have to come up with another explanation for the gravitation lensing:
You're absolutely right and this is the one thing about dark mayer that doesn't sound like "just keep sprinkling it on your problem till it works."
I don't know enough about the found examples of gravitational lensing to know whether all possible arrangements of black holes that could have the same effect, have been considered and found lacking. But I presume so.
Readit News
It's a common pattern with crackpots as well, that they focus on "big" but easy to understand things like special relativity, which doesn't involve more math than you learn in high school. No crackpot ever revolutionizes condensed matter physics or statistical mechanics.
Although the bullet cluster should be a clear indicator as well, the rest of the evidence of DM appears much more subtle to a layperson.
Anyway, cosmology is a weird topic which not only HN reveals tons of arrogance over. Lots of misplaced skepticism. (Probably similar to the Covid topic.)
There are infinitely many mathematical systems that could describe "reality" all with divergent behavior in the parts of the system we haven't observed yet. Trying to make authoritative claims by analogy to coincidentally correspondent mathematical systems is foolhardy and vain.
It's the curse of being clever. People believe that since they're clever and were able to be successful at what they do, it must be that all other topics are similarly easy. And with easy access (via the internet) to the most basic introductions for lay people to any topic, the Dunning Kruger effect really kicks in.
The more clever you are, the easier it is to fall into this trap. example: "Buy twitter and solve all its problems myself".
Well, the memory of water still has a lot of support, as the scientific explanation for homeopathy. Lots and lots of crackpots here.
I would not call this crackpot, but it took an awful amount of time for some to let go cold fusion.
As for statistics - my personal hell is when I have to read some biology or biology-like document and there are numbers that are tortured to make them sing the way they are expected to. I do not even talk about economy, psychology, sociology and other similar ologies.
These people are effectively asking about these objections. Perhaps many of them are phrasing it differently. But, with exceptions, I think people tend not to be angry or vitriolic. At best they are frustrated. Just treat these remarks as questions.
It might be annoying to get the same questions repeatedly, but that is a fault of wider communication about dark-matter.
Everything we see in the Universe outside our own galaxy is static. We don't have enough parallax to actually perceive objects (stars) moving outside our galaxy. Ok, there's an exception, people were able to perceive the movement of one star in one of our satellite galaxies in 2012. That's about it. So, when people talk about galaxy rotation curves, that's based on a sample of one, our own galaxy. Actually, we can plot galaxy rotation curves for other galaxies, but based on indirect evidence, such as the redshift. How accurate is that? Come to think of it, we can't estimate directly the distance to any galaxy beyond and including the Andromeda galaxy. But again, we can estimate indirectly. Only that estimation is based on a stack of estimations each with its own uncertainties.
Now, if you work in the area, stop me right here and tell me you went through that stack of estimates, and check all the uncertainties, and are confident there's no potential error anywhere.
Because, as an outsider, if I do a bit of Bayesian estimation, and I have to choose between the dark matter and the possibility that everything that can be explained with dark matter is actually a result of estimation errors, I'm somewhat on the fence. Every year that the hunt for dark matter brings nothing I'm more inclined to think there's a different explanation out there. Like the frame dragging explanation discussed in [1].
[1] https://physics.stackexchange.com/questions/618981/has-frame...
I'm not a cosmologist, but I have a degree in Physics. I won't be able to fill in the details, but all your objections sound pretty basic and unfounded even to my semi-amateur ears. About the cosmic distance ladder (as if parallax were the only way. I guess we don't see Earth's curvature either), about how to propagate errors (literally learnt in the 1st week of Physics. Those bars around the scatter points are literally the error bars, and we propagate them from start to finish). And even about the scientific method itself. No one can claim "there's no potential error anywhere" in anything.
And most importantly, which was my original main point, galaxy rotation curves are not our only evidence.
BTW, I would encourage you to look at how fossils are dated. To me it sounds kind of similar in spirit to how distances are measured in astronomical scales.
In their textbook you'll find that galaxy rotation curve studies are spectroscopic. That is they are keenly interested in the relative redshift in the 21 cm neutral atomic hydrogen lines (among others like H2 and CO molecular lines) at the limbs of edge-on disc galaxies, and adapting that to spirals and other disc galaxies that are tilted away from edge on. [Binney 2e sec 6.1]. Practically invariably the relevant lines are relatively redshifted and relatively blueshifted at the limbs, leading to the interpretation that generically there is equatorial spin in disc galaxies.
In elliptical galaxies there is practically no equatorial spin to speak of; instead the spread of relative redshift across the face of the elliptical is interpreted as blobs of hydrogen gas moving radially, that is sinking deeper into the galaxy or rising out of the galaxy's depths. It is also useful to study a wide range of absorption lines given that the clouds are backlit by a galaxy's worth of starlight (and sometimes a quasar), in a process which grinds out surface densities.
These relative redshifts do not depend on cosmological redshift (the whole galaxy, or its whole cluster, is cosmologically redshifted identically for all practical purposes, so the opposite limbs in discs are affected similarly). It is also not sensitive to an isolated galaxy's peculiar motion within a cluster. It may matter for merging galaxies.
We can also look deeper than disc limbs and ellptical surfaces. Optical interferometry is highly sensitive in this application, and provides direct evidence of the motion of the various sources of emission and absorption lines from various gas clouds, dusts, and even starlight. The Large Binocular Telescope does some work in this area. Radio interferometry is useful for looking into the bulk motions within the more central regions of galaxies and clusters; dust obscures optical signals but millimetre signals cut through.
The investigated starlight is bulk and while the interpretation depends on assumptions about the bulk stellar chemistry of an observational target (maybe metallicity varies slightly in different parts of an elliptical which might have a history of galaxy mergers) there is in no way a dependence upon any single star and its meanderings through its galaxy. We're interested in the light generated by ~billions of stars, not the positions or momenta of single stars, mostly because we just cannot resolve the latter with current technology.
> as an outsider
You seem interested in the topic. Learning how observations are made (and the history of them) is probably not inaccessible to you given your comments here. Whether that leads you into any sort of conclusions about the structure and evolution of galaxies is up to you, but I think textbooks will be better for you than whatever ultimately led you to the stackexchange link in your comment. (I did notice however that the Ciotti preprint discussed later in your link cites Binney & Tremaine multiple times, as does the Ludwig paper in the stackexchange question).
Okay now do climate change and climatologists.
And most people just don't bother to read the article at all it seems. No better than reddit.
Changing gears, though, and trying to be more useful, if you want more coverage on this topic there's a note in Peter Woit's blog:
https://www.math.columbia.edu/~woit/wordpress/?p=13232
Points to another blog which attacks the "nightmare scenario":
https://tritonstation.com/2022/11/29/the-angel-particle/
I don't particularly buy that a gravity-only interacting particle "is not physics". If that is what exists, that is what exists. And we've already detected it via dark matter observations. So it is physics. What we can't do is ever really prove it other than by exclusion, but if we racked up a thousand years of failure, we'd have to accept that it was real (hopefully by then not dogmatically and we'd let any kid who wanted to tilt at windmill to go for it).
And Woit links to a paper on dark matter being only a right handed majorana neutrino, requiring no real physics beyond the standard model (plus three right handed non-interacting neutrinos): https://arxiv.org/abs/1803.08930
And links to a comment on that in John Baez's blog:
https://golem.ph.utexas.edu/category/2022/12/neutrino_dark_m...
http://backreaction.blogspot.com/2017/01/the-bullet-cluster-...
Another big problem is that all LCDM predictions (MACHO/WIMP) were disproven but it still remains a theory. Now we need energy too high to use in universe.
It's eerily similar to Phlogiston theory that at one point was considered to have negative mass.
There are more laws of Physics waiting to be discovered. At least two big ones from what we currently observe.
*) Except gravity.
I think this is why people get frustrated with the DM story. It's not the evidence that points to that. It's the lack of evidence that points to that. Specifically speaking, we keep hypothesizing limits on interaction in the hopes of finding it below some threshold but keep pushing those limits back. A multimodal observation would be clutch, but we keep confirming that a multimodal observation can't happen.
I fail to see how dark matter could ever be falsified. The best potential replacement would be a theory that's simpler, but even then dark matter would probably explain the same observations.
The second question is: Is dark matter even matter if it only interacts via gravity? Wouldn't it be better named a property of space-time? That is, dark matter is a name for a recursive property of gravity?
* The right primordial abundance of different elements
* The amount of structure in the cosmic microwave background.
* The rotation speeds/velocity dispersions in large number of galaxies
* Large scale structure/The amount of clustering of galaxies.
* Gravitational lensing observations.
And much much more.
It is certainly true that there are individual cases that look challenging for Lambda CDM paradigm (CDM stands for Cold Dark Matter), but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM. (there are currently some tensions with the Hubble constant measurements and some other cosmological parameters between different techniques, and it remains to to be seen whether those are systematic errors or point towards some modification in the theory).
Comment on the article itself. I think at some point I would be fine with dark matter being essentially a computing trick, i.e. if the behaviour of the universe is well reproduced when we add gravitationally interacting but not interacting otherwise matter. It would be up to theorists to see if somehow that can be written as some sort of extra-term in Einsten's Equations and show we don't really need an actual particle. I think there is a possibility of that, but personally I still think it's more likely there is a particle responsible for dark matter.
No, he's correct that each galaxy does need tuning. Specifically, the amount of DM and it's specific distribution is tuned to match each observation, which is what the OP said.
> but it is also true that anything that has to do with galaxy/star formation is immensely complicated as it involves a lot of physics at a variety of scales, so I do not think it there is a strong case against CDM.
I think this "strong case" for LCDM is often oversold. See this review of the evidence for LCDM compared to modified gravity:
From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm
DM can be falsified perfectly well. If the rotation curves of all observed galaxies behaved as predicted by Newton, there would be no DM and every reasonable physicist would consider DM falsified. DM simply passes a lot of falsification experiments.
If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation.
Differentiating it from MoND or similar theories will then be based purely on how well the theory fits.
However, while I'm only a cosmologist and not a particle physicist, I find the implied assertion by proponents of MoND, that the existence of non-interacting particles is impossible, questionable. Why shouldn't there be heavy particles that don't interact electromagnetically, like a heavy neutrino? Or particles that don't even interact weakly? Or at least, why should their abundance be low compared to baryons?
Edit: Regarding falsification of DM, it's not like DM isn't challenged anywhere. Like I mentioned above, we should be able to compute the abundance of galaxies of certain sizes (dark or not), and the dwarf galaxies are coming up short. It's called the dwarf galaxy problem and poses indeed a challenge to DM, albeit not as much as DM alternatives in the mind of most cosmologists. They think it will be solved eventually as our understanding of galaxy formation improves, but who knows, maybe it's the first glimpse at something bigger. https://en.wikipedia.org/wiki/Dwarf_galaxy_problem
I think this is a strawman. MOND proponents don't discount that some observations are better suited to a particle, they just assert that the particle dark matter sledgehammer is vastly overused, and that modified gravity has been unusually successful in cases where it had no right to be if gravity acts as it does under GR.
For instance, this review [1] of the astrophysical evidence by a MOND proponent makes a good case for MOND + sterile neutrinos. This review also covers numerous falsifications of DM, so if you're correct that DM can be falsified, why did these falsifications fail to convince the majority?
[1] From Galactic Bars to the Hubble Tension: Weighing Up the Astrophysical Evidence for Milgromian Gravity, https://www.mdpi.com/2073-8994/14/7/1331/htm
> DM can be falsified perfectly well.
I recall hearing that DM was attempted to be falsified based on gravitational lensing. Is that correct?
> If DM interacts purely gravitationally, then yes, considering it either matter or an artifact of space-time is merely a matter of preference. Simply move the DM part of the energy-stress tensor on the other side of the Einstein equation.
Would DM as matter not have inertia, whereas treating it like an artifact of space-time would leave it without inertia?
> Differentiating it from MoND or similar theories will then be based purely on how well the theory fits.
A simple question here would be: do all galaxies of similar size have the same amount of dark matter. If so, then the uniformity of dark-matter would need explanation. If not, then I think something like MoND would have a hard time explaining why these similar galaxies rotate at different speeds.
If it's purely gravitational then that implies infinite density and infinite force for any matter that occupies the exact same space.
The concept of a particle that only interacts via gravity is straightforward. Consider neutrinos, which only interact via gravity and the weak force. Trillions of them pass through your body every second. Take away the weak force interaction and add some mass, and you have a dark matter candidate particle.
> I fail to see how dark matter could ever be falsified.
You’d have to explain what should be done about that. It could nevertheless be a correct theory, and we might one day verify it more directly.
Naive falsificationism is an almost useless principle at the frontiers of modern physics. We’re long past the days when you could run some simple experiments and come up with a clear yea or nay. That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.
Well, there should be something that follows from the assumption that there are huge amounts of particles that only interact via gravity. What's the prediction that this model makes? If it doesn't make any useful prediction, it's pretty much worthless.
> That’s living in the 19th century, basically - which admittedly, many people still seem to want to do.
I think this attitude is inappropriate. The scientific progress made with 19th century methods, and that includes most of the stuff discovered until the 1950s, is massive. Maybe scientists should focus more on a clear nay or yea? I mean, the LHC is certainly a significant achievement, but compared to the detection of gravitational waves or even just exoplanets it doesn't seem to deliver as much bang for the buck.
To a layman like myself this implies both that the things being studied then are both unprovable and not potentially of practical use or significance. I certainly am not saying that is the case, but rather am looking for more informed opinions to the contrary.
The cynical side of me feels like study of something unfalsifiable is as the study of religion. A poor investment for public funds.
As I mentioned however, I am seeking opinions here to the contrary of my gut reaction as my gut is often dumb and reactionary.
Of course same thinking can't easily be applied to things that are not always true, like social behaviour. Part of the reason the replication crisis hit so hard in the social sciences.
And of course this is not really as absolute as I wrote above. No field works like this all the time. But it is a good guideline. If falsifiability is completely ignored, then a concept really is nothing more than a religious belief and should be considered as such.
BTW, Popper did not publish in the 19th century.
This occurs because most science is not the "the frontiers of modern physics". But even then, falsification in the frontiers of modern physics is still a useful tool.
Originally dark matter was hypothesized because of galaxy rotation mismatches. This allows for mapping where 'dark matter' should be. One falsification of dark matter that was tried is gravitational lensing. If there is indeed extra curvature of space-time in these galaxies, then they should bend light-rays, Potentially showing the same galaxy twice.
This was tested, and confirmed. Hypothesized dark matter, as detected by galaxy rotation, causes the expected gravitational lensing.
I suppose another potential falsification is by 'moving' dark matter through a gravitational tractor (very heavy mass with rocket engines) and then detect the resultant change in gravity. You could attempt that detection by gravitational lensing, orbit velocity, or perhaps something else.
Despite all this, dark matter still feels like our theory of gravity was falsified, and instead of concluding the theory was wrong, it was attempted to add random parameters to the theory. Its not like there is a better theory though (I believe MONS has trouble with explaining gravitational lensing), so perhaps this is a problem of PR much more than a problem of science.
As for the second question, for dark matter to be matter, I think it suffices if it has mass and inertia. My gravity tractor detection idea tries to work with that.
Edit: also see this: https://medium.com/starts-with-a-bang/7-independent-pieces-o... from a different reply in this HN discussion.
> I fail to see how dark matter could ever be falsified
I don't think science cares. Frankly if you could back up your opinion with some actual reasoned argument, it would be a better post.
Theory implies they found out what it is.
Not in any technical sense I can think of.
Has anyone considered modeling the effect currently explained via dark matter as a new kind of force or field? Like a distinct force generated by mass and its relationship to space time?
This wouldn’t be modified gravity but a proposal for an entirely new force whose effect is what we currently explain with the “fudge factor” of dark matter.
From the rough picture I do have of dark matter this force would be one whose relationship to distance is different from the usual inverse square law that governs forces. That would be weird but maybe no weirder than particles that can never be detected or MOND.
Of course we don't have a quantum field theory for gravity, so the above is hand-waving away a HUGE issue of what that new field would be like, how its particles would act, etc, since we don't even know what the quantum gravitational field is like and what its particles are like (assuming such a thing even makes sense).
They even think about dark chemistry: https://iopscience.iop.org/article/10.3847/1538-4357/ac75ef
Didn't have much support last time I checked.
Ah, yes, we also need to have some sort of bound energy, to balance Einstein’s equations. So, geons! https://en.wikipedia.org/wiki/Geon_(physics)
Hamlet
William Shakespeare
My understanding is that based on observations of what can be seen galaxies move as if there's move stuff in them??
If you hypothesis that this extra stuff is dark matter but can't detect it, how do you know it's real?
How have you ruled out all the normal stuff that doesn't give off light or stuff we can't see at the distances we're looking over? Like planets, rocks, dust, etc. Couldn't that stuff, which to my mind there would be vastly more of than stars, make up the extra gravity?
I'm aware I might be talking complete shit because I'm completely ignorant of the detail here. I would love an understandable explanation as to why that's wrong.
Yup [1]
> My understanding is that based on observations of what can be seen galaxies move as if there's move stuff in them??
Yup [2]
> If you hypothesis that this extra stuff is dark matter but can't detect it, how do you know it's real?
It produces certain effects in various different enviromennts or domains, which lends more support to that hypothesis. By certain different domains, I mean like Galaxy and cluster dynamics, gravitational lensing around galaxies and clusters and the precise fluctuations seen in the cosmic microwave background, these are all very different regimes but all seem to support the hypothesis of extra mass which is not interacting electromagnetically.
> How have you ruled out all the normal stuff that doesn't give off light or stuff we can't see at the distances we're looking over?
There are research efforts to look for exactly this kind of stuff; Massive compact halo object (MACHOS). However they don't see enough of it to account for the observations we see in galaxies and galaxy clusters. [3]
[1] https://lss.fnal.gov/archive/2022/conf/fermilab-conf-22-180-... [2] https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... [3] https://en.wikipedia.org/wiki/Massive_compact_halo_object#De...
https://kipac.stanford.edu/highlights/using-gravitational-le...
I don't know enough about the found examples of gravitational lensing to know whether all possible arrangements of black holes that could have the same effect, have been considered and found lacking. But I presume so.