The MOND favouring group is a fringe of the cosmology community. The vast majority feel that there is enough evidence to rule it out.
I mostly mention this because I don't like that fact that popular science magazines (or at least their content that I see posted here!) has a bias towards "new and possibly exciting" or "controversial" research. Which I understand - revolution is more interesting that "physicist reduces error bars by 50%. Big picture unchanged". But, if all you read is these articles, you will get a very skewed idea of what the consensus is.
Just so there is no confusion, modified gravity as an explanation for Dark Energy is very possible, as an explanation for Dark Matter, the consensus is that it is ruled out.
Edit: I clarified my point below but will do it here too so everyone see it. I don't have a problem with this paper, I'm glad people are writing papers with alternate explanations to the consensus, that is how science is done. But, your conclusion from reading this article shouldn't be, "ahh damn, I guess MOND is right and LCDM is wrong" and I think that is how pop-sci articles tend to frame these things.
Right or wrong, this is the definition of good science.
> "If I could have my pick, I would like to learn that Newton's laws must be modified in order to correctly describe gravitational interactions at large distances. That's more appealing than a universe filled with a new kind of sub-nuclear particle." 
A lot of these ideas really are "new and exciting." Science just moves at a slower pace than media. It's not a monthly. By exaggerating the pace they're.. uhm... bridging the gap or something.
Controversy is even more engaging than novelty. Maybe a pop science article gets you sucked into the pre-clovis camp of a paleoanthropology controversy. Now you are engaged in paleoanthropology. Three years later, a possibly butchered snake skeleton is found and you go "aha! I was right! They ate snake soup! Clovis people didn't even like snake!" It's a little cheap, but it's also fun. Why not.
People enjoy having opinions. No one loves sports without having a favourite team, strong opinions regarding training schedules and sports rehabilitation practices... an occasional gamble. Most know that they're not really experts, but they enjoy being in the fray nonetheless.
Pop science magazines aren't supposed to be textbooks or encyclopedias. We are fortunate to have wikipedia for that. Give me a little sauce, a little clickbait. I might not always admit to it... but sugar tastes good.
My real issue is when this reporting is on things where the general public's opinion does matter. Things that the general public might vote on. Economics, medicine, etc. Having seen this type of reporting in a field that I do know something about (and a field where there is no real incentive to mislead, again MOND vs LCDM, who cares), I'm a lot more distrustful of science reporting in fields I don't know much about (and where there are incentives to mislead).
If they had published the article exactly as is, giving you all the excitement, but just added a single line somewhere saying "this is new work that is up against a large body of previous work that points in the opposite direction. Let's see what happens, but its a cool idea" I'd be totally fine with it.
> But it’s important to keep in mind that so far the bulk of the evidence still points towards dark matter, and it’ll take much more work to topple that hypothesis entirely.
Even though it's light on detail, I'm actually impressed at how measured & factual it is. They even managed to resist a clickbait headline.
I know more than one person who has taken it to the extreme, to the point that they latch onto any and every controversial idea seemingly only because it's controversial. That's what you want to avoid.
I stopped reading popsci mags for this reason. But I do feel like I'm missing out too. Do you know of any good mags that mostly talk about mainstream stuff, but targeted towards "I studied some of that in college but I don't work in the field"?
It's not a magazine, but the weekly podcast The Skeptic's Guide to the Universe also covers interesting science topics in a balanced manner.
These are paper summaries, written by people in the field, where you are probably the target audience. I don't read it myself, but give it a go!
Young earth creationism isn’t valid science and thus doesn’t even share the same reality let alone being on the fringe of reality.
A red flag to me is how there doesn't seem to be large amounts of dark matter in areas closest to us where we can see best. It's all this far off enigma. And at least originally the theory existed to explain why the laws of gravitation didn't line up with visual evidence.
To me "half the matter is invisible" is just as outlandish as "maybe our math is wrong about gravitation". I don't think enough of the community questions the dark matter theory.
Everything seems outlandish from a prejudiced superficial POV.
Not sure what you mean with there not being any dark matter (DM) in our neighbourhood, the DM content of the Large Magellantic Cloud is estimated to outweigh the visual matter by a factor of 4, and the LMC is right up in our face.
(At least this is what I tend to find. This may just reflect my biases - US based, in the cosmology field.)
But just because something gets published doesn't mean it is right :) I think non-scientists don't know what "peer review" actually entails! First, as this was a MOND paper, it could well have been reviewed by someone who favours MOND. Second, even if the reviewer doesn't favour MOND, if the steps taken and the arguments given seem reasonable, I expect they would suggest it should be published.
I have no issue with the paper being published. It is important that theories have advocates who put forward the best argument for them. What I do take issue with is the skewed presentation in popular science. What sells is exciting and new, not slow and steady. And 99% of science is slow and steady.
I just think that it is very hard to understand the scientific consensus (average view of people who spend a lot of time thinking about this) when all you read are popular science articles that tend to focus on the exciting/new/possibly game changing edges. I'm just here letting people know what the consensus is.
Framing it as a result that flies in the face of consensus is fun and exciting, especially since people who are science-literate know that a single compelling result can overturn a consensus formed by a large body of previous work. But framing it in that sense is unhelpful, since the vast majority of publications that “fly in the face of consensus” do not. It’s a lot less exciting to view this as one more paper on a large pile supporting MOND, that is still small in comparison to the pile that support dark matter. The paper is obviously worthy of attention and isn’t being dismissed out of hand, it is published in The Astrophysical Journal. But would Hacker News be discussing it if not for the contrarianism embodied by the former framing, as opposed to the latter?
"The ________ theory/hypothesis is fringe, the consensus is ________" is argument by authority.
See the difference? Phrasing matters in this case. All too often it is phrased the second way. I don't expect someone to literally enumerate all the evidence for or against something, but there is a middle ground between that and just saying "well consensus is ______".
That is not an argument, so it can't be "argument by authority." It is a framing.
That's exactly what it is. The rest of your argument boils down to, "if a lot of prominent people in a field believe something, it must be because there is a lot of merit-based evidence". That assertion is completely false. Merit-based evidence is one of the ingredients that lead to consensus, but hardly the only one.
It's not, because is specifically argument based on authority. Consensus could have authority in democratic context, but that isn't the meaning of "argument by <X>" which is the distilled context of some basis of proof. You could just as easily link consensus to "Argumentum ad populum".
In fact, this is spelled out on wikipedia: The two are similar, but not the same - both are "Fallacies of relevance": https://en.wikipedia.org/wiki/Argumentum_ad_populum
This doesn't really apply either though, because we are talking consensus among a specialized group of experts/peers, which seems to me a lot more relevant that general consensus. Appeal to authority isn't always fallicious: https://en.wikipedia.org/wiki/Argument_from_authority
The vast majority of people would be more correct about the world if they aligned with scientific consensus.
One problem is that journalistic coverage of ideas isn't proportional to beliefs held by experts in that field. Which gives the impression science is constantly changing, and that fringe theories are more widely held than they are.
That's why it's so important for people familiar with the field to help the rest of us who aren't know what "the field considers this a wacky idea".
More specifically, attempting to make up our minds on everything will inevitably lead us to accept the simplest superficially plausible explanations (that fit our existing biases), and the world is too complex for all those simple explanations to be correct, so we'll mainly find ourselves in the local maxima that have already been explored and properly abandoned by specialists.
There comes a point where the consensus view is no longer seen as the most plausible explanation. This does necessary mean something crazy like modified gravity is true, but rather that there could be a better theory out there. However, in order to find that better theory all avenues need to be explored.
The Fine-structure constant has been discussed that it is not a constant but slightly varies in the observable universe. Would make no wonder, if gravitation is related to the fine-structure constant, that gravitation varies too.
“It’s an intriguing result, and it may lend some weight to the MOND hypothesis for further study. But it’s important to keep in mind that so far the bulk of the evidence still points towards dark matter, and it’ll take much more work to topple that hypothesis entirely.“
Science does not work by consensus opinion. I know many non-scienctists and even some scientists get pissed when you say this but it is the absolute truth and true to the scientific method.
This idea that the "consensus" has ruled out all other theories for a substance that we cannot even detect is an appeal to authority logical fallacies and a bias towards a known flawed paradigm. Authorities who have never detected Dark Matter say it cannot possibly be anything other then Dark Matter.
Thomas Kuhns The Structure of Scientific Revolutions details this mentality that leads to these sort of errors in thinking, which can be summarized as follows (taken from an outline of his book found here: https://www.uky.edu/~eushe2/Pajares/Kuhn.html) :
--Students study these paradigms in order to become members of the particular scientific community in which they will later practice.
----Because the student largely learns from and is mentored by researchers "who learned the bases of their field from the same concrete models", there is seldom disagreement over fundamentals.
----Men whose research is based on shared paradigms are committed to the same rules and standards for scientific practice.
----A shared commitment to a paradigm ensures that its practitioners engage in the paradigmatic observations that its own paradigm can do most to explain, i.e., investigate the kinds of research questions to which their own theories can most easily provide answers.
Even the author of this paper is an LCDM (AKA dark matter exists) advocate who had to admit that his preference does not fit the data observed:
"I have been working under the hypothesis that dark matter exists, so this result really surprised me,” Chae said. “Initially, I was reluctant to interpret our own results in favor of MOND. But now I cannot deny the fact that the results as they stand clearly support MOND rather than the dark matter hypothesis.”
The keyword there is "reluctance". This sort of mentality ripe with internal bias towards the existing paradigm is the number one thing that I think holds back scientific progress. The breakthroughs that push things forward rarely come from the consensus holders and those simply doing research that refines the current paradigm. Dark matter / Dark energy has never been detected / sourced - and the by the way the Standard Model of physics IS DEFINITELY WRONG and does not explain all observations. It simply our best approximation just like Newtonian Mechanics was, but we need to look beyond it rather then be scared to defy it.
How could you possibly rule out the theory that "the rules are different over there, somehow"?
In a period of a few hundred years we've gone from believing that there were a few basic elements and that the sun revolves around the earth, to understanding the deep nature of particle physics and the structure of the Universe. We turned a basic understanding of chemistry into an understanding of subatomic particles, and the ability to create entirely new elements.
We did all of this through a process of open and skeptical inquiry, which has been remarkably consistent in its ability to tear down unsupportable theories. The reason the Kuhnian critique exists is not because the scientific process failed, it's because the process worked but just took longer than people expected it to because people are human and imperfect. And the speed of scientific advances over the past decades has been higher than at any point in human existence.
The reason the term "scientific consensus" exists is because most fields are vastly too complex for a single human being to be able to evaluate the totality of the evidence by themselves, at least in a reliable way. So the process is necessarily decentralized and broken up among many experts, who share their opinions. This isn't some popularity contest that you should ignore, it's a critically necessary task that has to be performed in order to digest the research contributions of any field, and make progress on solving open problems.
You're absolutely right to point out that consensus evaluation can malfunction sometimes. You'd be equally right to point out that sometimes experimenters produce invalid results. You're wrong that the answer to the former is to reflexively ignore the scientific consensus process, just as you'd be wrong to say that "don't do experiments anymore" is the correct response to a few experimental errors.
We know that a conductor moving in a magnetic field produces a voltage, and knowing the strength of the field and the motion, we know absolutely the voltage produced. Applied to moons of Jupiter and Saturn, we expect forces much larger than surface gravity of the moons, and therefore material leaving the poles. But when we find it occurring, we talk about "volcanoes" and "geysers". We carefully ignore that the volcanoes drift about like rubber ducks in the bath. We carefully ignore collimation that would need for the geysers to be shot from perfect paraboloid-shaped nozzles. It is easiest to just agree not to talk about perfect collimation, because it doesn't lead in a comforting direction.
Socially, people like a consensus. A challenger needs "extraordinary" evidence to displace it. But Nature doesn't play favorites: any alternative that accounts for all the established evidence is on equal footing. A consensus in the absence of compelling evidence, or in the presence of incompatible evidence, should make us suspicious that the consensus is a product not of evidence, but of biased preference. Seeing evidence carefully ignored should make us suspicious.
I am not aware of carefully-ignored evidence in the case of galaxy rotational anomalies, but this paper may be rubbing our noses in examples.
Nature is just as happy for all the leading theories to be wrong, and for us not to have invented the right one yet. The consensus can be wrong without any of the alternatives being right.
It is discomforting to find yourself wrong, but science isn't about comfort.
I have no idea what the situation is with current theories on the motion of the moons of Jupiter and Saturn. I'm guessing you also have an incomplete picture of the evidence, but you've got an alternative pet theory that explains some inconsistencies in the consensus theory. I'm also guessing your theory isn't a slam dunk, i.e., that there's good evidence against it and/or there's a distinct lack of evidence in favor of it. But I strongly suspect you're not going to present me with all of the negative evidence for your own theory in an HN comment: I would have to get the impressions of other people in the field in order to actually get a fair evaluation of the evidence. That's what scientific consensus is supposed to offer, and as imperfect as it is, it usually works better than trusting the opinions of a single enthusiast.
You illustrate my point better than I could ever explain.
matthewdgreen>> ... I have no idea what the situation is with current theories on the motion of the moons of Jupiter and Saturn....
ncmncm> Motions of the moons of Jupiter and Saturn? Opinions of enthusiasts with pet theories? What are you talking about?
It was pretty clear to me what @matthewdgreen was talking about.
(Unless you mean whatever your pet theory is about gravity or magmatism or whatever, and he addressed why that isn't worth discussing too).
Electro-"magma"-tism has not been anybody's pet theory for going on 150 years.
I think that perhaps you think people are paying a lot more attention to your theory than you think.
We are talking about scientific consensus, and you seem to be talking about something else.
Why do you reply to things that you cannot even bring yourself to take the time to read and comprehend? Who do you imagine you are fooling?
There are all sorts of minor details that can be matched to all sorts of fine theories, but there is almost always overwhelming evidence in other places that contradicts them. Trying to reinvent physics from the ground up by picking a few details is a fool's errand.
In particular, EU can't explain gravitational lensing, it can't explain the equality of gravitational mass and inertial mass, and these are just some of the most obvious.
But perhaps I misinterpreted your post. I took it initially to mean that you believe electrical interactions to be a better explanation than gravity for the movement of those moons - a theory that actually exists out there, called 'Electrical Universe'.
If instead you simply meant something much more specific, that there are electrical interactions that could explain mass ejections seen on these moons better than some geological explanations, then I apologize for my tangent.
The point was a specific example of evidence (collimated fluid motion) not consistent with descriptions of the cause of the motion ("geysers") but avoided as a consequence of discomfort with its implications.
Your comments illustrated the phenomenon with impressive clarity: wholly avoiding mention of anything even peripherally relevant, while promoting prejudicial distractions.
And your talk about the process and malfunctions is frankly comical. These are human beings, not machines. The average person would rather tell a socially sanctioned lie that led to the death of a million people rather than risk his next promotion.
There have been 3 major cases in the 20th century that I'm aware of where well-established theories turned out to be wrong.
1) Plate tectonics (dismissed until 1962)
2) The Bohr model of the atom, confirmed by the colours of light absorbed and emitted by ionised helium - which turned out to be wrong
3) The Sommerfeld extension of Bohr's model.
Both (2) and (3) were thought to be correct because they gave predictions correct to 4 decimal places. This turned out to be a co-incidence(!!) and their theories turned out to be wrong and replaced by Dirac's model.
Given the amount of science done in the 20th century, I think that's a pretty good record.
Often wrong, sure - but still right in the large majority of cases.
The other poster has alluded to the bullet cluster. The bullet cluster is a pair of colliding galaxies. We can measure the distribution of gravitational lensing in a region of space, and in doing so, indirectly measure the mass distribution. We can directly measure the light output of colliding gas clouds in the two galaxies. We can measure the distribution of stars within them.
Analysis of this data shows us that dark matter has momentum.
This tells us that if you want to reformulate dark matter in terms of a modification of variations of F = Gm1m2 / r^2 you'll need to do some really ... interesting things. On the other hand, we know neutrinos have mass, do not interact via the strong nuclear force, and do not interact via the electromagnetic force. We either have to assume really, really ugly math in order to give a fundamental force a momentum, or we can assume there are new particles that are sorta like neutrinos but different.
There are other problems that I don't fully understand. Apparently MOND is very, very difficult to reconcile with the rest of known theory if the speed of gravitational waves travel at the speed of light. Which the 2017 neutron star merger taught us that they do.
Explain the bullet cluster observations.
And same thing applies to LCDM. How does it explain bullet cluster, and several others, existing in the first place?
Disclaimer: I'm not a cosmologist even if I do have a background in physics. I do like following the field. It's just not quite so clear as you imply.
And if we go into things like superfluid dark matter then it doesn't really matter if its dark matter or some kind of modified gravity. One has bunch of equations and what matters is how they behave.
Nothing wrong with researching MOND, but then implying that you're from the opposite camp and this new result has been some sort of Road to Damascus moment made me sceptical as to this being anything new in what is still a minority theory.
I would more refer to it as a false flag conversion. Pretend to be the enemy and come out with a fake story about how some compelling argument “forced you” to convert to the side that really is your true colors.
Reluctance to interpret results in a certain way is how you end up with tainted data.
Every cosmologist worth their salt who thinks Dark Matter is the One True Way and that MOND (or anything else) is malarkey should re-evaluate what it means to be a scientist and start trying to falsify Dark Matter immediately.
That dark matter exists as well as a modified gravity thing.
As I understand gravity warps spacetime which seems to imply that the masses of all objects in the neighbourhood would be taken into account because they all exert a warping effect on the same spacetime. Could somebody clarify what the difference is that they’re referring to? (or possibly the error in my understanding)
 https://arxiv.org/abs/1211.0188 (2012)
 https://arxiv.org/abs/1812.03152v3 (2018)
I’m not a MOND theorist though. That’s just my generous interpretation of that statement.
So bolstering means something like 1 to 11, 10 more to go.
Maybe I don't understand your usage but it is rather confusing that you call MOND heavily backfitted and not dark matter. The traditional complaint about dark matter is that it is highly parameterized, requiring an empirically fit factor for each galaxy observed. Am I missing something?
Similarly dark matter has to be fit to each galaxy. However, before doing the fitting, dark matter suggests that galaxies would behave not just heavier than their matter content would suggest, but also that they would be lopsided. That is the first prediction you'd make as soon as you were told to consider invisible matter, even without seeing that the data fits.
More importantly, dark matter also elegantly explains the behavior of other massive objects, like galaxy clusters, which MOND struggles with. The figures that roughly work for galaxies don't predict anything like what is seen in clusters, suggesting that they work by coincidence on the more common types of galaxy. That's not to say we don't have more to learn about gravity; to my admittedly limited understanding weaker flavors of MOND could explain the effects of Dark Energy. Dark Matter is just a more elegant explanation of the extra mass issue.
No, it's not. It's explicitly multimodal. We estimate material distribution of baryonic matter by measuring light output as a proxy and then derive their presumed gravitational effects by observing their motions, which is a totally different method. Now you can argue that those estimates are poor, but it's categorically different from dark matter, where there is only one measurement that drives both its distribution and its presumed effects.
> More importantly, dark matter also elegantly explains the behavior of other massive objects, like galaxy clusters, which MOND struggles with...
This is a bit like saying a seventh-order polynomial adjustment to this 1st order fit "elegantly" explains these data points, so we should throw out the 2nd order fit because it makes us question the 1st order law we started working with.
Now to your points specifically. Dark Matter needs 0 modification to explain superclusters, normal clusters, and individual galaxies all at once. The basic principle is hard to swallow, obviously, but it made predictions about the sort of deviations we'd see that turned out completely correctly. While certainly if you could just plop dark matter anywhere you wanted and hold it 'stationary' you could overfit to any data you wanted, the models that work have dark matter right alongside regular matter, driven by the same forces. The only reason its possible to derive such models at all is that galaxies behave so much like there's more 'stuff' there that we just have to fill in the blanks. Brute forcing a dark matter solution to a world driven by MOND would be at best a feat, and more likely close to impossible.
Total side note, I was snooping in your profile and saw that you wrote Zigler. I actually just today tried it out, funny coincidence no?
Once predicted, it's easy to see how the motions of galaxies works perfectly with some extra stuff in the mix. The differing behavior of spiral vs. irregular galaxies, as well as scaling to clusters and superclusters, falls right out of dark matter. You could predict pretty much everything we've observed the minute someone told you about dark matter, even though the concept was formed before we had all the data we have now.
1 - give me a prediction, not just an explanation. Can you take epicycles and project the motions of any given body better than newton? No.
2 - Give me a multimodal observation. Imagine the world before we unified terrestrial gravity with heavenly motion - these were two separate phenomena. Could epicycles explain things outside of the category which it is designed to explain (heavenly motion)? No. Newton's theory also explains why if you throw a book across the room it will fall to earth, which epicycles do not.
One interesting new model for Dark Matter is called "Superfluid dark matter" which seems to be getting more attention and maybe will supplant LambdaCDM
Edit: from the FA "(3) we detect a systematic downward trend in the weak gravity part of the radial acceleration relation at the right acceleration predicted by the EFE of the MOND modified gravity. Tidal effects from neighboring galaxies in the Λ cold dark matter (CDM) context are not strong enough to explain these phenomena. They are not predicted by existing ΛCDM models of galaxy formation and evolution, adding a new small-scale challenge to the ΛCDM paradigm"
Yes, that specific part can be explained by MOND but ΛCDM is one of several models of dark matter (as MOND is one of several possible modified gravity models)
The idea that there exists some "stuff" we can't measure (except weakly via observed gravitational effects) that is ~5-6 times the stuff we can measure and interact with seems a bit problematic.
Likewise, the idea that fundamental forces aren't fundamental and function differently in different circumstances/environments seems to contradict much of what we've learned/deduced about the physical properties of the universe.
As such, it seems likely that our theories (in the scientific sense rather than the vernacular) are lacking in one or more respects.
Since these discrepancies focus on the effect we call gravity, I'd posit that until we can solve the Hierarchy Problem, there won't be a clear understanding of why we see these anomalous effects.
That assumes we don't find actual particles which fit the predictions of LCDM or clear evidence that the strength of gravity is dependent upon MOND principles.
Since many of the hypotheses that address the Hierarchy Problem are (at least currently) non-falsifiable, that's a big issue.
I'm sure that in the above statements I've ignored various observed evidence (I am a layman, after all), but IMHO it doesn't hurt to speculate.
This seems to be a common intuition, but is it founded at all? Why should the idea that there's more mass that's weakly-interacting than we're used to be a point against it? Should we have any expectations about the amount of mass that's interactable with us in regular ways we've evolved to deal with?
If we evolved in dark caves to be blind creatures with sensitive hearing and no concept of light, then when we first considered the possibility of outer space, would we be right to write it off just because it supposes that most matter couldn't possibly be heard by us?
If we imagined that all particles we know about may be made up of some smaller building block particles connected together, or concentrations of some kind of stuff, wouldn't it actually be more surprising if all of that building-block stuff had ended up becoming concentrated into normal particles? This might not be literally how it works, but it's interesting that it seems easy to imagine plenty of possible ways the world could be where it turns out that most building-block stuff is never pushed to combine into normal mass, and instead most of it stays in an uncombined form that's only weakly-interacting.
Also, dark matter is able to pull regular matter to be closer to it. If a galaxy's dark matter is moving too far away from a galaxy, it might pull the galaxy with it. (Or it might separate entirely, as happened with the Bullet Cluster.)
Also dark matter is gravitationally affected by light matter, which does interact. My intuition tells me that the interaction of light matter that creates clumps of light matter will also create clumps of dark matter indirectly.
* note: 90% my dark matter knowledge comes from HN. I'm just spitballing here
Isn't this always true, dark matter or MOND? There are two masses in F = G M1 M2 / d^2
Because of this there are a few scenarios that may validate MOND or at least post Newtonian/GR physics where G is no longer a constant but needs to be modified based on specific predictions of MOND.
If 'time' is not continuous and gravity and other effects share 'time quanta' then the more gravity from any source a thing experiences the less of other change can occur - just like time dilation where your 'time quanta' would be consumed for positional change with fewer remaining for other changes.
Then it wouldn't be that the systems are orbiting more slowly, or gravity is no longer constant, but that time itself is actually slower for them.
I'm sure actual physicists have a thousand observations that prove it's not a possible explanation.
But to me the concept is interesting in that it explains why there's time dilation, reference frames, warping of space, relativity, MOND, etc without any magic - epicycles that only exist from the viewpoint of speed of light being the center of the universe. They can never be reconciled with quantum if they don't actually exist.
Though kudos to the original site for providing the link.
I really like the idea of MOND as requiring an invisible unknown particle to explain galactic rotation always smacked of the old ether theories to me. But my understanding is it's nearly impossible for a mond theory to encompass all of the current evidence for dark matter without being totally contrived.
So it's basically being able to add whatever number you want to one side of an equation to get it to balance out.
I take the "faith in human ignorance" view that there are many things extant that we have no inkling of. Its easy to mistake quantified and classified ignorance for knowledge, especially when it is all we have.
But those models aren't interesting, precisely because of that.
The interesting models have as few parameters as possible, allowing them to be constrained in their behavior.
Thered have to be a lot of dark matter for that, and I would expect it to have clumped at the center of the earth, not be floating in your room
As far as I know, the mond models still require something equivalent to dark matter in order to work
If dark matter were a cloud of slow neutrons, we would know quite a bit about it. Actually we wouldn't because we'd all be dead, but that's a different story.
Yes the chance of doing so is much less due to it having an effectively smaller radius, but on the other hand there would have to be a _lot_ of them to explain dark matter.
There's also a lot of stuff for them to bump into. The interstellar medium is filled with diffuse gas, and there's there's highly energetic events like jets from black holes which when slamming into the neutrons surely would lead to some observable effect.
For one, I'd be very surprised if they didn't bump into each other, cooled down and clumped up like normal matter, which is precisely the opposite of what we expect from dark matter based on observations.
Secondly we know from nuclear experiments what happens when neutrons interact with matter. I'm certain a nuclear scientist could list a bunch of effects we'd see if a whole lot of neutrons were hanging around black holes and similar.
The whole point of dark matter is that observations show it interact extremely little, if any at all, with regular matter.
The story is different in the interstellar space. The only matter there is sparse gas and neutrons, even lots of them, would have little opportunity to bounce into each other or those gas atoms.
Those neutron clouds could eventually form asteroid-like rings around galaxies: invisible, but massive. For the same reason, every massive object, including our planet, would have such a ring. That's a falsifiable hypothesis.
As for the black hole argument, there could be two explanations: first is that neutrons that are nearby don't get to stay too long and either transform into protons and become visible gas, or fall into the black hole; second is that neutrons that are really far, light years away, could happily orbit around the black hole unnoticed and if there's enough of them, we would see subtle gravitational lensing effects.
There's another valid point against neutrons: their lifetime is only 10 minutes. However neutrons and protons are two particular compositions of quarks. Are there reasons why quarks can't form a stable neutral particle in the interstellar space? That particle would be much like a neutron, but stable.
The reason I'm pushing for this idea is that there's an odd flyby effect: it causes spacecrafts to accelerate and decelerate slightly. We could use those observations to calculate the precise shape, mass and velocity of that "neutron ring", make a satellite fly in the opposite direction and observe if there's strange heating of water or other reactions that could be caused by neutrons. Since it can visibly accelerate a spacecraft, there should be a good deal of those neutral particles and thus they should visibly heat up water.
I'm not a physicist, but this seems a way simpler explanation of dark matter than "axion fields" or whatever is currently trending.
Space is vast and has been around for a long time. Improbable events happen a lot more often due to that. The interstellar medium has roughly 1 hydrogen atom per cm^3. You're saying maybe 10x as many neutrons. Multiply that by kiloparsecs and millions of years and those neutrons would most definitely interact.
> I'm not a physicist, but this seems a way simpler explanation of dark matter than "axion fields" or whatever is currently trending.
The reason "axion fields" or whatever is currently trending, is because simpler explanations have been ruled out.
You not only have to explain the rotation curves, but also the properties your dark matter candidate need need.
How did these neutrons get made? How do you explain your over-abundance of neutrons given that the neutron-proton ratio after the freeze-out is roughly 6 protons for every neutron?
Why are the neutrons so cold? Did they start that way, then that's odd since everything else was very hot back in the days. If they're cooling down, where is that radiation and why haven't we observed it?
Why aren't the neutrons decaying? They can't be free, since the free-neutron lifetime is something we've measured.
: https://sites.astro.caltech.edu/~george/ay127/kamionkowski-e... (page 5)
In this model, neutrons aren't decaying: dark matter consists of "stable neutrons" that have a different quark structure, but are very similar to regular neutrons in all other aspects: no electric charge and a small magnetic moment.
I don't think we need to explain everything before testing this hypothesis. Launching a sat to the supposed dark halo responsible for the flyby anomaly seems a relatively trivial task these days.
The problem is that "stable neutrons" seems to be a unicorn, not a stone.
After all, if a "stable neutron" is just somehow the same quarks in a different configuration, then how come it has eluded detector experiments ala LHC and friends?
Especially in light of the neutron freeze-out in the early universe? That is, how come the early universe ended up with way more "stable neutrons" than regular neutrons, yet somehow our experiments which regularly make neutrons has not noticed them? The missing mass should be very obvious.
You should at the very least have a plausible answer for that before spending the considerable amount of money and effort it takes to make and launch a probe.
It has been eluding lhc experiments for the same reason dark matter has been eluding them: detecting neutron like particles is extremely difficult when there are only 5 of them per cm3. I suppose that in some experiments those neutral particles get hit by accident and physicists see some discrepancies in the results, but since those discrepancies aren't reproducible, they get swiped under the rug.
As for the early universe model, well, it's a nice theory and it's backed by a few indirect evidences, but it's still just a theory we can't verify directly. This theory invented before the dark matter came to light, right?
The experiment we're talking about would cost maybe 10 billions - a rounding error in a world where some people make multiples of that in just a few months.