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Evidence of “modified gravity” in 150 galaxies bolsters dark matter alternative (newatlas.com)
348 points by bookofjoe 25 days ago | hide | past | favorite | 179 comments

I only skimmed the paper past the abstract, but for what it is worth.

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.

It's seems that the authors of the paper all come from the LCDM/Dark Matter observational testing community, this isn't a paper by MOND theorists. They were quite likely looking for evidence of dark matter with the ESO survey and found something else.

Right or wrong, this is the definition of good science.

Vera Rubin would approve. Despite making a lot of the critical rotation rate measurements on spiral galaxies [1,2,3], it seems as though she was never particularly happy with the interpretation of this result as dark matter:

> "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." [4]

[1] http://articles.adsabs.harvard.edu/full/1970ApJ...159..379R/...

[2] http://adsabs.harvard.edu/full/1980ApJ...238..471R7

[3] https://www.annualreviews.org/doi/abs/10.1146/annurev.astro....

[4] https://scientificwomen.net/women/rubin-vera-86

The OP was about how pop-sci reports the paper, not the science itself.

TFA ends with, "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."


I didn’t find it boring or useless. I found it quite interesting. No need to be so negative bosswipe

As of right now it's the top comment even though it's actually misleading, thus contributing the dumbing-down phenomenon its author complains about.

The other comment threads in here are discussing the actual experiment and theories. But this thread is at the top taking up a bunch of the oxygen and it's just repeating tired old media criticisms that we've all heard before. The actual experiment is way more interesting.

on the topic of "repeating tired old media criticism": there are people who might have learned it today https://xkcd.com/1053/

Confirmed. I did.

Just to play devil's advocate... sometimes a little spectacle is OK.

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.

That's a fair point. I love reading computer hardware rumours, most of which are probably total garbage (and probably obviously so to anyone in the field). And in this case, whether the general public thinks the universe in MOND or LCDM really doesn't matter at all.

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.

The final sentence of the linked article is this:

> 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.

Eek! I guess I'm more OK with this now...

The funny thing is that I agree with you that there are a lot of bad pop-sci articles that are misleading in the ways you're talking about. This particular article just isn't one of them.

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.

Lol, TLDR; strikes again. Guess OP should've read more than the abstract if they "would've been fine with the article if it had contained a sentence like this one"

I find this within the sciences also. A lot of the more provocative scientists are willing to be wrong while promoting strong versions of their hypotheses. While this can be criticized as unscientific, I tend to believe that it serves a scientific purpose. Presenting a strong version of a hypothesis pushes people to contend with it, gather empirical evidence, and then incorporate a more moderate version into the idea-sphere.

I think OP's point is that pop science reporting is more than "sometimes." Keeping up with controversial theories is fine, often ideas that turn out to be revolutionary are controversial, but it's important not to get a warped perspective about what the broader scientific consensus is.

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.

It's true the LCDM is the consensus, so this result is even stronger from an outsider's perspective since the lead author apparently comes from the LCDM community, not the MOND community.


Specifically: “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.”

> 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.

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"?

I think Ars Technica does a pretty great job of science reporting without being over the top.

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.

The best suggestion I have is https://astrobites.org/

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!

I think physicists throw around the word "fringe" too easily. Most physicists think that MOND is not true, but "fringe" implies that the physicists who advocate for it are wackos, like they're Young Earth Creationists or something.

To me fringe means has little support but still in the realm of science. In other words little support but not yet disproven.

Young earth creationism isn’t valid science and thus doesn’t even share the same reality let alone being on the fringe of reality.

Yup, by fringe I meant "On the edge of what reasonable people are working on". But maybe it's not a great term if people misunderstand.

physicists are usually quite blunt what you call 'fringe' is labeled 'crackpot'

MiHsC/wuantized inertia would be fringe

MOND may be a fringe but wholesale support of Dark Matter by mainstream physics has always seemed cargo culty to me. Over a hundred years and we still don't have a good understanding of dark matter.

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.

Well, maybe the thing is you just don't know anything about it. Current dark matter concepts are 40 years old, not over a hundred (again, Wikipedia is not a substitute for an education). And we have detected indeed a very common kind of actual dark matter, predicted 26 years before you could "see" it simply because it explained the data, we call it neutrinos. The universe is brimming with cold neutrinos btw.

Everything seems outlandish from a prejudiced superficial POV.

This comment could reduce the snark (to zero) and still be just as informative.

Probably, but then HN commenters could reduce this tendency of stating that whole scientific disciplines don't know what they're doing/are full of deluded morons. Particularly when it comes to physics. At some point you have to mention the other possibility.

Who gave you the idea that it's over a hundred years old? The solid results are from the 70's, while a hundred years ago it was still an open question if galaxies really existed...

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.

Is the journal that the paper [1] is published in also fringe? Or not refereed?

[1] https://iopscience.iop.org/article/10.3847/1538-4357/abbb96/...

The astrophysics journal (ApJ) is a really good journal. A majority of good astronomy papers are published in ApJ or in MNRAS. Big things go in Nature/Science + then there are other smaller journals.

(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.

Also, peer review in astronomy generally consists of correspondence with a single reviewer. There may be multiple rounds of review but a significant majority of papers submitted by professional astronomers doing serious research to journals are ultimately published. It is not the norm in astronomy to be rejected outright from a top journal and then submit to a lower tier journal, instead almost everyone publishes in a few popular journals. This is a tradeoff, but definitely in astronomy at least this means some iffy papers are ultimately published, even in ApJ, and you must always be aware when reading the literature that the paper you are reading is not infallibly correct just because it was published in a journal.

ApJ is one of the most respected journals in astronomy.

Argument by authority is bad science too.

No-one is making an argument by authority. Here is a nice popular article that outlines some of the issues with MOND https://www.forbes.com/sites/startswithabang/2018/03/06/only...

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.

Most people on HN are aware that the consensus, a form of argument by authority, is dark matter. It is also known that the dark matter consensus has problems despite billions spent on research over decades. When there is an experimental result that contradicts the leading theory, that is interesting and worthy of media attention. If this experimental result was only published in popular media, then I too would likely ignore it. However it was also published in a major journal. Like you said elsewhere, that doesn't mean it is right. But I think it is worthy of attention and discussion and should not be dismissed out of hand because it doesn't agree with consensus.

The existence of a consensus opinion isn’t an argument from authority. To claim so is an interesting form of the post hoc ergo propter hoc fallacy ('after this, therefore because of this'). The argument for agreeing with the consensus isn’t the existence of the consensus, but rather the same argument that caused the consensus to exist, the underlying body of research.

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?

"Here is why the consensus is _______" is not argument by authority, correct.

"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 ______".

> "The ________ theory/hypothesis is fringe, the consensus is ________" is argument by authority.

That is not an argument, so it can't be "argument by authority." It is a framing.

It's not an explicit argument. Most arguments people make have lots of implicit steps and subarguments though. This is one of those cases.

It is clearly a case of argument by authority.

>The existence of a consensus opinion isn’t an argument from authority.

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.

I see your point, but I think the person you're responding to is trying to say that he is not saying "if a lot of prominent people believe something then it must be because there's a lot of evidence" but rather "there is a lot of evidence, that is why so many people believe it." The difference there is in actually being able to show you the evidence rather than just telling you to trust a priesthood. I don't think that the person you're responding to is using the consensus in place of the evidence, but rather telling you that if you look into it deeply you'll realize why the consensus exists.

The consensus is not always correct, but unless one becomes an expert in the field, betting on the consensus being right is probably the way to go.

> consensus, a form of argument by authority

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

Authorities are really useful ways to understand the world.

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".

If you are in the field, attempting to contribute, then I don't think authority is a good rubric. But for those of us who aren't, I think it's necessary. The world is too complex for all of us to make up our own minds on everything.

> But for those of us who aren't, I think it's necessary. The world is too complex for all of us to make up our own minds on everything.

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.


I think the bigger issue is that LCDM has no produced much fruit in the last several years. Despite many attempts to detect it, no one has found anything. Furthermore, the main motivation for LCDM was super-symmetry, but the latter has also failed to produce much evidence in favor of it.

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.

Are gravitation and the Fine-structure constant https://en.wikipedia.org/wiki/Fine-structure_constant related? I am absolutely no physicist but if yes:

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.

Here a scientist was so sick of the journalist bias towards sensationalism that he trolled the international media into reporting that chocolate helps you lose weight. Sample size n = 4. https://www.npr.org/sections/thesalt/2015/05/28/410313446/wh...

From the bottom of the article:

“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.“

>> the consensus is that it is ruled out

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.

Until Dark Matter/Dark Energy theories have been "solved," other theories will continue to exist. That's the way this works.

> 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.

How could you possibly rule out the theory that "the rules are different over there, somehow"?

Scientific consensus is often wrong. Science advances one funeral at a time. Most “scientists” are conformists looking for their next grant. Outside of the tenured professor elite, most of them are looking for the next job. And the tenured are just one step higher on the pyramid, motivated by prestige and admiration, not pure motives. There’s nothing wrong with that, it’s called being human. But don’t put too much stock in consensus, because many of the great breakthroughs have been ignored and ridiculed before finally being accepted.

While it's fun to criticize science this way, this line of criticism misses an important fact: namely that open scientific inquiry has been one of the most successful enterprises in the history of humanity.

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.

The danger is that the apparent heights of "the piles" are badly skewed by bias: the evidence for what you favor naturally feels more solid, while evidence that contradicts it, or that it entirely fails to predict, proves very easy to ignore indefinitely.

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.

The counterweight to this is that there's an enormous prestige benefit for any scientist who conclusively overturns the established scientific consensus. This is why Einstein became so famous in his lifetime: not just because he had some elegant theories, but because he had theories that unambiguously matched the experimental evidence (and even offered new predictions) in ways the previous scientific consensus could not. And even though a few skeptics tried to resist his ideas, the scientific consensus worked in exactly the ways we'd expect it to as the evidence came in.

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.

Motions of the moons of Jupiter and Saturn? Opinions of enthusiasts with pet theories? What are you talking about?

You illustrate my point better than I could ever explain.

ncmncm>>> ... Applied to moons of Jupiter and Saturn ... [implications that there is something scientists are missing about "material leaving the poles"]

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.

Although clearly nothing about what I wrote.

He addressed exactly what you wrote.

(Unless you mean whatever your pet theory is about gravity or magmatism or whatever, and he addressed why that isn't worth discussing too).

You illustrate my point, as well, and so forcefully!

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.

You have no faint idea what you are talking about. I don't have any "theory".

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?

Please try to explain.

If this is some electrical universe support, I think that's a good example of why it's important to know and understand the scientific consensus.

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.

Gravitational lensing??? Inertial mass??? Are Jovian moons supposed to be involved in that now? What are you talking about?

I'm saying that, even if it were possible for the electrical force to explain the motion of some moon, it still can't explain other things that gravity can, and I gave two examples of phenomena that general relativity explains that en electrical universe doesn't.

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.

Apology welcome.

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.

I'm responding to a comment that pooh-poohs serious consideration of alternative theories because they are contrary to the "scientific consensus." No one is going to pursue alternative theories if people don't believe in them. Some of us go against the grain, and that's a good thing. But you appear intent in making any true belief in unpopular alternatives into a sin. God forbid someone have a fringe belief! Fringe is BAD and consensus is GOOD. No independent thought. No room for dreamers and speculators and hunch-havers in your world. At most they can coldly look at evidence for alternative theories but never believe them (and resist all cognitive biases with superhuman ability). Have you even considered that fringe belief, and obsession and cognitive bias in its favor, is essential for progress too?

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.

Scientific consensus is rarely wrong.

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.

> Scientific consensus is often wrong.

Often wrong, sure - but still right in the large majority of cases.

The most insightful comment is buried at the bottom of the list. Two kinds of people in this world, those who think truth lies where the crowd is and those who think it's where the crowd isn't. The conformists won't see the blindingly obvious until it knocks them over. Unfortunately the genes that make us good for a farming society make us a little sheep like ourselves. Here's to the hunters who've struggled through this far into the age of the farmer! cheers!

Yes, except the evidence for dark matter is really an absense of evidence. So "new and exciting" in that context isn't some grand transgression. There's no experimentally supported theory of dark matter to transgress!

The evidence that dark matter is particulate in nature that obeys the gravitational force, but not the strong nuclear force or electromagnetic force does have actual direct evidence for it.

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.

You're confusing direct evidence with circumstantial evidence. I'm not saying there aren't indications of such a particle. Of course there are. But what we have are a small number of measurements that require invoking a massive particle in order to explain discrepancies in the measurements. There is no experimental support for the existence of this massive particle.

The job of astrophysicists is to make models that explain observation.

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.

“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.”


I saw that article the other day and decided not to submit it here on the basis I found that exact quote to be somewhat disingenuous considering his prior work includes very MOND-friendly stuff, eg.


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.

That is a general pattern that I see a lot of these days: people pretending to have changed their minds by the sheer power of the evidence, when really they were always on side. Maybe we need a new word for this.

Confirmation bias?

Confirmation bias is an existing concept and may be a driver for the conviction but is not what’s happening here. It’s more akin to someone coming out and saying “I used to be a strong supporter for the democrats and voted for Hillary in the election, but the covid pandemic has made me a firm Trump supporter due to his beautiful handling of it!” And then someone finds a series of Tweets proving the person was openly supporting Trump during the election.

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.

Losing sight of falsification as one of the primary drivers of good science has not been good for the scientific community.

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.

Based on the article it looks like it only looks at the rotations of galaxies and not the higher order organization of galaxies, which dark matter accounts for and MOND does not.

As Stacy McGaugh likes to put it in his blog - the question is why MOND (a relatively simple theory with just one free parameter) is able to make successful predictions at all? Predictions like this one, which you have no reason to expect to be true in CDM models?

Probably MOND is a "good enough first approximation" to the whole story, which might come out later, and include more terms. I think it's interesting some people are doing the complex (to me, anyway) maths from a theory point of view to find ways to get it to spit out MOND. I have almost zero understanding of all this, but none of the theories I read seem physically insightful. They say MOND a_0 constant is close to cH_0, but not exact. I wonder if some of the difference (if that is significant) could be c altering.

Occam's razor aside, why can't both theories be correct?

That dark matter exists as well as a modified gravity thing.

most MOND theories do include some sort of dark matter, there is just less of it and it could possibly be less exotic (think brown dwarfs and such)

> A side effect of this is that the motions of objects would depend not just on their own mass, but all other masses in their neighborhood.

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)

MOND is a modification of Newtonian gravity and thus doesn't (yet?) take relativistic corrections into a account (see the last paragraph in [0]). So it's hard to predict what the fully relativistic picture is going to look like. I'm not a MOND theorist, though, so I might be wrong. (There are some relativistic generalizations[1] of MOND but it doesn't appear the MOND community has settled/agreed on one approach yet.)

[0]: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Ou...

[1]: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics#Re...

There's conformal gravity, which replaces the Einstein field equations with ones derived from a conformally invariant action. Like MOND, this allows you to fit rotation curves nicely without invoking dark matter, see eg [1][2].

[1] https://arxiv.org/abs/1211.0188 (2012)

[2] https://arxiv.org/abs/1812.03152v3 (2018)

I think what it is saying is that the gravitational effect of a mass depends on the gravitational environment in which it exists. So a given sample of mass warps space more or less depending on whether it is near other masses or not.

I’m not a MOND theorist though. That’s just my generous interpretation of that statement.

There is bunch of independent evidence towards the dark matter. https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... Alternative theories usually get one observation supporting them, but can't explain the rest.

So bolstering means something like 1 to 11, 10 more to go.

Most of them are gravitational observation, so calling them "independent evidence" is bit begging the question since gravitational observations are the motivation ("initial evidence") for the model.

I think its more that if gravity is just acting weird than God for some reason decided that gravity should act exactly like lots of invisible matter in clumps. For some galaxies we've worked out where dark matter would have to be, right down to how it's rotating and where its lopsided. Gravity would have to be both weird and dedicated to seeming like dark matter for that to hammer out.

You're defining dark matter to be "however gravity looks" which is my point. That's not necessarily helpful. You can't be sure that by winnowing down the parameter space, you're aren't just making an equivalent statement saying like "we are fitting these points to a nth order polynomial and the derivative must be allowed to exceed this value instead of being constrained". In other words, constraining the properties of DM, especially in the direction of lowering the upper bound on it's granularity and lowering the upper bound on it's interaction with normal matter, could artificially increase its independent explanatory power.

That is true in general, but the theory "hey what if there was lots of invisible stuff" was developed long before further study confirmed that "gosh it sure looks like there's lots of invisible stuff". So you'd need a really contorted idea of gravity (at this stage heavily backfitted) to get the results that just fall right out of dark matter, even as conceived of before we got further confirmation.

I'm sympathetic with the idea of "invisible stuff" - classical energy is famously "invisible stuff" that has eventually gotten explained away in different ways, and has a couple of corners that haven't been battened down. But it's also never been categorically described as something with X properties, it's always been accepted as "something unaccounted for for which we need SOME OTHER model to explain", even after it became Noetherian (you still need to define the field).

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?

I think that where we disagree is in that last bit, where dark matter needs to be fit to each galaxy. That's absolutely true, but that complaint goes for regular matter too! Every time the location of some body was predicted from the motions of what we could already see, the theory of light matter was fit exactly to the data. What makes it stay within the bounds of Occam's Razor is that before ever noticing oddities in the motion of planets, we could say that probably matter would be clumped up, and could affect other matter.

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.

> that complaint goes for regular matter too

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.

To shore up my point from earlier, I was specifically talking about how we spotted Neptune without ever seeing it by observing the stuff we could see, just like how we identify Dark Matter. Looking back I definitely didn't make that clear, plus the fact that we already knew that normal matter existed muddies the waters further.

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?

You make it seems like we would need to make adhoc change to gravity which would result in a hard-coded equation with magic numbers that would fine tune gravity for our universe. On the contrary, MOND is a simple modification while dark matter has a very weak epistemological strength in comparison of added premise

MOND is an entire family of theories, each of which accounts for maybe one or two edge cases by tuning specifically to them while handwaving the rest away. On the contrary, dark matter is just matter that is dark. There are plenty of questions about why it formed, or why there's so much of it, but in principle stuff that doesn't interact with the forces we work with isn't out of left field.

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 for the correct use of the phrase "begging the question."

There are multiple separate weird gravitational observations besides galexy rotation such as galaxy clusters structure formations etc and while dark matter explains them all MOND does not

explaining more does not necessarily mean better. I can explain all of the motions of all of the celestial bodies (better even than newton-einstein gravity can!) using epicycles, but that's trivial because really epicycles are just fourier transforms under another name and all continuous-valued functions have well-defined fourier transforms.

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.

Off topic and minor question: Aren't epicycles using trigonometric functions, which is a different paradigm than fourrier transform? I don't remember the difference with Taylor series btw

I misspoke because epicycles are actually fourier series, but both are trig functions. Fourier's initial formulation of the transform was with trig functions remember e^it = cos t + i sin t

Would you categorize the bullet cluster in the same manner?

It's just "another group of galaxies, another empirical distribution of DM". A direct observation of DM in its (rare) non-gravitational interaction would absolutely qualify as an independent, multimodal observation.

Here's a video from Sabine Hossenfelder (2018) which explains the subject in a simple way.


It's not evidence for MOND, it's evidence contradicting the LambdaCDM model

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)

From a layman's (an interested one, at least) perspective, both MOND and LCDM seem to be lacking in some respects.

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[0], 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.

[0] https://en.wikipedia.org/wiki/Hierarchy_problem

>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.

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.

I think that it is supposedly a lot of something that hasn’t been predicted or detected in any other way that is considered problematic. It has to interact at least with other dark matter in some other form than via gravity to clump together.

Dark matter doesn't clump together though, that's part of what makes it different to light matter.

How come it stays in the vicinity of other matter? It needs to lose kinetic energy somehow.

Dark matter halos are often found around galaxies, extending far past them. The dark matter fails to stay within the galaxy because it's not slowed down by regular interactions.

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.)

The universe didn't form uniformly, so if a bunch of dark matter formed as a clump, it would remain like that.

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

What happens to a comet that enters the solar system if it doesn’t hit anything? It leaves the solar system with the same speed it entered it with. Dark matter cannot hit anything.

> A side effect of this is that the motions of objects would depend not just on their own mass, but all other masses in their neighborhood.

Isn't this always true, dark matter or MOND? There are two masses in F = G M1 M2 / d^2

The article doesn't explains what EFE is well. The ELI5 of MOND is that acceleration due to gravity works in a non-linear the result is that the actual force may be weaker or stronger than what you would expect with Newtonian mechanics.

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.

Or gravity is constant but time is not.

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.

So is it like in a boardgame with a maximum number of actions per turn? If you use all your actions for movement you reach the maximum speed in the game (speed of light here) but you can't do anything else (produce, consume, buy, sell, etc.)

Like Conway's Game of Life.

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.

That sounds like Mach’s principle


Sabine on the matter (pun intended): https://youtu.be/2VNcDoLNJk8

Actual link https://iopscience.iop.org/article/10.3847/1538-4357/abbb96/...

Though kudos to the original site for providing the link.

I'm very skeptical of this because there is plenty of evidence that's explainable with dark matter but not with MOND. I believe one such thing is how the dark matter can seem to separate from the host galaxy in a collision with another galaxy. Someone can correct me as I might not be remembering that quite right. That's tough to explain with just gravity alone.

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.

the difficulty of dark matter is that you can explain anything with dark matter. If the oxygen in my room moved to the upper left corner and suffocated me, there is a dark matter field that could account for that.

You could explain anything with dark matter, but it's a lot harder to explain multiple things with the same dark matter. The different observations (galaxy rotation curves, the Bullet Cluster collision, CMB power spectrum) all put different constraints on what the explanation has to be, and a particle that feels gravity but not electromagnetism is consistent with all of that.

I think the argument is that you can put exactly as much dark matter anywhere you want to explain whatever you want. Sure it has to be "a particle that feels gravity but not electromagnetism", but that just means it's invisible and the only effect it has is the exact effect you want.

So it's basically being able to add whatever number you want to one side of an equation to get it to balance out.

Nicely expressed. Put that way, it becomes obvious the problem isn't that we have a class of unknowns gathered under the label "dark matter", its that the label comes to imply things unrelated to the real use.

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.

Yes, and you can put exactly as much invisible air as you want to explain why branches move the way they do. If the branch is moving twice as much, isn't it so convenient to claim that there just happens to be twice as strong a wind?

Sure if your model is complicated enough you can add unicorns and Santa Claus.

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.

also "constraints" on DM values can be misleading. When you say "a DM particle cannot be more clumpy than X" given an average density of DM particles in the universe, you are saying "we must LOOSEN a constraint on the types of things that DM can explain", because for that particular parameter, there is an inverse relationship between constraint tightness and explanatory power.

Sorry, could you please define "same" here? That "same" dark matter could also explain the room evacuation. Should I stretch it further to include poltergeists?

Can it?

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

you'd also expect two galaxy clusters to be travelling at each other at high speed with frequency less than one universes.

To anyone who is interested in this field I suggest to look up MiHsC aka Quantized Inertia. The theory is based mostly on known physics. It is not too hard to understand. If proven true it may lead to us building new kinds of flying machines or energy sources. It also has a way of explaining gravity in terms of information theory.

The theory of quantized inertia has been criticized in articles online as being pseudoscience.[2][28][3] https://en.wikipedia.org/wiki/Quantized_inertia

If you believe the many worlds interpretation of qm then there is a much simpler explanation. To better explain it I will return to Abbott’s flatland. Consider a flat universe. Consider an imaginary grid on it that represents the ‘curvature of space’. Planets in this flat-verse look like solid circles. They bend space so the grid lines are more closely bunched together near them. Now consider another universe next door. Nobody in one universe can know anything about the other universe because their senses are 2d. Even light in their universe vibrates the gridlines of space in the plane of their universe. Now the flatlanders have figured out that gravity operates in 3dimensions. Now consider another universe(B) nearby with a black hole in it(a black circle to us). In space A the flatlanders are struggling to understand a phenomenon they have called dark matter. We on the other hand see the black hole in universe B and it’s effect on universe A. Meanwhile, we are struggling to understand our own version of dark matter. In ‘reality’ there is a black hole in a universe a little bit ‘kata’ from us, bending our space.

Can anyone point to a reference on the math for the expected galactic rotation curves should look like? This whole thing is based on a discrepancy between prediction and observation. What is the prediction in detail?

Ok, but how much dark matter does MOND have? My understanding is that it just renames dark matter, but still can't describe gravity based solely on visible matter

MOND isn't a model, it's an umbrella for models that change `F=ma`. Seems like most MOND models still don't account for a large amount of observations, but it's probably worthwhile to compare the person-hours devoted to fitting dark matter models vs. MOND models, and give the MOND approach some benefit of the doubt.

Are they changing f=ma though? Or F=Gm1m2/r^2?

As far as I know, the mond models still require something equivalent to dark matter in order to work

Is it possible that dark matter is a cloud of slow neutrons? They wouldn't be able to firm electron clouds and without those they would be invisible.

Free neutrons decay on the order of 10 minutes

Are there neutron-like particles that are stable?


Wimps seem to be exotic particles of unknown structure. My guess is that regular neutrons are enough, so long as they are "fixed" to be stable.

In addition to what andreareina said: Neutrons also interact with light while dark matter is called "dark" for a reason.

Interact enough to drop a noticeable shadow? Iirc, a neutron is 50e3 times smaller than a hydrogen atom.

Neutrons bonk into stuff. If you're at a rave in a 100ft by 100ft warehouse, you look around and see four other people, but you keep bonking into ... something ... that you can't see every inch you move, you would surmise something is up.

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.

How would they bonk onto anything? A neutron is a tiny particle 50000 smaller than a hydrogen atom. Without an electric charge, it would float straight thru anything and we wouldn't know.

Neutrons have a nucleus, just like a proton, and that would bonk into things.

Yes the chance[1] 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.

[1]: https://en.wikipedia.org/wiki/Neutron_cross_section#Typical_...

Iirc, we'd only need 10 neutrons per each hydrogen atom to explain the dark matter gravity. OK, if neutrons would be observable, are there any weird visual effects in galaxy cores that could be attributed to such neutron clouds?

You say "only" but that's a _lot_ of neutrons, and space is vast.

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[1] from nuclear experiments[2] 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.

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

[2]: https://en.wikipedia.org/wiki/Neutron_diffraction

Neutrons visibly interact with matter, but only when there's a lot of that matter, e.g. reactors use neutrons to heat water. I'm yet to read the two links you provided, though.

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.

> even lots of them, would have little opportunity to bounce into each other

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[1] after the freeze-out[2] 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[3].

[1]: https://physics.stackexchange.com/a/217357

[2]: https://sites.astro.caltech.edu/~george/ay127/kamionkowski-e... (page 5)

[3]: https://phys.org/news/2020-09-mystery-neutron-lifetime.html

Dark matter made of neutrons would take more time to form a disk. Ballpark estimate: neutrons are 50e3 times smaller, but we need 5x and of them, so dark matter is 10e3 slower at forming disks.

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.

It seems you want to kill two birds with one stone: flyby anomaly is caused by dark matter, which is actually "stable neutrons".

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.

That stable neutron can be a stable tetraquark with zero electric charge, or something of that sort.

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.

Along this line, another shower thought: Could it be delocalized (ie very widely spread out) electron wave functions?

Electrons are charged and thus interact with electromagnetic fields (light), so they don’t fit the requirement for “dark” matter. Huge amounts of unaccounted for matter being in the form of electrons would also skew the charge balance of the universe locally and globally into the negative, which conflicts with observations that the universe has a neutral net charge.

Thank you for the explanation.

Dark matter, the Clovis Culture of astronomy.

I'm not well versed on archeology, can you explain what the Clovis Culture is?

At this point any modified gravity proposal which doesn't offer a full explanation for the bullet cluster observations can be dismissed out of hand.

It will be hilarious if far in the future we observe some totally unprecedented phenomenon with the Bullet Cluster that renders it irrelevant to this argument. Something like the gas clouds forming into a giant paperclip, mega-Dyson Sphere, or "2001: A Space Odyssey" style space fetus.

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