Heavy particles don’t explain gravitational lensing oddities | Hacker News Reader

Heavy particles don’t explain gravitational lensing oddities

arstechnica.com

130 pointsisaacfrond94 comments

Sticking to TFA, and not ancillary information, I found this frustrating:

> Instead, the research team ran 75 different models with the initial conditions chosen at random. By chance, some of these created distortions similar to the ones seen in the real-world data, typically affecting only one of the four lensed images. So, the researchers conclude that the distortions in the lensed images are consistent with a dark matter halo structured by the quantum interference of axions.

Essentially the best contender aside from WIMPs is, as subject of TFA, axions. And axions have near-random interference and standing waves, which essentially scatters about light bent by gravitational lensing due to uneven gravity in the lensing object.

Essentially, if you randomize the field enough, you can eventually find, after enough trials, something that seems to match reality. And it just so happens axions are the nice random variables we needed to inject.

That's where we're at: Just randomly select some parameters until the data fits, and lo and behold, it fits.

Its not just you: https://www.youtube.com/watch?v=lu4mH3Hmw2o

I knew this was Sabine Hossenfelder before clicking the link and not having seen the video before (I like her, fwiw)

Wild guess: Sabine is not a fan.

I can't tell if she's a legitimate scientist or a crackpot. Does anyone know?

(I haven't actually watched this video, but I've seen others of hers. They seem convincing in general to a nonspecialist.)

Sabine is very legit

They claim the parameters that go with canonical particle dark matter won't lead to a fit. This is the point, not necessarily that they could pinpoint axions in particular.

So they searched for a set of parameters for axions that would essentially be a counter example to WIMPs? That makes more sense. But the article is structured as this being supporting evidence for axions, which doesn't make much sense to me.

At least the headline says "No WIMPS! Heavy particles don’t explain gravitational lensing oddities" and also inside the article (near the two colorful figures) they write

"On cosmological scales, WIMPs continue to fit the data extremely well. But once you get down to the levels of individual galaxies, there are some oddities that don't work quite as well unless the dark matter halo surrounding a galaxy has a complicated structure. Similar things seem to be true when you try to map the dark matter of individual galaxies based on its ability to create a gravitational lens that warps space so that it magnifies and distorts background objects."

and later

"It's a relatively simple thing to do with the WIMPs, since there's only one pattern we'd expect: the gradual fall off of dark matter levels as you move away from the galactic core. The lensing predictions based on that distribution do a poor job of matching the real-world data of where the lensed images show up."

So I guess the real article (which I haven't read) has the hypothesis that the WIMPs fit badly and they try to make a case for the axions. Still it is kind of a nice clue to show that the WIMP predictions fit badly even if their axion solution is weak..

>That's where we're at: Just randomly select some parameters until the data fits, and lo and behold, it fits.

When I read that it felt like a Monte-Carlo simulation. A legit approach in modeling. And indeed - the link to the paper: https://arxiv.org/abs/2304.09895

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

This seems fine? Planks law was originally just a line fit with parameter tuning. It wasn't till afterwards that theoretical backing was developed with statistical mechanics.

Knowing what types of models and configurations match experiments is useful even if these models themselves aren't derived from first principles.

This 2014 Fermilab post on why physicists are hopeful that dark matter has some kind of weak-force interaction is a good read:

https://www.fnal.gov/pub/today/archive/archive_2014/today14-...

> "So why would scientists postulate that dark matter experiences the weak force? One answer is that if it doesn't, we'll never detect it. But that's not a very good answer. A better answer involves the Higgs boson. Because the Higgs field gives mass to ordinary matter..."

Current status of the Higgs boson decaying to invisible particles is, no such evidence has yet been found (2020):

https://atlas.cern/updates/briefing/probing-dark-matter-higg...

This apparent failure (so far) of that hypothesis seems to have caused scientists in the field to cast around for other ideas, like primordial black holes:

https://news.yale.edu/2021/12/16/black-holes-and-dark-matter...

I'd like to see more work done with investigating field self-interaction.

This 2020 paper by Alexandre Deur is very interesting. "[W]e find that relativistic corrections to the rotation curves of disk galaxies are significant at large galactic radii."

"Relativistic corrections to the rotation curves of disk galaxies" - https://arxiv.org/pdf/2004.05905.pdf

And a blog covering the paper: https://astrobites.org/2020/08/17/the-alternative-to-dark-ma...

Yep, there's a lot of stuff that properly using general relativity would address in a much cleaner way than dark matter.

Ignoring the relativistic nature and nonlinearity of GR is a very large problem with FLRW and other toy models, but they have become embedded in cosmology to the point where it's heresy to question them.

The fun part about this is that gravity has to either be self interacting or capable of propagating faster than light.

If gravity wasn't self interacting it wouldn't follow the shortest path that gravity itself creates. Light does follow this path. Gravity would move in different directions and at different velocities to light across the universe which is full of gravitational field if it wasn't self interacting. We have seen zero evidence that gravity does propagate differently to light. In fact ligo and other gravity experiments have only confirmed it's the same.

So gravity is almost certainly self interacting. What does this mean? Well it wouldn't change much at small scales. There's not enough effect at say planetary scales to really notice. But what happens when you have a field self interacting across vast distances and involving vast galaxy sized masses? Well your link looks at this and yes it matches some of the anomaly's we are at large scale.

It's a good theory and I suspect despite being hard to argue against it will face opposition since it means a lot of scientists were chasing a dead end.

In my mind, gravity must self interact due to moving at exactly the same speed as light (and light is affected by gravity). To put this in quantitative terms, the paper "Gravitational Waves and Gamma-rays from a Binary Neutron Star Merger: GW170817 and GRB 170817A" (https://arxiv.org/abs/1710.05834) constrains the "difference between the speed of gravity and the speed of light to be between −3×10^−15 and +7×10^−16 times the speed of light."

Yep I feel like ligo and Virgo are the Michelson Morley experiment of this century.

They have shown something that is not well accounted for in current theories. Just as MM showed light is always observed travelling at the same velocity Ligo has shown gravity is self interacting. Now we need to change our models and build mathematical frameworks to account for this. It is not acceptable to have a theory that doesn't account for this post Ligo.

I'm not convinced of that interpretation of MM. MM found a signal below the margin of error. I think if they spun the table faster the signal would grow in strength. But subsequent experiments stopped rotating the assembly at all and just relied on the rotation of earth, and thus had even slower rotation and even smaller signals.

But at this point nobody is going to revisit MM, attempting to do so would immediately ruin your scientific reputation.

Some of us have no scientific reputation at all, so we are free from the reputation trap.

I would love to revisit MM myself, but I don't have the expertise to actually setup the experiment. I might be able to resolve that gap, but I definitely don't have the funding either, and that is a gap I don't know how to solve.

> MM found a signal below the margin of error

Wiki has their data: https://upload.wikimedia.org/wikipedia/en/thumb/d/d1/Michels...

In Michelson's own words (per wiki):

> The expected deviation of the interference fringes from the zero should have been 0.40 of a fringe – the maximum displacement was 0.02 and the average much less than 0.01 – and then not in the right place.

The data doesn't look anything like a signal, and wiki also says:

> Roberts (2006) has pointed out that the primitive data reduction techniques used by Miller and other early experimenters, including Michelson and Morley, were capable of creating apparent periodic signals even when none existed in the actual data.

> But subsequent experiments stopped rotating the assembly at all and just relied on the rotation of earth

This is also untrue. Per https://arxiv.org/abs/1002.1284 (found via references in wiki):

> Our measurement compares the resonance frequencies of two orthogonal optical resonators that are implemented in a single block of fused silica and are rotated continuously on a precision air bearing turntable

Thank you for providing that research, I wasn't aware of that paper.

But when I read it, I found something interesting.

"The final precision could be reached by integrating over more than 130 000 rotations relying on a careful suppression of systematic effects caused by the turntable rotation"

I wondered what effects they were suppressing, and while they discuss that to some extent they also say:

"While even faster rotation would have allowed to acquire more data and thus improve statistics, it resulted in increased residual systematic effects presumably due to modulated centrifugal forces and was thus not implemented"

And they state:

"active rotation potentially causes a systematic modulation of the beat frequency and might thus mimic an anisotropy signal. For exam- ple, gravitational or centrifugal forces that act on the resonators may get modulated with the turntable rota- tion and therefore modulate the length of the resonators."

So I would ask, how do we distinguish between new physics and systematic errors that look like new physics and need to be excluded from the experiment?

My bet is that difference between gravitational waves and gamma rays due to the effect known as "surfing". A gravitational wave pushes light back from peak, where light is slower, to bottom, where the speed of light is normal again.

The difference between gravitational wave and gamma rays is more dependent on initial magnitude of gravitational wave, than on distance traveled.

I like this gedankenexperiment: Imagine you're in the vicinity of a supergiant black hole orbited by a close binary pair of neutron stars about to merge with each other.

The neutron stars are close enough to the black hole that you see multiple images of them, and the images are highly distorted. E.g.: You can see them even when they're "behind" the black hole due to the highly bent path of the light.

You have hundreds of probes scattered over a large volume of space, each equipped with an optical telescope and a gravity wave sensor similar to LIGO.

What would you expect to observe?

Would the frequency of the final "ringdown" chirp before the merger be observed identically in the optical and gravity sensors? Would they be observed as "coming from" the same spots in the sky? Would you expect to feel a higher contribution to gravity by the neutron stars due to their multiple images? Etc...

It's interesting that our intuition about the consistency of the rules related to the speed of causality imply that the answers ought to be: Yes, identical, yes, yes, etc...

But that means that gravity is definitely self-interacting, and it also means that gravity can be focused, just like light can be focused by gravitational lensing.

This implies that stars at the edge of a galactic disk "see" more gravity from the opposite side of their galaxy due to the lensing effect of the central bulge!

> If gravity wasn't self interacting it wouldn't follow the shortest path that gravity itself creates.

I don't know if this is the right way to think about this. Gravity follows all paths. In a sense, it is the path. Or, as Hawking described it[0]:

> [Gravity] shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background.

Back to your comment:

> So gravity is almost certainly self interacting.

I'm confused by the "almost certainly". I think this has long been established[1], but for entirely different reasons: The Einstein field equations are highly non-linear, so of course there will be self-interaction.

Or, if you prefer to think of the Einstein-Hilbert action: The Lagrangian density L = R · sqrt(det|g|) behaves as L ~ g⁻¹ · Ric · g², and the Ricci tensor is Ric ~ (∂g)∂(g⁻¹) + g⁻¹ ∂²g + (g⁻¹)²(∂g)², so if we view gravity as a quantum field theory (effective or whatever), there are plenty of self-interaction terms that will prevent the N-point function from vanishing for N > 2. Already in the weak field approach (where you approximate g = 1 + h) you get a triple graviton vertex, see p. 21 of https://arxiv.org/abs/1702.00319 .

Maybe we are talking about different definitions of "self-interaction here"?

[0]: Hawking: The Nature of Space and Time (1994), https://arxiv.org/abs/hep-th/9409195

[1]: See e.g. https://physics.stackexchange.com/questions/293873/do-gravit...

There are some papers on this subj from Alexandre Deur. Here’s a good explanation - http://dispatchesfromturtleisland.blogspot.com/p/deurs-work-...

It really is a beautiful theory if correct

A solved problem doesn’t attract funding for the next financial year, which is why his “too simple” explanation is being systematically ignored.

Does "WIMPS don't explain observed data" also exclude "matter in other M-Theory branes exerts a gravitational influence on matter in our own" as an explanation for dark matter?

The main argument I've seen against this is that matter interacting gravitationally across branes would cause a sort of n-body problem condition where everything in a particular sphere of influence would orbit continuously around the center of mass instead of settling into a lump, but maybe that explains why some planet-scale (or larger) objects have spinning cores?

I don't believe anyone's casting around too hard for an alternate explanation for conservation of angular momentum, which is a consequence of rotational symmetry of physical laws, a relatively intuitive and desired property.

"When you hear hoofprints, think horses, not zebras."

You can always contrive a more complicated structure to explain some observation. There is no evidence branes exist. We are looking for an explanation to the "dark matter problem" that doesn't require asserting that a ton of new and unproven physics exists.

This may point to a key problem in the hypotheses, theories and conclusions. When you are in Africa, a "think horses" paradigm, may lead you very astray in your thinking.

I always wonder if dark matter isn't simply made of wrinkles in space-time that just don't go back to flat after some weird event happens

It would have to be a galaxy sized, smooth spherical wrinkle that happened to coincide with practically every galaxy. And also to fail to interact with other galaxies when they collide.

While we don't know the details of dark matter, we do know a lot of things about it. Putative explanations have to cover a lot of ground.

That wouldn't be so weird as it sounds. From simulations we know that it was dark matter that came first, then came the galaxies.

So if these wrinkles (they don't have to be very smooth) simply had a (large) preferred size and clumped up faster than normal matter in the early universe, that would be enough.

Neither of those would be particularly weird. When dark matter clumped up normal matter was essentially a very low density hydrogen cloud, devoid of stars, much less dense than a cloud in the sky, which would take (without dark matter) some 15-50 billion years to clump up by itself.

As for the size of the dark matter structures, well we don't know what they are, so you can postulate any size for them.

In the case that dark matter doesn't interact via any force other than gravity, then there will be an epistemological crisis in physics as your model will be indistinguishable from one in which dark matter is a particle.

It's the classic failing of everyone's casual "alternative to dark matter". It's not X because we can't detect it, what if instead it's an undetectable form of Y?

Far too many people get hung up on the idea that in science what things "are" matters: because it doesn't. What matters is predictive power for future observations - if whatever you're proposing doesn't predict anything different to "invisible matter" then it's exactly as bad an explanation as "invisible matter" which so far is a perfectly good explanation for current observations.

Then you just get fancy regression formulas. With enough layers and "bars", epicycles "work" in terms of predictive power. It's usually better to find out the underlying mechanism rather than merely satisfy prediction. Otherwise we'd still be using epicycles. Plus, gravity theory has Occam's Razor over epicycles.

Right but that's the point: there's nothing wrong with "invisible matter" as an explanation if it explains everything. Even if it's actually not invisible matter, provided it behaves like it at all times, then that's good enough until it doesn't.

The universe has no requirement to satisfy the day-to-day intuitions of African-plains originated upright apes, with senses optimized to spot large predators, in how it works. Quantum mechanics is a great example of that.

So WIMPS don't explain the pattern of lensing caused by one particular galaxy, if you assume a vanilla distribution of the WIMPS. But we already have situations (the Bullet Cluster, e.g.) where we don't assume that. So... I'm not sure that this is all that strong an argument?

And, they argue for axions, because it would be more normal for them to assume a more complicated distribution. But they can't predict which distribution they would assume. So this looks like "it must be A rather than B, if we assume the distribution of B, because we can't assume the distribution of A". That's not all that convincing.

What happened with general relativity somehow explaining the "dark matter" observations?

It didn't? Dark matter in its modern conception was discovered after general relativity was established. Dark matter was hypothesised because general relativity alone couldn't explain some observations.

I was under the impression that it was not fully rigorously accounted for because its not exactly a simple calculation. So galaxies were still being modeled with newtonian physics in at least part of the calculations which neglects field self-interactions.

Ordinary matter in the galaxies cannot explain the rotation curves. Something dark (that doesn't interact with electromagnetic radiation) is needed.

Also, the mass in the universe cannot be all baryonic, since that would make primordial nucleosynthesis inconsistent with observed abundances of various light isotopes.

You are repeating the same dominant theory and don't address that at least some of the excess can be accounted for by more rigorous computation...

I should be clear: ordinary matter cannot explain the rotation curves with conventional gravity. One can come up with ad hoc modified theories of gravity, but none appears to be consistent with all the evidence.

See my comment above. Watch the whole video. Once you understand it, which you will, you will never talk about galaxy rotation curves again. Dark Matter may still be necessary, as a lot of unexplained has been dumped into the theory, but it is no longer necessary to explain galaxy rotation curves as a much simpler explanation exists, namely, gravitational tidal forces created by other galaxies in a cluster, which is very well understood and entirely explains curves of galaxy rotation and solves the winding problem.

Is there a paper on that? The talk was ten years ago, so I assume the idea would have appeared in print by now, if the idea actually worked.

I looked under his name on arxiv and didn't find anything on this topic.

Bro just trust youtube kkek

Will a paper be more convincing? I think there's a book. Write the author and ask him.[1] But the simple and unambiguous explanation of galaxy cluster tidal forces is devastating to Dark Matter Theory in regards to galaxy rotation curves. Why anyone is still talking about Dark Matter as a viable explanation for galaxy rotation curves likely has to do with the paradigmatic nature of science which resists any change, plus that there are myriads that have wasted entire academic and scientific careers on Dark Matter.

[1] alex@SensibleUniverse.net

A paper would tell one the thing passed peer review, and also let us look for papers publishing rebuttals. If it sank without a trace in the refereed literature, that's a screaming red flag.

I am very dubious the idea is correct. If objects at large distances from the galaxy center have these large relative velocities induced by external objects they wouldn't be bound to the galaxy.

> If objects at large distances from the galaxy center have these large relative velocities induced by external objects they wouldn't be bound to the galaxy.

Right. Tides must be caused by something other than the Moon because it's just too far away. No, wait, large masses have do have an effect across large distances. There are no isolated reference frames without gravity. Galaxy clusters do orbit around their center of gravity. Would you like to know how many theories in theoretical physics been proven? Precisely none of them, including Dark Matter Theory. This dubiousness is specious and, fwiw, a bandwagon argument.

You still haven't engaged on the idea that we haven't been doing full fidelity calculations based on vanilla general relativity and doing so yields non-negligible impacts on the lensing apparently. If you are using models which were meant to have their numbers crunched on an 80s pc then its possible it got too dumbed down. The assertion is that the theoretical error bars were improperly computed and should have been much larger.

> Ordinary matter in the galaxies cannot explain the rotation curves.

The observations of Fritz Zwicky and Jan Oort were of spiral galaxies in isolation, which led to the need for Dark Matter to explain the rotation curives. Once the entire galaxy cluster is considered, it becomes evident that a spiral galaxy is not an isolated reference frame and that Dark Matter is no longer necessary to explain galaxy rotation curves.[1]

[1] https://www.youtube.com/watch?v=PL0ewiwqoTw

Hey I recognize that name. He's the guy who wrote a book about some new cosmology he came up with from making a small modification to relativity that he claimed, among other things and IIRC, explained the Ganymede Mass Anomaly, that black holes were time-reversed white-holes from the other (time reversed from us) side of a hyperspherical universe, that the expansion of the universe is an illusion created by the curvature of said hpyersphere, and probably some other stuff I don't remember.

His work is attractive to laymen like myself, but any rational person has to wonder why, if his claims have any validity, he has been ignored completely by the wider physics community (which is to say, people who actually know shit, unlike most of us).

Your call to "watch the video and you'll never talk about galaxy rotation curves again" sounds a whole lot like similar statements made about such nonsense as "There are no forests on Earth", so if you honestly believe this guy should be taken seriously for some reason I would suggest a change in approach.

Near as I can tell, Alexander Franklin Mayer is an MIT educated physicist that was associated with Stanford's Department of Physics, where he lectured.[1] He published a book titled On the Geometry of Time in Physics and Cosmology and the Fall of the Canonical Cosmological Model[2] I can find no evidence or even remote hints of his connection to anything you have listed. But even as generous as your comment is, fundamentally you are making an ad hominem argument, effectively saying, "this guy is a crackpot, why should we listen to him?" It doesn't matter who he is. One must examine the assertions and findings detailed in his lecture, and only if flaws can be found there can his tidal theory explaining galaxy rotation curves be defeated. As such, between you and dubious commenter pfdietz, Mayer's lecture, assertions, findings and conclusions stand entirely unopposed by HN commenters. There seems to be a strong sense of bandwagon among HN members, that because Dark Matter Theory is now paradigmatic, Dark Matter has to be proven not to exist for it to be defeated. But proving things don't exist is not the realm of science while finding simpler explanations for physical phenomena often is. Mayer's explanation for galaxy rotation curves is simple, elegant, and compelling, and it is interesting that no one at HN wants to take it head on but instead will employ the all too common hand wave and other fallacious arguments. Look at it. Figure out why it is wrong or don't. But the argument that if it was true we would all know about it and Mayer would be famous is not in the slightest way persuasive.

[1] https://web.archive.org/web/20060207030527/http://www.stanfo...

[2] https://www.sensibleuniverse.net/pages/book.html

> I can find no evidence or even remote hints of his connection to anything you have listed

It took a while, but I think I found the PDF of the book I was talking about [0].

> fundamentally you are making an ad hominem argument, effectively saying, "this guy is a crackpot, why should we listen to him?"

Kinda, not really. I'm saying that if my recollection is correct this guy has expressed wild ideas before that were also ignored by the physics community and therefore it is worth viewing his works with suspicion because we are not qualified to evaluate it, but people who are have apparently decided it wasn't worth much.

> It doesn't matter who he is. One must examine the assertions and findings detailed in his lecture, and only if flaws can be found there can his tidal theory explaining galaxy rotation curves be defeated.

Sure, admittedly. So, why aren't any physicists giving this idea the time of day?

As I was pretty clear about in my post, I do not consider myself (nor you) qualified to assess his work in anything more than an armchair manner. The fact that his ideas don't seem to be something other physicists take seriously is something of an indicator that maybe I shouldn't either.

> Mayer's explanation for galaxy rotation curves is simple, elegant, and compelling, and it is interesting that no one at HN wants to take it head on

Because we are not physicists, as I assume you aren't either. Go to a forum full of physicists if you want a real assessment of its worth. We, being not physicists ourselves, can only judge its merit by proxy and intuition. The latter of those is especially dubious and especially prone to liking simple, elegant, compelling, and utterly wrong ideas.

> but instead will employ the all too common hand wave and other fallacious arguments. Look at it. Figure out why it is wrong or don't.

Strong words for someone who's response to all of this is "watch the video and you'll understand". Hand wave indeed.

[0] entitled The Many Directions of Time: http://www.5ision.org/GTR/amayer_the-many-directions-of-time...

> I think I found the PDF of the book I was talking about

Thank you for taking the time. My search fu is admittedly weak. That you've consumed this is also impressive.

> So, why aren't any physicists giving this idea the time of day?

Bandwagon.

> As I was pretty clear about in my post, I do not consider myself (nor you) qualified to assess his work in anything more than an armchair manner.

Ad hominem.

> Strong words for someone who's response to all of this is "watch the video and you'll understand". Hand wave indeed.

First, recognize the fallacies you're employing.

The video lecture and slides use high school level Newtonian physics to explain galaxy rotation clusters. If you understood the physics in his book, the physics in the video should be child's play. It is not remotely difficult to understand. And it is possible for a layman to evaluate it for rigor without referring to the theorist or other physicists.

> Thank you for taking the time. My search fu is admittedly weak. That you've consumed this is also impressive.

It was years ago, and I only gleaned the high concept really, but it has been around for nearly two decades now (though mysteriously not mentioned at all by Mayer's site) and, you will note, makes some pretty big claims about the fundamental nature of the cosmos including the ones I mentioned.

> Bandwagon.

Hand waving. You say that others are not doing the work by looking at the video you posted yet insist on this dismissive justification for ignoring that the qualified professional physicists of the world don't seem to think Mayer has accomplished what he claims.

> Ad hominem.

Hand waving, and wrong. I'm not insulting you, I'm pointing out the facts: we are not qualified to give an informed appraisal of this material. Drawing conclusions about its validity because it makes sense to us is pretty much exactly the Dunning-Kruger effect.

Look at what you're saying: somehow this guy has explained galactic rotation curves with high school level physics while practically all other physicists over the last nearly a century clung to some obviously wrong idea instead? And your explanation for this is that they're all in a bandwagon? This is the same community that said high-temperature superconductors couldn't exist, and then someone proved them wrong and won the fastest nobel prize in history.

Please explain how that is even remotely a rational conclusion, because it sounds like the same level of discourse one gets from flat earth conspiracy theorists.

Frankly, the idea that you are arguing in good faith is becoming harder to believe.

> You say that others are not doing the work by looking at the video you posted yet insist on this dismissive justification for ignoring that the qualified professional physicists of the world don't seem to think Mayer has accomplished what he claims.

No, I was identifying your argument, made again after I already identified it, as a bandwagon fallacy.

> Hand waving, and wrong. I'm not insulting you, I'm pointing out the facts

I did not suggest there was insult, only that you were employing the ad hominem fallacy by ignoring the material and focusing on me and you. Any argument focusing on anyone's expertise, in this case mine and yours, is entirely irrelevant and fundamentally an ad hominem argument, which is necessarily fallacious.

> And your explanation for this is that they're all in a bandwagon?

No. Your's and pfdietz's argument, paraphrasing, "if it's correct or worth our time, why hasn't the physics community acknowledged it?" or whatever, is fundamentally a bandwagon argument, which is fallacious.

You're right that what I'm saying about our level of expertise says nothing about the validity of Mayer's assertion, but what you can't seem to grasp is that even if I did watch the video, and even if I did understand everything it was saying and that made total sense to me, that doesn't mean that his assertion is valid because I'm not qualified in the appropriate domain.

When you're not actually well versed in a domain, all sorts of completely simple, elegant, compelling, and utterly wrong ideas make sense. I couldn't find any fault with The Many Directions of Time, but that doesn't mean it is a good model of cosmology[0], it's just the result of me not really understanding the domain sufficiently. It is completely rational to doubt my judgement under these circumstances.

Therefore, knowing that I lack the ability to judge the work's validity, I employ a heuristic: do people who are qualified to judge it say it is valid? And the answer appears to be "they don't even consider it worth their time to look at", which I find to be particularly strong evidence that I shouldn't take it seriously either.

The alternative is employing my own flawed understanding to judge validity, which is of course going to produce garbage results.

As a proponent of this idea, you have asserted that you think it is valid, and despite not disagreeing with the assertion that you yourself are not a domain expert, continue to assert that because you understand it, it is valid. You are free to believe that all you wish, of course, but it isn't going to sway any of us. Especially when, heuristically, the things you're saying are a close match the language employed by conspiracy theorists, flat earthers, and the like.

If your goal is not to convince us... then what is the point of this discussion?

If your goal is to convince us, I'm telling you that you're doing a particularly unimpressive job of making your case by refusing to engage with the heuristic reasoning in any way, or even offering an alternative other than "watch the video and it will convince you". I will note that watching a video of a conman or magician can be pretty convincing too, so that's not exactly good evidence either.

If what you're trying to say is to trust in scientific method, then we still can't trust our own reasoning with the argument and need to rely on the objective analysis of others including testing of hypothesis... none of which has happened. If it had, I'm sure you'd have brought it up by now.

So, again, this all heuristically looks like bullshit. We have a supposed "one neat trick solves one of the most mysterious things in modern physics" that, unsurprisingly, "physicists don't want you to know about" or whatever, and the resident proponent of the concept refuses to do anything but point people at the source video they aren't qualified to objectively analyze, and said proponent refuses to explain why it all looks like bullshit even if it supposedly isn't.

Since we seem to be miscommunication to a significant degree, I will try and summarize this:

  -Mayer has an idea that looks compelling to laymen
  -The experts in cosmology have apparently not even bothered to engage with it
  -While this does not mean Mayer is wrong, for those of us who are not experts in cosmology it certainly looks like we shouldn't take it seriously
  -You're asking us to ignore that and pretend that our layperson judgement is somehow good enough

[0] It might be, but again the heuristics are against it for the same reasons.

> that doesn't mean that his assertion is valid because I'm not qualified in the appropriate domain.

You are very eloquently and thoroughly attempting to justify your ad hominem argument, and to be more specific, you are presenting a credentials fallacy.

Let's assume for the sake of argument that I am an expert, and more so, that I am more qualified to evaluate the lecture than any scientist that ever lived or ever will live, and I tell you, it is correct, and I know, because I'm an expert, then regardless whether I or the presentation is correct or not, I'd be committing an appeal to authority and a fallacious argument.

Regardless of whether you are an expert in physics or not has no bearing on the validity of the theory, nor whether you are capable of deciding whether the theory is valid or not. You've speculated whether I was arguing in good faith, and now I must do the same. You're obviously intelligent and capable of deep consideration. Either you watch the video and draw your own conclusions, or you can not speak to it. It is that simple. Do or do not, those are your only choices. There is no valid argument that you can make to weasel out of them.

Does that mean some WIMPS Heavy particles do explain gravitational lensing oddities?

I think it means that the HN software auto-removes exclamation marks from titles, and in this case the original title began "No WIMPS!".

(I don't know that the HN software does this. Maybe the submitter made a typo.)

Dark matter is really a joint venture between God and ET to bleep with us.

Variable speed of light must be considered

I’ve heard this theory for years but never understood why it should be considered viable / possible. Part of me wants to believe because it would be really cool, but the rest of me is skeptical :)

Can you explain why it’s reasonable and what it answers or how? Ie why must it be considered here?

When watching the history of how the idea of dark matter came to be, I do not understand how they jumped directly to matter being the solution to the gravitational oddities that were noticed.

So here goes the story as I recall. A scientist noticed the galaxies spinning faster than they would have expected for the amount of gravity produced by the expected matter in that galaxy. The galaxies are spinning in a more uniform way throughout... aka the outside of the spiral galaxy is spinning faster than expected. Immediately I see this as being a gravitational issue. Maybe gravity works in ways we do not understand at this level. But instead scientists jump directly to "there must be matter here that we can't see".

My question is, has the marketing of the phrase "dark matter" got ahead of the actual problem and has stuck and really is a synonym to "strange gravity issues we are seeing" or have we really jumped to matter being the culprit without taking a look at gravity behaving in unexpected ways? Is the issue matter or gravity, and if it is matter, why does the story of dark matter jump from fast spinning galaxies directly to "missing matter" without a detailed explanation of why it must be matter, and not gravity acting strange?

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

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

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

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

Because it's the most economical assumption.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

I also find that pretty compelling.

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

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

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

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

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

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

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

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

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

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

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