1/12/2025 at 3:07:03 PM
Dark energy and dark matter aren't really theories. They are sort of the default solution to a set of problems that exist in cosmology. In a sense, every physicist wants to ditch dark matter or dark energy (or at least our current understandings of them), but they just don't know what to replace them with.by pclmulqdq
1/12/2025 at 3:52:53 PM
What annoys many was the pretense of a theory. At some point in the history of physics we stopped calling open problems, puzzles and (yet) unresolved paradoxes as what different but similarly unexplained phenomena were called in the past and pretended we resolved them.It's simply unnecessary to pretend its a theory, it is possible to name things without pretending they are theories.
by DoctorOetker
1/13/2025 at 1:45:03 AM
> pretended we resolved themWho did? Dark matter and energy are famously-unsolved problems in physics.
by JumpCrisscross
1/13/2025 at 2:03:19 AM
There is an overton window of acceptable dark matter theories which really should be a lot bigger. Many of the current big frameworks agree on too many details that essentially just arise from curve fitting.And no, I am not referring to the various MOND theories. They still belong on the same edge of that overton window that it currently occupies.
by pclmulqdq
1/13/2025 at 10:52:33 AM
well the correct theory will have to explain what we actually observe, which is going to limit the nature of what these theories can be. I'm not sure if you have an example of something that should be accepted when it's currently not, but usually that's the case when something is clearly broken with regards to basic facts we already know to be true.by fartsucker69
1/13/2025 at 4:08:02 PM
My problem with how the physics community is having this discussion is actually the converse of what you are suggesting: None of the existing theories explain every observation we have, and yet we are sticking to the same general frameworks that don't work.by pclmulqdq
1/13/2025 at 4:31:28 AM
> an overton window of acceptable dark matter theories which really should be a lot biggerOverton windows pertain to mainstream opinions [1]. Particle physics shouldn’t be subject to mainstream opinion.
by JumpCrisscross
1/13/2025 at 7:17:21 AM
Something can still be mainstream in the particle physics communityby geysersam
1/13/2025 at 8:50:21 AM
Who in the particle physics community claims dark matter or energy are solved?by JumpCrisscross
1/13/2025 at 12:43:24 PM
You are taking this too literally. I’m not GP, but I can guarantee they were not claiming any position like what you’re insinuating.by DiggyJohnson
1/13/2025 at 2:42:45 PM
> You are taking this too literallyThey’re calling out a “the pretense of a theory” of dark matter that we “pretended we resolved” unlike the “different but similarly unexplained phenomena were called in the past.” Where do you see the figurative gap?
In reality, we have General Relativity and the Standard Model. Both theories which have defied falsification to the limits of our instruments. Their unification, unfortunately, demands certain discrepancies be resolved. Dark matter and dark energy being possible solutions to those discrepancies. There is no “theory” of dark matter or dark energy, just a compendium of hypotheses.
OP is wrong. Literally, not figuratively.
by JumpCrisscross
1/13/2025 at 6:22:27 PM
In the past, unresolved questions were aptly named, like "UV-catastrophe" etc. as opposed to "dark energy" etc.One clearly indicates scientists being baffled (a catastrophic conclusion), versus a linguistically tangible substance "dark energy", unnecessarily specific in quantity (energy vs position vs momentum vs ...).
For unexplained phenomena, doesn't it sound alarmingly specific?
Wouldn't specific issues getting their own names not have been more desirable? like "anomalous rotation curves", ...
by DoctorOetker
1/13/2025 at 12:33:30 PM
Calling something dark energy is giving impression of claiming existence of something. While the reality is that equation of cosmological model is just wrong but could potentially be made right by arbitrarily adding, for no reason at all, a term that has unit of energy (density?). So what's unsolved is not really a problem of dark energy, but a problem of not having correct equation that models prior cosmological observations. You could add a factor to any wrong equation and call it "dark flexum" and fine-tune it as new observations come in.New observations point to possibility that dark energy might not be fixed in time nor isotropic in space. It strongly reeks of epicycles.
by scotty79
1/13/2025 at 2:46:58 PM
> Calling something dark energy is giving impression of claiming existence of somethingThere is an existence of something. Unknown variables aren’t improperly named. The solution to dark energy may be matter, it may be modified gravity (almost certainly not), it might be time bubbles à la timescape (which requires re-figuring the Big Bang, possibly less parsimonious than lambda CDM).
There are terribly-named conventions in science. Dark matter is only one to the degree of explaining confident popular ignorance.
by JumpCrisscross
1/13/2025 at 4:17:35 PM
My bet is on timescape as it doesn't use any unknown magic. Just requires applying GR correctly instead of in the simplest conceivable way that CDM does.A lot of things might clear up once you remove the assumption that Big Bang was true begining of matter, energy and time, with everything isotropic instead of just temporary state of high temperature (kinetic energy) and density with its own internal structure.
by scotty79
1/13/2025 at 4:38:43 PM
> lot of things might clear up once you remove the assumption that Big Bang was true begining of matter, energy and time, with everything isotropicSure, but now you’ve created a magic variable the literal size of the universe. Why do we have the timescape that we do?
by JumpCrisscross
1/13/2025 at 5:52:47 PM
Assuming zero is no less magic.We have the timescape that we have because of what the distribution of mass and kinetic energy is. And why the matter is distributed like that is because it never used to be isotropic, just dense and hot. Asking why this specific distribution is what we got is like a grain of sand thrown up by exploding landmine asking why it observes this specific distribution around itself. By the way it would observe a Hubble law because faster moving grains (relatively) would move farther away during the time that passed since the blast. The correct answer to question about why it's this specific distribution definitely isn't assuming that before the blast there was no space, time or matter even though that could be the simplest mathematical thing to model.
by scotty79
1/12/2025 at 3:29:44 PM
Dark matter in particular strikes me as… “yes, obviously”.There’s about a dozen quantum fields corresponding to particles. These form a graph, which is by no means fully connected; the fields each interact with a subset of each other, and neutrinos in particular only interact with gravity and the weak force.
If the connections are in some sense random, then it should come as no surprise whatsoever that the graph has disconnected subsets. In fact dark matter theory is effectively stating that the subset we’re a part of is one of many, which also agrees with the copernican principle.
by Filligree
1/12/2025 at 3:39:32 PM
That's all essentially true, but it doesn't necessarily mean that there's a quantum field that matches the properties needed to explain the phenomena dark matter is supposed to explain.It just means that it's plausible, and wouldn't be surprising, if there were such a field.
by antonvs
1/12/2025 at 9:22:51 PM
How are they so beyond direct detection if they similarly permeate the entire universe?by MichaelZuo
1/12/2025 at 10:57:58 PM
Every quantum field only interacts ("couples with") a subset of other quantum fields. Undetectable quantum fields would be ones that don't couple with any fields we interact with.We already see something like this with the neutrino: they only interact via the weak force and gravity, so we need massive detectors measured in kilometres and buried underground to detect them.[1,2]
It's estimated that about 100 trillion neutrinos pass through your body every second, traveling close to the speed of light.
Now imagine a particle that doesn't even interact via the weak force. The only way we would be able to detect it is via gravity, but gravity is an extremely weak force at the individual particle level. That's a possible candidate for dark matter. (Even particles that interact only via the weak force and gravity are candidates, but it's generally believed they'd have to be more massive than neutrinos.)
There's also the possibility that, if it turns out that gravity is an emergent property arising from quantum interactions or something along those lines, that there could be fields that don't participate in the gravitational interaction. But that's highly speculative territory.
[1] Ice Cube neutrino detector: https://icecube.wisc.edu/science/icecube/
[2] Super Kamiokande neutrino detector: https://www-sk.icrr.u-tokyo.ac.jp/en/sk/
by antonvs
1/12/2025 at 11:14:39 PM
> Now imagine a particle that doesn't even interact via the weak force. The only way we would be able to detect it is via gravity, but gravity is an extremely weak force at the individual particle level. That's a possible candidate for dark matter.I think even calling it a particle is presupposing things we don't observe. All we know is that there are places where spacetime curves in ways our theories didn’t predict from the matter we observe.
This could be due to a particle of a field that only interacts via gravity… or it could be that there are things other than particles that bend space-time. (Like maybe there’s an entirely separate dimension of space we can’t see, but its gravity affects ours…) Or it could be that spacetime warpage propagates in different ways than we thought (“gravity is different at large distances” etc), or it could be something else entirely we haven’t even thought of.
Quantum mechanics gives us a mental framework (fields and their corresponding particles) which has been extraordinarily useful for understanding the visible universe so far, but I think it’s important to remember that it’s just a model. It isn’t reality itself.
by ninkendo
1/13/2025 at 12:12:51 AM
> I think even calling it a particle is presupposing things we don't observe.Right, that's why I said it's a possible candidate for dark matter.
Your philosophical point about quantum field theory isn't really relevant here. The point is that, as the original commenter pointed out, we observe evidence of the presence of a set of quantum fields that act and interact in very well-defined ways. Given the nature of those interactions in the fields we can observe, it shouldn't be surprising if there are also other fields we can't observe - all it takes is a different set of coupling parameters. And we already see examples of this - we're virtually blind to interactions besides electromagnetism and gravity, and have to build enormous multi-billion dollar machines to explore other such interactions.
No-one is saying that this must be the explanation for dark matter, or that there must be other invisible fields. It's simply a natural prediction that arises from a successful theory.
by antonvs
1/13/2025 at 4:19:30 AM
I agree no one seems to be saying that dark matter must be part of a quantum field, but my point is more to lament the overgeneralizing of “quantum fields” as a concept to explain unobserved things.I’m absolutely a layman here, so apologies if any of this is misguided or flat out wrong… but its something that’s always gnawed at me about the constant tendency to “quantize” everything including gravity and dark matter… I’ll try my best to articulate:
Fields and particles are a mathematical construct we use to describe our observations. Quantum field theory seems to be a consistent mathematical framework you can use for particle interaction once you’ve observed the properties of the particles and how strongly force is carried, etc. But each field has to have its parameters set via observation; we don’t have a good reason why particle strengths are what they are (the fine structure constant doesn’t have an understood “cause”, for example.)
But my problem with this is that it can be so general that you can postulate a new field every time you don’t understand why something works a certain way. Like, we have the em field with photons, the weak field with neutrinos, the strong field with gluons… so dark matter? Must be another field with some new particle. Gravity? Must be another field with its own graviton or something…
It’s like string theory’s 10 dimensions… I certainly don’t have the expertise to understand string theory or why the extra dimensions are required, but something rubs me the wrong way if you can just postulate another dimension every time the theory is having trouble describing reality.
I’m not saying there aren’t multiple fields or that QFT is wrong, but I will say that QFT is a model we have that usefully describes parts of reality, and we shouldn’t confuse the model with reality itself. Some subset of the phenomena we observe can be described and predicted by it, but there are many phenomena that are not predicted by it (gravity is the big one.) I’m not convinced that the fix is to just narrow down the parameters of a graviton, but instead to accept that we simply don’t know if QFT and the standard model can ever be a truly unified theory of everything or if it’s just a useful tool for a few categories of phenomena. It’s possible that we simply don’t have (and might never have) a better theory.
So when we have a problem like dark matter, to say “maybe it’s a particle in its own field” can feel a bit like physicists going back to the same old well again.
by ninkendo
1/14/2025 at 4:43:53 PM
You may be assigning too much meaning to the term "field".All that a field is in physics is a physical quantity that has a value at every point in spacetime.
So if we observe an electron, and we believe that electrons can appear anywhere in spacetime, then we define a field with the relevant properties, as an abstraction that allows us to model electron activity in spacetime. Aside from the mathematical details of the representation, that's all there is to it.
(Clarification: "anywhere in spacetime" can actually be a subset of spacetime, e.g. many particles could not have appeared in the early universe before the electroweak phase transition. We say the universe then had a different vacuum structure, which is equivalent to saying it had a different set of fields.)
This seems to me to negate your objection about "so general that...", at least for particles we observe. We define the fields to match the observed properties, and that's our model of that type of particle.
The second issue I see with what you're saying seems to be a kind of conflation of established theories supported by evidence, with theoretical developments in progress. Many scientists would agree that gravitons may not be the right approach - particularly those working on alternate theories, such as emergent gravity, AdS/CFT correspondence, loop quantum gravity, string theory, etc. (The full list is much longer!)
Those people working on the theories of quantum gravity are of course going to talk about gravitons, but they can't claim we know gravitons exist because we can't observe them and there isn't even a complete and consistent theory that describes them. That's still being worked on. But it's certainly an obvious avenue of research.
The same thing goes for what you said about the "constant tendency to quantize everything". For quantum objects we observe, we observe that they're quantum so there shouldn't be any controversy there. For possible objects we haven't yet observed, like dark matter or gravitons, exploring the possibility that they're quantum just makes sense, if the behavior of what we're looking for is consistent with that. It doesn't prevent research on other possibilities.
> So when we have a problem like dark matter, to say “maybe it’s a particle in its own field” can feel a bit like physicists going back to the same old well again.
If we're looking for missing mass, there's already a whole theory of how mass works, which is QFT. The theory predicts that anything with mass must be a quantum particle. Of course the theory could be incomplete, but it wouldn't make sense not to explore the possibility that other massive particles could explain our observations.
Besides, everything in the physical universe we've ever observed fits under either QFT or GR. As far as we know, that's how the universe works. It's natural to explore new phenomena from that perspective.
> (the fine structure constant doesn’t have an understood “cause”, for example.)
Funnily enough what was being discussed in this subthread can completely explain this. Imagine a very large, if not infinite number of quantum fields, each coupled to others in all sorts of possible ways. In that case, the fine structure constant we observe can be explained by the weak anthropic principle: somewhere in that large possibility space, there are likely to be fields with properties that can support the existence of observers like us.
The idea of "hidden" quantum fields is known as hidden sectors: https://en.wikipedia.org/wiki/Hidden_sector . According to the Copernican principle, we should take it seriously. There's no real reason to think that the particular set of quantum fields we're able to interact with are the only ones, just as it turned out we didn't live on the only planet, or in the only solar system, or in the only galaxy. (Although, the nature of gravity's interaction with quantum objects could constrain the possibilities here.)
by antonvs
1/13/2025 at 6:55:33 AM
> All we know is that there are places where spacetime curves in ways our theories didn’t predict from the matter we observeI think we can say more than that. For observations to be compatible with the hypothesis that dark matter is particles or fields that interact gravitationally but not electromagnetically or strongly enough with itself to cause too much friction with itself,
It doesn’t just need to be the case that “if there was such matter at these locations at this time, then the gravitational effects on what we do see would be like what we see”. There is an additional major constraint: the dynamics of the dark matter particles/field(s) have to work with the distribution we see.
Like, if we have two regions which each have some normal matter and some dark matter, and they ran into one another, the dark matter shouldn’t be slowed by the friction in the way the visible matter is,
Etc. etc.
Like, in order for the observations to be compatible with the hypothesis that dark matter particles are the thing, the distribution of “where the dark matter would have to be in order to explain the gravitational effects” would have to be a distribution that would make sense for it to be in, under the assumption that it is dark matter particles.
by drdeca
1/15/2025 at 7:00:37 AM
Would such a particle not accumulate after a slowdown at a la grange point? its acummulation altering then other properties of that space we could measure?by ashoeafoot
1/15/2025 at 6:11:26 PM
Such particles would be frictionless and collisionless - they don't interact via the electromagnetic force. As such, they would just sail through a Lagrange point on whatever geodesic they're on.There's no particular reason to expect them to collect there, except possibly a small subset of particles that has a low enough velocity to actually be gravitationally captured. But that's unlikely if they're coming from some distance from the Lagrange point. The expected quantity of particles that could be captured like this is too low to be measurable.
Keep in mind that much of the nature of what we observe in the solar system is a consequence of friction or collisions having occurred in the past: the flattened disk shape, with all the planets orbiting on a plane, the accumulation of matter into objects like stars, planets, asteroids, and comets, and the parameters of their orbits were all heavily influenced by friction and collisions. Particulate dark matter would experience none of that, so doesn't end up being as organized as ordinary matter.
by antonvs
1/12/2025 at 11:41:15 PM
However the connections are not at all random, but they observe certain strict symmetries.For instance, in each of the 3 "generations" of quarks and leptons, separately for particles and for antiparticles, all the different kinds of charges and spin sum to zero, e.g. for the 8 particles that are the 3 kinds of u quarks + the 3 kinds of d quarks + the electron + the electronic neutrino. Moreover, in a 3-dimensional space of the "color" charges and electric charge, the 8 particles and 8 antiparticles of a "generation" are located in all the corners of 2 cubes, not in arbitrary positions.
So the set of elementary particles that we know is complete, there are no random locations where there could be extra particles.
Any so-called "dark matter", if it would exist, would have to be something completely different and not related in any way with the known elementary particles.
by adrian_b
1/13/2025 at 9:05:21 AM
Not true, right-handed neutrinos are a dark matter candidate. They don't experience the weak force, so would only interact through gravity.See recent work by Neil Turok. This podcast is one of the best popular accounts of his work:
The (Simple) Theory That Explains Everything
by mikhailfranco
1/13/2025 at 12:55:17 AM
> Dark matter in particular strikes me as… “yes, obviously”.Why? Historically there have been two "dark matter" theories prior to this one (we speculated non-visible mass in a place to explain motion of celestial bodies). One turned out to be neptune. The other was vulcan, which turned out to be general relativity.
So historically, the inclination to invoke some form of "dark" matter is batting 50/50. im not sure i would put 50/50 into "obvious" territory
by throwawaymaths
1/13/2025 at 10:27:17 AM
The "batting average" is a bit higher than that. For example, measurements of the proper motion (motion across the sky) of Sirius led to the prediction in 1844 that it was in an orbit with an (observed) faint or dark companion; the latter (the white dwarf Sirius B) was not directly observed until 1862, when better telescopes were available.One could also argue that detections of planets from spectroscopic observations of stars is another example. The first observations of transiting exoplanets -- where the planet blocks some of the light of the star -- were actually cases where the existence of the planet had been previously inferred from Doppler shifting of the parent star (e.g., https://en.wikipedia.org/wiki/HD_209458_b).
As another example, the first evidence for dark matter came from observations in the 1930s of the Doppler shifts of galaxies in galaxy clusters, which suggested much more mass in the clusters than could be explained by the masses of the individual galaxies. Some of this "missing mass" was actually observed in the 1960s and 1970s, when orbiting X-ray telescopes showed X-ray emission from very hot, dilute gas within the clusters (unobservable from the ground because the Earth's atmosphere blocks X-rays). It turns out that the hot, X-ray-emitting gas has about five times the mass of the (stars in) the individual galaxies. So some of the missing mass has been found -- though you still need significant, as-yet-undetected extra mass in clusters to explain why they haven't flown apart long ago.
by Keysh
1/13/2025 at 3:31:21 PM
Good examples.by throwawaymaths
1/13/2025 at 9:57:36 AM
Could there then be some particle that would interact electromagnetically, but not via gravity? And should we not be able to observe this?by Ekaros
1/13/2025 at 11:00:29 AM
the current thinking is no because quantum fields still act within spacetime, which is curved by gravity. there could be no particle that exists outside of this, so every particle has to interact gravitationally.but obviously, this is a concept from general relativity which is currently not compatible with quantum field theory at all. for the purposes of QFT every particle exists within some magical separate space where all those considerations are ignored because nobody really knows how to incorporate them without breaking predictions.
by fartsucker69
1/13/2025 at 11:34:19 AM
I don't think it could be possible. IANAP, but as far as I understand, anything that contributes to the stress-energy tensor "gravitates". So electromagnetic interaction per-se already "gravitates".by gpderetta
1/13/2025 at 9:51:14 AM
Waaaiiit a minute... Neutrinos interact with gravity ? How did anyone manage to prove THAT ?by euroderf
1/14/2025 at 7:04:01 PM
I think one way to prove that would be to observe electromagnetic activity of the sun and neutrinos originating from it. If you manage to find correlation that looks like neutrinos and photons arrive at roughly the same time it would indicate that neutrinos are affected by gravity. I think it was done but I'm not sure.by scotty79
1/13/2025 at 7:28:01 PM
They have energy and exist in our universe. GR does all the heavy lifting.by itishappy
1/12/2025 at 3:36:44 PM
I've been doing a deep-dive in the past few weeks of papers and data sets regarding to rotation curves, mass densities etc. (SPARC, papers describing the rotation curves of various dwarf galaxies etc). The impression I get is that most of the authors are not critical of a CDM dark matter halo explaining rotation curve data. The papers I'm reading span from ~1990 to present.by the__alchemist
1/12/2025 at 4:15:46 PM
That seems really time consuming.One question I’ve had for a while but didn’t seem worth the look is if there any consideration for local gravitational interaction between stars exchanging momentum between them and thus flattening the rotation curve.
I’m assuming that’s one of the first things looked at but couldn’t find a paper on the subject. Remember any references on the topic?
by Retric
1/12/2025 at 10:08:33 PM
The problem isn't so much the flatness of the rotation curve, but its continued high value: as you go farther and farther out in distance, it should drop rapidly because most of the visible matter is concentrated toward the center of the galaxy, but it doesn't. This implies that there is more matter, less centrally concentrated than the visible matter.Note that most "rotation curves" are actually measured from gas, not stars, and also that strong gravitational interactions between individual stars are extremely rare except in very dense star clusters and galactic nuclei, due to the increasingly large distances between stars as you go out from galactic centers. The time required for individual stellar interactions in the main or outer parts of galaxies to significantly affect their motions is much larger than the age of the universe (see, e.g., https://en.wikipedia.org/wiki/Stellar_dynamics).
Finally, this wouldn't address other evidence for dark matter, like the halos of hot (millions or tens of millions of K) intergalactic gas in galaxy clusters. The pressure of the gas should have driven the gas to expand way billions of years ago, if you assume that only the gravity of the individual galaxies and the gas itself is restraining it.
by Keysh
1/13/2025 at 12:09:26 AM
Edited.> strong gravitational interactions between individual stars are extremely rare except in very dense star clusters
We’re talking 225 million years for the sun to orbit the galaxy, rare events become commonplace on those timescales. Anyway, I’m sure someone has actually done this kind of simulation I’m just curious about what the result is and how they did it.
by Retric
1/13/2025 at 9:10:37 AM
You don't need a simulation; you just need an understanding of Newtonian gravity, basic algebra and a bit of calculus, and some knowledge of stellar masses, velocities, and space densities. This is a standard part of the grad school curriculum (even the advanced undergrad level) in astronomy; here's an example with the math in some lecture notes from an undergrad course at Caltech (by George Djorgovski): https://sites.astro.caltech.edu/~george/ay20/Ay20-Lec15x.pdfThe mean time for the orbit of a star to be significantly randomized by weak, intermediate-distance interactions (e.g., the kind the Sun is experiencing now from neighboring stars) is the relaxation time, and for a star like the Sun it's of order several trillion years.
The mean time between strong gravitational interactions, where the gravity of a single nearby star significantly changes the orbit of a star (perhaps more like what you were imagining), is of order one quadrillion (10^15) years.
(Note that the numbers are for the density of the stars at the Sun's orbit; further out, where you start to get to the point where dark-matter effects really show up, the density is lower, and so these times would be even longer.)
Those are examples of "extremely rare" even on timescales of the age of the universe.
by Keysh
1/13/2025 at 11:09:35 AM
I appreciate that link, but Dynamic relaxation is a much larger impact on velocity than required to be significant here. It’s still large enough to probably make such interactions meaningless on these timescales but it’s close.by Retric
1/12/2025 at 4:03:55 PM
Have you seen any work from Stacy McGaugh? https://tritonstation.com/new-blog-page/by uoaei
1/12/2025 at 4:04:28 PM
No; checking it out. Ty!by the__alchemist
1/13/2025 at 12:58:04 AM
not saying MOND is correct, but the list of predictions it made without fine-tuning are stunning:- keplerian rotation curves ("no dark matter") in elliptical and lenticular galaxies
- EFE (a group tried to find evidence against EFE and found it instead)
- early galaxies in the universe
- renzo's rule
- keplerian descent in the milky way (due to the effects of the magellanic clouds)
There's also explaining the tully-fisher relation, which is a post-hoc rationalization, so not really a prediction, but the model wasn't fine tuned to obtain the exact numerical solution.
by throwawaymaths
1/13/2025 at 12:55:57 PM
It's kind of amusing that you present "keplerian rotation curves" as a "prediction" of MOND, given that the whole point of MOND is a kludge to produce non-Keplerian rotation curves. That is, by definition MOND cannot produce Keplerian rotation curves. This is why the (small) number of dwarf galaxies (not "elliptical and lenticular galaxies"!) which apparently lack dark matter -- and which do not follow the Tully-Fisher relation-- are serious problems for MOND.by Keysh
1/13/2025 at 2:19:37 PM
> by definition MOND cannot produce Keplerian rotation curves.thats a common misunderstanding of MOND and means you did not stop to understand the definition. look at the formula carefully; in high gravitational regimes it looks newtonian (obviously, e.g. solar system). MOND, by definition predicts keplerian curves when the galaxies are rotating quickly (high gravitation). You will find that ellpitical and lenticular galaxies are almost always rotating fast. it also can through EFE but i dont fully understand the math enough to explain that.
by throwawaymaths
1/13/2025 at 9:31:55 PM
"Keplerian" is a term of art based on the rotation curve in Newtonian gravity when all the mass of a system is concentrated at its center. For the Solar System, the rotation curve outside the radius of the Sun is pretty much pure Keplerian: the velocity decreases proportional to the square root of the radius.For galaxies, which are extended objects, the rotation curve is not Keplerian when you are well inside the galaxy: it first rises to larger radii, then levels off. But since the baryonic matter (stars, gas, dust) in galaxy is rather centrally concentrated, the rotation curve should start looking more and more Keplerian as you get further and further into the outskirts and outside the galaxy.
But that is not what we see. Instead, we see the rotation curves staying roughly constant with radius ("flat"); we call this "non-Keplerian". This is true for almost all galaxies, including ellipticals and lenticulars (this is a recent study of three lenticular galaxies: https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-.... Note the rotation curves in the bottom panels of Figure 3: they do not at any point start decaying, let alone decaying as fast as R^(-1/2).)
Figure 5 of that paper (https://www.aanda.org/articles/aa/full_html/2020/09/aa38184-...) shows the observed rotation curves; it also shows the predicted curves if just the stars and standard Newtonian gravity were operating, with the dotted red lines. Note how these lines first rise to a local peak at small radii, and then decline to larger radii: this is a (quasi-)Keplerian decline. It fails to match the actual rotation curve at large radii.
The conventional response is to postulate some additional form of matter, distributed in a much more extended fashion than the baryonic matter (this produces the dashed black lines in Figure 5 of that paper). The MOND response is to modify gravity (or: to modify the acceleration due to gravity) such that it doesn't show anything like a Keplerian falloff at large radii, even at radii where the gravitating matter (assumed to be just the visible baryonic matter) is well inside.
In the case of the Solar System, the Keplerian decline starts right outside the Sun, where the acceleration is strong enough to be above the MOND threshold. But if you went far enough out and could measure the circular orbital speed, MOND would start to deviate from Keplerian. In the case of galaxies, the outer radii where the rotation curves appear "flat" are where the acceleration due to gravity is low enough for MOND to matter, and so the predicted MOND curves will not be Keplerian.
(I should perhaps point out that I'm a professional astronomer whose been studying galaxies, including lenticulars and ellipticals, for almost 30 years, so attempts to mansplain my field to me won't really impress me.)
by Keysh
1/14/2025 at 2:38:15 AM
[flagged]by throwawaymaths
1/13/2025 at 2:57:49 AM
The typical CDM response to this is "What about the Bullet Cluster?" (Gravitational lensing not correlated with visible mass). What is your thought on that? I think a convincing answer there would go a long way towards convincing people. I believe Milgrom's speculation is undetected matter.by the__alchemist
1/13/2025 at 4:29:40 AM
I don't know enough about the statisical image processing to measure weak gravitational lensing to have a cogent answer, but also it's totally ok to have one or two things that don't quite match up.Some of the "there's no DM" or "there's almost entirely DM" diffuse galaxies are likely due to measurement error in the distance to the diffuse galaxy, for example.
by throwawaymaths
1/13/2025 at 12:55:10 PM
Thanks for your original post as well. Petitioning anyone else to understand GGPs list and attempt an explanation of GP’s observation of the Bullet Cluster.Cheers all. I think it’s interesting how many of us are seriously interested laymen on this topic. I think it’s fun and fascinating.
by DiggyJohnson
1/12/2025 at 9:13:34 PM
They shouldn't have named them then. What they really are is an approximation of what they think they do not know.by willmadden
1/12/2025 at 10:03:04 PM
Yeah, the best analogy for people that know a little physics is aether - it’s obviously ridiculous now, but there was a time when it filled in some unknowns and people took it seriously. It would be nice if it weren’t presented as fact in tv and planetarium shows, but what can you do?by plasticchris
1/12/2025 at 4:11:48 PM
Maybe that's the evidence that dark energy and dark matter are the reality which our dark side refuses to admit.by begueradj
1/12/2025 at 4:02:33 PM
Dark matter is and always has been curve-fitting to residuals between theory and data. There is no there there, every map you see is nothing more than subtracting theory from data and having residuals left over. Dark energy is similar except much more coarse, in that the "model" is just a single parameter with a very simplistic interpretation.Neither are theories, but good luck coming away unscathed when mentioning this in the presence of ΛCDM dogmatists.
by uoaei
1/13/2025 at 12:52:00 AM
This is nonsense. Dark energy is a theory. Dark matter is a theory.As soon as you have at least two observations that you put together into a batch, you are at a minimum suggesting "these observations are causally connected". You have theorized that suggestion. You did not arbitrarily group those observations. (it's not "my dog pooped this morning" and "the car started when I hit the gas this afternoon") That makes it a theory.
by throwawaymaths
1/13/2025 at 1:40:23 AM
This is generally not how scientists, if they are being careful, use the word "theory".What you describe, the idea that two things are connected, is not a theory. That's a hypothesis. A hypothesis is a claim about the world, and it might not (yet) be equipped with an acceptable explanation for why it should be believed.
A theory would be a formula or equation or perhaps a process which is consistent with a set of information and allows scientists or mathematicians to calculate more information. It's a system whose consequences you can work out on paper, or in a computer, or in your head. But notably, a theory need not have any bearing on reality. You can develop a robust theory in all its mathematical glory and never find or expect to find anything like it out in the universe. It is a theory nonetheless, because you can work with it and explore it for it's sake.
Now, certainly, we have developed some theories of dark matter over the years and hypothesized that they are candidates for explaining the real world. There are many such theories. And, for each one, some scientists hypothesize that that one might be an accurate description of the world.
But, no, the idea of dark matter is not a theory.
by cvoss
1/13/2025 at 2:11:24 AM
as you stated:a hypothesis is the model (alone) for how things might be
a theory is the hypothesis + corroborating observation
if you have a set of observations and put forth a hypothesis from it (versus a hypothesis that has no observations backing it), it is automatically a theory. it may not be a good one but it is one nonetheless.
by throwawaymaths
1/12/2025 at 10:03:41 PM
> just don't know what to replace them with.Which is fine, but every physicist seems to just assume that it must be a "force", a "particle", or a "field".
It can be other things, including errors in the math, errors in the models, errors in the observations, invalid assumptions, etc, etc...
It's rather irritating to see the n-th experiment "searching" for some previously unseen particle while literally only one team thought of revisiting the maths to see if something was missed.
by jiggawatts