Astrophysicist here.
This is the same shtick from from a known anti-DM guy.
A few points.
* DM particle haven't yet been found.
* The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations.
* There is a broad variety of Dark matter models that are consistent with simulations.
* It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO.
* There are a few cases where you can find tensions with the existing DM based paradigm. I.e. the is test pointed in the article involving galactic bar in the paper "Fast galaxy bars continue to challenge standard cosmology". Note the toned down title. In this paper they just compared one specific feature of galactic disks in the simulations to the data and show that it doesn't match. I'd argue there are many reasons that could be the case that doesnt' involve killing DM. The same applies to other tensions.
* A final point. Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background.
> * The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations.
Dark matter models are fine-tuned to reproduce certain observations. These don't really count as evidence in favour of DM.
> * It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO.
MOND has more points in favour for it as a preferred theory than DM models. See [1].
> Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background.
I'm not sure why I should find your claim that DM is "essential" as persuasive given MOND has received orders of magnitude less attention and less development than DM models. I would certainly not bet any money on MOND being incapable of reproducing these observations.
> That's the definition of physics. Models are fine-tuned to match the observation and be predictive.
That's not what "fine-tuned" means. Epicycles can be fine-tuned to reproduce the motion we see from elliptical orbits. However, we should clearly prefer the theory of elliptical orbits because it reproduces observations without any extra parameters needed to fine-tune it.
In physics, a (free) parameter is finely-tuned when the model is highly sensitive to different values at high precisions. If you get radically different results for some parameter k = 0.0000000010 and k = 0.0000000011, and observations match only the first case but clearly do not match the second (or any value much larger or smaller than either of these), then the value of parameter k is finely-tuned.
One may need to add more free parameters to a theory, but those parameters need not be finely tuned.
A free parameter is one that must be measured or estimated, as opposed to dependent parameters which are fixed by the theory (and possibly its free parameters).
The pointwise energy-density of dark matter, or its contribution to the stress-energy tensor at a point -- a free parameter -- is presently only estimated, but that estimate is not finely-tuned. In fact, that's part of the practical problem of proving particle dark matter: the pointwise density is underdetermined by observation. As a result, we don't know whether the DM particles must all have very small masses, or whether there can be a mix of masses right up to masses comparable to that of Jupiter. We also don't know the density-gradient very precisely within galaxy clusters and individual galaxies. However, we get coarsely-similar galaxies and clusters in simulations for a wide variety of choices in these areas. So therefore, the dark-matter density (and its gradient, if any) are not finely-tuned. We must therefore search the parameter space to exclude many families of possibilities as significant contributors to dark matter energy-density. (Which has happened: many types of MACHO have been ruled out by observation; various supersymmetry candidates for WIMPs have been ruled out by experiment; most "warm" dark matter has been ruled out; and so forth).
If only the distribution of dark matter was finely-tuned, we could quickly narrow down the search space : <https://en.wikipedia.org/wiki/Dark_matter#Composition> (the last box in the table is for the cases where one has the mathematical freedom to choose to move some or all of the non-metric-tensor additional field(s) from the curvature side of the Einstein Field Equations to the matter side (where the stress-energy tensor is found), without prejudice as to whether it is more naturally (in a conceptual sense) a function on the curvature (which is why the cosmological constant appears on that side, where it could just as easily be written down as a function on stress-energy)).
> One may need to add more free parameters to a theory, but those parameters need not be finely tuned.
You're right I've been too loose with how I've been using the term "fine-tuning". Adding a parameter is not fine-tuning so much as just tuning.
The point I was trying to make is that both adding parameters to a theory to fit observations that it didn't predict, and fine-tuning it's parameters to fit should count against its plausibility, since they're both forms of tweaking the theory to match observations after the fact rather than making accurate predictions that follow naturally from the model being proposed.
Famaey & McGaugh's review <https://arxiv.org/abs/1112.3960> (Stacey McGaugh is <https://tritonstation.com/>, a frequent critic of particle dark matter) §7 is all about adding whole new fields ("parameters") to standard General Relativity in order to capture higher-order terms in post-Newtonian expansions (in 1/c^n or in the metric tensor) of theories that preserve the central characteristics of Milgrom's MOND.
Those characteristics are: no "non-luminous" or "hidden" matter sources -- the glowing, refracting, and obscuring dusts are the sole sources of gravitation -- and in circumstances where post-Newtonian corrections are vanishingly small, strong concordance with Newton and Kepler except at the lowest accelerations.
The orbits of galactic blobs of slowly-moving molecular gas that produce characteristic spectral lines amenable to Doppler redshift study are almost always in those special circumstances, and they have higher accelerations "corewards" in galaxies and low accelerations "outwards" in galaxies. These gas blobs are most interesting at the leading and trailing limbs of edge-on discoid/spiral galaxies, or face-on in large elliptical galaxies with negligible bulk rotation (the Doppler-shifting blobs move in and out, almost entirely radially), precisely where the accelerations are smallest.
The question we then ask is: what if we have a blob or some other spectrally well-defined object in a galaxy is on a very fast orbit? We can no longer ignore post-Newtonian corrections, or we lose the MONDian match with spectral lines. This is what drives §7 ibid.
If we think of the expansion as:
Total gravitation = empty flat background + MONDian matter relation + higher-order terms (h.-o.t.s. or HOTs)
the relativistically moving but still-MONDian matter is the generator of the HOTs, which has been measured (by among other people Pavel Kroupa, the author of the fine article at the top of all this discussion).
Abandoning the MONDian matter relation is easy. Just recast:
Total Einstein gravitation = empty flat background + unknown stress-energy tensor + higher-order terms
and adapt the stress-energy tensor. Nothing in the theory of gravitation has ever suggested strongly the nature of matter which generates the stress-energy, but we can get constraints from other areas of physics like the standard model of particle physics (or even classical electromagnetism, as was the case when General Relativity was new), and we can get candidates from programs seeking to expand or alter the Standard Model to solve open non-gravitational problems. Thus: various types of electrically-neutral particles like axions or supersymmetrical sparticles have been considered because "someone else" (i.e., not a relativist) hypothesized them. (Observational cosmologists can find evidence to constrain such theories, and in practice helped kill off several supersymmetry extensions of the standard model, for example). Low-mass primordial black holes were a candidate too, because there were non-galactic-dynamics reasons to suspect they could exist in significant number. But those black holes (or very dim tiny stars or isolated cold Jupiters) could maybe generate the unknown stress-energy tensor.
Retaining the MONDian approach can be done in a couple of ways. Hold to the idea that the "luminous" stuff is the only source of gravitation, and then ask whether relativistically-moving MONDian matter is coupled to an unexpected background:
(where all that's left in HOTs is basically gravitational waves from relativistically-moving MONDian matter)
or alternatively whether there is a further field or fields to which slow-moving MONDian matter couples only very weakly, but fast-moving MONDian matter experiences quite strongly. That means the HOTs imply the existence of one or more new fields.
Are these fields free parameters? Good (and open) question.
What types of fields are allowed? §7 ibid. explores this largely in terms of adding fields to the Einstein Field Equations (EFEs) of general relativity. The fields conceptually modify Einsteinian gravitation, so are written on the curvature side of the EFEs. However, as the authors of the survey note, in most cases for purely mathematical reasons the fields could be written exactly equivalently as a function on the stress-energy tensor (the matter side of the curvature = matter relation).
Why are we thinking in terms of an adaptation of the Einstein Field equation in the first place? Because it works well for length scales much shorter than ~ten kiloparsecs, and might work well for length scales much greater than megaparsecs. In particular, it works so well in the solar system that it cannot be ignored: it is the effective theory of gravitation near us. So however one might eventually write down a relativistic MOND theory, it must be possible to re-write it into a modification of the Einstein Field Equations for use in the solar system and in systems like Hulse-Taylor or triple-pulsar J0337+1715.
Additionally, analysis through tools like the Parameterized post-Newtonian formalism <https://en.wikipedia.org/wiki/Alternatives_to_general_relati...> is extremely useful if one recasts some arbitrarily-different-from-General-Relativity gravitational theory into something for which PPNF variables can be found. The discipline of submitting to PPNF by rewriting a theory for PPNF friendliness often is personally useful for the theorist keenly interested in her or his not-like-General-Relativity theory of gravitation.
One might say, aiming for neutrality, that if one adapts the EFEs in such a way that the adaptation can appear on either side of the EFEs, the "conceptual" weight of the adaptation is perhaps more aesthetic than physical.
However, that's not quite fair: outright adding extra stress-energy dark-matter-style lets one decouple the "adaptation" of the galactic relation from galaxies themselves. One could put particle dark matter in parts of the universe where there are no electrons or protons. For instance, before electroweak decoupling and big bang baryogenesis, or in the extreme distant future where we have basically isolated black holes and relic microwaves (and neutrinos) that are undetectably cold/long-wavelength. Even around "today"'s epoch, dark matter could be put willy nilly well away from clusters of galaxies.
A modified gravity theory, relativized along the lines of Famaey and McGaugh's surveyed options, might also be able to do this. Strange curvature just existing apart from any matter. (One can do this by a choice of a strange background in a post-Newtonian correction formulation of standard General Relativity too.) But to be MONDian I think that you would want attractive gravitation to appear only where there is matter that interacts electromagnetically, because far from such matter (attractive) accelerations will be very low, and the central feature of MOND is that Newtonian gravitation needs adjustment at very low accelerations.
So for a fully-relativistic MONDian theory of gravitation, although maybe mathematically we could treat the low-acceleration as a field that we could drop just about anywhere, doing so would violate MOND's spirit. There is thus no vacuum + MOND field(s) gravitational solution that is remotely physical. This is certainly not the case for particle theories of dark matter. Those basically insist that it is perfectly reasonable to drop dark matter just about anywhere. One can easily expect to simulate a vacuum + cold dark matter solution to the Einstein Field Equations, and that the simulation might accord with some part of our actual universe.
Finally, without further diving into the aesthetics or bets about what particle physics at CERN and the like might discover about mechanisms that generate stress-energy, I think the "fight" between MONDian modified gravitation and Einsteinian gravitation with dark matter is irrelevant to astrophysicists (as opposed to theorists who deal with non-astrophysical systems), even those who work on galaxy dynamics. It is almost certain that it will be possible to find an initial values formulation for either type of "final conceptual" answer to the observed Milgrom relation. It's just a question then of finding out how to populate the initial values surface, and then letting the dynamical laws go to work on those.
(The initial data and laws for each theory (MOND or GR+DM) reformulated this way will necessarily differ, perhaps by a lot, but the approach of evolving a set of initial data will be the same (this is done in many areas of physics having little/nothing to do with gravitation, after all)).
That is, the core of the MOND/particle DM fight might be more about bets on whether the single minimally-coupled metric tensor approach in General Relativity is not always suitable, or whether the Standard Model of particle physics is incomplete in relevant ways, than about whether one is really more suitable for calculations than the other. (The bet is also not strictly either/or!)
I'm honestly not sure who will really care in practice, if it's ever "finally" decided one way or another.
> So however one might eventually write down a relativistic MOND theory, it must be possible to re-write it into a modification of the Einstein Field Equations for use in the solar system and in systems like Hulse-Taylor or triple-pulsar J0337+1715.
This is the contention I have with the "fine-tune" argument against DM: there's still tuning needed from the MOND side to both make it a predictable tool across the universe and also ensure it still acts like EFE at home. Everybody's tuning/tweaking because no one has anything novel.
To bring in an interesting analogy: it's almost like the debate over functional and OO programming paradigms. MOND seeks to tune the "function" or theory to better fit all observations. DM seeks to tune the "objects" or observations to better fit the theory.
> This is the contention I have with the "fine-tune" argument against DM: there's still tuning needed from the MOND side to both make it a predictable tool across the universe and also ensure it still acts like EFE at home.
The whole point of MOND is that our gravitational theory is inadequate, and exploring what modifications are needed to match observations is the way move forward, rather than assuming GR is correct and searching for the missing mass it says should be there. We actually already know GR must be wrong because of its singularities, so this insistence on looking for DM despite the constant failures is doubly strange.
MOND's process of exploring what modifications are needed isn't "fine-tuning" in the way that DM has been tuned because MOND is not a final or complete theory, and nobody has presented it as such.
> We actually already know GR must be wrong because of its singularities
This is not why we "know" GR must be wrong.
You keep making very strong assertions that any expert -- including those not hostile to MOND, or even positively in favour of properly capturing the empirical MOND relation into some other theory (whether that's the standard one or one like Bekenstein's) -- simply would not make.
I do not know what you are trying to do here on HN, but it does not seem to be an attempt to honestly help people who are interested in dark matter but who lack expertise; it doesn't seem to be an attempt to acquire expertise you clearly lack; and it doesn't seem to be an attempt to work out how you yourself might better understand difficult concepts through a process of ELI5/ELI12. It seems, frankly, like you just enjoy being rude on hackernews.
In order to make up for the observations above that might be fairly taken as leading off with insults, I'll supply some related real, standard, uncontroversial physics discussion.
The principal problem with General Relativity is that the stress-energy tensor when generated by real matter (which we know can behave quantum mechanically) must in principle be able to encode superpositions of (at least) spin, momentum, and position. However, the Einstein Field Equations are fully classical, so how to do this encoding is not known. As a result "hacks" like taking the expectation value of the stress-energy T_{\mu\nu} -> <T_{\mu\nu}> are used to make the EFEs work at all. Unfortunately this averaging generally destroys superpositions, so one runs into problems like: if we took a fairly large molecule and put it into a superposition of position in a laboratory full of sensitive precision-stabilized accelerometers, what should we expect them to show? To which the answers are unattractive (and hopefully will be tested soon enough by
direct experiment (at e.g. <https://www.npl.washington.edu/eotwash/node/1>)). A little more detail in <https://en.wikipedia.org/wiki/Semiclassical_gravity> or for more advanced readers e.g. <https://journals.aps.org/prd/abstract/10.1103/PhysRevD.47.45...> which corresponds with <https://arxiv.org/abs/gr-qc/9304008>.
This is far far far from the strong gravity regime near gravitational singularities. It's a problem we could test in a spacecraft in deep space, or in a laboratory here on Earth. No black holes required.
Singularities, while annoying for developing Cauchy problems as one likes to do in many areas of physics, are not immediately fatal to the theory for several reasons, including the probable correctness of the cosmic censorship hypothesis and the long term stability of realistic black holes. Indeed, at least General Relativity is a complete theory that makes a prediction about what happens at its highest energy levels (measured in curvature invariants, like Kretschmann's). The Standard Model of Particle Physics is not presently "UV-complete". (More details @ <https://en.wikipedia.org/wiki/Physics_beyond_the_Standard_Mo...>, "The Standard Model is inherently an incomplete theory.") The converse is that while General Relativity is an inherently complete theory, it requires care in encoding the stress-energy tensor, any "background" curvature, boundary conditions, constraints, energy and coordinate conditions, and so forth. It is not an easy theory at all, especially when one does not understand all the mechanisms that generate the stress-energy tensor. Its successes as an effective field theory in astrophysical applications are legion, and completely contradict the remainder of your second paragraph.
All of the above equally applies to relativistic MOND (except the last sentence of the preceding paragraph: relativistic MOND generically gets the acoustic peaks of the CMB temperature power spectrum wrong, and usually badly wrong for the odd-numbered peaks, principally and importantly because of how different MOND is, by design, at assigning pressure in equations-of-state, and how that matters before the formation of the first galaxies). Milgrom's work has afaik always been in the regime where quantum weirdness has been negligible (and indeed, for most of MOND's history was even wholly in the regime where special-relativistic weirdness was negligible).
MOND has been explored for more than fifty years, and Relativistic MOND for more than twenty years, by some of the biggest brains in gravitational physics, including Mordehai Milgrom <https://arxiv.org/abs/1310.3373>. There are endless publications about MOND even today (tiny sample: <https://arxiv.org/search/advanced?advanced=&terms-0-operator...>). Nobody working in galactic astrophysics is ignorant of it, or of the phenomenological Milgrom's law. Most theorists working in gravitation have played with it so they already know:
> exploring what modifications are needed to match observations is the way forward
So I find myself wondering: do you genuinely not know that MOND "is a thing" already (and has been for decades) in the relevant academic and professional circles?
I don't at all dispute that GR is very well confirmed in many regimes, but it's still an indisputable fact is that GR is definitely wrong because of its singularities. Whether it's wrong in other regimes remains to be seen.
Some like to reframe its breakdown as GR being "incomplete", but that's just a cute euphemism for "wrong". This point couldn't be more obvious so I'm not even sure why you would dispute it.
You can of course argue that GR may only be wrong on small scales or very high energies where quantum effects become relevant, but that's just conjecture. A conjecture with good reasons given GR's empirical success so far, but still not established fact.
How GR fails may or may not impact the argument for DM, but we should acknowledge that the fact that GR is wrong actually might impact the justification for DM, which should already make us more skeptical of the core case for DM.
> So I find myself wondering: do you genuinely not know that MOND "is a thing" already (and has been for decades) in the relevant academic and professional circles?
Of course it's a thing. I'm not sure how I can be more clear on what arguing, so I'll try one last time: while I appreciate the interesting details you've put into your replies, nothing you've said changes the fact that DM's highly regarded status in this field is just undeserved, and that MOND still gets less attention than it deserves given its unexpected predictive successes [3,4].
DM was very plausible early on, but it has repeatedly failed to provide clear predictions and failed to match observations without post-hoc adjustments, where MOND has had some very surprising predictions that matched observations in ways it had no right to if DM were true. I don't believe I ever disputed that MOND also required tuning to match some results, so clearly neither theory is adequate to explain all of our observations in their current forms. At the very least we should be able to agree on that.
Other specific points:
TeVeS is one relativistic generalization of MOND, but hardly the only one possible is it? How much attention has been paid to MOND and relativistic generalizations when compared to tweaking DM theories to match results, would you say? Wouldn't you agree that all of the attention to DM, and the dogma that DM must simply exist [1], detracts from exploring other possible avenues, like questioning assumptions that underlie our theories, as in [2]?
On a final point on tone, I have no idea what you mean by me being enjoying being rude. I haven't said a single rude thing that I can see, I've only disagreed with some interpretations of the data and claimed that DM enjoys an undeserved status as preferred model, and that MOND deserves more attention, all positions with ample evidence.
I agree. I find it hard to argue against the claim that all physics is just curve fitting. We always end up with tuned parameters to make the maths work. For example the G in F = Gmm/r^2 (Newtonian mechanics).
I'm not an expert, but it seems like Dark Matter theories allow the DM distribution to be tuned. Is that the case? Can you say, "oh, there must be a clump of DM over here with mass X". If so, that would give you lots of parameters to tune per situation you're modelling. It may be that IS how the universe works. The problem is that the theory is hard to falsify if it has so much wiggle room to fit every situation.
sure, but you didn't get the point. just because a model can predict certain results doesn't mean its correct. early astronomers were able to very precisely predict the positions (w/ looping motions and all) of stellar bodies in the night sky despite using a geocentric model. if you make it complex enough, you can engineer quite a few models that can predict natural phenomenon pretty well.
Yes of course, but this applies to any model. We cannot know of any model is "correct", they are just tools to describe what we observe. If they are more predictive than other models, then we should use them. Or course it's better to use a simpler model that gives the same predictions.
> if you make it complex enough, you can engineer quite a few models that can predict natural phenomenon pretty well.
Well, yes. Next thing is to pick the easiest of these models: Occam's razor. But pondering whether such model is "correct" won't bring any further improvements.
I worked in this field for a minute (XENON1T) and I heard a few talks about MOND. We were all pretty open minded but it failed to stick because it didn’t convince us, for a number of reasons. Curious about your background here.
It was already falsified by well established limits on self interaction of DM from galaxy collisions. And from the already mentioned temperature fluctuations in the CMB. Every now and then some MOND supporter comes out if they found a way to tune their model to one specific observation, but particle DM remains the only approach that can fit all these observations at the same time. Ofc that doesn't guarantee that particle DM is the only explanation. After all, it could be a combination of DM+MOND. But MOND alone is just not possible, whereas DM alone might be.
By that argument, DM was already falsified by the observations pointed out in this article and the paper I linked. Your claim that DM could "fit all of these observations at the same time" is just wrong. DM has been tuned to correct its parameter-free predictions, so if you're going to compare like against like, then MOND should be permitted to be tuned to fit as well, and we can judge in the end which theory requires the least fine-tuning.
As for galaxies without DM, those observations are still being debated, but even if true, that doesn't entail MOND can't explain those results.
I don't think it's fair to say that it's "tuned" to fit observation. It's not like several different types of DM are introduced to fit the observation. Rather the observations add some constraints to the DM properties. That's how it's supposed to be.
If DM has to have a very specific initial distribution to reproduce what we see, for example, that too is a constraint but it's also fine-tuning. Another theory that is free of such parameters should be preferred. Some types of constraints really should decrease a theory's plausibility.
MOND needs some pretty crazy arguing to explain decoupling of gravity from baryonic matter distributions. And it also needs a varying length scale parameter to account for different galaxy types. Particle DM can explain all of these things at the same time without re-tuning the model every time.
> MOND needs some pretty crazy arguing to explain decoupling of gravity from baryonic matter distributions. And it also needs a varying length scale parameter to account for different galaxy types.
You mean the theory that has received 2-3 orders of magnitude less attention than DM has some conceptual holes to patch up? Colour me shocked. The fact that DM still fails despite all of that investment should worry you more.
> Particle DM can explain all of these things at the same time without re-tuning the model every time.
DM has already been tweaked and fine-tuned to patch up its own endless list of holes, many of which were recently reviewed by McGaugh [1]. Many more remain despite that, like the dynamical friction discussed in this article.
Sean's breakdown is ironic, because every point he makes against MOND also applies to DM theories, particularly given new evidence. I have no problem acknowledging that MOND is inadequate given the data, but apparently it's difficult for DM supporters to similarly acknowledge that DM has been refuted. The core argument for DM rests on a theory (GR) that we already know must be incorrect due to its singularities.
Finally, I'll just note that literally nothing in Sean's post suggests McGaugh's work is "dubious" in the least, so I have no idea where you got that notion.
The best thing this article can provide is that some galaxies work without DM. Only particle DM can explain the early universe observations from the CMB (which this article completely glosses over), decoupling from baryonic matter (which also isn't even mentioned) and galaxy rotation curves at the same time. The arguments in favour of DM are far more general and rightly accepted for what they are, whereas the sporadic argument for MOND that works in one domain is rightly shunned as long as its proponents blissfully keep ignoring the overwhelming incompatibilities in the remaining domains.
> Only particle DM can explain the early universe observations from the CMB
Are you actually arguing that DM is the only conceivable theory that can possibly explain those observations? Be serious. This obsession with DM when it's been clearly falsified numerous times is restricting imagination and attention on novel approaches.
I explicitly said it might very well not be the only one. But the data clearly shows that MOND is definitely not the only one. Since Occam's Razor tells us to go with the simplest solution, science goes for DM, instead of DM+MOND. If we ever discover DM (eg in the form of WIMPs) and it can't account for all the extra mass in the universe, then people will be able to look closer at modified gravity again. But right now these models are just not at a point where they could explain more than noise. If you're interested in how difficult it actually is to come up with a robust alternative theory of gravity that can fully explain all aspects of DM we see in the universe, I suggest this lecture here: https://www.youtube.com/watch?v=WECVq2YBduY tl;dr: it's hard. Very hard. Noone has done it yet.
> I explicitly said it might very well not be the only one.
Actually you didn't. Perhaps you claimed that in another thread, but I quoted exactly what you wrote in this thread, which is that only DM can explain those results.
> Since Occam's Razor tells us to go with the simplest solution, science goes for DM, instead of DM+MOND.
That's not a correct formulation of Occam's razor. Occam's razor tells us not to posit the existence of more things than are necessary to explain observations. Observations already tell us that DM either does not exist, or doesn't exist in the amounts most people think exists, ie. most DM models have been refuted. Therefore, even if DM exists, something else is still needed to explain our observations.
>Observations already tell us that DM either does not exist
This is just a plain lie and since you obviously have no interest in further educating yourself in any way, I will now stop wasting my time with replying to you.
As my understanding goes the models used are there to see what would be needed to replicate the current best know state of the universe, given what we know about its beginnings and as said its "current" state. Running very many different scenarios the best models need something the astro "community" came to call "dark matter".
As long as a opposing theory can't replicate these simulations from initial state to known "now" state without the use of some form of "whatever" it is quite interesting to call out that there is nothing like "dark matter" (whatever this something may be).
> This is the same shtick from from a known anti-DM guy.
Whether it's a "shtick" or not, his basic point seems like one that is worthy of some kind of substantive counter argument, if one exists. His basic point is that small galaxies orbiting larger galaxies should experience Chandrasekhar dynamical friction if dark matter exists, but no such thing is observed. Therefore dark matter cannot exist.
Has any astrophysicist who supports the dark matter hypothesis published a counter argument to this?
Edit: A published paper of Kroupa's that gives much more detail about his point is here:
How about therefore thing we've yet to observe if it exists fails to behave as we naively expect.
This is a grand claim from one piece of work and feels more like a straw man grab at further research funding rather than finding the heffalon at the LHC https://arxiv.org/abs/1303.7367
Or in other words Occam's razor says you have a stripey horse in northern Europe rather than a zebra.
> How about therefore thing we've yet to observe if it exists fails to behave as we naively expect.
As I understand it, the "behavior" of dark matter in Kroupa's analysis is the behavior that is claimed for it by dark matter proponents. So if dark matter doesn't behave the way dark matter proponents say it does, the burden would be on the dark matter proponents to modify their models to say how it does behave.
Trust me this is a straw man picking at a hole of a yet incomplete theory to claim the whole deck of cards is invalid.
AFAIK none of these alternatives offer a serious explanation of gravitational lensing for instance (yet, I'm happy for someone to make a theoretical breakthrough that explains away the need for an unknown new particle, but that just opens more questions around CP violation and so on)
The basic point I described is not proposing an alternative to dark matter, and I'm not asking for a refutation of Kroupa's well-known preference for alternative gravity theories. I'm just pointing out that Kroupa has described a prediction of the dark matter hypothesis that does not appear to match observations. I'm wondering if any proponent of the dark matter hypothesis has a counter argument to this: either an explanation of why the dark matter hypothesis doesn't make the prediction Kroupa is describing, or an explanation of why that prediction does match observations after all. This seems to me to be a perfectly reasonable question that does not require any discussion of alternative theories at all.
I'm not infallible but I do have a PhD in the study of CPV in particle physics so I understand this at a level enough to pass a doctoral viva.
Alternate gravity theories struggle with gravitational lensing. This is a proven observable optical phenomena in the study of the universe.
Kroupa is still trying to explain the rotational differences between observation and "normal" Newtonian/Einstein gravity. This is colloquially referred to as Dark Matter. A gravitational (matter like) effect, which doesn't interact optically via standard EM as we can see the galaxies and stars. Hence the name Dark Matter.
Are not what I'm asking about here. I know Kroupa pushes them, but that's not the part of Kroupa's work that I'm asking about.
> Kroupa is still trying to explain the rotational differences between observation and "normal" Newtonian/Einstein gravity.
Not in the specific item I'm asking about. In that specific item, he's not talking about any alternative theories of gravity at all. He's pointing out what he claims is a conflict between the predictions of the dark matter hypothesis and actual observations. I've already explained what a counter argument to that would look like. Do you have one?
Unfortunately you are getting a little crossed from your perspective. You're asking a question that to experts is structured oddly because you can't disentangle things you're trying to separate.
I'll try to explain.
Again dark matter is an attempt to explain a collection of observations that don't match current accepted theoretical models.
There "classical dark matter hypothesises" is just that. That these observations can be explained by a quantizable matter candidate of some form with some yet to be determined properties.
The only non particle descriptions of "dark matter phenomena" is a modified gravity framework. These attempt to address the "classical dark matter phenomena" whilst disentangling it from the "CPV phenomena". This is fine but breaks the big bang model at the stage of baryogenesis (matter formation from pure energy).
There is the possibility that you can explain "classical dark matter phenomena" without demanding it also be responsible for CPV. However, if you do that the CPV step of the big bang model demands additional particle candidates which then raises the problem of what do they look like and where are they and do they interact with the Higgs field and therefore have mass.
There are no counter arguments to "classical dark matter phenomena" because they are just that. Observed natural phenomena which do not fit into our current understanding of the universe.
A "complete dark matter theory" tends to explain CPV+DM. These again don't make many testable predictions other than a (potentially TeV scale) weekly interacting particle candidate. So the testable prediction is that a dark matter candidate exists and we can observe it.
> I've already explained what a counter argument to that would look like. Do you have one?
I'll choose to read that in a relaxed manner, albeit it's a bit direct.
There is no counter argument to natural phenomena. The main 2 theories are that dark matter particles exist and we can find them or we can't.
MOND et al make few predictions and struggle to observe defined "gravitational phenomena", hence why there is little interest in them.
There are differences between DM particle models but almost all agree that the major premise is that we might/should be able to see certain particle types given how they interact.
> You're asking a question that to experts is structured oddly because you can't disentangle things you're trying to separate.
To me this reads as the answer "no" to the question I asked. Nobody actually has a counter argument. "Experts" have simply chosen to dismiss the matter without any argument at all.
To expand on this somewhat: you appear to believe that I am asking for some kind of separation of observations, so that we consider only the particular observations Kroupa refers to (orbits of small satellite galaxies around large galaxies) and ignore all the other observations. That is not what I am asking. I am asking for a simple response from "experts" who support the dark matter hypothesis in general to these two questions:
(1) Is Kroupa correct in his claims about what dark matter models of "halos" around galaxies predict regarding the orbits of small satellite galaxies around larger galaxies? If not, why not?
(2) Is Kroupa correct that actual observations of the orbits of small satellite galaxies around larger galaxies do not match the predictions of dark matter models? If not, why not?
These seem to me like straightforward questions of the kind that scientists pose to other scientists all the time, and it does not seem credible to me that "experts" would not have responses to them along any of several obvious lines:
(A) Kroupa's computations of the predictions of dark matter "halo" models are wrong. Here are the correct predictions, which match observations.
(B) Kroupa's claims about the predictions are correct, but his comparison with the data is wrong. Here is the correct comparison with the data, which shows that the models match observations.
(C) Kroupa's claims that the predictions don't match observations in this particular domain are correct taken in isolation. However, when the entirety of all observations are considered, including ones that have nothing to do with individual galaxies (such as the evolution of the early universe), the dark matter models do much better at minimizing the overall error between model and observation than any of the alternative models, including the ones Kroupa favors.
You seem to be leaning towards something like (C), but if so, you could just say (C) and be done with it. Instead, you appear to be trying to avoid any possible admission that there is any discrepancy between dark matter models and observations at all, which IMO is not a good strategy. No model ever matches 100% of observations.
> dark matter is an attempt to explain a collection of observations that don't match current accepted theoretical models
I would not put it that way. I would put it that dark matter is an attempt to explain a collection of observations using currently accepted theoretical models (General Relativity and the relevant solutions to the Einstein Field Equation), by making use of a free parameter in the models, namely the assumed stress-energy tensor (or distribution of matter, if you like). By adding a "dark matter" component to the distribution of matter, which does not interact electromagnetically and therefore is not visible to us by any means other than its gravitational effects, many observations can be matched much better.
MOND, by contrast, proposes modifying the theory of gravity in general. That, to me, is a much bigger change than adding a component to the distribution of matter.
> There is no counter argument to natural phenomena.
So what? I wasn't asking for a counter argument to "natural phenomena". I was asking for a counter argument to a claim that a particular model makes predictions that don't match observations. Claims like this are made all the time in science, and the appropriate response is to give a counter argument along the lines I described. I have not seen one, and as I noted above, all your remarks have given me the strong impression that there isn't one, and that "experts" have no interest in providing one. That answers my question, although no doubt the inferences I draw from that answer will be very different than yours.
In my opinion, the big bang model, along with all of GR - LCDM is broken and can not be fixed.
Please take a look at the modified gravity proposal that I have put forward. This idea incorporates a mechanism that would explain why gravity might slightly deviate from Newtonian and/or GR in some circumstances.
This paper also adapts GR in such a way that it is consistent with galactic rotation rates, the anisotropies of the CMB, and cosmological expansion -- while showing that the simple operation of gravity is the cause of each of these phenomena.
This idea, Cyclic Gravity and Cosmology (CGC) predicts that there are discrete specific sizes allowable for macro-objects. The instability of Bennu and the fact that it behaves more like loosely held scree rather than a compact mass -- is an example of a mass that is not exactly at one of the discrete allowable sizes. Please also refer to the link I included wherein I uploaded a video simulation of the formation of a solar system using this type of force law. (This is in section 18 of the paper)
I would greatly appreciate any comments on this idea. A copy may be downloaded here:
Tl;Dr c is closest to what you're asking but it's more like.
(D) Kroupka is making bold claims that have to be verified first in the scientific arena very loudly and publically. That is odd.
He is claiming that DM doesn't exist which is effectively saying the particle solution to the problem can't exist. This breaks the big bang model as we understand it as I've stated. And is offering a gravity based solution which explains some small phenomena which he claims that not only that this isn't modelled accurately currently but that it can't be using the DM models. That claim requires huge effort to correctly simulate the matter field as it _might_ exist and that requires more assumptions. This could mean certain particle solutions are ruled out or that we need to revise the particle model (fantastic we learned something), but it's much less likely that it's completely dead just because one test says so.
His model may be valid. His data might be good. His conclusion has little to do with either.
Just because a theory doesn't match all observations doesn't mean people stop working on it. If that happened the foundations leading to the Higgs scalar boson wouldn't have been researched as at the time everyone was interested in understanding other field theories.
Ok some background.
DM research was introduced as a solution to the problem of the distribution of mass within galaxies. I.e. orbital velocities of stars within a single galaxy.
In order to try and understand this phenomena models were used to simulate the distribution of "matter fields" within galaxies.
Now modern modelling has evolved to simulate complex systems of multiple galaxies or even larger.
We now suspect and have strong evidence for huge mega structures (MPa mega parsecs in size) made of some "DM goop" in the universe.
Modeling of this is notoriously difficult and relies on numerous assumptions because we can't model everything exactly so we take sensible shortcuts which shouldn't impact the physics of interest for the simulation.
Large simulations have problems the further they evolve. This is a stochastic not model dependent statement. Modelling a thing we're yet to observe is even more difficult.
Most claims that DM can't exist are people modifying gravity in some way to remove DM as a physical particle that you can build a DM hammer out of in principle. These claims are not entirely baseless but really struggle when you try to match large gravitational structures we have seen so far in the universe (cosmic filaments).
Kroupa claiming his simulation has to be the only simulation to explain a problem (if he makes or infers this) would be false. Everything in science has requirement of reproducibility which goes beyond share the source and run the tool.
Kroupa claiming that his model perfectly describes this set of observations might be interesting, but is yet to be demonstrated at different scales. Again this has to satisfy orbital momenta of stars within a galaxy as well as demonstrating larger scale effects such as gravitational lensing.
Dark matter simulations _all of them_ will have some amount of disagreement or uncertainty. That is the nature of scientific computer modelling and comparison to scientificly collected data. Uncertainty and error estimation/understanding is a huge part of this field. Anyone claiming exact results better have a huge amount of data to back it up.
This is either a conversation that would have been easier by Skype. I'm guessing you don't have a strong background in a formal science. I'm not saying this from some pedastle, it's a joint question/observation of you focusing on the socialogical implications and exactness of a thing.
There is no lack of interest from experts to talk about their field. There is almost never a lack of wanting to move away from the "status quo" in the scientific community. Especially in blue sky particle and astro research at a professional level. The public perception of this existing comes from 2 issues. 1) "loud" individuals who shout loudly that they're not being heard will be ignored. That isn't discourse, that is someone acting like a small child within this area. Unfortunately they often get awarded by funding and we're stuck with them. As they get pushed aside for shouting rather than discussing they tend to get louder. 2) the media loves to paint the image of scientists sitting there as gatekeepers to some theoretical framework. This is fun in movies etc but mostly we talk about this in science education as a case of remember the mistakes from the last 100 years. As a result you will be looked at strangely for dismissing an idea out of hand without proof or testability.
> (D) Kroupka is making bold claims that have to be verified first in the scientific arena very loudly and publically.
Meaning basically that other experts haven't had time yet to check his work? How long is that expected to take? The paper I referenced was published in 2015.
> He is claiming that DM doesn't exist
Yes, on the basis of his claim that dark matter models make predictions about the orbits of small satellite galaxies around larger galaxies that don't match observations. As I've said, these kinds of claims get made in science all the time. Scientific models are supposed to be tested against observations. When someone sees a mismatch, they're supposed to say so. That's part of how science is supposed to work.
> This breaks the big bang model as we understand it as I've stated.
More precisely, it "breaks" the belief that we can construct a valid model of our universe using standard General Relativity, on the basis of the claim that we cannot find any assumption for the distribution of matter that gives a satisfactory fit against all observations. That's the larger claim he's making and the basis of his advocacy for alternative theories of gravity, yes. But, as I've already stated, that larger claim is not what I have been asking about.
> it's much less likely that it's completely dead just because one test says so.
That doesn't change the fact that there should be some answer to how that one test comes out: either it does come out the way he claims, or it doesn't. Which is all I was asking about. If the answer is "nobody else has had the time or resources to check as yet", then that's the answer.
> Ok some background.
While this is all valid information it's not information I really needed. I'm well aware of the difficulties involved in constructing these models and checking them against observations.
> I'm guessing you don't have a strong background in a formal science.
Your guess would be wrong. I don't currently do scientific research for a living, but that doesn't mean I don't have a lot of background in how science works.
> you focusing on the socialogical implications and exactness of a thing.
I have no idea where you are getting this from. I haven't asked about "sociological implications" at all. You're the one who keeps bringing that up. And I have already explicitly recognized that no scientific model matches observations with 100% exactness.
> The public perception
Has nothing to do with what I've been asking about. I haven't been asking general questions about how the scientific field of cosmology works or whether or not cosmologists are supporting a "status quo" or whether or not "dissenters" in science get a fair hearing. I've been asking a very specific question which I've already described several times.
> Meaning basically that other experts haven't had time yet to check his work? How long is that expected to take? The paper I referenced was published in 2015.
Likely never, as I have explained. Someone loud will be pushed aside from the community and simulations of this type/scale take too long to 'jsut test' or 'just repeat'.
> Yes, on the basis of his claim that dark matter models make predictions about the orbits of small satellite galaxies around larger galaxies that don't match observations. As I've said, these kinds of claims get made in science all the time. Scientific models are supposed to be tested against observations. When someone sees a mismatch, they're supposed to say so. That's part of how science is supposed to work.
This is not a reasinable assertion based on his work so, in simple terms, no.
> More precisely, it "breaks" the belief that we can construct a valid model of our universe using standard General Relativity, on the basis of the claim that we cannot find any assumption for the distribution of matter that gives a satisfactory fit against all observations. That's the larger claim he's making and the basis of his advocacy for alternative theories of gravity, yes. But, as I've already stated, that larger claim is not what I have been asking about.
No you cannot make claims in isolation that break the rest of science that's not how it works. Ever.
> While this is all valid information it's not information I really needed. I'm well aware of the difficulties involved in constructing these models and checking them against observations.
This is _essential_ to the discussion to underatand what is being discussed. To throw the topic aside is showing a lack of focussing on the article, but I think
> That doesn't change the fact that there should be some answer to how that one test comes out: either it does come out the way he claims, or it doesn't. Which is all I was asking about. If the answer is "nobody else has had the time or resources to check as yet", then that's the answer.
Again, a loud person jumping up and down with a simple test over a single overvation will not get the community to divert funding, time and effort. Reasonable arguments for diverting of limited resources are to approach the community with fact based evidence and reason.
> Your guess would be wrong. I don't currently do scientific research for a living, but that doesn't mean I don't have a lot of background in how science works.
Please stop being blunt and defensive, I'm trying to be polite.
> I have no idea where you are getting this from. I haven't asked about "sociological implications" at all. You're the one who keeps bringing that up. And I have already explicitly recognized that no scientific model matches observations with 100% exactness.
You brought it up in a paragraph with the theme of "how the community should change" which is the main shouting point from this article.
> Has nothing to do with what I've been asking about.
as above.
OK, to avoid this going further I think I've in good faith discussed everything in reasonable detail. You may feel different, you're entitled to and you're entitled to reply. But to move on I feel I've given you the large benefit of an experienced insight that is for you to make of what you chose. Have a great day/week/year, I won't be replying further.
Ok, that answers my question. I won't comment on the rest except to say that I don't think I have the same views about how science works (or should work) that you do. But scientists all have the freedom to choose what to work on, so ultimately that will determine how all this plays out.
> You brought it up in a paragraph with the theme of "how the community should change"
To be clear, nothing I have posted in this thread was intended to make any such claim. As I have said all along, I was only interested in a specific answer to a specific question, which you have now made clear. As I noted just above, scientists are all free to choose what they will work on, and I have no issue with that at all. Kroupa is free to express his opinions about more general questions like "how the community should change", and other scientists are free to ignore him, which is what seems to be happening. All of that is beyond the scope of what I was asking about.
Interestingly, a similar argument (ie. “if it exists, it should behave like this, and it doesn’t”) was used to disprove the existence of the aether[1].
However, "dark matter" is an admittedly lose framework it could be due to 1 new type of particles, 10, or higher dimensional effects and new physics we don't understand yet.
Saying "therefore it doesn't exist" is ignoring the mountain of decades old solid data showing gaps in our knowledge exist.
And as for dark energy... "The universal expansion is accelerating". Well done you now know as much as 99% of the physicists in the world on this strange unexplained problem.
We consider particles with oscillatory behaviour sufficiently different from aether to abandon the name and the concept as useless. But contrary to aether, dark matter is still a new, vague and complicated concept: there is ample room to evolve dark matter theories without giving up on the idea of dark matter.
In any case competing theories will need to converge towards new experimental evidence and therefore towards each other, and then names and starting principles will be unimportant.
Your statement implies dark matter cannot be falsified by a contradicting observation, because it (dark matter) hasn't yet been observed. But if it doesn't exist, then it will never be observed. So it, therefore, can never be falsified?
If I'm straw manning you, it is unintentional. How would you propose dark matter be falsified?
Dark matter as a particle has to have certain properties in order to satisfy what we know about a) it's EW interactions and b) the fact we haven't seen it yet.
Over the next 20 years or so we will likely cover a huge amount of the phase space of direct observation of dark matter like candidates assuming they weakly interact with the standard model.
If we observe it, great we now know a huge chunk more than ever before of the universe.
If we fail to observe it then we have to go away and explain the Dark Matter problem another way. Axions are my personal favourite here because I claim nature for her beauty is sometimes horrible. But this could (in the most scary scenario) potentially be non quantisable interactions between unknown aspects of the universe. At that point we need to further our understanding of potential graviton candidates to evolve our theories of spacetime.
DM isn't a solid prediction like an aether it's a statement of a big gap in our knowledge that has implications to asto and particle physics models.
The big problem is then CP violation (why more matter than not, real why are we here at all stuff). This can't be explained by the standard model and stuff we can see and touch as far as we know. So we assume that this huge amount of it that we can't explain must come from interactions with new particles somewhere in some way at some energy scale.
Physicists try to narrow down that phase space of where and how but a lot of this stuff makes the search for the higgs, look like cheating and watching where your parents hid the Easter eggs. There is scary little to directly go on so theoreticians have to get clever.
Edit:
There is also little to say that extra-SM CP violation _must_ be from the same source as Dark Matter astro candidates. The combination of the 2 fields is much more a car of. "Hmm we both have a missing piece in our jigsaw that has 3 of the same sides and a 4th we've never seen. That's odd let's look together to try and find it."
Modeling of the early universe to today relies on some unknown amount of CP violation occurring and we know we see it in particle physics which describes everything we see eat and touch. So why the amounts are so different (orders of magnitude different) keeps some scientists up at night.
Unfortunately a lot of the models try to remove DM completely from either a gravity or CP perspective and both have direct consequences as to what sort of stable structures in the universe that allow to form. And the models often conflict with other observations showing theyre no better.
It's asif Newton observed an extra solar comet and therefore decided the Principia must otherwise all be wrong. It's a big step to say shinny thing in sky makes the Apple not fall as we observe it.
We cannot simulate evolution of galaxies using equations of General Relativity. So at best the simulations are done using Newton mechanics with minimal corrections like accounting for finite speed of light. And often it is not even done using Newton mechanics per see, but numerical models use artificial constrains like a minimal distance between stars to avoid dealing with numerical instabilities.
Yet there is no proof that such simplifications or constrains are valid on the scale of a typical galaxy. Moreover, when people managed to use full equations of General Relativity, then it was possible to account the effects previously thought to require the dark matter hypothesis.
So until it is mathematically proven that the current simulations are valid approximations, I will remain very skeptical about the dark matter.
Like the GP, I'm not in love with DM hypothesis and wouldn't mind something else. I also used to think the same as you. But DM explains a lot of disparate effects, in ways that MOND does not.
The best-well known effect is the "missing mass" in rotating galaxies. Well recently we found galaxies that look like they have no dark matter in them - i.e. they behave as you would expect from classical gravity/centripetal force arguments [1]. So while you can tweak General Relativity to match some of the effects in typical galaxies, you would then expect the results to hold in all galaxies. The "outliers" are easily explained by Dark Matter - they just don't have it for whatever reason.
The simulations of Universe formation, sure have their limitations, but look much worse without DM.
Galaxy clusters look like they have unexplained mass in them. Crucially, this isn't only from observing their rotational speeds (so like galaxy rotations) but also from looking at the strength of gravitational lensing they induce. This is as compared to fluid-dynamics-y calculations of gas temperature, pressure etc.
Bullet cluster is another interesting case [2]. It is the result of a collision of two galaxies. In such cases, you expect (a) the stars to "fly through" each other, as they are basically point masses and only interact gravitationally, but (b) the gasses and dust to collide. In this case, most of the mass is in fact gas and dust, so it should be distributed in the middle of the cluster (having just had an inelastic collision). But in fact observed mass is where the stars are, further out of what looks like center of mass.
The point is that there is no need to tweak GR. For example, [1] shows how properly accounting for GR and using rather detailed data about Milky Way allows to explain the rotational curves.
But GR is complex enough to allow very different behavior depending on the initial conditions of galaxy formations. So it would not be surprising to see different scale of deviations from Newtonian mechanics depending on Galaxy history or age.
One more thing. It is not true that stars simply fly through. Already in the second-order approximation in GR there are forces that behave like Lorentz force of electromagnetism perpendicular to the line of movement. Granted this a small effect, but on the scale of millions of years it matters. That alone can explain a lot of subtle behavior.
I don't understand this: "Without such theory, claiming DM doesn't exist is simply stupid IMO."
You are saying that without another theory to explain something, the current theory must be true because ... How does this logic work exactly?
In the middle ages, there wasn't a theory that explained headaches, so dismissing the idea that we should cut hole in people's skulls to let out the evil spirits is "stupid"?
Dismissing the idea that lighting is caused by Zeus being angry is "stupid" because there is no other theory yet?
Theories are not necessarily absolutely true. They are models that describe and help us understand problems. In most (all?) cases they are approximations. We know that there are problems with them but they are the best tool we have to understand something. Until another tool comes along that works better, the current tool will sufficient.
We know that General Relativity is incomplete. It cannot describe the work at high energies or very small scales. It is useful as far as it goes but it is not a perfect match.
Quantum Mechanics does describe the world of the small and the hot, but we cannot apply it to the larger world. QD, too, is incomplete. We look for a new theory that can combine the features of both but it evades us, so far.
Dark Matter is a similar theory. It describes a world. It seems to describe this work better than other theories like MOND. Looking for actual dark matter will let us see if DM is a good enough theory or not. If another theory can be found that is a better match, we can switch to that. In the meantime, every time we continue to try to verify/disprove DM.
We've been looking for dark matter for a long time without success. Until we actually find it, I think it is premature to dismiss out of hand other theories that explain the same phenomena. I'm not sure MOND is any better than DM, but I think it is worth at least pursuing the idea that maybe DM doesn't exist and there is something about gravity that isn't fully explained by general relativity.
> Until we actually find it, I think it is premature to dismiss out of hand other theories that explain the same phenomena. I'm not sure MOND is any better than DM, but I think it is worth at least pursuing the idea
“Dark matter” is just a phenomenological name to whatever mechanism that might explain all the observations consistently. Under this banner, people have studied models of everything from axions to small black holes. Speaking as someone who’s worked in the field, nobody is “ignoring” alternatives to dark matter — if they could think of ways to make up new alternatives (leveraging MOND, or otherwise) they totally would do it — the problem is that most such alternatives raise more complicated questions like breaking the consistency our theory of gravity, or introducing bizarre quantum behavior of certain cosmological fields, etc. Maybe all those problems can also be solved — but nobody knows how! That’s why it appears like they “ignore” it.
Further, any new theory going to supplant dark matter needs to explain not just galactic rotation curves, but also beat the Lambda CDM model in its ability to model/predict details of the cosmic microwave background radiation. AFAIK, MOND or whatever else is not even close, when you try to put all that together.
> the problem is that most such alternatives raise more complicated questions like breaking the consistency our theory of gravity, or introducing bizarre quantum behavior of certain cosmological fields, etc. Maybe all those problems can also be solved — but nobody knows how! That’s why it appears like they “ignore” it.
To me as a lay-man, this seems like you are indeed ignoring non-dark matter solutions, as dark matter allows you to add various magical elements to explain up whatever is missing in other models. This seems very un-scientific.
> dark matter allows you to add various magical elements to explain up whatever is missing in other models.
It’s not like physicists are looking for every little possibility to be fanciful. Here’s the irony — The “DAMA” experiment has been claiming to have discovered dark matter particles for years, but most physicists don’t trust that the experiment is understood well enough to call it a victory. The process of “adding particles” is an extremely rigorous one (the various detailed checks and stringent requirements involved are unsexy content ignored by pop science). Adding particles is how we discovered the Higgs boson, the weak nuclear force carriers and the internal structure of the proton. It is a battle tested process that underpins the tremendous success of particle physics in the twentieth century.
> To me as a lay-man, this seems like you are indeed ignoring non-dark matter solutions
Seems like there is a massive disconnect with how one actually “does research”. Let’s say I completely agree with you and want to come up with alternative models, and I’ve read the literature — but don’t know how to solve the hairy outstanding problems. What do I actually do when I get up and go to work tomorrow morning? :-P
To me as a lay-man, it is unclear what you expect them to do instead. Which part of "nobody has come up with a theory that explains nearly as much as DM can explain" constitutes "ignoring"?
>but I think it is worth at least pursuing the idea that maybe DM doesn't exist and there is something about gravity that isn't fully explained by general relativity.
I don't think anyone is disagreeing with this. However, the current observations are best explained by dark matter. The various MOND models don't fit as well. We either actually detect and confirm dark matter or someone comes up with a MOND model that fits as well or better and has testable predictions.
When people deride MOND, it's not to say that it can't be an answer later but people who are pushing it now are all ignoring some gaping hole in their theory.
Continuing research is fine but, confidence wise, dark matter is in the lead by a lot.
Dark matter ratios are not free parameters, because galaxies did not form through independent processes. The evidence for a highly homogeneous early universe is overwhelming, and any dark matter candidate that required large-scale inhomogeneity would be rejected out of hand.
If they aren't free parameters then why do we assign arbitrary dark matter content and distribution to galaxies like the bullet cluster/ultradiffuse galaxies with "too much/almost no/no DM" to "make it work out"?
If dark matter distribution weren't arbitrarily assignable, why would reputable physicists suggest that some galaxies might have a dark matter clump at the center and not a supermassive black hole?
The evidence for a highly homogeneous early universe has basically been refuted, unless systematic difficulties in assigning galaxy distances (certainly a possibility) JWST is observing galaxies that are forming WAY too early for a standard gravitational model; MOND has predicted early galaxy formation for more than a decade now.
> If they aren't free parameters then why do we assign arbitrary dark matter content and distribution to galaxies like the bullet cluster/ultradiffuse galaxies with "too much/almost no/no DM" to "make it work out"?
Something has to explain why galaxies aren't all exactly the same right? Whatever other theory out there will have some "free parameters" that explain that difference won't they? The observable matter differs, why shouldn't dark matter?
I think you're being really cynical about the scientific process here. The process is a new observation comes along that's a bit out of line with the norm and scientists look at the parameters and see if Dark Matter is still a candidate to explain it. Do you think it's unrealistic to assume that different galaxies might have different amounts of dark matter (should it exist)? They have different amounts of observable matter. If there was some galaxy that just didn't make sense by just adding some arbitrary amount of dark matter, it'd put a real damper on the theory but that hasn't happened yet. As of today, the only thing that seems "off" about galaxies is the amount of gravity which is very simply explained by adding in some amount dark matter.
I think my intuition of that is probably the same as your, that it seems kind of ridiculous to just add some invisible matter to the equation but it does actually do a really good job of explaining a few different phenomenon. I suspect the process for the acceptance of dark matter initially began as a "these galaxies are behaving _as if_ they had more matter" and a lot of people trying to explain why that might be to no avail until they kind of just accepted that maybe there just _is_ more matter that we don't see. At least that's how it's been for me. I'm still holding out some kind of hope/expectation that there's a better theory out there that actually explains dark matter/energy in a more satisfactory way.
It's not rediculous to add invisible matter. This is how we discovered Neptune, and Uranus. But it didn't work for Vulcan and Pluto was super sketchy. But to scientist's credit they gave up on dark matter Vulcan when nothing was found, over and over again, and GR explained mercury's orbit.
The analogy would be if we didn't figure out GR, used dark matter to explain mercury's precession, then used dark matter to explain aberrant voyager probe kinetics, then rejected GR because "it failed to explain the weird motion of voyager"
Adding invisible matter is not crazy, but over time you make your model increasingly unfalsifiable as the observations fail to meet the expectations. Ideally that would decrease confidence in the model, but for dark matter it seems to have entrenched the conviction of mainstream astro.
No? Those galaxies are ultradiffuse galaxies and if their distance estimates are off by about 25%, which is reasonable -- we don't have good models for distances to such cosmic entities -- then the calculation for their rotations bring them in line with expected.
People were being a bit stupid when they dogmatically stuck with the idea that "the earth is the center of the universe" and had to come up with ridiculously complex epicycles and systems of motion to describe the movement of the planets, when really if they had just change their assumptions and frame of reference, it was quite simple to see the planets were orbiting around the sun rather than in some weird epicycle involving the earth.
The impression a lot of people have about dark matter is I think similar -- dark matter / dark energy basically comes up as an explanation for why theory doesn't match what we observe. Thus a lot of people assume there is simply a more reasonable explanation, like a hidden variable we haven't considered, that would result in a theory that matches our observations. When I was in high school physics this was my assumption as well. I think if that were really the problem, though, someone would have solved it by now. That said, never underestimate an entire school of thought's inability to see things differently.
> (b) We need to scientifically understand why the dark-matter based model, being the most falsified physical theory in the history of humankind, continues to be religiously believed to be true by the vast majority of the modern, highly-educated scientists. This is a problem for the sociological and philosophical sciences and suggests a breakdown of the scientific method [18].
There are few hot takes I agree with more. I don't care if the opponents of DM/DE are proponents of MOND, or proponents of something else involving pixies and turtles. We should continue to investigate DM/DE, but not with the religious fervor that we do today.
> Dismissing the idea that lighting is caused by Zeus being angry is "stupid" because there is no other theory yet?
It's not true that there are no other theories; here in Norway we believe that it's caused by Thor hammering away up there. Some say he's hammering away at dark matter...
> In the middle ages, there wasn't a theory that explained headaches, so dismissing the idea that we should cut hole in people's skulls to let out the evil spirits is "stupid"?
> Dismissing the idea that lighting is caused by Zeus being angry is "stupid" because there is no other theory yet?
I think you could have picked far better examples.
My general relativity professor claimed the need to postulate the existence of dark matter disappears when using basis vectors for derivations. To be honest, as the only undergraduate in the class, it was mostly way over my head.
Hopefully I'm not misrepresenting his views, I think the paper is DOI:10.1140/epjp/i2011-11032-x
Clickable: Goedecke - Global embedding via coordinate basis vectors; http://doi.org/10.1140/epjp/i2011-11032-x. I am skeptical of the idea that the need for dark matter can be "conventioned away", but, while I am a mathematician, I'm not even glancingly familiar with the physics.
I don't have access to the paper right now, but the abstract is off to a very bad start.
> any curved Riemannian space must be a subspace of a larger flat host space
This is not even wrong. Yes, you can always embed a Riemannian manifold in a Euclidean space. In fact, there are infinitely many ways to do so. But this is extra structure that you can choose to impose, if you want to - it's not intrinsic.
You might as well say that every string must be a substring of a larger "host" string.
> the 4D Riemann-Christoffel curvature tensor is identically equal to a geometrical tensor associated with the complementary subspace of the host space
Impossibly vague. Yes, sometimes things are equal to other things. What tensor, and associated how?
> Einstein’s field equations are automatically geometrized, with the stress-energy tensor expressed in terms of the contracted complementary tensor
This is a tautology. The Einstein field equations relate the Ricci tensor to the metric and the stress energy tensor, and the Ricci tensor is a contraction of the Riemann curvature tensor. This remains true even if you rename the Riemann curvature tensor "the complementary tensor".
But the deeper problem is this: the entire point of general relativity is that physics does not, in fact, care what coordinates we use. And this is built into the mathematical structure of the theory - there can't be any such thing as the "the coordinate basis vector approach to tensor calculus", because tensors (properly speaking: tensor fields) are, by definition, coordinate-independent objects.
Sounds like a misunderstanding, I’d recommend reading the paper before jumping to conclusions. Certainly we covered the various tensors mentioned here in the class I took - if you want details, you have to read past the summary.
General Relativity largely describes how geometry is nature - what may seem obvious from a mathematical perspective isn’t always so from a physical one. The results of using basis vectors for derivations aren’t riddled with artifacts of any particular coordinate system, as it would become immediately clear when expressing Christoffel symbols, for example - they wouldn’t be equal with other approaches.
One of the other students who was getting her PhD specializing in GR said that using a vector approach gave her intuition about concepts she had previously considered incomprehensible.
> The results of using basis vectors for derivations aren’t riddled with artifacts of any particular coordinate system, as it would become immediately clear when expressing Christoffel symbols, for example - they wouldn’t be equal with other approaches.
But the Christoffel symbols are artifacts of a particular coordinate system - the real object is the connection, of which the Christoffel symbols are just basis-dependent components.
I must admit I haven’t touched the subject for twelve years. It sounds like you have a vested interest in dark matter. I think you’d have better luck talking to the author or reading the paper.
The arguments against it sounded plausible at the time.
This Pavel Kroupa guy is popping up a lot across the futurist and credulous science fandom forums. To cut to the chase, his theory only applies to galactic dynamics and it doesn't even do a very good job at it. There are many other aspects that MOND doesn't address. Kroupa doesn't even bring interesting critiques of DM to the table, just claims about his theories and then concluding DM doesn't exist.
> To cut to the chase, his theory only applies to galactic dynamics and it doesn't even do a very good job at it. There are many other aspects that MOND doesn't address.
There are many more that DM doesn't address, or only addressed by extending it with parameters to fine-tune the result, a process which can also be done with MOND. I hate to keep posting this, but it's a good thorough review of the evidence for/against both DM and MOND, and DM does not fair as well as you imply:
May I get your thoughts on this idea of mine:
We all know the marbles on a rubber sheet experiment. Masses produce "dents" in spacetime that "pull" on other objects.
Couldn't it be that that these dents are balanced out by bulges between the masses, that would additionally "push" smaller masses (like solar systems) towards larger masses (like the centers of galaxies)?
In an experiment it would look like this: the rubber sheet seals the top of a container filled with water. Now if you push down at some point, the increasing water pressure will push up all around that point.
If you now added a marble onto that bulge, it would roll down a steeper angle than it would in the normal experiment ... just like if there was extra mass ("dark matter").
Sabine Hoffensteder had apparently the same intuition as you and set out to develop a theory of dark matter as being anti-gravitating matter. It didn’t work out…
> Because I had this idea that anti-gravitating matter could surround normal galaxies and push in on them. Which would create an additional force that looks much like dark matter. Normally the excess force we observe is believed to be caused by more positive mass inside and around the galaxies. But aren’t those situations very similar? More positive mass inside, or negative mass outside pushing in? And if you remember, the important thing about dark energy is that it has negative pressure. Certainly if you have negative energy you can also get negative pressure somehow.
> So using anti-gravitating matter to explain dark matter and dark energy sounds good at first sight. But at second sight neither of those ideas work. The idea that galaxies would be surrounded by anti-gravitating matter doesn’t work because such an arrangement would be dramatically unstable. Remember the anti-gravitating stuff wants to clump just like normal matter. It wouldn’t enclose galaxies of normal matter, it would just form its own galaxies. So getting anti-gravity to explain dark matter doesn’t work even for galaxies, and that’s leaving aside all the other evidence for dark matter.
She goes into a lot more detail in the video (the blog post is just a transcript), but hearing her describe the thought process is interesting because she spent a lot of time on this theory (I’m assuming after she got her phd), so you’re in good company for coming to this initial conclusion at least :)
Thanks.
I don't see what would clump, because it's not negative-mass matter floating between galaxies that I'm suggesting.
Just like entangled particles that are lightyears apart, when you measure the angular momentum of one, you know the other's because of conservation laws and not because of some hidden object nudging the particle the right way either.
Spacetime bulging the other way ("anti-gravity") would simply be a feature of spacetime to conserve some other property. In my example that would be the water pressure that wants to stay constant.
Some commenter below mention that this is like a waterbed. I like that analogy.
If you put a large marble on a waterbed, not only will it create a trough, it will also ever so slightly lift everything around it.
Analogies are great for explaining versions of complex theories, but they rarely prove or disprove the theory because they aren't a 1:1 mapping to the theory, they're analogous. Similar. But similar is not "is the same"
Has anyone in the MoND world looked into the James Webb observations that seem to contradict our current theories of galaxy formation in the early universe?
ie the observations seem to show more mature structures than predicted, we're finding.
I've been wondering if some MoND / non-DM kind of theory may better explain these new JWST observations.
- I'm not anything close to an astrophysicist, so give me some rope :-)
Specifically this paper[0] is mentioned as predicting it
> Remarkably, the data roughly follow the green line, which is an L* galaxy magically put in place at the inconceivably high redshift of z=10. Galaxies seem to have gotten big impossibly early. This is why you see us astronomers flipping our lids at the JWST results. Can’t happen.
> Except that it can, and was predicted[0] to do so by Bob Sanders a quarter century ago: “Objects of galaxy mass are the first virialized objects to form (by z=10) and larger structure develops rapidly.”
> Galaxies seem to have gotten big impossibly early.
A MoND-like theory that allows the strength of gravity / inertia to vary over time can explain that. Granted, that sounds farfetched, but then, so does DM. Of course, it's easy to spend money looking for DM, and hard to spend money looking for a better theory of gravity.
Mond doesn't need that to explain early galaxies. When the matter is sparse gravitational contributions are weak and thus overwhelmingly in the O(1/r) regime and not the O(1/r^2) -- this makes gravity "seem stronger", and so stuff clumps together faster.
I believe 99.99% of new discoveries in astrophysics create more problems for cosmic models than they had before. Pluto shocked the scientific world in 2015 when we got up close photos of what was supposed to be a cold frozen stone, being so small and so far from the sun, but it's still geologically active, something no model could account for at the time, nor since afaik.
Adams et al., authors of the preprint linked in that blog entry, are all well-known astronomers. Adams is a research associate and Observational Astronomer at <https://en.wikipedia.org/wiki/Jodrell_Bank_Observatory> specializing in high-redshift galaxies, and is in several large-telescope international collaborations (ESO, SKA, MIGHTEE, LADUMA).
The punchline: those seemingly super-far-away galaxies are getting much much closer as JWST's calibrations proceed according to plan.
I would like to toss my own idea into this discussion. This idea incorporates a mechanism that would explain why gravity might slightly deviate from Newtonian and/or GR in some circumstances.
This paper also adapts GR in such a way that it is consistent with galactic rotation rates, the anisotropies of the CMB, and cosmological expansion -- while showing that the simple operation of gravity is the cause of each of these phenomena.
Cyclic Gravity and Cosmology (CGC) predicts that there are discrete specific sizes allowable for macro-objects. The instability of Bennu and the fact that it behaves more like loosely held scree rather than a compact mass -- is an example of a mass that is not exactly at one of the discrete allowable sizes. Please also refer to the link I included wherein I uploaded a video simulation of the formation of a solar system using this type of force law. (This is in section 18 of the paper)
I would greatly appreciate any comments on this idea. Copies may be downloaded here:
I have personally no problems with epicycles. Those were effectively the Fourier decompositions of the elliptical orbits while viewed from the Earth. At some point we found a simpler and a more predictive theory and that naturally produces the right shapes (kepler laws + classical gravitational dynamics) and we choose that instead.
I think many people would be happy (myself included) if there is theory that wouldn't require dark matter and dark energy. But at the moment there is none and to me (and many astrophysicsts) the universe with dark matter is the best description we've found so far of what we observe.
from the angle of "explaining observations" a fundamentally incorrect theory can work well. but it breaks down as soon as you interact with the object of the theory in new ways. in epicycles' case that means traveling to a satellite that doesn't orbit the earth. usually the theory is disproven before it kills people, but you know, possibility's there
I don't believe in fundamentally correct theories. All theories have limitations. They work until they do not. Is general relativity fundamentally correct ? I wouldn't call it that (others may disagree), because it doesn't explain quantum effects or very early universe.
So to me theories are just tools that explain what we see and predict the future.
I'm with you - "All models are wrong, but some models are useful".
I do think dark matter is tantalizing because it hints strongly that our model needs a fundamental change in understanding at some other level. It just feels like too much wall paper to be adding to the model in an attempt to cover the holes.
Doesn't mean I find any of the current alternatives more viable then our current models.
exactly, so when we attempt to put general relativity into practice at the limit of its predictive capability, we may see something is very wrong, then extrapolate a better model, and a few decades later everyone will look back on general relativity as an example of science's ability to carry us away from grossly false beliefs. as with epicycles. and maybe dark matter.
Yes there are facts. I.e. there is a particle called electron, there is a Sun, Cosmic Microwave background, Milky Way galaxy. I'd say those are facts. But I do think the boundary what's a 'fact' vs model/theory dependent statement can be different among different people.
I treat them as true because they are true. I guess there is more than one definition of truth.
But I think what you mean that you treat some theories as axioms (temporarily, I hope). It's a great aid in thinking but a great source of confusion for newbies as well.
There are absolutely multiple definitions of truth. "1 + 1 = 2" is not true in the sense that "Paris is the capital of France" or "Water boils at 100C" are true.
I think the OP's point is that we should understand our current models of natural phenomenon to be just that, models. Outside of their predictive capabilities we should remain humble about how much "truth" they contain and be skeptical of their ability to tell us what's "really going on".
Kepler and Newton's models produce better predictions over a long period than Ptolemaic models, which is great. But it's a bit of delusion to think that necessarily means they are analogous to the cosmological mechanisms that produce orbital mechanics.
Or dark matter - the new germ theory of disease. Both introduce a new, hard-to-observe-with-current-technology entity which explains observations better than any other theory we've came up with. MOND doesn't explain structure formations or lower rotational speed galaxies; miasma didn't explain non-airborne illness.
It's incredibly hard to know until we've either observed DM particles or made good alternative hypothesises which also fit the data.
Dark matter sounds suspiciously like aether to non-experts. I’m not saying it is, but do recall that the concept of aether filling the universe was an immensely popular one prior to the turn of the last century, so not too long ago. We don’t believe it any more, it’s hogwash of course.
Aether seemed to be necessary conceptually because the theory of electromagnetism was a theory of waves, and no would could conceive of waves except in some medium, hence aether. Einstein's breakthrough was to show that the concept of aether was not necessary to explain empirical observations about electromagnetism and the speed of light.
Dark matter, on the other hand, is not a conceptual necessity but an empirical one. Whereas aether was something people clung to because they couldn't imagine a world without it, dark matter is something scientists have been forced into even though the world would be more conceptually elegant without it.
The luminiferous ether theory met the basic criteria of being testable and falsifiable, hence the Michaelson-Morley experiment. Isn't that the basic objection to dark matter, that there's nothing in it that's falsifiable?
The reason we teach the Michaelson-Morley experiment in basic Physics classes is because it's a bedrock example of Science functioning correctly. The reason people believed in the ether theory is because in the wake of Maxwell's revolutionary work, there needed to be an explanation of how light propagated that could be reconciled with Newton's mechanics which had been laid out 200 years earlier. In hindsight, we credit Einstein with finding the way out, but in the late 1800's it was in no way obvious that it was going to work out that way.
> Isn't that the basic objection to dark matter, that there's nothing in it that's falsifiable?
I don't think that's case, at least to me it seems that the basic objection is some vague discomfort with the idea, not an issue with the falsifiability. And dark matter is clearly falsifiable, it has already survived many observations.
The CMB power spectrum would look entirely different if there was no dark matter.
Gravitational lensing in certain areas in the sky wouldn't look as strong as it does without dark matter either.
Until Michaelson-Morley’s landmark experiment, we hadn’t found a way to falsify aether either. Is there physical proof that dark matter is not falsifiable? If not, we still have a bit further to go.
There is always a tension in Physics between physical proof and observability on the one hand, and having the right idea, the right intuition on the other.
Going back to my Einstein example, there were other Physicists who contributed to Relativity (Lorentz, Minkowski, Poincaré, Fitzgerald) but we give Einstein the most credit because he had the right idea, that it's not enough to treat the speed of light as a constant in your math, you actually have to believe the Universe works that way. One of the stories that gets told is the reason Einstein never got a Nobel prize for Relativity is because the Nobel committee just wasn't ready to give it up and credit Einstein with discovering a new, fundamental truth about the Universe.
Another story that gets told is that when Einstein was an undergrad at the ETH in Zurich, Minkowski was one of his professors and he dismissed Einstein for being a "Lazy Dog" because the math never came easy to him. But he looked at how Physics was taught at the time and knew something had to change. And he was right about that.
I'll just leave this here, Feynman explained it better than I ever will.
Dark Matter in totality is hard to disprove (as it's really not one theory, but a rather large set). But large parts of that parameter space is testable, and is tested. See WIMPs for example, where we searched extensively. Have we ruled out everything? No. But we have ruled out many variants, mass and coupling ranges.
Michaelson-Morley didn't falsify aether. At the time, their results were interpreted within the aether framework in terms of compression of the aether yielding the Lorentz transformations.
We absolutely do not have an aether! Light does not travel through a medium like ripples in water.
The aether makes specific predictions. Like the fact that we're all traveling through it because it is a background medium. These were clearly refuted.
Fields are not aethers. Fields do not have a reference frame.
Lots, and more than we can afford, assuming particle colliders could even produce it.
It's popular to look for types of Dark Matter that interact very weakly with forces other than gravity (WIMPs). That's a lot like the joke of the man searching for his keys under a street light even though he dropped them away from the light, because the light is better there. It's perfectly possible that Dark Matter ONLY interacts gravitationally, and so will never be detected by any of the current approaches. Detecting the gravitational force of an elementary particle is ridiculously difficult, well beyond the sensitivity of any current detectors. Even if we could, there would be a huge noise floor from all the normal matter around!
I thought that a purely gravitationally interacting dark matter would not produce the right clumping, i.e. there must be an additional force -- either one of the known ones, or a new one.
True, a pure GIMP alone can't do it. But they could account for most of the observed effects of Dark Matter, meaning we'd be very far from ever detecting any. And an extra force (if one exists) might interact with Dark Matter but not to any detectable degree (or at all) with normal matter, leaving things in about the same position (you'd never produce Dark Matter from colliding normal matter).
Germs are observable _today_, because we have powerful microscopes. For most of its existence, germ theory relied on unobservable entities, and it had to be supported by the fact that the theory fits the data, not through direct observation. Similarly, dark matter is observable in principle, just not with our current technology.
I don't know which aspects you think dark matter shares with miasma. Dark matter fits all the data; miasma doesn't.
miasma largely fit the data for a millennium, the same way dark matter largely fits the data. if you believed in germs in the 1600s, which is about where we are now with dark matter, you would have been considered insane, so i get where you're coming from.
the fact that dark matter is widely accepted without proof, and that the current model continues to encounter new anomalies, puts it about on the same level as strings for me—very cool and i hope it's real, but likely to be explained away when we better understand quantum physics and higher dimensions
Sorry, but you're just wrong. You can't look at a disease outbreak and explain the transmission among the population with a miasma-based statistical model. In other words, it doesn't fit the data.
And dark matter is not accepted without proof. It fits the data extremely well. That is the proof. It explains many unrelated phenomena and isn't ruled out by anything; just like gravity. The fact that we can explain why things fall down, why the moon orbits around the earth, why the earth orbits around the sun, why stars form, why planets form and a whole bunch of other phenomena with one force is the proof; we haven't seen a gravity particle, but that doesn't matter, because that's not how proof works in science.
It's not about thinking it's cool and hoping it's real either. It's simply the only explanation we have for a whole bunch of stuff, and it's a damn good explanation that's extremely simple.
That's not to say it's necessarily correct. It's just the only explanation we have, and we have no reason to believe it not to be correct.
in AD 700 one would absolutely have been able to explain an outbreak with a miasma-based model, and the explanation absolutely would have become consensus among the best and brightest. you can't apply what we know now to what was known at a given point in the past
> we can explain why things fall down
that's pretty new too! and it's not semantically correct, but was considered so for quite a while. this is a pretty good analogy for why i'm skeptical of dark matter
> It fits the data extremely well. That is the proof. It explains many unrelated phenomena and isn't ruled out by anything;
Suppose you had six frames of a ball moving in a parabolic trajectory. A sixth order polynomial would fit the data incredibly well, but I don't think you would accept it as a good model for the motion of the ball.
You can't say "it isn't ruled out by anything":
EFE rules out LCDM.
Galaxies are redshift >~ 7 rules out LCDM (we now have galaxies at redshift > 13).
> Both introduce a new, hard-to-observe-with-current-technology entity which explains observations better than any other theory we've came up with. MOND doesn't explain structure formations or lower rotational speed galaxies
DM's failures are more numerous, and it has to be extended with new assumptions to account for other observations. MOND's predictions fair better as favourable evidence actually [1].
Epicycles aren't wrong. In fact, the Copernican model of the solar system is less correct than the Ptolemaic epicycle model that was being used because Copernicus still viewed orbits as circular.
More to the point, since epicycles are just a Fourier decomposition, they're actually perfectly correct. The problem, though, is that they are a nightmare to compute with unless you have a modern computer.
Once Kepler decided to use an ellipse to model solar orbits; however, things were vastly easier to compute. At that point, the epicycle model got swiftly pushed aside.
Epicycles aren't wrong, they're not even wrong. You could model an orbit that looks like homer Simpson's face with epicycles, but the ancients weren't sophisticated enough to understand that. The problem is that the technique of epicycles this has diminishing explanatory power, as you add more free variables (how many epicycles you are "allowed" to have.
The reason why LCDM gets called epicycles is that there is charitably one free parameter per galaxy (dm: bm ratio -- but actually more, because it's an arbitrary distribution)... So the fact that it has high explanatory power is unsurprising. And what's should be dismaying is the number of things it can't explain, given just how many free parameters you're allowed to have in a DM model.
Not a physicist, but it feels more like the orbit of Mercury. Newtonian physics didn't quite predict it correctly, but it was still the best theory available until relativity came along. That seems to be the thrust of the GP comment: "you got a better idea?"
This is a constant bug/feature of human thought. When we try to see beyond the limits of our technology we find it irresistible promote explanations that can't be proved yet. Often these placeholder theories turn out to be complete nonsense (epicycles). Often they are incredibly convincing (Dark Matter), and sometimes they even turn out to be mostly correct - so this process is generally a good thing.
Where we run into trouble is by assigning too much credence (or too little skepticism) for that which has not been proven yet. This can be counter-productive as it may stifle research into other areas (in this case gravity). When we take the "best current theory" and deride skeptics of it to the point that it discourages other areas of investigation, that's a bug, not a feature.
Our current understanding of gravity is woefully inadequate, as shown by our inability to reproducibly measure the gravitational constant as well as we should be able to with current technology. The flyby anomaly is another example where DM does not help. I hope that the current popularity of DM does not stifle research into gravity.
Epicycles had low predictive power and was needlessly complicated compared to later theories, but it wasn't "nonsense"; the predictions the theory made about the relative positions of the Earth and planets were pretty accurate.
And thanks to our knowledge of Fourier Series we could come up with an even more accurate epicyclic model today! It's only when you try to explain tides or Lagrange points that a geocentric epicyclic model falls apart.
I feel like scientists today have forgotten a fundamental truth about science and understanding: We can never _know_ when we are right about our theories. We can only ever _know_ when we are wrong about them. New evidence can arrive at any time, and that evidence will either bolster our theories, or prove one or more theories incorrect. New evidence is always arriving.
I mean do we really believe that we know everything about this? Somehow, we A) know we have large knowledge gaps, AND B) believe that we understand things we have unanswered questions about.
We have a LOT of confidence about dark matter and exactly zero direct evidence.
It seems much more likely to me (I am not a scientist) that we are wrong about things that we assume to be hard facts and those error(s) have artificially produced the need for dark matter in order for things to make sense.
> We can only ever _know_ when we are wrong about them.
I think this is what the OP is saying though: There are many ideas about what dark matter is or could be, but astrophysicists model the early universe using only baryonic matter, the results _are wrong_.
As a theory, it's quite simple: in its absence, the physics doesn't work. If you add matter that behaves in a particular way, simulations align to observation.
What is dark matter? shrug I don't think we can call any answer to that question a theory yet, we don't have any way to falsify it. We have many hypotheses, but those aren't theories.
> There are many ideas about what dark matter is or could be, but astrophysicists model the early universe using only baryonic matter, the results _are wrong_.
That doesn’t make any particular theory correct. It is only proof that our current model is wrong.
Filling the gaps with math, and calling the math “dark matter” does not make dark matter correct; one or more of our existing models is wrong.
The name dark matter is not a specific thing, rather, it's a collection of hypotheses whose characteristics are bounded by results in high energy experiments and astronomical observation.
> As a theory, it's quite simple: in its absence, the physics doesn't work. If you add matter that behaves in a particular way, simulations align to observation.
They actually don't, numerous extra parameters have to be added for DM to account for observations. DM was invented to fix one problem with observation, and 15 more have cropped up which don't fit.
See the paper [1] and the discussions of the times DM has been falsified. Extra parameters were then added to account for the new observations, where MOND makes certain parameter-free predictions that match observations. Clearly neither theory is a perfect fit for observations but the strong preference for DM seems unjustified.
It’s not shtick, it makes perfect sense to me - unlike your post which has no substance.
Group-think does not equal truth.
Just because there is no compelling alternative theory does not make the Dark Matter Theory correct.
It’s probably nonsense, like many theories in Physics such as the Big Bang. Our level of intelligence and understanding, at the moment, is simply too small to comprehend the mysteries of the Universe that we live in.
Does this guy also reject DM models based on MACHOs?
I think they expect particles to be thrown out and result with slowdown but dark objects as massive as stars wouldn't be thrown at such a huge rate to observe slowdown, am I right?
People actually believe that there is an yet-undiscovered substance that has mass but interacts very weakly (or not at all) with light, and that there are vast quantities of this substance in the universe. They call this "dark matter".
The former will tend towards a defensive attachment to an idea which leads to, as Planck put it, "Science [advancing] one funeral at a time." The openness of the latter leads to less certainty, but greater ability for progress.
Consider the heliocentric/geocentric argument some centuries passed. Contrary to pop-science portrayals this wasn't an issue of Man against Church. The heliocentric view initially had numerous flaws and invalid assumptions. But because scientists of the time genuinely believed in the geocentric universe, they obsessed on these flaws and trying to use those to discredit the view, instead of impartially considering the idea and seeing if it could be patched and whether it would ultimately make more sense.
The "right" answer, with negligible refinement, can often look much worse than the wrong answer which has had decades, if not centuries of mass refinement. And that can drive those with a "belief" to the contrary, to want to attack it. The analogs outside of science are as endless as those within it.
QI (quantized inertia) is a theory created by Mike McCulloch in the UK. It predicts galaxy rotations, including wide binaries extremely well, amongst other things. It is well worth a look.
Why? Because you say so? With a dismissal like that you ought to elaborate with some evidence. Especially since you seem to be attacking the man, along with his theory.
> * The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations.
The stories we like to tell require dark matter. Otherwise our stories are wrong
> * There is a broad variety of Dark matter models that are consistent with simulations.
We have a lot of stories using dark matter. We really like it
> * It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO.
Our stories are just stories but it is all we have
> * There are a few cases where you can find tensions with the existing DM based paradigm.
Our stories are not internally consistent.
> * A final point. Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background.
We really like stories with dark matter
I love Astrophysics! Pure math with constraints. Do not confuse it with reality
All theories are stories. All communicated information is stories. Stories is just a word, it has pejorative meaning in context. Bible Stories are not like physics stories. A lot of physics stories are subject to test, null hypothesis, disproof.
The stories which persist are useful. Calling them theorems or theory helps us rei-fy stories into propositions which can be used to do science. Science is about modelling things, understanding things, and testing things. It is a valid proposition to argue DM is defined by stories which remain untestable, but the proposition there are better alternatives ignores that to construct stories without DM which are better will demand addressing the problems DM has to exist, to define the blank spaces in other, testable stories. You think Absolute zero exists? It's just a story. We haven't got there, we got close, we observe what happens, but this mythical zero point.. What good is it, if we can't get there? I love temperature, the whole thing depends on a fantasy...
Do not confuse your projection of "what is reality" with reality. I love HN, where people make asserts which don't mean what they think they mean!
I'm not an astrophysicist. But I note, had Oppenheimer not died before it was confirmed at least one of his theories predicted black holes. He would have got a Nobel on it.
To me, thats a strong indication the fundamentals of mass, energy as modelled, were projected in advance of radio-telescopes confirming the theory. isn't that what Astrophysics does?
Perhaps there are better examples. Personally, I find the main sequence circular logic reasoning, but its a model for spectral colour, distance, age and size. I'd kind of like it if we flew a mission to hold a Pantone colour wheel up close, and measure the damn thing properly for size and colour but age is very hard. You're back in models of decay of mass/energy outcomes and how we believe half-life in C14 analysis.. except done as spectral lines at a distance based on one solar mass we have close by to model things on. Plus, we don't know how to make pantone colour stable across 20,000 years of flight time to measure a star up close and by then Humans will have 29 eyeballs seeing into the infrared like mantis shrimps.
Gravitational lensing is pretty cool. I think it was initially theoretical, and it was a long time being proved.
Gravity waves defy detectors. If we can get detectors working that will be immensely cool. I may have stepped off the scienting engine by wanting an outcome, which arguably is not aligned to null hypothesis testing. Maybe we can't detect gravity waves and they remain theory?
Look, its rude. I apologize in advance. I do kinda think you're trolling. Are you trolling? Maybe Dang will be upset at my asking. It's ok to ignore.
Wat... "Like" implies preference. You're using the word "story" to imply someone pulled it out of their imagination. What they said was "we currently have no plausible theories that doesn't require dark matter". These theories have to be consistent with other theories and observations.
If they don't agree with the hole in the other theories which use DM to reconcile observations, thats an issue worth exploring. If they just don't like the DM label on those holes, its a fine point. What do you want to call the emerging shape in theory which is a hole, in otherwise very fine, testable propositions?
No, but we have a theory that predicts several things, including something like dark matter. It has been tested for several of it’s large scale predictions involving the movement of matter. Now we are testing it’s prediction of physical dark matter.
We do know that dark matter, as constituted, in all its variations, requires observations grossly incompatible with what we, in fact, find.
Pop science cosmology presentations never, ever mention these contradictions. But it is, exactly, embracing contradictions like these that would make it science, not religion. Presentations without them actively mislead the public as to what science is and is for.
Trotting out details found (or made) consistent with DM amounts to indulging confirmation bias. An honest presentation describes the current favored theory, which summarizes a collection of observations, and then whatever is inconsistent with it. Sweeping inconsistent facts under the rug is the opposite of science.
Every substantial advance has come from embracing contradictions of current favored theories. Young people are inspired not by sweeping, broad-brush stories of how the universe is, but by mysteries that remind us how much is left to be discovered.
Biologists understand this. It is very easy to get a biologist to say "nobody knows". Every non-moribund field revels in what is still wholly unknown. Only cosmology circles the wagons against contradictory evidence.
I don't think i have ever heard anyone claim we know for sure. I have heard lots of physicists claim that its the best theory they are aware of given evidence they have seen, which is a really different claim.
> Biologists understand this. It is very easy to get a biologist to say "nobody knows"
Expressing flaws doesn't require a better theory, no. By all means, do that.
Expecting others to give up their theories that work well for 99% of the problem, or even only work well for the 10% they care about, just because there's a problem with it DOES require something better AND a reason to switch.
For example, architects (technically stability engineers. Architects don't really design buildings except in the artistic sense) are still simulating Newtonian dynamics on plates that fly through empty space held up by nothing. You want them to change? No problem, but you'll have to make something better and point out why they'd want to use something better ... but, truthfully, it works pretty well.
The piece says, thing X doesn't exist, here's how it was already figured out, etc. We should find something better. I don't think it says we aren't allowed to keep it around when it is sometimes useful as a thought device.
It also reduces the religious aspect, demoting DM to simply a tool used in specific situations, "we don't have anything better at the moment."
What if the "dark matter" is actually ions not observable from our telescopes? Plasma Physics hold ions permeating the Universe as a premise & has physical experiments reproducable in the lab to back up their claims...And can use classical EM equations explain how the stars & galaxies work instead of some novel Physics which needs constructs such as "Dark Matter" to balance the equations...
> What if the "dark matter" is actually ions not observable from our telescopes?
We have radio telescopes that can see all the way down to a few tens of MHz. This is far colder than the CMB, and certainly much too cold for the intergalactic medium.
> Plasma Physics hold ions permeating the Universe as a premise & has physical experiments reproducable in the lab to back up their claims
Yes, plasmas exist. There are lots of them in space. This is completely uncontroversial.
> And can use classical EM equations explain how the stars & galaxies work
Absolutely not. Galaxies are firmly in the nonclassical regime.
> We have radio telescopes that can see all the way down to a few tens of MHz. This is far colder than the CMB, and certainly much too cold for the intergalactic medium.
PV = nRT
What if the P is very low & V is very big...like it is in space? What would the plasma in dark discharge radiate?
> Absolutely not. Galaxies are firmly in the nonclassical regime.
According to some models. Do these models have reproducible experimental evidence? If not, I don't see how you can credibly be so sure of yourself...
Sure, exotic & speculative math is way more fun than reconciling with physical reality. Without reproducible experimental evidence, the model is non-falsifiable (& non-provable).
In contrast, the Plasma model does have physically reproducible experiments that are able to create galactic phenomena in the lab. Live physical experimentation...not computer simulations using math that has never been verified by physical experiment.
* DM particle haven't yet been found.
* The current best theory of the structure formation and galaxy evolution involves dark matter and critically depend on it to reproduce observations.
* There is a broad variety of Dark matter models that are consistent with simulations.
* It is possible there is some theory that somehow explains everything without the need of dark matter, but it doesn't exist (now), and very likely it would work effectively like dark matter. Without such theory, claiming DM doesn't exist is simply stupid IMO.
* There are a few cases where you can find tensions with the existing DM based paradigm. I.e. the is test pointed in the article involving galactic bar in the paper "Fast galaxy bars continue to challenge standard cosmology". Note the toned down title. In this paper they just compared one specific feature of galactic disks in the simulations to the data and show that it doesn't match. I'd argue there are many reasons that could be the case that doesnt' involve killing DM. The same applies to other tensions.
* A final point. Even Modified Newtonian Dynamics theories require DM, because without it you cannot form enough structure early in the universe (as dark matter start to collapse earlier) and is essential to reproduce the amount of structure we see in the cosmic microwave background.