In medicine, they often call it a "syndrome". A collection of symptoms that together form a known pattern that is often closely associated with a disease or disorder but has no understood cause or origin (although some syndromes continue to be called that after they are better understood).
How do you think we arrived at terms like "planets"? We noticed that some stars in the sky moved around while the rest didn't, and we called them "wandering stars". We still call them "atoms" even though that word means "indivisible".
Weather is full of such names: rain, thunder, lightning, wind, morning glory, dew, tornado, rainbow, etc. All these words for material phenomenon existed long before we knew how they worked.
So what's wrong with "dark matter" as a name?
>>That is, an interrim scientific theory that will eventually be completely replaced by revisions of our physics.
How about "missing matter"? To non-astronomers, dark matter sounds like it has already been found and it doesn't emit light.
It's a bit late to suggest new nomenclature for something that's been around for many decades.
it encodes a normative, biased prejudgement about what we think explains the syndrome.
Until the point where the observations are multimodal. Then I'd be more comfortable assigning provisionally normative names. Till then it should probably explicitly reference the fact that all observations are gravitational in nature.
It needs to be "matter" in the cosmological sense [1] because of the ratio of protons and neutrons to photons produced during Big Bang nucleosythesis. We know the ratio of the number of protons to number of photons. We know the number of photons in the universe (most of them are in the CMB), and we therefore know the number of protons. That means we know the amount of "normal" matter in the universe, and it is far less than the total amount of matter in the universe (that must have survived from baryogenesis). This requires there to be "dark matter" in the universe.
The other effects like galaxy formation and stellar motion in galaxies are just things that might be caused by dark matter (though that's come into doubt).
Sean Carroll discusses it here: https://youtu.be/tZQadPmTd84?t=4642
[1] "Matter" in the cosmological sense is "stuff" that moves slower than the speed of light, as opposed to "energy" which moves at the speed of light.
X-rays.
Black hole.
Dark matter.
I think it’s going to stick as a name even when the details have been largely worked out, especially considering the other examples I listed.
In any case, it's a placeholder, not a specific theory. There are many candidate theories, but so far they're either untestable or the tests have had negative outcomes.
In the Multiverse/Multidimensional universe theory, all of these dimensions aren't necessarily in different universes. Some are of different dimensions, laid over top of ours, but we can't see them because we can't perceive the dimensions they exist in. The only dimension that bleeds into ours, that we can track so far, is gravity. This is what we call "Dark matter". It is actually gravity leaking over from an "Nth" dimension laid over our own that we can't perceive.
Again, as to the veracity I can't say. And I wish I remembered to author, but I don't =/ maybe I'll look it up later and make an annotation.
[1] https://www.twitter.com/seanmcarroll/status/1371141258236764...
No, we don't. We have observations that show that the motion of stars in galaxies is not what we would predict if we used our current laws of gravity with a distribution of matter that matches just what is visible to us.
There are two possible hypotheses to explain these observations:
(1) There is matter present that is not visible to us;
(2) Our current laws of gravity are wrong and need to be modified.
"Dark matter" is basically a name for #1; "MOND" and similar models are in category #2.
You are mis-stating the real issue here.
The distribution of matter is already a free parameter in our current theory of gravity, General Relativity. GR in no way requires that all matter be visible to humans.
The "no one has been able to find" thing is not a problem for GR, it's a problem for the Standard Model of particle physics, which does not contain any particles that dark matter could be made out of. But most physicists already believe that there are more kinds of particles than the SM describes, we just haven't figured out how to make them in accelerators so we can study them in a controlled experimental fashion, and we haven't figured out any "smoking gun" signature that will tell us if such particles exist out there in the universe. "Dark matter" is really just a name for those kinds of particles that are already believed to exist anyway, we just can't detect them with the technology we currently have. But the technology we currently have is very limited.
So the real question you should be asking is, what do you think is more likely, that there are particles that aren't included in the Standard Model of particle physics--which is already believed to be the case anyway--or that General Relativity is wrong in its description of the dynamics of galaxies, even though this is a domain in which GR has already passed stringent experimental tests with predictions accurate to many decimal places?
I'm going with the former.
What do you think is more likely, there is a theory that makes all observations look exactly like there is invisible matter without actually being invisible matter, or that there is invisible matter? I am going with the latter.
I also do not understand why laypeople are of the opinion that all massive particles must interact electromagnetically. If you are open to the idea that there is a massive particle that doesn't, you got dark matter.
It’s surprising how often this topic pops up. I think it’s based on a misjudgment that simply missing something in gravity follows Occams Razor better but it actually seems to be the opposite.
I think the killer is that dark matter seems to not just hold galaxies together (given their “too high” rotational speeds), but also clump together, and also sometimes be missing in galaxies and other times overwhelm galaxies.
It’s super hard to adapt the well proven theory of gravity to especially localized phenomena.
If something like MOND gains popularity and there’s no longer a need to postulate invisible matter, it would be fair to say that dark matter is incorrect and outdated, just like ether or phlogiston.
No, it doesn't. See my response to your other post upthread about the two hypotheses. "Dark matter" is one hypothesis; "MOND" and friends are the other.
MOND is characterized by an interpolating function between its fundamental constant a_0 and acceleration, such that in the "deep-MOND" regime, F = m \frac{a^2}{a_0} rather than Newton's F = ma.
Both MOND and Newton work in everywhere-vacuum, and in everywhere-vacuum perturbed by a point mass. Neither the vacuum case nor the point mass case qualifies as a "dark matter theory" any more than the vacuum Schwarzschild solution of General Relativity does. These are model universes where one can calculate a Gauss law for gravity.
MOND would behave differently for deep deep extragalactic space with small icy body moving around a large gassy body than Newton would, or than we would get from e.g. a roughly Einstein-de Sitter solution.
Milgrom has pursued several relativistic corrections to the simple MOND formula, and they generally allow for solutions where the stress-energy tensor is everywhere 0, and everywhere 0 except for one point. Milgrom originated MOND. He has collected numerous thoughts and many links at http://www.scholarpedia.org/article/The_MOND_paradigm_of_mod...
When applied to large systems like galaxies and especially galaxy clusters, Milgrom's corrections may be insufficient, and other corrections (catalogued well in https://arxiv.org/abs/1112.3960 section 7). Several of these are very hard to make work with the stress-energy tensor everywhere zero, and those (certainly not all of them) one might ungraciously call "just another dark matter theory". On the other hand, several of these theories admit plausible gravitational radiation in the total absence of matter, just as does General Relativity. (Gravitational radiation is not a feature of Newton or the simple original Milgrom formula.) McGaugh runs tritonstation and is a well known sceptic of particle dark matter.
Relativistic dark matter theories deliberately introduce nonzeros into the stress-energy tensor. You need a distribution of dark matter and trace out how that generates curvature, or you don't have a (relativistic) dark matter theory. You can also of course set out a distribution of cold dark matter and see how it behaves under F = ma or F = m \frac{a^2}{a_0}. Neither MOND nor Newton requires dark matter; likewise neither MOND nor Newton is incompatible with dark matter. Likewise, General Relativity doesn't require dark matter. But one gets more physically realistic solutions at a wide range of scales when one adds dark matter to a defined large-scale distribution of matter and radiation. MOND or the various attempts at generally covariant Milgrom gravitation would also behave differently when one adds dark matter to a large-scale distribution of matter and radiation. That is unlikely to produce more physically realistic solutions to a wide range of scales, however.
Indeed, Milgrom et al's most concrete argument is that their formula for gravitation is worse if one adds in a distribution of dark matter, and that their formula is better than e.g. a Lemaître-Tolman-Bondi solution equipped with a distribution of cold dark matter. That's as much a non-dark-matter theory as I think it is possible to have!
As in computing, naming things is hard.
The redshifting things are mostly molecular gas clouds which we would expect would orbit in a Keplerian fashion given the overall distribution of luminous matter and occluding/extinguishing dust visible in galaxies. Astrophysicists love hydrogen gas clouds because they ionize easily from background emitters (stars, quasars) producing very sharp spectral lines, and you can pick up their changes by looking on different sides of an edge-on spiral or are at nearly perpendicular angles to the rotational axis of spheroidals. For giant ellipticals, which have practically zero axial rotation, an instrument that gets a sweeping view of them will see a more chaotic red-and-blue shifting of the Lyman-alpha hydrogen line, rather than an obvious gradient between advancing and retreating edges. (Face-on spirals often also have gas clouds on excursions back and forth in front of the disk plane, where "forth" means "towards us" means "relatively blueshifted" and that's hard to explain without a halo rather than a ring of dark matter).
So really it's a question explaining the non-Keplerian https://en.wikipedia.org/wiki/Peculiar_velocity of structures from ionized hydrogen to molecular clouds to supernovae with highly predictable light curves.
While basic MOND can explain edge-on spirals and axis-perpendicular spheroidals and lenticulars, it does a bad job with face-on spirals, ellipticals, and lots of other unusual shapes in the galaxy zoo. It also doesn't do well with entire clusters, where there are enormous gas clouds between the member galaxies, and often a bright quasar (or at least central galaxy) really lighting them up. Corrections to basic MOND patch some of these up, mostly starting by adding extra terms to the initial basic Milgrom formula which modified F = ma.
A ball-like distribution of dark matter around these very large structures (galaxies, clusters) on the other hand is a decent explanation, with the major drawback that the only sufficiently transparent stuff we've produced in labs is simply too light to stay constrained within these structures (electron neutrinos and their antis would fly away to infinity, for example, rather than stick around in a hierarchy of halo/shell/ball structures; free neutrons aren't stable). We also don't have a really good explanation about why these proposed dark matter structures don't have a strong density gradient leading to grossly non-Keplerian orbits of stars in the cores of galaxies (including our own). That's why you get wildly speculative ideas like phase transitions, "dark chemistry", dark-matter/dark-matter annihilation, and so on. There are similar questions about why the outer reaches of dark matter halos don't blow off into infinity rather than lingering around. ( See the excellent III A and III E of Adams & Laughlin https://arxiv.org/abs/astro-ph/9701131 or the Binney & Tremaine textbook https://press.princeton.edu/books/paperback/9780691130279/ga... )
Good explanations should cover the entire lifetime of a galaxy including the things that lead up to it and its ultimate decay. This also means it should be good for the oldest galaxies that we can see. It also means that as we get better at simulating initial value problems we can take a large number of observed quantities of a highly-redshifted galaxy and evolve that initial snapshot step by step such that after trillions of steps we end up with something that looks remarkably like a galaxy with low redshift. We're not really there yet. Work harder, supercomputer builders! :D :D
We do. Structure formation is part of the standard cosmology. https://www.esa.int/Science_Exploration/Space_Science/Planck... https://kipac.stanford.edu/research/cosmic-structure etc etc.
Matter fell into place wherever it could, forming overdense areas. More matter tended to fall into overdense areas from underdense areas, and this allowed the underdense areas to expand more easily while the overdense areas were constrained to expand more slowly or even to collapse. Dark matter, not being able to collide and radiate away the collision-energy collapses much more slowly than heavy matter.
Dark matter connects fluctuations in the cosmic microwave background with large scale structures -- the cosmic web -- extremely well, as long as it moves nonrelativistically (that's why it's "cold" compared to electron neutrinos which move ultrarelativistically) and collapses very slowly.
> ultradense ball of lambda-CDM
Mu?
Lambda (capital greek letter) is the term that encodes the expansion of space and the consequent separation of clusters of galaxies from one another. It's irrelevant within clusters of galaxies, let alone individual galaxies. The solar system is not expanding at all, and neither is Manhattan.
Lambda is also a constant. It doesn't form balls, there are no overdensities or underdensities, it has the same tiny value at every point in spacetime.
CDM is cold dark matter, and your phys.org link discusses evidence for a possible explanation about why there isn't a high density "ball" of dark matter in the galactic centre compared to around our solar system: maybe dark matter can interact electromagnetically under conditions found in galactic cores. The electromagnetic interaction in this study would produce a gamma ray and the vanishing of a dark matter particle. This could be some complicated way dark matter can self-annihilate in the presence of very hot standard model gas like that around black holes or produced by supernovae, for instance. Whatever the specifics, this conversion of dark matter to gammas would keep the central dark matter relatively sparse.
(Other mechanisms for avoiding high dark matter densities in galaxy cores are available. These do have a good collective name: galactic outflow, of which dark matter heating and dark matter decay are subcategories that are at the root of your phys.org link. Generally dark matter turns into hot matter, or drags hot matter along with it for a ride. Outflow must enrich the metallicity of stars far from galactic cores, and quench core star formation, and these generate observables for individual proposed mechanisms, and astronomers are already happily gathering up data that probe those observables. Good proposals for outflow mechanisms also work on the scale of structures like the Coma Cluster).
Or 'caloric' or 'phlogiston'
https://en.wikipedia.org/wiki/Superseded_theories_in_science
No, it wasn't, because, as you note, that observation was accounted for by adopting new laws of gravity, not by adding more matter to the distribution of matter we used in our model. Those are two different things.
By analogy suppose we find out in 35 years that some non-lambda-cdm, non-particle theory is correct? It would be revisionist to say we didn't believe in dark matter today.
If you mean that nobody considered modifying the theory of gravity to explain the precession until Einstein and General Relativity, that is not correct. By the 1890s pretty much everyone had given up on finding another planet and a number of astronomers were considering modifications to the law of gravity.
In any case, you are shifting your ground from the comments you have made elsewhere in this thread. Elsewhere in this thread, you have been claiming that "dark matter hypothesis" includes the hypothesis that we should change the laws of gravity instead of changing the matter distribution in our models. Now you are admitting that those are two different things, which is the point I have been trying to make all along.
> suppose we find out in 35 years that some non-lambda-cdm, non-particle theory is correct? It would be revisionist to say we didn't believe in dark matter today.
"Believe in" is the wrong term. Dark matter is a hypothesis (just as suggesting there was another planet causing Mercury's excess orbital precession was a hypothesis), which appears to be the one most cosmologists currently think will end up being the right one. But that does not mean anyone claims that it is the only hypothesis, or that that hypothesis includes the hypothesis that we should modify the laws of gravity. The latter is understood by all cosmologists to be a separate hypothesis. Just as modifying the laws of gravity to explain Mercury's excess orbital precession was understood by all at the time to be a separate hypothesis.
> you have been claiming that "dark matter hypothesis"
No. I have claimed that the class of dark matter theories includes those which reject the existence of "dark matter" under the (possibly incorrect) assumption that the historical term "dark matter theory" refers to any theory that explains galactic rotation curves, as coined by Zwicky and cataloged by Rubin. In general, "A theory of X" can certainly reject X, and replace it with something else.
And this claim is incorrect. "Dark matter" is a particular hypothesis for explaining the discrepancy in galaxy rotation curves (and other observations as well, such as features of the CMB that cannot be explained with a model that only includes the matter visible to us). It is not just another name for the discrepancy itself, or for any theory that claims to explain the discrepancy by any mechanism whatever.
> under the (possibly incorrect) assumption that the historical term "dark matter theory" refers to any theory that explains galactic rotation curves
And this is an incorrect assumption. That's what I've been saying all along. See above.
Thank you for introducing your comment like this so I can give it the true weight it deserves.