7 ms·

Heavy particles don’t explain gravitational lensing oddities

130 points by isaacfrond 4y ago

jstanley 4y ago
Does that mean some WIMPS Heavy particles do explain gravitational lensing oddities?
- gjm11 4y ago
  I think it means that the HN software auto-removes exclamation marks from titles, and in this case the original title began "No WIMPS!". (I don't know that the HN software does this. Maybe the submitter made a typo.)
jmartrican 4y ago
When watching the history of how the idea of dark matter came to be, I do not understand how they jumped directly to matter being the solution to the gravitational oddities that were noticed. So here goes the story as I recall. A scientist noticed the galaxies spinning faster than they would have expected for the amount of gravity produced by the expected matter in that galaxy. The galaxies are spinning in a more uniform way throughout... aka the outside of the spiral galaxy is spinning faster than expected. Immediately I see this as being a gravitational issue. Maybe gravity works in ways we do not understand at this level. But instead scientists jump directly to "there must be matter here that we can't see". My question is, has the marketing of the phrase "dark matter" got ahead of the actual problem and has stuck and really is a synonym to "strange gravity issues we are seeing" or have we really jumped to matter being the culprit without taking a look at gravity behaving in unexpected ways? Is the issue matter or gravity, and if it is matter, why does the story of dark matter jump from fast spinning galaxies directly to "missing matter" without a detailed explanation of why it must be matter, and not gravity acting strange?
- rich_sasha 4y ago
  I believe the rotational speed discrepancy in galaxies is simply the most intuitive example. There are many more cases where mass appears to be missing: https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... . Crucially, these occur at vastly different scales, from individual galaxies to clusters and whole observable universe. > ...have we really jumped to matter being the culprit without taking a look at gravity behaving in unexpected ways? This is called Modified Gravity, or MOND: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics and is well-researched. It stands as a niche but actively pursued alternative to Dark Matter. For the most part, Dark Matter fits observations better and more parsimoniously. But of course we don't have definitive proof/disproof for either.
- eterevsky 4y ago
  Because it's the most economical assumption. We know that there are various different particles and there are some that we haven't discovered yet. There's no reason why some particle wouldn't only interact via gravity and weak interaction. Adding some "dark" mass in the simulations paints the picture that matches what we see. Some galaxies were found without these gravitational anomalies, which all but rules out a general systemic issues with the theory of gravity.
  - wholinator2 4y ago
    I'll add, it paints the _entire_ picture (including smaller gravitational bodies and new found anomalies) much better than other theories
- plaguepilled 4y ago
  I wasn't aware of the galaxy rotation argument for DM - I have always heard it argued for using the bullet cluster observation for galaxy collision (https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster). That's not to say DM is the only way to explain such data, but it is a good motivation for it.
  - Sharlin 4y ago
    Bullet cluster came much later and indeed the mass/visible matter discrepancy was discovered specifically because they were looking for evidence for dark matter. It’s just one piece of additional evidence for the theory that dark matter is specifically matter.
    - omneity 4y ago
      Seen this way it seems DM theories are a failure of applying Occam's Razor on a scientifically complex topic. Whereby our current tools and theories cannot explain some observed discrepancies, we aim for a more complicated explanation for them (special types of matter) rather than limitations of our current techniques. At the same time science doesn't progress by just shrugging and saying "we can't know", so maybe it's an inherent conflict of interest at play here?
      - Sharlin 4y ago
        More complicated? That makes zero sense and is some sort of a strange cognitive bias against particles (or possibly against things we can’t directly see?) When it was found that experiments didn’t support Thomson’s "raisin pudding" model of the atom, the solution was not to modify electromagnetism but to introduce a hitherto unknown particle, the proton. When it was found that the mass of a nucleus wasn’t simply the sum of the masses of the protons, the solution was not to modify theories of inertial mass and gravity, but to introduce a hitherto unknown particle, the neutron. When it was found that in a beta decay some of the mass-energy was unaccounted for, the solution was not a modification to Einstein’s equations but to introduce a hitherto unknown particle, the neutrino, whose existence was only experimentally proven quite a bit later. All our evidence from distinct sub-disciplines of astronomy suggest that some specific well-constrained type of matter that interacts via gravity but not via EM is needed to make predictions match observations. No competing hypothesis has been proposed that is as good a match. Scientific method 101 says that if something walks like a duck, swims like a duck, and quacks like a duck, we should assume that it is indeed a duck until and unless additional evidence rules out the duck model.
- wholinator2 4y ago
  The issue is that a modified gravitational theory means getting rid of general Relativity or at least _significantly_ modifying it. As i understand, no proposed modified gravity theory has ever been able to be more accurate than GR. Every theory that's been proposed (very many for over 100 years) to resolve galaxy rotation has then failed at some other scale or situation. Most of those theories still require dark matter to explain those discrepancies. The single most accurate and explainable model currently is general relativity with some extra dark mass hiding in extremely large structures. As well, to do away with GR means ridding ourselves of the most accurate gravitational predictions ever made. GR explains basically _everything else_. It predicts gravitational waves, black holes, the orbit of mercury and the precession of rotating bodies, as well as many other things I'm sure. And so far every big prediction has come out to be true. In physics, predictive power is everything. Most of the modified gravity theories can't even explain everything we already see let alone make accurate predictions about other things. It just comes down to math and data. I've had this conversation with many people and while i don't want to discourage questioning or curiosity, none of these people were very versed in the actual topics of modified gravity or modern dark matter theories (not saying this about you, but I've had this conversation _a lot_). You can Google a Wikipedia article about problems with modified gravity, MOND, etc. And read about it freely if curious. The issue i think is that it's too complicated. The science and especially the math of these theories is graduate level physics and these theories are being proposed and discounted by doctorates who've dedicated their entire lives to learning about this. It's extremely difficult for regular people with jobs and lives to have the time to truly understand the issues here, at least until someone comes along who can explain it much better than I. I must say, I'm also not a doctorate in gr or anything, but I've been around it a while and have seen the developments unfold over time. The short answer is that modified gravity theories have more problems than gr + dark matter, at least right now.
  - naveen99 4y ago
    What about another force that shows up at larger scales that is to gravity what gravity is to the strong force ?
    - empyrrhicist 4y ago
- magicalhippo 4y ago
  > Immediately I see this as being a gravitational issue. Maybe gravity works in ways we do not understand at this level. But instead scientists jump directly to "there must be matter here that we can't see". You go for the most likely explanation first. When I go to my car and my keys aren't in my pocket, I go and check the counter where I usually put them when I get home. I don't start checking behind the toilet or similar. In this case we had a theory, general relativity, which worked very well for all the tests we'd done so far, and there was nothing in the theory to indicate it shouldn't be the same all over. Meanwhile astronomical observations are difficult and we knew, and still know, there's more to particle physics. Hence more mass is a more likely explanation. Keep in mind that trying to replace an existing, validated theory can be very hard, since you need to not just explain the new stuff, but also pass all the checks the existing theory did. So far nobody has been able to come up with a modification of general relativity that can do this. And it's not been for lack of trying[1]. [1]: https://en.wikipedia.org/wiki/Alternatives_to_general_relativity https://en.wikipedia.org/wiki/Alternatives_to_general_relati...
- deleted 4y ago
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- mkl 4y ago
  There's a lot more to it than galaxy rotation curves. https://en.wikipedia.org/wiki/Dark_matter https://en.wikipedia.org/wiki/Dark_matter is worth a read, especially the Observational Evidence section. Also, general relativity has had considerable predictive success, so replacing it with something else that works the same in some situations but differently in others seems more complicated than "there's mass here in some form we don't understand yet", though people have of course tried to find such replacements. Physicists have been trying all the hypotheses they can think of and dark matter currently fits the evidence best, they didn't jump straight to dark matter or fixate on it without considering anything else.
  - Beldin 4y ago
    > general relativity has had considerable predictive success, so replacing it with something else that works the same in some situations but differently in others seems more complicated than [...] First: you're very correct - we shouldn't expect an easy solution. Second: precisely the same argument applies to Newtonian gravity. It was phenomenally successful. The mismatch between observations and its model were more easily explained by lack in observations - and they were! We found Neptune because of that! That role (of plugging holes in observations) is currently being played by dark matter. So chances for dark matter existing in some form are good from a historical perspective. On the other hand, from that same perspective, chances of gravity theory being slightly(1) updated for extreme cases are also good. (1) "slightly" in the sense that the difference with the current theory, GR, would be nil or negligible in most cases.
  - the__alchemist 4y ago
    >general relativity has had considerable predictive success, so replacing it with something else that works the same in some situations but differently in others seems more complicated than "there's mass here in some form we don't understand yet", though people have of course tried to find such replacements. Do the models use GR? My understanding is that they make the assumption that Newtoninan gravity models are sufficient.
- deleted 4y ago
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- Sharlin 4y ago
  If I had a euro every time someone is skeptical of dark matter because they think it only predicts galaxy rotation curves, I’d have enough money for several beers (beer is expensive in Finland!) GRC observations were the original motivator for the DM hypothesis, but in the following decades its predictions (specifically, those of a specific cold dark matter model well-constrained by evidence) have been in entire agreement with many observations in several distinct subfields of astronomy, whereas no competing hypothesis has been able to achieve the same. Indeed DM, or something very much like it, is a required ingredient in cosmology to get a universe even remotely similar to ours! As far as we know, we wouldn’t even have galaxies, much less the intricate foam-like structure of voids and filaments, without DM, and no competing hypothesis can predict the observed large-scale structure of the universe while also explaining galaxy rotation curves and everything else, for instance the Bullet Cluster, or the specific observed anisotropies in the cosmic microwave background.
- quantum_mcts 4y ago
  I've long noticed that this idea - that scientists just "jumped to conclusion" that there must be some dark matter to resolve some observational inconsistencies - is very prevalent among non-specialists. It could not be further from the truth, of course - as soon as the rigorous statistical study of galaxy rotation curves was provided by Vera Rubin et. al. in the 80s, the MOND classes of alternatives to gravity started to appear. I tried to understand why this narrative is so prevalent, and it looks like it is due to its very appealing nature - both to the listener and to the speaker: "all those egghead scientists can't see an obvious solution to a problem they are fighting with for decades". I also noticed that when you get deeper into it, then you find that the source - the "speaker" part in the whole narrative - is some kind of science popularizer. Who at the same time (surprise) is a proponent of his own flavor of modified gravity theory. Truth is - everyone, including me, had this idea as soon as they first heard about the DM. If you actually study all the evidence and discussions around the subject, you'll see that introducing an extra "dark" particle is the most obvious solution to the whole collection of observational inconsistencies. Modifying gravity in such a way so that it is self-consistent and consistent with all the observations... I actually don't think anyone completely finished that project so far.
  - xeonmc 4y ago
    can it be possible that the galaxy just appears to be rotating faster due to time dilation, so from the POV of the galaxy itself it doesn't seem like there is anything out of the ordinary?
    - gus_massa 4y ago
      [It's not my branch, so I have to guess.] They are using special and general relativity to make a lot of corrections, like red shift and gravitational lens, so there is 0% chance that all the community forgot to add that correction.
    - kmm 4y ago
      Galactic rotational velocities are on the order of 100 km/s, which, though blazingly fast by human standards, is so slow compared to the speed of light that special relativistic effects are completely negligible. Moreover, it's not just the absolute magnitude of the velocity that is the problem, it's also the shape of the curve. Newtonian mechanics (even with the extremely tiny relativistic corrections taken into account) predict that the rotational speed of a galaxy would decrease the further you go out from the center, and we in fact find that this speed remains more or less constant.
- bena 4y ago
  I mean we're trying to determine the mass of objects many light years away. The concept that we might not be able to perceive everything isn't a real huge leap. Far shorter a leap than "everything we know about gravity is wrong".
- ajross 4y ago
  > I do not understand how they jumped directly to matter being the solution to the gravitational oddities This never happened. "Dark Matter" is just a name for a phenomenon. Galaxies rotate as if they have a bunch of matter in them that can't be accounted for by looking at the light coming from them. So it's "dark" matter. Get it? No one has a "solution" here. It's a problem, and it needs a name. What would you prefer?
  - dragonelite 4y ago
    Once again "naming things is hard" strikes again :p
01648599160 4y ago
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peteradio 4y ago
What happened with general relativity somehow explaining the "dark matter" observations?
- mkl 4y ago
  It didn't? Dark matter in its modern conception was discovered after general relativity was established. Dark matter was hypothesised because general relativity alone couldn't explain some observations.
  - peteradio 4y ago
    I was under the impression that it was not fully rigorously accounted for because its not exactly a simple calculation. So galaxies were still being modeled with newtonian physics in at least part of the calculations which neglects field self-interactions.
    - pfdietz 4y ago
      Ordinary matter in the galaxies cannot explain the rotation curves. Something dark (that doesn't interact with electromagnetic radiation) is needed. Also, the mass in the universe cannot be all baryonic, since that would make primordial nucleosynthesis inconsistent with observed abundances of various light isotopes.
      - peteradio 4y ago
        You are repeating the same dominant theory and don't address that at least some of the excess can be accounted for by more rigorous computation...
        pfdietz 4y ago
        I should be clear: ordinary matter cannot explain the rotation curves with conventional gravity. One can come up with ad hoc modified theories of gravity, but none appears to be consistent with all the evidence.
AnimalMuppet 4y ago
So WIMPS don't explain the pattern of lensing caused by one particular galaxy, if you assume a vanilla distribution of the WIMPS. But we already have situations (the Bullet Cluster, e.g.) where we don't assume that. So... I'm not sure that this is all that strong an argument? And, they argue for axions, because it would be more normal for them to assume a more complicated distribution. But they can't predict which distribution they would assume. So this looks like "it must be A rather than B, if we assume the distribution of B, because we can't assume the distribution of A". That's not all that convincing.
blincoln 4y ago
Does "WIMPS don't explain observed data" also exclude "matter in other M-Theory branes exerts a gravitational influence on matter in our own" as an explanation for dark matter? The main argument I've seen against this is that matter interacting gravitationally across branes would cause a sort of n-body problem condition where everything in a particular sphere of influence would orbit continuously around the center of mass instead of settling into a lump, but maybe that explains why some planet-scale (or larger) objects have spinning cores?
- addaon 4y ago
  I don't believe anyone's casting around too hard for an alternate explanation for conservation of angular momentum, which is a consequence of rotational symmetry of physical laws, a relatively intuitive and desired property.
- icapybara 4y ago
  "When you hear hoofprints, think horses, not zebras." You can always contrive a more complicated structure to explain some observation. There is no evidence branes exist. We are looking for an explanation to the "dark matter problem" that doesn't require asserting that a ton of new and unproven physics exists.
  - sleeplessworld 4y ago
    This may point to a key problem in the hypotheses, theories and conclusions. When you are in Africa, a "think horses" paradigm, may lead you very astray in your thinking.
d--b 4y ago
I always wonder if dark matter isn't simply made of wrinkles in space-time that just don't go back to flat after some weird event happens
- jfengel 4y ago
  It would have to be a galaxy sized, smooth spherical wrinkle that happened to coincide with practically every galaxy. And also to fail to interact with other galaxies when they collide. While we don't know the details of dark matter, we do know a lot of things about it. Putative explanations have to cover a lot of ground.
  - candiodari 4y ago
    That wouldn't be so weird as it sounds. From simulations we know that it was dark matter that came first, then came the galaxies. So if these wrinkles (they don't have to be very smooth) simply had a (large) preferred size and clumped up faster than normal matter in the early universe, that would be enough. Neither of those would be particularly weird. When dark matter clumped up normal matter was essentially a very low density hydrogen cloud, devoid of stars, much less dense than a cloud in the sky, which would take (without dark matter) some 15-50 billion years to clump up by itself. As for the size of the dark matter structures, well we don't know what they are, so you can postulate any size for them.
- throwawaymaths 4y ago
  In the case that dark matter doesn't interact via any force other than gravity, then there will be an epistemological crisis in physics as your model will be indistinguishable from one in which dark matter is a particle.
  - XorNot 4y ago
    It's the classic failing of everyone's casual "alternative to dark matter". It's not X because we can't detect it, what if instead it's an undetectable form of Y? Far too many people get hung up on the idea that in science what things "are" matters: because it doesn't. What matters is predictive power for future observations - if whatever you're proposing doesn't predict anything different to "invisible matter" then it's exactly as bad an explanation as "invisible matter" which so far is a perfectly good explanation for current observations.
photochemsyn 4y ago
This 2014 Fermilab post on why physicists are hopeful that dark matter has some kind of weak-force interaction is a good read: https://www.fnal.gov/pub/today/archive/archive_2014/today14-08-15_NutshellReadMore.html https://www.fnal.gov/pub/today/archive/archive_2014/today14-... > "So why would scientists postulate that dark matter experiences the weak force? One answer is that if it doesn't, we'll never detect it. But that's not a very good answer. A better answer involves the Higgs boson. Because the Higgs field gives mass to ordinary matter..." Current status of the Higgs boson decaying to invisible particles is, no such evidence has yet been found (2020): https://atlas.cern/updates/briefing/probing-dark-matter-higgs-boson https://atlas.cern/updates/briefing/probing-dark-matter-higg... This apparent failure (so far) of that hypothesis seems to have caused scientists in the field to cast around for other ideas, like primordial black holes: https://news.yale.edu/2021/12/16/black-holes-and-dark-matter-are-they-one-and-same https://news.yale.edu/2021/12/16/black-holes-and-dark-matter...
Zamicol 4y ago
I'd like to see more work done with investigating field self-interaction. This 2020 paper by Alexandre Deur is very interesting. "[W]e find that relativistic corrections to the rotation curves of disk galaxies are significant at large galactic radii." "Relativistic corrections to the rotation curves of disk galaxies" - https://arxiv.org/pdf/2004.05905.pdf https://arxiv.org/pdf/2004.05905.pdf And a blog covering the paper: https://astrobites.org/2020/08/17/the-alternative-to-dark-matter-may-be-general-relativity-itself/ https://astrobites.org/2020/08/17/the-alternative-to-dark-ma...
- nimish 4y ago
  Yep, there's a lot of stuff that properly using general relativity would address in a much cleaner way than dark matter. Ignoring the relativistic nature and nonlinearity of GR is a very large problem with FLRW and other toy models, but they have become embedded in cosmology to the point where it's heresy to question them.
- AnotherGoodName 4y ago
  The fun part about this is that gravity has to either be self interacting or capable of propagating faster than light. If gravity wasn't self interacting it wouldn't follow the shortest path that gravity itself creates. Light does follow this path. Gravity would move in different directions and at different velocities to light across the universe which is full of gravitational field if it wasn't self interacting. We have seen zero evidence that gravity does propagate differently to light. In fact ligo and other gravity experiments have only confirmed it's the same. So gravity is almost certainly self interacting. What does this mean? Well it wouldn't change much at small scales. There's not enough effect at say planetary scales to really notice. But what happens when you have a field self interacting across vast distances and involving vast galaxy sized masses? Well your link looks at this and yes it matches some of the anomaly's we are at large scale. It's a good theory and I suspect despite being hard to argue against it will face opposition since it means a lot of scientists were chasing a dead end.
  - deleted 4y ago
    [deleted]
jvanderbot 4y ago
Sticking to TFA, and not ancillary information, I found this frustrating: > Instead, the research team ran 75 different models with the initial conditions chosen at random. By chance, some of these created distortions similar to the ones seen in the real-world data, typically affecting only one of the four lensed images. So, the researchers conclude that the distortions in the lensed images are consistent with a dark matter halo structured by the quantum interference of axions. Essentially the best contender aside from WIMPs is, as subject of TFA, axions. And axions have near-random interference and standing waves, which essentially scatters about light bent by gravitational lensing due to uneven gravity in the lensing object. Essentially, if you randomize the field enough, you can eventually find, after enough trials, something that seems to match reality. And it just so happens axions are the nice random variables we needed to inject. That's where we're at: Just randomly select some parameters until the data fits, and lo and behold, it fits.
- zapov 4y ago
  Its not just you: https://www.youtube.com/watch?v=lu4mH3Hmw2o https://www.youtube.com/watch?v=lu4mH3Hmw2o
  - jeroenvlek 4y ago
    I knew this was Sabine Hossenfelder before clicking the link and not having seen the video before (I like her, fwiw)
  - kakapo88 4y ago
    Wild guess: Sabine is not a fan.
  - mkehrt 4y ago
    I can't tell if she's a legitimate scientist or a crackpot. Does anyone know? (I haven't actually watched this video, but I've seen others of hers. They seem convincing in general to a nonspecialist.)
    - voldacar 4y ago
signalToNose 4y ago
Variable speed of light must be considered
- vintagedave 4y ago
  I’ve heard this theory for years but never understood why it should be considered viable / possible. Part of me wants to believe because it would be really cool, but the rest of me is skeptical :) Can you explain why it’s reasonable and what it answers or how? Ie why must it be considered here?
tabtab 4y ago
Dark matter is really a joint venture between God and ET to bleep with us.

130 points by isaacfrond 4y ago

jstanley 4y ago

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

gjm11 4y ago
I think it means that the HN software auto-removes exclamation marks from titles, and in this case the original title began "No WIMPS!". (I don't know that the HN software does this. Maybe the submitter made a typo.)

jmartrican 4y ago

When watching the history of how the idea of dark matter came to be, I do not understand how they jumped directly to matter being the solution to the gravitational oddities that were noticed. So here goes the story as I recall. A scientist noticed the galaxies spinning faster than they would have expected for the amount of gravity produced by the expected matter in that galaxy. The galaxies are spinning in a more uniform way throughout... aka the outside of the spiral galaxy is spinning faster than expected. Immediately I see this as being a gravitational issue. Maybe gravity works in ways we do not understand at this level. But instead scientists jump directly to "there must be matter here that we can't see". My question is, has the marketing of the phrase "dark matter" got ahead of the actual problem and has stuck and really is a synonym to "strange gravity issues we are seeing" or have we really jumped to matter being the culprit without taking a look at gravity behaving in unexpected ways? Is the issue matter or gravity, and if it is matter, why does the story of dark matter jump from fast spinning galaxies directly to "missing matter" without a detailed explanation of why it must be matter, and not gravity acting strange?

rich_sasha 4y ago
I believe the rotational speed discrepancy in galaxies is simply the most intuitive example. There are many more cases where mass appears to be missing: https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence https://en.wikipedia.org/wiki/Dark_matter#Observational_evid... . Crucially, these occur at vastly different scales, from individual galaxies to clusters and whole observable universe. > ...have we really jumped to matter being the culprit without taking a look at gravity behaving in unexpected ways? This is called Modified Gravity, or MOND: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics and is well-researched. It stands as a niche but actively pursued alternative to Dark Matter. For the most part, Dark Matter fits observations better and more parsimoniously. But of course we don't have definitive proof/disproof for either.
eterevsky 4y ago
Because it's the most economical assumption. We know that there are various different particles and there are some that we haven't discovered yet. There's no reason why some particle wouldn't only interact via gravity and weak interaction. Adding some "dark" mass in the simulations paints the picture that matches what we see. Some galaxies were found without these gravitational anomalies, which all but rules out a general systemic issues with the theory of gravity.
- wholinator2 4y ago
  I'll add, it paints the _entire_ picture (including smaller gravitational bodies and new found anomalies) much better than other theories
plaguepilled 4y ago
I wasn't aware of the galaxy rotation argument for DM - I have always heard it argued for using the bullet cluster observation for galaxy collision (https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster). That's not to say DM is the only way to explain such data, but it is a good motivation for it.
- Sharlin 4y ago
  Bullet cluster came much later and indeed the mass/visible matter discrepancy was discovered specifically because they were looking for evidence for dark matter. It’s just one piece of additional evidence for the theory that dark matter is specifically matter.
  - omneity 4y ago
    Seen this way it seems DM theories are a failure of applying Occam's Razor on a scientifically complex topic. Whereby our current tools and theories cannot explain some observed discrepancies, we aim for a more complicated explanation for them (special types of matter) rather than limitations of our current techniques. At the same time science doesn't progress by just shrugging and saying "we can't know", so maybe it's an inherent conflict of interest at play here?
    - Sharlin 4y ago
      More complicated? That makes zero sense and is some sort of a strange cognitive bias against particles (or possibly against things we can’t directly see?) When it was found that experiments didn’t support Thomson’s "raisin pudding" model of the atom, the solution was not to modify electromagnetism but to introduce a hitherto unknown particle, the proton. When it was found that the mass of a nucleus wasn’t simply the sum of the masses of the protons, the solution was not to modify theories of inertial mass and gravity, but to introduce a hitherto unknown particle, the neutron. When it was found that in a beta decay some of the mass-energy was unaccounted for, the solution was not a modification to Einstein’s equations but to introduce a hitherto unknown particle, the neutrino, whose existence was only experimentally proven quite a bit later. All our evidence from distinct sub-disciplines of astronomy suggest that some specific well-constrained type of matter that interacts via gravity but not via EM is needed to make predictions match observations. No competing hypothesis has been proposed that is as good a match. Scientific method 101 says that if something walks like a duck, swims like a duck, and quacks like a duck, we should assume that it is indeed a duck until and unless additional evidence rules out the duck model.
wholinator2 4y ago
The issue is that a modified gravitational theory means getting rid of general Relativity or at least _significantly_ modifying it. As i understand, no proposed modified gravity theory has ever been able to be more accurate than GR. Every theory that's been proposed (very many for over 100 years) to resolve galaxy rotation has then failed at some other scale or situation. Most of those theories still require dark matter to explain those discrepancies. The single most accurate and explainable model currently is general relativity with some extra dark mass hiding in extremely large structures. As well, to do away with GR means ridding ourselves of the most accurate gravitational predictions ever made. GR explains basically _everything else_. It predicts gravitational waves, black holes, the orbit of mercury and the precession of rotating bodies, as well as many other things I'm sure. And so far every big prediction has come out to be true. In physics, predictive power is everything. Most of the modified gravity theories can't even explain everything we already see let alone make accurate predictions about other things. It just comes down to math and data. I've had this conversation with many people and while i don't want to discourage questioning or curiosity, none of these people were very versed in the actual topics of modified gravity or modern dark matter theories (not saying this about you, but I've had this conversation _a lot_). You can Google a Wikipedia article about problems with modified gravity, MOND, etc. And read about it freely if curious. The issue i think is that it's too complicated. The science and especially the math of these theories is graduate level physics and these theories are being proposed and discounted by doctorates who've dedicated their entire lives to learning about this. It's extremely difficult for regular people with jobs and lives to have the time to truly understand the issues here, at least until someone comes along who can explain it much better than I. I must say, I'm also not a doctorate in gr or anything, but I've been around it a while and have seen the developments unfold over time. The short answer is that modified gravity theories have more problems than gr + dark matter, at least right now.
- naveen99 4y ago
  What about another force that shows up at larger scales that is to gravity what gravity is to the strong force ?
  - empyrrhicist 4y ago
magicalhippo 4y ago
> Immediately I see this as being a gravitational issue. Maybe gravity works in ways we do not understand at this level. But instead scientists jump directly to "there must be matter here that we can't see". You go for the most likely explanation first. When I go to my car and my keys aren't in my pocket, I go and check the counter where I usually put them when I get home. I don't start checking behind the toilet or similar. In this case we had a theory, general relativity, which worked very well for all the tests we'd done so far, and there was nothing in the theory to indicate it shouldn't be the same all over. Meanwhile astronomical observations are difficult and we knew, and still know, there's more to particle physics. Hence more mass is a more likely explanation. Keep in mind that trying to replace an existing, validated theory can be very hard, since you need to not just explain the new stuff, but also pass all the checks the existing theory did. So far nobody has been able to come up with a modification of general relativity that can do this. And it's not been for lack of trying[1]. [1]: https://en.wikipedia.org/wiki/Alternatives_to_general_relativity https://en.wikipedia.org/wiki/Alternatives_to_general_relati...
deleted 4y ago
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mkl 4y ago
There's a lot more to it than galaxy rotation curves. https://en.wikipedia.org/wiki/Dark_matter https://en.wikipedia.org/wiki/Dark_matter is worth a read, especially the Observational Evidence section. Also, general relativity has had considerable predictive success, so replacing it with something else that works the same in some situations but differently in others seems more complicated than "there's mass here in some form we don't understand yet", though people have of course tried to find such replacements. Physicists have been trying all the hypotheses they can think of and dark matter currently fits the evidence best, they didn't jump straight to dark matter or fixate on it without considering anything else.
- Beldin 4y ago
  > general relativity has had considerable predictive success, so replacing it with something else that works the same in some situations but differently in others seems more complicated than [...] First: you're very correct - we shouldn't expect an easy solution. Second: precisely the same argument applies to Newtonian gravity. It was phenomenally successful. The mismatch between observations and its model were more easily explained by lack in observations - and they were! We found Neptune because of that! That role (of plugging holes in observations) is currently being played by dark matter. So chances for dark matter existing in some form are good from a historical perspective. On the other hand, from that same perspective, chances of gravity theory being slightly(1) updated for extreme cases are also good. (1) "slightly" in the sense that the difference with the current theory, GR, would be nil or negligible in most cases.
- the__alchemist 4y ago
  >general relativity has had considerable predictive success, so replacing it with something else that works the same in some situations but differently in others seems more complicated than "there's mass here in some form we don't understand yet", though people have of course tried to find such replacements. Do the models use GR? My understanding is that they make the assumption that Newtoninan gravity models are sufficient.
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Sharlin 4y ago
If I had a euro every time someone is skeptical of dark matter because they think it only predicts galaxy rotation curves, I’d have enough money for several beers (beer is expensive in Finland!) GRC observations were the original motivator for the DM hypothesis, but in the following decades its predictions (specifically, those of a specific cold dark matter model well-constrained by evidence) have been in entire agreement with many observations in several distinct subfields of astronomy, whereas no competing hypothesis has been able to achieve the same. Indeed DM, or something very much like it, is a required ingredient in cosmology to get a universe even remotely similar to ours! As far as we know, we wouldn’t even have galaxies, much less the intricate foam-like structure of voids and filaments, without DM, and no competing hypothesis can predict the observed large-scale structure of the universe while also explaining galaxy rotation curves and everything else, for instance the Bullet Cluster, or the specific observed anisotropies in the cosmic microwave background.
quantum_mcts 4y ago
I've long noticed that this idea - that scientists just "jumped to conclusion" that there must be some dark matter to resolve some observational inconsistencies - is very prevalent among non-specialists. It could not be further from the truth, of course - as soon as the rigorous statistical study of galaxy rotation curves was provided by Vera Rubin et. al. in the 80s, the MOND classes of alternatives to gravity started to appear. I tried to understand why this narrative is so prevalent, and it looks like it is due to its very appealing nature - both to the listener and to the speaker: "all those egghead scientists can't see an obvious solution to a problem they are fighting with for decades". I also noticed that when you get deeper into it, then you find that the source - the "speaker" part in the whole narrative - is some kind of science popularizer. Who at the same time (surprise) is a proponent of his own flavor of modified gravity theory. Truth is - everyone, including me, had this idea as soon as they first heard about the DM. If you actually study all the evidence and discussions around the subject, you'll see that introducing an extra "dark" particle is the most obvious solution to the whole collection of observational inconsistencies. Modifying gravity in such a way so that it is self-consistent and consistent with all the observations... I actually don't think anyone completely finished that project so far.
- xeonmc 4y ago
  can it be possible that the galaxy just appears to be rotating faster due to time dilation, so from the POV of the galaxy itself it doesn't seem like there is anything out of the ordinary?
  - gus_massa 4y ago
    [It's not my branch, so I have to guess.] They are using special and general relativity to make a lot of corrections, like red shift and gravitational lens, so there is 0% chance that all the community forgot to add that correction.
  - kmm 4y ago
    Galactic rotational velocities are on the order of 100 km/s, which, though blazingly fast by human standards, is so slow compared to the speed of light that special relativistic effects are completely negligible. Moreover, it's not just the absolute magnitude of the velocity that is the problem, it's also the shape of the curve. Newtonian mechanics (even with the extremely tiny relativistic corrections taken into account) predict that the rotational speed of a galaxy would decrease the further you go out from the center, and we in fact find that this speed remains more or less constant.
bena 4y ago
I mean we're trying to determine the mass of objects many light years away. The concept that we might not be able to perceive everything isn't a real huge leap. Far shorter a leap than "everything we know about gravity is wrong".
ajross 4y ago
> I do not understand how they jumped directly to matter being the solution to the gravitational oddities This never happened. "Dark Matter" is just a name for a phenomenon. Galaxies rotate as if they have a bunch of matter in them that can't be accounted for by looking at the light coming from them. So it's "dark" matter. Get it? No one has a "solution" here. It's a problem, and it needs a name. What would you prefer?
- dragonelite 4y ago
  Once again "naming things is hard" strikes again :p

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peteradio 4y ago

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

mkl 4y ago
It didn't? Dark matter in its modern conception was discovered after general relativity was established. Dark matter was hypothesised because general relativity alone couldn't explain some observations.
- peteradio 4y ago
  I was under the impression that it was not fully rigorously accounted for because its not exactly a simple calculation. So galaxies were still being modeled with newtonian physics in at least part of the calculations which neglects field self-interactions.
  - pfdietz 4y ago
    Ordinary matter in the galaxies cannot explain the rotation curves. Something dark (that doesn't interact with electromagnetic radiation) is needed. Also, the mass in the universe cannot be all baryonic, since that would make primordial nucleosynthesis inconsistent with observed abundances of various light isotopes.
    - peteradio 4y ago
      You are repeating the same dominant theory and don't address that at least some of the excess can be accounted for by more rigorous computation...
      - pfdietz 4y ago
        I should be clear: ordinary matter cannot explain the rotation curves with conventional gravity. One can come up with ad hoc modified theories of gravity, but none appears to be consistent with all the evidence.

AnimalMuppet 4y ago

So WIMPS don't explain the pattern of lensing caused by one particular galaxy, if you assume a vanilla distribution of the WIMPS. But we already have situations (the Bullet Cluster, e.g.) where we don't assume that. So... I'm not sure that this is all that strong an argument? And, they argue for axions, because it would be more normal for them to assume a more complicated distribution. But they can't predict which distribution they would assume. So this looks like "it must be A rather than B, if we assume the distribution of B, because we can't assume the distribution of A". That's not all that convincing.

blincoln 4y ago

Does "WIMPS don't explain observed data" also exclude "matter in other M-Theory branes exerts a gravitational influence on matter in our own" as an explanation for dark matter? The main argument I've seen against this is that matter interacting gravitationally across branes would cause a sort of n-body problem condition where everything in a particular sphere of influence would orbit continuously around the center of mass instead of settling into a lump, but maybe that explains why some planet-scale (or larger) objects have spinning cores?

addaon 4y ago
I don't believe anyone's casting around too hard for an alternate explanation for conservation of angular momentum, which is a consequence of rotational symmetry of physical laws, a relatively intuitive and desired property.
icapybara 4y ago
"When you hear hoofprints, think horses, not zebras." You can always contrive a more complicated structure to explain some observation. There is no evidence branes exist. We are looking for an explanation to the "dark matter problem" that doesn't require asserting that a ton of new and unproven physics exists.
- sleeplessworld 4y ago
  This may point to a key problem in the hypotheses, theories and conclusions. When you are in Africa, a "think horses" paradigm, may lead you very astray in your thinking.

d--b 4y ago

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

jfengel 4y ago
It would have to be a galaxy sized, smooth spherical wrinkle that happened to coincide with practically every galaxy. And also to fail to interact with other galaxies when they collide. While we don't know the details of dark matter, we do know a lot of things about it. Putative explanations have to cover a lot of ground.
- candiodari 4y ago
  That wouldn't be so weird as it sounds. From simulations we know that it was dark matter that came first, then came the galaxies. So if these wrinkles (they don't have to be very smooth) simply had a (large) preferred size and clumped up faster than normal matter in the early universe, that would be enough. Neither of those would be particularly weird. When dark matter clumped up normal matter was essentially a very low density hydrogen cloud, devoid of stars, much less dense than a cloud in the sky, which would take (without dark matter) some 15-50 billion years to clump up by itself. As for the size of the dark matter structures, well we don't know what they are, so you can postulate any size for them.
throwawaymaths 4y ago
In the case that dark matter doesn't interact via any force other than gravity, then there will be an epistemological crisis in physics as your model will be indistinguishable from one in which dark matter is a particle.
- XorNot 4y ago
  It's the classic failing of everyone's casual "alternative to dark matter". It's not X because we can't detect it, what if instead it's an undetectable form of Y? Far too many people get hung up on the idea that in science what things "are" matters: because it doesn't. What matters is predictive power for future observations - if whatever you're proposing doesn't predict anything different to "invisible matter" then it's exactly as bad an explanation as "invisible matter" which so far is a perfectly good explanation for current observations.

photochemsyn 4y ago

This 2014 Fermilab post on why physicists are hopeful that dark matter has some kind of weak-force interaction is a good read: https://www.fnal.gov/pub/today/archive/archive_2014/today14-08-15_NutshellReadMore.html https://www.fnal.gov/pub/today/archive/archive_2014/today14-... > "So why would scientists postulate that dark matter experiences the weak force? One answer is that if it doesn't, we'll never detect it. But that's not a very good answer. A better answer involves the Higgs boson. Because the Higgs field gives mass to ordinary matter..." Current status of the Higgs boson decaying to invisible particles is, no such evidence has yet been found (2020): https://atlas.cern/updates/briefing/probing-dark-matter-higgs-boson https://atlas.cern/updates/briefing/probing-dark-matter-higg... This apparent failure (so far) of that hypothesis seems to have caused scientists in the field to cast around for other ideas, like primordial black holes: https://news.yale.edu/2021/12/16/black-holes-and-dark-matter-are-they-one-and-same https://news.yale.edu/2021/12/16/black-holes-and-dark-matter...

Zamicol 4y ago

I'd like to see more work done with investigating field self-interaction. This 2020 paper by Alexandre Deur is very interesting. "[W]e find that relativistic corrections to the rotation curves of disk galaxies are significant at large galactic radii." "Relativistic corrections to the rotation curves of disk galaxies" - https://arxiv.org/pdf/2004.05905.pdf https://arxiv.org/pdf/2004.05905.pdf And a blog covering the paper: https://astrobites.org/2020/08/17/the-alternative-to-dark-matter-may-be-general-relativity-itself/ https://astrobites.org/2020/08/17/the-alternative-to-dark-ma...

nimish 4y ago
Yep, there's a lot of stuff that properly using general relativity would address in a much cleaner way than dark matter. Ignoring the relativistic nature and nonlinearity of GR is a very large problem with FLRW and other toy models, but they have become embedded in cosmology to the point where it's heresy to question them.
AnotherGoodName 4y ago
The fun part about this is that gravity has to either be self interacting or capable of propagating faster than light. If gravity wasn't self interacting it wouldn't follow the shortest path that gravity itself creates. Light does follow this path. Gravity would move in different directions and at different velocities to light across the universe which is full of gravitational field if it wasn't self interacting. We have seen zero evidence that gravity does propagate differently to light. In fact ligo and other gravity experiments have only confirmed it's the same. So gravity is almost certainly self interacting. What does this mean? Well it wouldn't change much at small scales. There's not enough effect at say planetary scales to really notice. But what happens when you have a field self interacting across vast distances and involving vast galaxy sized masses? Well your link looks at this and yes it matches some of the anomaly's we are at large scale. It's a good theory and I suspect despite being hard to argue against it will face opposition since it means a lot of scientists were chasing a dead end.
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jvanderbot 4y ago

Sticking to TFA, and not ancillary information, I found this frustrating: > Instead, the research team ran 75 different models with the initial conditions chosen at random. By chance, some of these created distortions similar to the ones seen in the real-world data, typically affecting only one of the four lensed images. So, the researchers conclude that the distortions in the lensed images are consistent with a dark matter halo structured by the quantum interference of axions. Essentially the best contender aside from WIMPs is, as subject of TFA, axions. And axions have near-random interference and standing waves, which essentially scatters about light bent by gravitational lensing due to uneven gravity in the lensing object. Essentially, if you randomize the field enough, you can eventually find, after enough trials, something that seems to match reality. And it just so happens axions are the nice random variables we needed to inject. That's where we're at: Just randomly select some parameters until the data fits, and lo and behold, it fits.

zapov 4y ago
Its not just you: https://www.youtube.com/watch?v=lu4mH3Hmw2o https://www.youtube.com/watch?v=lu4mH3Hmw2o
- jeroenvlek 4y ago
  I knew this was Sabine Hossenfelder before clicking the link and not having seen the video before (I like her, fwiw)
- kakapo88 4y ago
  Wild guess: Sabine is not a fan.
- mkehrt 4y ago
  I can't tell if she's a legitimate scientist or a crackpot. Does anyone know? (I haven't actually watched this video, but I've seen others of hers. They seem convincing in general to a nonspecialist.)
  - voldacar 4y ago

signalToNose 4y ago

Variable speed of light must be considered

vintagedave 4y ago
I’ve heard this theory for years but never understood why it should be considered viable / possible. Part of me wants to believe because it would be really cool, but the rest of me is skeptical :) Can you explain why it’s reasonable and what it answers or how? Ie why must it be considered here?

tabtab 4y ago

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