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James Webb Space Telescope finds evidence for alternate theory of gravity
- jchanimal 2y agoWhat’s MOND really mean? Here’s the Wikipedia entry https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
- uoaei 2y agoI follow the lead author, Stacy McGaugh, via his blog where he posts discussions and musings about the latest research into the dark matter vs MOND debate: https://tritonstation.com/new-blog-page/ https://tritonstation.com/new-blog-page/ His arguments are very convincing and relatively clear. I am not an astrophysicist but I have two degrees in physics and have always found the dark matter theory to be lacking -- in absence of any evidence of causation whatsoever, dark matter can only be described trivially as "where we would put matter if we could to make our theory of gravity make sense," which is totally backwards from a basic scientific perspective. Predictions based on modern MOND postulates are shown to be more and more accurate as our observational instruments continue to improve in sensitivity.
- simonh 2y agoI don’t think that’s quite fair. That approach is exactly how we find planets. Here’s an unexpected variance in the motion of a planet or star. It could be explained by a planet over there. Oh look, there’s a planet over there.
- russdill 2y agoIt's actually a better example than you think. This exact theory led to long and protracted searches for the planet Vulcan, which would explain Mercury's strange behavior.
- TheOtherHobbes 2y agoPlanets are visible when you look for them. Dark matter - so far - isn't.
- drdeca 2y agoWhat do you mean by “visible when you look for them”? Like, with light? Does gravitational lensing count as “visible” to you?
- MarkusQ 2y agoRight, which is why it quickly led to the detection of dark matter...hmm. I think a better analogy would be "that approach is exactly how we explain failing to find planets like Vulcan; we hypothesize that they are made of as-yet-unknown stuff that you can't see, touch, hear, smell, or in fact detect at all. But we know they're there because our calculations say they are."
- solid_fuel 2y agoHypothesizing that a planet might be over there is a testable hypothesis. Have we found a way to verify the presence of dark matter yet? Or is it still an untestable hypothesis sprinkled around distant galaxies so their acceleration curves look right?
- User23 2y agoI’m particularly amused by the hypothesis that spacetime can be bent without the presence of matter. We can’t detect dark matter because there’s no such thing, it’s just a brute topological fact.
- mr_mitm 2y agoDark matter predicted lensing effect which were successfully tested. Same for the baryonic acoustic oscillations in the CMB.
- MattPalmer1086 2y agoThat's not quite true. General relativity predicts gravitational lensing, not dark matter. Lensing has been used as an experimental probe for the presence of dark matter.
- elashri 2y agoMOND is an alternative theory of gravity competing with GR. People usually forget that while MOND started to present a different explanation for Dark Matter, it is a theory of gravity. Dark Matter is not a theory of gravity and is compatible with GR.
- zeroonetwothree 2y agoDark matter isn’t much of a theory in the first place.
- mr_mitm 2y agoWell of course. What I clearly meant was that DM predicts lensing effects in a magnitude that cannot be explained with ordinary matter. See bullet cluster or weak lensing observations.
- griffzhowl 2y ago> which is totally backwards from a basic scientific perspective This is not right, because if we have a situation where our theories and observations don't cohere, it's not given whether the theory requires modification or we're missing something in our observations (or both). A classical illustration is the orbit of Uranus being observed in the nineteenth century to be contrary to the predictions of Newtonian theory. Calculations were made assuming the truth of the Newtonian theory and that we were missing something in our observations - the position of Neptune was predicted and it was subsequently discovered. On the other hand, the orbit of Mercury diverged from the prediction of Newton's theory. Again, a previously unobserved planet closer to the sun was postulated as being responsible, but in this case it really did require a modification to the theory of gravity: general relativity, which accurately predicted the 43 arcseconds per century of perihelion precession by which Mercury's orbit diverges from Newtonian predicitions. GR has obviously made many other predictions, such as the gravitational bending of light, black holes, and gravitational waves, which have been vindicated. So there's obviously a problem of the theory and observations not cohering, but whether the solution is a modification of the theory or a new form of matter is not clear in advance, and the latter is not unreasonable and certainly it's not unscientific to make as a hypothesis, to see where it leads. The difficulty is in coming up with a theoretical framework that retains all the successful predictions of GR while also accounting for the galactic rotation curves.
- bbor 2y agoWell put, thanks for sharing! Never saw it phrased in such a clear narrative. As a novice, it seems like there's one big difference between those anecdotes and the current situation, though: sample size. Sure, if we were observing Andromeda spinning too slowly I'd be open to our instruments not capturing some massive objects/clouds, but we're actively observing, what, ~1E5-6 galaxies? In the case of a missing planet there were accidents of history/solar system makeup that led to our otherwise solid frameworks missing a key piece of information. But that clearly couldn't happen millions of times; whatever explains the inconsistencies we're seeing has to be a fundamental misunderstanding. Once we've arrived at this point, we can compare the two theoretical re-workings on their own terms: one is that we're glossing over some important detail of how gravitational relations in spacetime work, and the other is that we're failing to observe some new class of matter. I mean, right? There's no way this conundrum will be solved by "whoops turns out there was more plain ol' dust than we thought" at this point, right? In those terms, I feel parsimony clearly favors one possibility over the other. Every hypothesis is worth exploring (I mean, QM and GR are dumb as hell, yet nonetheless turned out to be correct), but when funding is on the line it's also not out of line to favor one explanation explicitly. That's already happening anyway, just in the other direction. But also I'm just some kid who's awed and grateful to be living in times of such profound mystery and discovery. Could be totally off base -- I barely passed physics I!
- antognini 2y ago> where we would put matter if we could to make our theory of gravity make sense Dark matter behaves in a fundamentally different way from baryonic matter. We can constrain the total amount of matter in the universe (both dark and baryonic) from the observed abundances of baryogenesis. But dark matter has a different effect on the relative amplitudes of peaks in the CMB. As far as I can tell, MOND has never really had any success outside of modeling galaxy rotation curves. The skepticism I've seen towards dark matter vs. MOND has always been strange to me. Dark matter doesn't really require much in the way of new physics --- there's just a new particle to add to the standard model. But most MOND theories violate Lorentz invariance which is a vastly more radical departure from standard physics. (And in my mind, the more sophisticated MOND theories that maintain Lorentz invariance like TeVeS are really a theory of dark matter dressed up in the language of MOND.)
- MattPalmer1086 2y agoThere are more successful predictions than just rotation curves. For example, see: http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html
- antognini 2y agoThese successful predictions are all generally variants on modeling galactic dynamics, though. The trouble is that galaxies and galaxy clusters are very messy places, so it's hard to make sure you've incorporated all the relevant physics. By contrast something like baryon acoustic oscillations are very simple to model, so you can be quite confident that you've incorporated all the relevant processes. And in that regime LCDM performs beautifully and MOND completely fails. So it's reasonable to suspect that in more complicated environments the problem is that we're not modeling the systems correctly rather than that there's new physics going on.
- gus_massa 2y agoVery interesting. Do you know an article that ELI25 this?
- halgir 2y agoI usually understand "dark matter" to be shorthand for the discrepancy between theory and observation. The explanation might indeed be matter that is dark, or it might be solved by entirely unexpected observations and/or changes to theory.
- mr_mitm 2y agoNot really. You might think this after watching Angela Coulliers video, but when you read something like "25% of the universe's energy content is made of dark matter", they do not mean changes to some theory. They literally mean non-baryonic matter.
- OutOfHere 2y agoNope. It can mean change to some theory, without a need for matter. It is the difference between relativistic gravity and the corresponding observed mass.
- zeroonetwothree 2y agoEnergy content not only comes from matter but also from fields.
- samsartor 2y agoMy hangup with MOND is still general relativity. We know for a fact that gravity is _not_ Newtonian, that the inverse square law does not hold. Any model of gravity based on an inverse law is simply wrong. Another comment linked to https://tritonstation.com/new-blog-page/ https://tritonstation.com/new-blog-page/, which is an excellent read. It makes the case that GR has never been tested at low accelerations, that is might be wrong. But we know for a fact MOND is wrong at high accelerations. Unless your theory can cover both, I don't see how it can be pitched as an improvement to GR. Edit: this sounds a bit hostile. to be clear, I think modified gravity is absolutely worth researching. but it isn't a silver bullet
- MattPalmer1086 2y agoMOND isn't pitched as an improvement to GR. It was always a Newtonian theory - it's in its name! There are relativistic versions of MOND, for example, TeVeS [1], but they all still have some problems. [1] https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%93scalar_gravity https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%...
- samsartor 2y agoTeVeS is definitely interesting, but it still has problems like you said. AFAICT gravitational wave observations are particularly bad for TeVeS theories. TeVeS isn't dead, but if dark matter theories are criticized for being patched up post-hoc, that standard should also apply to modified gravity.
- gliptic 2y agoThe weirdest thing about TeVeS IMO is that it adds additional fields that warp spacetime, so how is it not a dark matter theory?
- MathMonkeyMan 2y agoFor the fields to be considered particles, they have to be freely propagating in space. TeVeS adds a vector field, a scalar field, and some lagrange fields that are part of their coupling. The degrees of freedom aren't consistent with one or more particles.
- Bengalilol 2y ago« Stunning evidence » … then later on: « Instead, the readings _seem_ to support a basis for MOND, which _would_ force astronomers and cosmologists to reconsider this alternative and long-controversial theory of gravity. » What’s conditional evidence? I may be missing the overall picture, but I view such writing as non precise at its best.
- MattPalmer1086 2y agoIt's just typical pop sci journalism, with a click baity headline. Read the paper instead.
- Bengalilol 2y agoThanks, I will. https://iopscience.iop.org/article/10.3847/1538-4357/ad834d https://iopscience.iop.org/article/10.3847/1538-4357/ad834d
- joe_the_user 2y agoNot entirely typical. MOND proponents seem to be trying more and more sell their approach to the public. It annoys me but I suppose every theory has to do that now, "the mouse trap must go to market now" and all.
- akvadrako 2y agoWell you have to convince somebody to pay researchers for their time, which ultimately means selling your idea to non-experts.
- bbor 2y agoWell, it’s evidence that a) must be verified on a mathematical and empirical level, and b) (arguably) fits better with a currently unpopular theory than the dominant one. There’s so many unknowns in physics that opponents can easily reply “well your theory doesn’t explain XYZ yet, so we likely just need to tweak our theory”. In other words, reasonable minds do disagree. AFAIU as an amateur.
- astroH 2y agoIn my opinion, this article is misleading at best. "...scans of ancient galaxies gathered by the JWST seem to contradict the commonly accepted predictions of the most widely accepted Cold Dark Matter theory, Lambda-CDM." --> LCDM doesn't predict what galaxies should look like, it simply predicts how much mass is in collapsed structures and that dark matter haloes grow hierarchically. In contrast, with JWST we see light and need to infer what the underlying properties of the system are. It was shown very early on that the theoretical upper limit (i.e. taking all of the gas that is available in collapsed structures and turning it into stars) predicts a luminosity function (i.e. number of galaxies per unit luminosity) that is orders of above what JWST has observed (e.g. https://ui.adsabs.harvard.edu/abs/2023MNRAS.521..497M/abstract https://ui.adsabs.harvard.edu/abs/2023MNRAS.521..497M/abstra...). This means that there is plenty of space within the context of LCDM to have bright and seemingly large and massive galaxies early on. Based on current JWST data at these early epochs, there are really no convincing arguments for or against LCDM because it's highly sensitive to the galaxy formation model that's adopted.
- uoaei 2y ago> with JWST we see light and need to infer what the underlying properties of the system are Every theory of dark matter is based exclusively on light-emitting objects. There is no "contrast" between JWST's methods and those of others. Casting aspersions on JWST because it can only see light is like casting aspersions on Galileo because he could only build telescopes. If we could teleport to the things we study and get more information that way, it would be nice, but we live in reality and must bend to its rules. > highly sensitive to the galaxy formation model that's adopted I should only need to remind the reader of the classic idiom "cart before the horse" to remind them that this line of reasoning is invalid.
- astroH 2y agoThis is a misrepresentation of what I am saying. By no means am I casting an aspersion on JWST. I am casting an aspersion on this particular observation as a test of MOND and LCDM. Also I highly disagree about your comments on my line of reasoning. The fact that you can obtain a huge range of possible galaxy properties in the context of LCDM indicates that in general, tests of LCDM and MOND that rely on galaxy formation model are in usually not strong tests. This is the key issue with using the abundance of high-z galaxies (or even their masses -- despite the fact that these aren't measured) as a test. In the context of LCDM, you need haloes to form galaxies but it has been shown many times that there are enough haloes to solve the problem (see the paper linked) by a huge amount.
- RicoElectrico 2y agoWaiting for Angela Collier to make a video on this, I'm sure many people will forward her this article. MOND is actually a niche in cosmology despite its PR.
- mgraczyk 2y agoAre any of the MOND theories consistent with this new data also consistent with recent gravitational wave observations? My understanding is that gravitational wave detectors have recently ruled out most plausible MOND theories. The linked paper doesn't seem to discuss this.
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- verzali 2y agoWhy why why do people share articles with sensational headlines like this? Its no wonder science journalism gets a bad rap. This kind of thing really undermines all the people who are actually trying to communicate science properly.
- muglug 2y agoWithout this article and HN discussion I’d never have known about MOND, which is (at the very least) a fun theory.
- trimethylpurine 2y agoPersonally, I think it would be better that way. Science works in pursuit of truth, not towards the obfuscation of it for personal and selfish financial gain. That should hopefully explain the outage that scientists have towards articles like this one. In place of relying on articles like this, you might try searching scholarly articles or subscribing to them.
- prof-dr-ir 2y agoThe trouble is that MOND is just not worth your time. In fact, I would even object to calling it a 'theory' in the first place. MOND is just some wild idea, but a little thought should convince every physicist of its uselessness. It has major issues both in explaining experimental data and in its theoretical consistency. It justifiably receives next to no attention from the vast majority of (astro)physicists. In popular science the idea however does not seem to want to die, perhaps because it is so easily explained to a layperson. Of course this is a little frustrating for the community, but perhaps we should look at the upsides: more attention for science is probably a good thing, and explaining to people why MOND is so useless provides a good opportunity to discuss some proper physics.
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- ogogmad 2y ago
- docflabby 2y agoDark matter is just made up bs if you replace "magic" for dark whenever its mentioned its the same difference - theres no tangable evidence it exists at all.
- XCSme 2y agoThere is no evidence that anything exists...
- anon291 2y agoOne can use multiple different instruments to corroborate our own senses that matter does indeed exist, whereas the explicitly inferred properties of dark matter / energy make it impossible to detect. That honestly seems more religious than anything. Yes... we can claim that the gravitational effects are what let us 'observe' it, but this is like the former view of geocentrism and then using various orbital corrections to make things work. That is to say, one can choose almost any axiom and then fit predictive models to work around it, but it doesn't mean that the axiom itself is more accurate, and indeed we should always be looking to vet our axioms anyway.
- XCSme 2y ago> own senses that matter does indeed exist What about people with schizofrenia? They can also use their senses and say something exists, when it actually doesn't.
- akvadrako 2y agoDark energy is literally this - it just means something is different than predicted by current leading theories. There is plenty of evidence that either dark matter or an alternative is needed and CDM is just the most popular take.
- smolder 2y agoIt represents a discrepancy between our models and observations of the universe, which imply there is a lot of unaccounted-for non-interactive mass out there. Dark matter generally is less a theory and more a question: Where is all this mass? Does it really exist? What can explain it? What is missing from or wrong with our understanding of physics that explains our observations? If you want to complain about a specific theory of dark matter like lambda-CDM or challenge our understanding of gravity or whatever, it'd be more correct to name the actual theory.
- I_am_tiberius 2y agoWhat I find implausible about MOND is the constant a0 (~1.2). Why stick with a measurement based constant instead of exploring a parameter that varies with distance?
- pikseladam 2y agoIt shows early galaxies forming way faster and bigger than expected, which kind of shakes up the whole dark matter idea. Seems like it supports the MOND theory—that gravity might not work the way we think. Pretty wild, but it’s still up for debate.
- steve_adams_86 2y agoThat would be a fun surprise to me as a lay person who doesn’t actually understand these things, because I see a lot of disparagement towards the MOND theory.
- maronato 2y agoFor a dark matter researcher’s take on MOND, see this video: https://youtu.be/qS34oV-jv_A https://youtu.be/qS34oV-jv_A
- Glyptodon 2y agoIs there a quantized version of MOND where the increased acceleration is because a quantized unit of gravity will exert force across distances that would otherwise suggest that that the force would be less than a "g quanta" or because maybe quantization "ceilings" more than floors at very large distances? If gravity does have some kind of particle or fundamental quantization like a photon, and basically still exerts at huge or "infinite" distance, does it make sense that it's more likely there's some kind quantization floor or maybe quantization bands or something? Or is it thought that quantization of gravity imposes a limit on distance for the exertion of gravitational attraction? (Or is it thought that that with quantized gravity that what's happening is a decreased rate of "gravitons" interacting between the objects?) Anyway, a bit clueless about this, just curious what gravitons are supposed to mean for either theory (MOND, LCDM, etc.).
- librasteve 2y agoHere is what Sean Carroll has to say about MOND… https://www.preposterousuniverse.com/blog/2011/02/26/dark-matter-just-fine-thanks/ https://www.preposterousuniverse.com/blog/2011/02/26/dark-ma... I find this treatment more compelling.
- nathan_compton 2y agoI wish science reporters would stop using MOND to stand in for all theories for which MOND is the low curvature limit. MOND itself is not covariant and has a lot of other really well known issues which make it obviously a non-starter and more sophisticated theories in the family of General Relativity reproduce MOND like behavior but are better behaved and more plausible. At the very least the term Modified Gravity or MOG should be used instead of MOND to avoid a lot of pointless back and forth about MOND.