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These articles always miss the mark because they get the history backwards. Modified gravity theories were taken much more seriously in the 1970s and earlier, w
by knzhou 6y ago
These articles always miss the mark because they get the history backwards. Modified gravity theories were taken much more seriously in the 1970s and earlier, where the evidence for dark matter was shakier. Cosmological and astrophysical observations from the 1980s to 2000s have vastly strengthened the case for dark matter, which is why it's the leading hypothesis now.
Popular articles won't tell you this, because without exception they laser focus on galaxy rotation curves, a piece of evidence that's nearly a century old and by far the weakest one. But by neglecting to mention the actual evidence we base our conclusions on, they (purposely or not) make us look like fools.
Furthermore, dark matter hasn't been ruled out. One particular candidate of what it could be (a WIMP) has been studied and mostly ruled out over the past 20 years, but by the nature of the business there are many possible candidates. And of course we've shifted attention towards those other options, which is exactly how science is supposed to work!
- sincerely 6y agoYou’ve also neglected to mention the evidence that the articles are leaving out ;0
- Filligree 6y agoFor example, galaxy collisions. Dark matter by its nature doesn't collide, so it behaves differently than the galaxies it's in when there are physical collisions, and that's visible. Basically, the galaxies stop; the dark matter keeps going. We can't directly see it, but we can see the gravitational lensing it causes. Dark matter isn't as invisible as believed. Then there are cases of galaxies which don't contain any dark matter, and therefore act the way we were expecting them to from simple gravitational mechanics on visible matter. That rules out any solution involving changes to the laws of gravity, since such a change would be universal. The fact that galaxies can exist which contain none means it must be a substance of some kind which can exist more in one spot than another.
- lostmsu 6y agoWhy does the dark matter stop? Don't colliding galaxies stop due to gravity, which dark matter is equally affected by?
- yongjik 6y agoGalaxies are drawn toward each other due to gravity, but when they meet ordinary matters "stop" because lumps of gas plow against each other, causing them to lose energy and stop. Dark matter doesn't interact with anything else (or even itself) so they pass right through each other.
- chadcmulligan 6y agoSo does that mean there are some galaxies without dark matter - because of a previous collision? And then some dark matter only galaxies, because they left their galaxy behind? I suppose this is unknown - but does the dark matter reappear in the galaxy later somehow, or it's left behind forever? Edit: well there are galaxies without dark matter (maybe) [1] and there are dark matter galaxies [2] [1] https://www.space.com/19-galaxies-missing-dark-matter.html https://www.space.com/19-galaxies-missing-dark-matter.html [2] https://www.space.com/33850-weird-galaxy-is-mostly-dark-matter.html https://www.space.com/33850-weird-galaxy-is-mostly-dark-matt...
- corty 6y agoreappearance is possible in collision sims, but the timescale is too long to be easily observable.
- The_rationalist 6y agoI remember that a few years ago a galaxy had been discovered with a claimed ~>90% of dark matter. Personally I believe the theory is epistemologically weaker than MOND
- nwallin 6y agoSame reason cars stop when they hit a brick wall. Most of the mass of galaxies isn't in their stars (which, for the most part, keep moving with the dark matter) it's in gas clouds and dust formations. When two galaxies collide, the gas and dust effectively halts. What we see when galaxies collide is that most of the mass of the galaxies move straight through each other without appreciably decelerating. And most of the matter, the gas clouds and star forming regions, halt in the middle. MOND was taken seriously as a theory from its inception in the '70s through the end of the 20th century, but because of the bullet cluster, it is effectively dead. MOND is falsified by observational evidence. For this article to discuss MOND so thoroughly, but not mention the bullet cluster once... It doesn't pass the smell test. This is pseudo-scientific trash. There's other evidence against MOND and various alternative-gravity models (there are several of them, it's a large umbrella) but most of them do a very poor job at explaining the edge cases, such as the bullet cluster and ultra diffuse galaxies. Some ultra diffuse galaxies have a conspicuous absence of gravitational lensing and stars which orbit extraordinarily slowly; ie, they have very little dark matter. If the explanation for dark matter was that we had the laws of physics wrong, the newly revised gravitational theories should apply equally well to these galaxies- which they don't.
- knzhou 6y ago- gravitational lensing measurements - velocities in galaxy clusters - getting the fine details of the complicated, wiggly cosmic microwave background spectrum right to extreme precision - getting the fine details of cosmological structure formation (as measured from matter density fluctuations on very large scales) right - galaxy collisions, like the Bullet cluster (hard to explain with modified gravity) - the fact that some galaxies have much less dark matter than others (very hard to explain with modified gravity)
- rocqua 6y agoFor the last point. Where do we get our confidence in the (light) mass estimation of a galaxy? Couldn't we be off by a factor of 10?
- knzhou 6y agoMostly the confidence comes from half a century of trying to find such "extra" visible mass! There's a long history of people looking for extra dust or gas or compact objects using a tremendous variety of clever techniques, and yet the deficit has remained. At this point, if you believe that there indeed is extra mass around, it kind of has to be dark matter by definition because it's been so damn hard to see.
- NohatCoder 6y agoThe question is how the f do we know the mass of a galaxy in the first place? We can't have the discrepancy until we have two distinct methods of measuring the mass of a galaxy.
- astroH 6y agoThe best evidence for dark matter is arguably the CMB. The strengths of the individual peaks are highly suggestive of dark matter, in particular the third peak. Furthermore, most MOND simulations on cosmological scales still require dark matter (if they evolve the Universe following the Friedmann equation), for example an 11eV sterile neutrino, because there has yet to be an underlying theory that accurately describes the evolution that can reproduce the mass function and galaxy clustering with MOND. This isn't to say that dark matter doesn't have its problem, one of which is the fact that we don't know what it is; however, there still remain numerous candidates that have yet to be sufficiently explored experimentally.
- prox 6y agoIs there a scenario where both could be right in some way, so part due to DM and part due to modified gravity?
- astroH 6y agoModified gravity is still being actively explored and I would argue taken very seriously but not in the context of dark matter, rather in the context of dark energy. So in a sense, yes, modified and dark matter combined are both possible in the same theory. However, at the moment, there is no real reason to suggest both dark matter and modified gravity are responsible for things like galaxy rotation curves because one of these works just as well without the other. In fact, many modified gravity theories apply "screening mechanisms" so they don't impact galaxy scales
- kwbauson 6y agoSabine Hossenfelder has a great series of videos about both dark matter and modified gravity theories (https://www.youtube.com/watch?v=FN2d2cmi_Gk https://www.youtube.com/watch?v=FN2d2cmi_Gk, https://www.youtube.com/watch?v=2VNcDoLNJk8 https://www.youtube.com/watch?v=2VNcDoLNJk8, and https://www.youtube.com/watch?v=468cyBZ_cq4 https://www.youtube.com/watch?v=468cyBZ_cq4). I think she does a very thorough (and relatively easy to understand) job of explaining what both of the types of theories are, the motivations for developing them, and their strengths and weaknesses. The first video is about dark matter, the second is about modified gravity (mostly focusing on MOND), and in the third she talks about how she thinks it makes sense to accept both. Even if you don't agree with her position, her videos are still very informative for anybody who wants to know more.
- mehrdadn 6y agoQuestion: as a layman, knowing nothing about the quantitative nature of the evidence, neutrinos seem like the perfect candidates for dark matter, since we've already detected them and we know interact so poorly with anything. Why are they not the obvious candidate for dark matter? Is there any evidence against them?
- hwillis 6y agothey are obvious. The evidence against it is that there are not that many of them (or we would have detected more), and while we aren't sure how much they all weigh we're pretty sure it's orders of magnitude less than what would be required for dark matter.
- Trasmatta 6y agoI believe I heard on a podcast that there's a theory that dark matter might actually be multiple things, and that neutrinos could just account for a small percentage of that. Is that correct?
- exmadscientist 6y agoThat's correct. In the standard ΛCDM cosmology, neutrinos would provide about 0.5% of the required matter. See the Particle Data Group's dark matter review: http://pdg.lbl.gov/2020/reviews/rpp2020-rev-dark-matter.pdf http://pdg.lbl.gov/2020/reviews/rpp2020-rev-dark-matter.pdf
- scythe 6y agoNeutrinos move too fast. Dark matter ought to move at nonrelativistic speeds to generate the observed effects. Another way to put this is that neutrinos are "hot" dark matter and we expect "cold" dark matter.
- knzhou 6y agoThat's a perfectly good question, and that's exactly why for a time in the 90s they were one of the leading candidates for dark matter. Unfortunately it fell apart when we got a better understanding of structure formation, which determines the large-scale distribution of matter in the universe today. If the 3 neutrinos have masses around where we think, then they'd be too light and move too fast in the early universe, smoothing the distribution out and thus suppressing small-scale clumping, contrary to observations. On the other hand, if the 3 neutrinos were a lot heavier (possible we can only detect their mass differences, not the absolute masses) they would screw up the CMB. The qualitative case was great, but quantitatively it just didn't work out.
- raverbashing 6y agoWe need better Modified Gravity theories, because 99% of the time people say "waa MOND was disproven already", yeah, actually "The MOND theory" (the modified gravity that everyone calls MOND) doesn't work out. Doesn't mean any other formulation wouldn't work out.