https://tritonstation.com/2024/06/18/rotation-curves-still-flat-after-a-million-light-years/ Skip to content Home Menu Rotation curves: still flat after a million light-years tritonstation in Dark Matter, Laws of Nature, MOND, Rotation curves June 18, 2024 1,639 Words Recent Comments [e10ab] tritonstation on Rotation curves: still flat af... [3946f] Jonathan K on Rotation curves: still flat af... [e10ab] tritonstation on Rotation curves: still flat af... [e10ab] tritonstation on Rotation curves: still flat af... [d284f] George H. on Rotation curves: still flat af... Previous Posts Previous Posts [Select Month ] Categories * Climate Change * commercial * Cosmology * Dark Matter * Data Interpretation * Dwarf satellite galaxies * Emergent Gravity * Galaxy Evolution * Galaxy Formation * JWST * Laws of Nature * LCDM * MOND * particle physics * Personal Experience * Philosophy of Science * politics * Rotation curves * Sociology * Stellar Populations * Uncategorized * Wide binaries Recent Posts * Rotation curves: still flat after a million light-years * Updated WIMP Exclusion Diagram * Aurora Over Ohio * The MHONGOOSE survey of atomic gas in and around galaxies * The Eclipse Experience Search this blog Search for: [ ] [Search] That rotation curves become flat at large radii is one of the most famous results in extragalactic astronomy. This had been established by Vera Rubin and her collaborators by the late 1970s. There were a few earlier anecdotal cases to this effect, but these seemed like mild curiosities until Rubin showed that the same thing was true over and over again for a hundred spiral galaxies. Flat rotation curves took on the air of a de facto natural law and precipitated the modern dark matter paradigm. Optical and radio data Rotation curves shouldn't be flat. If what we saw was what we got, the rotation curve would reach a peak within the light distribution and decline further out. Perhaps an illustration is in order: [ngc6946blogillustration]The rotation curve (data points, left) of NGC 6946 (right). The red line shows the expected rotation curve for the detected normal matter, which includes both the stars (yellow, from 2MASS) and atomic gas (blue, from THINGS). This provides a good description of the inner rotation curve but falls short further out. The excess observed rotation leads to the need for dark matter or MOND. Also noted is the extent of the rotation curve measured optically to the effective edge of the stars (Daigle et al. 2006; Epinat et al. 2008) and that measured with radio interferometric observations of the gas (Boomsma et al. 2008). An obvious question is how far out rotation curves remain flat. In the rotation curves traced with optical observations by Rubin et al., the discrepancy was clear but modest - typically a factor of two in mass. It was possible to imagine that the mass-to-light ratios of stars increased with radius in a systematic way, bending the red line above to match the data out to the edge of the stars. This seemed unlikely, but neither did it seem like a huge ask. Once one gets to the edge of the stellar distribution, most of the mass has been encompassed, and the rotation curve really should start to decline. Increasing the mass-to-light ratio of the stars ceases to be an option once we run out of stars*. Fortunately, the atomic gas typically extends to larger radii, so provides a tracer further out. Albert Bosma pursued this until there were again enough examples to establish that yes, flat rotation curves were the rule. They extended much further out, well beyond where the mass of the observed stars and gas could explain the data. How much further out? It depends on the galaxy. A convenient metric is the scale length of the disk, which is a measure of the extent of the light distribution. Some galaxies are bigger than others. The peak of the contribution of the stars to the rotation curve occurs around 2.2 scale lengths. The rotation curve of NGC 6946 extends to about 7 scale lengths, far enough to make the discrepancy clear. For a long time, the record holder was NGC 2403, with a rotation curve that remains flat for 20 scale lengths. Twenty scale lengths is a long way out. It is observations like this that demanded dark matter halos that are much larger than the galaxies they contain. They also posed a puzzle, since we were still nowhere near finding the edge of the mass distribution. Rotation curves seemed to persist in being flat indefinitely. Results from gravitational lensing Weak gravitational lensing provides a statistical technique to probe the gravitational potential of galaxies. Brouwer et al. did pioneering work with data from the KiDS survey, and found that the radial acceleration relation extended to much lower accelerations than probed by the types of kinematic data discussed above. That implies that rotation curves remain flat way far out. How far? Postdoc Tobias Mistele worked out an elegant technique to improve the analysis of lensing data. His analysis corroborates the findings of Brouwer et al. It also provides the opportunity to push further out. Weak gravitational lensing is a subtle effect - so subtle that one must coadd thousands of galaxies to get a signal. Beyond that, the limiting effect on the result is how isolated the galaxies are. Lensing is sensitive to all mass; if you go far enough out you start to run into other galaxies whose mass contributes to the signal. So one key is to identify isolated galaxies, and restrict the sample to them. KiDS is large enough to do this. Indeed, Mistele was able to show that while neighbors^+ were a definite concern for elliptical galaxies, they were much less of a problem for spirals. Consequently, we can trace the implied rotation curve way far out. How far out? In a new paper, Mistele shows that rotation curves continue way far out. Way way way far out. I mean, damn. [weaklensingrotationcurve_pr]The average rotation curve of isolated galaxies (blue points) inferred from KiDS gravitational lensing data. This remains flat well beyond a million light-years with no end in sight. The width of the figure is the distance between the Milky Way and Andromeda. For comparison, the rotation curve of a single galaxy, UGC 6614, is shown in red. An image of the galaxy is shown to scale centered at the origin. UGC 6614 was selected for this illustration because it has a comparable rotation speed to the KiDS average and because it is one of the largest galaxies known: the red points are already a very extended rotation curve. Image credit: Mistele, Lelli, & McGaugh 2024. Optical rotation curves typically extend to the edge of the stellar disk. That's about 8 kpc in the example of NGC 6946 given above. Radio observations of the atomic gas of that galaxy extend to 17 kpc. That fits within the first two tick marks on the graph with the lensing rotation curve. UGC 6614 is a massive galaxy with a very extended low surface brightness disk. Its rotation curve is traced by radio data to over 60 kpc. It is one of the most extended individual rotation curves known. The statistical lensing data push this out by a factor of ten, and more, with no end in sight. The flat rotation curves found by Rubin and Bosma and everyone else appear to persist indefinitely. So what does it mean? First, flat rotation curves really are a law of nature, in the same sense of Kepler's laws of planetary motion. Galaxies don't obey those planetary rules, they have their own set of rules. This is what nature does. In terms of dark matter halos, the extent of isolated galaxy rotation curves is surprisingly large. Just as we come to the edge of the stellar disk, then the gas disk, we should eventually hit the edge of the dark matter halo. In principle we can imagine this to be arbitrarily large, but in practice there are other galaxies in the universe so this cannot go one forever. In the context of LCDM, we now have a pretty good idea of how extended halos should be from abundance matching. A galaxy of the mass of UGC 6614 should live in a halo with a virial radius of about 300 kpc or less. There is some uncertainty in this, of course, but we really should have hit the edge with the lensing data. There should be some sign of it, but we see none. One complication is the so-called 2-halo term. In addition to the primary dark matter halo that hosts a galaxy, when you get very far out, you run into other halos. Isolated galaxies are selected to avoid this to the extent possible, but eventually there will be some extra mass that causes extra lensing signal that would cause an overestimate of the rotation speed. I'll forgo a detailed discussion of this for now (see Mistele et al. if you're eager), but the bottom line is that it would require some unnatural fine-tuning for the 1+2 halo terms to add up to such flat rotation curves. There ought to be a perceptible feature in the transition from the primary halo to the surrounding environment. We don't see that. In the context of MOND, a flat rotation curve that persists indefinitely is completely natural. That's what an isolated galaxy should do. Even in MOND there should be an environmental effect: the mass of everything else in the universe should impose an external field effect that eventually limits the extent of the rotation curve. How this transition happens depends on the density of other galaxies; by selecting isolated galaxies this effect is put off as much as possible. Hopefully it will be detected as the data improve from projects like Euclid. The primary prediction of MOND is an indefinitely extended rotation curve; the external field effect is a subtle detail. Yet again, that is what we see: MOND gets it right without really trying, and in a way that makes little sense in terms of dark matter. Sometimes I wish MOND had never been invented so we could claim to have discovered something profoundly new, or at least discuss the empirical result without concern that the data would get confused with the theory. MOND predictions keep being corroborated, yet the community persists in ignoring its implications, even in terms of dark matter. It's gotta be telling us something. We have a press release about this result, so perhaps you will see it kicking around your news feed. --------------------------------------------------------------------- *We could, of course, invoke dark stars, but that's just an invisible horse of a different color. ^+There is a well known correlation between morphology and density such that elliptical galaxies tend to live in the densest environments. This means that they are more likely to have neighbors that interfere with the lensing measurement, so finding that identifying isolated ellipticals with a clean lensing signal is more challenging that finding isolated spirals comes as no surprise. Isolated ellipticals do exist so it is possible, but one has to be very restrictive with the sample. Share this: * Twitter * Facebook * Like Loading... [e10abfeb8f10e] tritonstation Stacy McGaugh is an astrophysicist and cosmologist who studies galaxies, dark matter, and theories of modified gravity. He is an expert on low surface brightness galaxies, a class of objects in which the stars are spread thin compared to bright galaxies like our own Milky Way. He demonstrated that these dim galaxies appear to be dark matter dominated, providing unique tests of theories of galaxy formation and modified gravity. Professor McGaugh is currently the chair of the Department of Astronomy at Case Western Reserve University in Cleveland, Ohio, and director of the Warner and Swasey Observatory. Previously he was a member of the faculty at the University of Maryland, having also held research fellowships at Rutgers, the Department of Terrestrial Magnetism of the Carnegie Institution of Washington, and the Institute of Astronomy at the University of Cambridge after earning his Ph.D. from the University of Michigan. Published June 18, 2024 Post navigation Updated WIMP Exclusion Diagram 43 thoughts on "Rotation curves: still flat after a million light-years" 1. [47504] jeremyjr01 says: June 18, 2024 at 6:56 pm "yet the community persists in ignoring its implications" The typical signature of cultist behavior. They're too much ideological and economic interests behind the "dark matter" thing to be discarded easily. LikeLike Reply 1. [47504] jeremyjr01 says: June 18, 2024 at 9:13 pm That they're new, irreducible natural laws at galaxy complexity level is obviously a direct challenge to the ever present reductionist mindset in mainstream scientific thinking, that is even more unacceptable to particle physicists naive reductionism, their WIMPs "hypothesis" can't be more weak. Particle physicists assume that they are the only ones making "fundamental" research and only laws found at that level are fundamental, but as P. A. Anderson already said: "at each level of complexity entirely new properties appear, and the understand- ing of the new behaviors requires re- search which I think is as fundamental in its nature as any other". LikeLike Reply 2. [4cd3d] Tom says: June 18, 2024 at 7:08 pm Wow! (As in better than that wow) LikeLiked by 1 person Reply 3. [c8980] budrap says: June 18, 2024 at 9:37 pm Maybe you should just ignore dark matter. There's a lot more justification for that than ignoring MOND. MOND at least demonstrates that the problem lies with our gravitational model (s). MOND doesn't fix the problem though, it just patches the old models and it shares with them a central flaw - it only describes the gravitational effect but does not describe the causal physical mechanism. All physical effects have to have physical causes, don't they? LikeLiked by 1 person Reply 1. [47504] jeremyjr01 says: June 18, 2024 at 10:01 pm "All physical effects have to have physical causes, don't they?" Not necessarily if that "effect " is a new fundamental, irreducible property. Like an independent axiom that is irreducible from other axioms. Reductionism is intrinsically limited by assuming that almost anything should have "physical causes", the reality of new irreducible properties/behaviors is ignored by this approach. LikeLike Reply 2. [127a5] David Merritt says: June 19, 2024 at 8:12 am Sorry, but I don't understand the question. For years after Newton proposed his theory of gravitation and motion, many scientists could not accept it because it included (what we would now call) "action at a distance." Like you, apparently, these scientists demanded a "cause" -- something physically in contact with the planets, pushing them along in their orbits. (Angels maybe?) Newton's theory predicts elliptical orbits for the planets. That is all the "cause" you get and all you need. And no, the "cause" of the observed motions is not simply the force from the Sun. There are many possible non-Newtonian theories that include a central force but do not predict elliptical orbits (e.g. post-Newtonian theories). And many theories that do not include central forces as Newton understood them but which do predict elliptical orbits (e.g. Einstein's!). Likewise, Milgrom's theory predicts flat rotation curves for galaxies. That's your "cause" and that's all you're ever going to get. LikeLiked by 1 person Reply 1. [c8980] budrap says: June 19, 2024 at 11:15 am Milgrom's theory predicts flat rotation curves for galaxies. That's your "cause" and that's all you're ever going to get. As a summary of the mathematicist philosophy that's pretty good. It gets right to the heart of the problem that has turned modern theoretical physics into a wasteland of mathematical fantasies. The two "standard models" of academic physics do not bear any resemblance to empirical reality - because they are full of the modern equivalent of angels and devils - dark matter, dark energy, quarks, gluons, etc.. So we don't know the cause of the gravitational effect and therefore we can't know the cause and anyway some math is the cause? That's mathematicism or, if you like, instrumentalism alright, but those are philosophies not science. Science is supposed to be the study of physical reality which excludes angels and devils and should exclude their modern undetectable variants because those things (ancient and modern) are not present in physical reality. Scientists do want to know how gravity works, mathematicists don't care. BTW, you do know that Newton himself was dissatisfied with the "action at a distance" account of gravity, don't you? That's what makes him still a great scientist. He didn't hide behind his math. He was evaluating his model in terms of physical reality and found it wanting. He understood that his model's failure to explain the cause of the gravitational effect was a shortcoming. No such thoughts intrude upon the mathematicist belief system of course. And so the ongoing crisis in physics will continue until mathematicism is returned to the philosophy and mathematics departments for a full refund and the scientists, like Dr McGaugh, who actually study physical reality are put in charge of theoretical physics, not as it is now, the other way around. Modern theoretical physics has devolved over the last 40+ years into the study, not of physical reality, but of a pair of "chosen models" which are treated as unassailable truths that can be tinkered with at the margins but whose central axioms and conceits cannot be questioned. Forty years have been wasted chasing mathematical fantasies and all we have to show for it are two absurd standard models in which empirical reality is only described in terms of entities and events that make no appearance in physical reality. That's forty years too long. Math is not physics. LikeLike Reply 1. [c2c73] Maarten Havinga says: June 19, 2024 at 11:36 am It is fitting to be content with Merritt's answer. The universe's laws are very complicated. While locality in a set of laws can give certainty and confidence of having formulated proper laws, it is way too easy to theorize a local theory above a not yet well understood mathematical action at a distance - it's something people can understand. Flogiston is a good example: some substance burns in whatever is burning. The mathematics of oxidation were instead way too abstract in the beginning, before chemical reactions began to be widely studied. But I agree that things have to work in some way. LikeLike Reply 2. [3946f] Jonathan K says: June 19, 2024 at 3:49 pm It didn't matter that Newton didn't have a cause - the basic mathematics was plenty at the time. And it went on like that, mathematics but often no picture. In the last century theories got so weird that some argued no picture existed. But in other areas, as we learn more, we find causes. Now the data has got so good that to make real progress this century, we need the underlying picture Einstein and Wheeler both said would be found in the future (several quotes from each of them on it). If there were courses on conceptual physics, we'd probably have found it by now, but as you say, mathematicism is everywhere. I've found a way to shorten the evidence for one possibility, helical path refraction, down to a few lines. I hope this ultra-compressed version is of interest. Talking of causes, this would be one. (from Kerr, 2023) ---------------------- In well known refraction, a light beam in a graded or layered medium has all points on its path linked via Snell's law. For two points any distance apart, if 4 numbers are put into Snell's law (two angles to the normal, two local speeds for light), the sides of the equation will always agree, and their agreement shows refraction is at work. sin (theta[1]) / sin (theta[2]) = S[1]/S[2] This can also be applied to orbits, with a slight variation on the same equation. It is applied to matter, assuming it is like light at a very small scale. The 4 terms are replaced by terms for the angle to the normal, and the local speed of light: sin (arccos [v[1]/c]) / sin (arccos [v[2]/c]) = (1 - [2GM/r[1]c^2])^1/2 / (1 - [2GM/r[2]c^2])^1/2 Numbers are put in for any orbit: open, closed or radial, around any spherical mass M. v[1] and v[2] are speeds at radii r[1] and r[1]. The two sides always agree, to ~ 16 decimal places. Again, their agreement shows refraction is at work. Only on radial paths is this a near-proof, as there's no need to bring in the dimensions, only a graded medium surrounding the mass, and matter on helical paths, so it's straightforward. Any comment, particularly from Stacy, would be appreciated. LikeLike Reply 1. [c8980] budrap says: June 19, 2024 at 5:23 pm I think that is indeed a good approach - but it begs the question, what is the physical nature of the graded medium? LikeLike Reply 1. [341b0] shermanlj says: June 19, 2024 at 6:00 pm Space has mass of it's own. And is distorted ("warped" "concentrated") by matter. LikeLike Reply 2. [3946f] Jonathan K says: June 19, 2024 at 6:29 pm It's very small-scale waves in space itself. At a scale like the Planck scale, a mass, say a planet, is a lot of rotating disturbances in the circular dimensions. The mass ultimately consists only of vibration, and it gives off weaker vibrations that dissipate in the radial direction. That makes a graded refractive medium surrounding the mass, which moves away, thinning out, so the field keeps its shape, and from a distance could look like a solid object. The transmission speed of space increases radially, but is constant at any point in the field. Matter nearby, being like rotating light, gets refracted along helical paths. At large scales an excess of the medium builds up, and is what is taken to be DM. In a different situation, the main drawback of PSG would be one has to assume this invisible medium exists - in a gravity theory it can seem 'excess baggage'. But because of the present need for DM, similar assumptions are widely being made anyway, and it might be said that the medium in PSG has 'more reason to be there' than DM. LikeLike Reply 2. [c2c73] Maarten Havinga says: June 19, 2024 at 11:19 am Indeed it is enough in order to be content, at the current state of potency of scientific analysis. A graviton particle would be great, however the details of the MOND law cannot really be deduced from how much gravitons and so on (it would anyhow on first-order perturbations be a inverse square law). It's a beautiful mess. We'll have to settle on being content with 'spooky action at a distance' like Newton did for now. It's clear that MOND is what the data tell, figuring out how (how and to what extent locality is preserved) will be up to the coming generations. LikeLike Reply 3. [d284f] George H. says: June 20, 2024 at 12:08 pm Huh, well I guess this is just a matter of taste. But I don't think of Mond as a theory, but more as some sort of observational fit to the data. Kinda like Kepler's laws, or the ideal gas law. And I do hope some day that we will find the underlying 'thing' that explains it. Though this may just be a pipe dream and may never come true. Still I find it fun to think about.... LikeLike Reply 4. [57459] Laurence Cox says: June 19, 2024 at 7:51 am OK, let's look at this from a dark matter viewpoint and see if we can prove a falsehood. The original idea was that each galaxy was surrounded by a (spherical?) dark matter halo, which might extend to, say, 20 scale lengths at most. Now we have a requirement for a 'halo' that extends for perhaps 100 or more scale lengths. So, we are looking at a 'dark matter' mass that might exceed 5 cubed times the previous estimate (say a factor of 100). At what size of halo does the total 'dark matter' imputed around galaxies exceed the total amount of 'dark matter' allowed in the universe in the ^CDM model? LikeLike Reply 1. [e10ab] tritonstation says: June 19, 2024 at 5:09 pm A flat RC implies an enclosed mass that increases linearly without limit - hence one reason why it can't persist forever. But the community wrapped its head around that a long time ago, so as usual, good luck proving a negative. LikeLiked by 1 person Reply 1. [47504] jeremyjr01 says: June 19, 2024 at 9:31 pm It took thousands of years to recognize that the parallel axiom in Euclidean geometry was irreducible from the other axioms leading to the discovery of non Euclidean geometries by Gauss, Bolyai and Lobachevsky. It's intrinsic human nature to look for underlying reasons, for "explanations", but sometimes they're not hidden reasons or hidden variables (remember Einstein and quantum mechanics), nature can't care less about humans wishful thinking and if mathematics is any guide irreducibility is the norm not the exception, "understanding" is overrated, many times knowing is the best that we can do. Hopefully we don't have to wait thousands of years for the "community" to acknowledge the presence of irreducible properties at any level of nature complexity hierarchy, including cosmic hierarchies. LikeLike Reply 5. [8dd3d] srobinson551 says: June 19, 2024 at 6:48 pm A couple more questions please. (1) How actually does one determine the rotation velocity of (i) the orbiting stars, (ii) the gas beyond the stars? (2) Spiral arms are often less tightly wound towards the edge of the disc. In the case of NGC 6614, the extreme ends of the two major arms are so far unwound that the stars associated with them - and any gaseous extrapolation of them - cannot be rotating round the nucleus at all; the arms are at too much of a tangent. Is all this taken into account, and could you perhaps adumbrate? LikeLike Reply 1. [e10ab] tritonstation says: June 19, 2024 at 7:23 pm 1. Velocities are measured via the Doppler effect. 2. Spiral structure is a rich and interesting topic quite beyond the scope of this reply. What we perceive as spirals are not physical objects but rather a collective phenomenon like waves. Individual stars can orbit in nearly circular orbits that take them in and out of the spirals arms without regard to their pitch angles. We see this directly in the Milky Way where we have enough information from Gaia to work out orbits for many stars; the typical star like the sun is on a low eccentricity orbit that does not keep it in a given spiral arm indefinitely. We are not in one at the moment, but presumably have been in the past. LikeLiked by 1 person Reply 1. [8dd3d] srobinson551 says: June 20, 2024 at 7:37 am I wasn't meaning to go into the question of 'density waves', whose significance for spiral arm formation is contested (Sellwood & Masters 2022), and I appreciate that stars do not entirely align with the arms. But surely that is where most stars occur. Stars are seen in the optical, and the most obvious feature of galaxies beyond the MW are the optically visible arms. I was asking about the trajectory of stars in the arms near the disc's optical edge, where the arms, composed of stars and gas, drift tangentially away from the nucleus and cease to rotate around it. How is one to understand the 'rotation' curves of stars and gas from that point on? LikeLike Reply 1. [e10ab] tritonstation says: June 20, 2024 at 8:50 am Yes, spiral arms are a concentration of stars. That's all I meant by the analogy to waves, I was not specifically advocating nor requiring that spiral arms be density waves. No, stars and gas do not cease to rotate around the nucleus near the optical edge. I don't understand where you get that idea. LikeLike Reply 1. [8dd3d] srobinson551 says: June 20, 2024 at 10:09 am I was trying to avoid making general assertions, my query being illustrated by the specific instance of NGC 6614, the galaxy in your figure and a larger image of which I linked to in case my description was not clear enough, as it appears not to have been. Let me try another tack. Armed galaxies are for the most part spiraliform rather than a series of concentric circles, so the distance from r = 0 to any point along the arm will always be greater than the radial distance to that point. In some cases arms expand so much that at their extremes they seem almost to escape the gravitational field (e.g. NGC 6614, 2336, 2442, 6872). If there is any relation at all between stellar orbits and the arms, then the orbital velocities will be different than if the orbits were circular. Since it is not possible to image the motions of individual stars in faraway galaxies, are rotation curves based on the assumption that orbits are in fact circular? Regarding very faraway galaxies, I note that according to one study (Genzel, Nature 2017) the rotation curves of high-redshift discs show an ultimate decrease in rotation velocity, ascribed partly to less DM around the outer disc and partly to greater velocity dispersion. 'Our analysis leaves little space for dark matter in the outer disks (and inner halos) of massive, high-redshift star-forming galaxies' - but by the same token suggests that MOND is not universal? This evolutionary aspect must have been researched a good deal more in the 7 years since, but as the study has attracted >300 citations, I'm a little loath to follow up. LikeLike Reply 1. [e10ab] tritonstation says: June 20, 2024 at 10:31 am Orbits are of course not perfectly circular, but the deviations therefrom are measured to be small in spiral galaxies. There is an effect called asymmetric drift, in which the eccentricities of stellar orbits grow over time - this is observed in detail in the Milky Way. We know how to correct for it in other galaxies; in most cases it is a small effect. I discussed the claim of declining rotation curves that you cite in https:// tritonstation.com/2017/03/19/ declining-rotation-curves-at-high-redshift/ The short answer is that this claim is overblown. For all the metrics that I can measure in the same way for both samples, there is no apparent difference between local galaxies and those at high redshift. LikeLike Reply 6. [f5ffc] Arun says: June 19, 2024 at 10:31 pm If we had a single galaxy in a contracting universe, would we expect to see flat rotation curves? Once the contraction was more significant than the galaxy's gravity, wouldn't the rotation curve flatten? LikeLike Reply 1. [e10ab] tritonstation says: June 20, 2024 at 8:53 am That is a hypothetical I hadn't considered. The universe being much much larger than a single galaxy, it shouldn't matter to a galaxy's rotation whether it is expanding or contracting. It the contraction gets to the point where it is more significant than the galaxy's gravity, then we are close to a Big Crunch and have bigger problems than the shapes of rotation curves. I see no reason why this situation would lead to flat rotation curves. LikeLike Reply 7. Pingback: Xuan Zhuan Qu Xian :Bai Mo Guang Nian Hou Reng Ran Ping Tan - Pian Zhi De Ma Nong 8. [88679] question says: June 20, 2024 at 9:29 am If the analysis is using weak gravitational lensing (an effect of general relativity), how does it fit together with MOND? If lensing is used in the analysis, do we already assume something about gravity that would prevent us from making conclusions about whether or not gravity needs to be modified into the direction MOND? Thx for clearing up my confusion LikeLike Reply 9. [e10ab] tritonstation says: June 20, 2024 at 9:53 am Good question. We basically assume that lensing gets the same boost as kinematics. That was a hang-up in writing a GR extension of MOND for a long time. TeVes (Bekenstein 2004) was the first theory that could do this. TeVeS wasn't the final theory, but showed how it can be done. I consider this to be a requirement for theory, either as an extension of GR (lensing and kinematics must both be boosted in the low acceleration regime as in MOND) or for a dark matter theory (one has to construct a halo that knows about the baryon distribution and continues to large radii indefinitely with the density falling as 1/r^2). LikeLike Reply 10. [45a3c] JB says: June 20, 2024 at 10:39 am I would imagine that MOND's "why?" is just as interesting a question as what kind of particle could comprise dark matter. At the very least, it could be simply that the prior assumptions about the nature of spacetime are incomplete. LikeLike Reply 1. [e10ab] tritonstation says: June 20, 2024 at 10:45 am Indeed. Why does MOND happen? That is does happen is well established. How it can possibly be so is the head-scratcher. That's what makes it such an important question. LikeLike Reply 11. [ce087] Stephane Dubedat says: June 20, 2024 at 11:25 am I suppose such observation is also consistent with the minimal acceleration from quantized inertia? LikeLike Reply 1. [e10ab] tritonstation says: June 20, 2024 at 12:03 pm Anything that reproduces MOND will also get this right. I have seen versions of quantized inertia that come close, but none that are quite right. Usually the issue is around a0; the behavior converges to MOND at low accelerations so is probably OK here. LikeLike Reply 12. [d284f] George H. says: June 20, 2024 at 11:28 am Wow, this is great. Congrats to all the authors. So two questions (for which I'm too lazy to try and look up the answer for myself.) Your previous post on weak lensing data (from Brouwer et al) extended the acceleration range down to about 10^-12.5 m/s^2. What's the new range? And second, there is some length/ acceleration scale for which Mond fails... galactic clusters. What is that acceleration? And is there any hope to get to that range in the lensing data? (Three questions; Our three weapons are fear, surprise, and ruthless efficiency, and fanatical devotion to the pope... :^) LikeLike Reply 1. [e10ab] tritonstation says: June 20, 2024 at 12:30 pm Thanks. The range of accelerations is the same, but the credible portion is larger. How much larger remains a matter of judgement. 10^-13, perhaps. MOND fails in clusters because their accelerations are too high by a factor of ~2. This happens in the vicinity of a0; see https://arxiv.org/abs/2303.10175 I hope to write a post about all these things soon. Yes. Soon. Tobias is looking into what can be done with lensing for clusters. Not clear yet what we can add, so definite maybe. LikeLike Reply 1. [d284f] George H. says: June 20, 2024 at 1:27 pm Re clusters: Ahh thanks, I was confused. Clusters have the same acceleration range a_0, it's just that the effect is stronger... or that we are missing some of the mass. https://tritonstation.com/2024/02/06/ clusters-of-galaxies-ruin-everything/ LikeLike Reply 1. [e10ab] tritonstation says: June 20, 2024 at 2:10 pm yes. Just so. LikeLike Reply 1. [3946f] Jonathan K says: June 20, 2024 at 2:25 pm The 'distinct RAR' in clusters paper https:// arxiv.org/abs/2402.12016 showed the large-scale pattern with more certainty - similarites and differences to individual galaxies. Is it possible that the equivalent of a[0] in clusters is around 2e-9? In galaxies, the transition (if one takes it that way) often starts around the same place, 2e-9, and ends at a[0]. Is it a valid approach to say both transitions may start at 2e-9, but in clusters it's quick, while in galaxies it's gradual? LikeLike Reply 1. [e10ab] tritonstation says: June 20, 2024 at 2:48 pm Yes, insofar as one ends up with two parallel sequences like that. There is a lot more scatter in the cluster data, so one might also say the relation gets fuzzy at that point, with a band of higher acceleration points. Any way you look at it, it is hard to explain in any paradigm. LikeLike Reply 13. [ce087] Stephane Dubedat says: June 20, 2024 at 11:54 am Isn't this weak lensing in contradiction with the more robust GAIA astrometry? (at least for the milky way) https://arxiv.org/abs/2309.00048 LikeLike Reply 14. Pingback: Rotation curves: still flat after a million light-years xqcgrek2 on June 20, 2024 at 06:04 Hacker News: Front Page - Bharat Courses 15. [e10ab] tritonstation says: June 20, 2024 at 12:21 pm IF the Milky Way rotation curve declines in a Keplerian fashion, then the Milky Way is different from every other galaxy in the universe. So yes, this aspect of the Gaia result you cite is in contradiction with more robust data of many varieties, including the lensing reported here. It's not that the Gaia data aren't robust, it's that they are much more involved to interpret. For starters, it is not correct to say there is a contradiction between our result and Gaia; there many aspects of the Gaia data that conform well to the radial acceleration relation. It is only the portion of the Gaia data at large radius where there are few stars where there is a potential discrepancy. I've discussed this extensively in a series of posts: https://rogue-scholar.org/posts/10.59350/ yj9c7-kvs82 | https://rogue-scholar.org/posts/10.59350/ nr1qw-99t29 | https://rogue-scholar.org/posts/10.59350/ ndyxv-dng39 | https://rogue-scholar.org/posts/10.59350/ 1hskp-b5a62 We've reached the point with Gaia data where we have to worry about all the cross terms in the Jeans equation. The apparent problem arises where these go whacky - see Fig. 3 of https:// arxiv.org/abs/2405.19028. So I suspect the apparent discrepancy will resolve when all this is taken into account. It is on my to-do list, but it will take some time to get to. LikeLike Reply 16. [4beb5] Poul-Henning Kamp says: June 20, 2024 at 1:14 pm It used to be that neutrinoes "have no mass", but now we know they do in fact have mass. We also know they dont interact much with anything else. So doesn't it follow that most neutrinoes must still be around? If so, they must more or less have distributed their mass uniformly throughout the universe? Is everybody still ignoring the collective mass of all the neutrinos? LikeLike Reply 1. [e10ab] tritonstation says: June 20, 2024 at 2:09 pm Yes. A remarkable turn of events. Yes. Yes. Yes. No. Neutrinos have to be considered when fitting the Planck CMB power spectrum, for example. This provides the strongest formal upper limit on the sum of neutrino masses. It also provides a potential test of LCDM, as I've previously pointed out: if a laboratory experiment (like KATRIN) were to measure a neutrino mass larger than the Planck limit, it would [in principle] falsify the LCDM structure formation paradigm. LikeLike Reply Leave a comment Cancel reply [ ] [ ] [ ] [ ] [ ] [ ] [ ] D[ ] Blog at WordPress.com. Menu Menu * Science Blog * Triton Station * Comment * Reblog * Subscribe Subscribed + [croppe] Triton Station Join 297 other subscribers [ ] Sign me up + Already have a WordPress.com account? Log in now. * + [croppe] Triton Station + Customize + Subscribe Subscribed + Sign up + Log in + Copy shortlink + Report this content + View post in Reader + Manage subscriptions + Collapse this bar Loading Comments... Write a Comment... [ ] Email (Required) [ ] Name (Required) [ ] Website [ ] [Post Comment] %d [b]