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Dark Matter Sighting Gets a Reprieve in New Analysis
- nonbel 8y ago>"The galactic center shines too brightly, like the glow of a metropolis at night where maps show only a town. [...] The new study, along with two others that came out in March, reopens the possibility that space-based instruments have found the first direct evidence of the elusive “dark matter” thought to pervade the universe." Please give an example of what dark matter cannot explain.
- nonbel 8y agoI am serious. This is what I have seen: Galactic rotation curves exactly as predicted: explained by dark matter Galactic rotation curves not as predicted: explained by dark matter Area of sky brighter than predicted: explained by dark matter Area of sky dimmer than predicted: explained by dark matter.
- yorwba 8y agoX exactly as predicted (by a theory that includes dark matter): explained by dark matter X not as predicted (by a theory that doesn't include dark matter): explained by dark matter Not exactly contradictory if you add the context of how the predictions are made.
- nonbel 8y agoI never said it was contradictory. I think what they've done is set up a system to filter out all theories except those that are flexible enough to be consistent with all possible observations. Specifically, they measure some value for each galaxy: x +/- e. Where e is calculated for a 95% (or whatever) statistical interval. In each paper they study another galaxy and check whether the prediction is within x +/- e, if they find one that is not there are two branches: 1) If it is a flexible theory: they adjust the theory, arbitrarily fit some parameters, etc. 2) If it is a very exact theory that cannot be adjusted: they reject the theory. Obviously this practice is flawed because the 95% interval says if you look at 100 galaxies and the theory is correct you should expect 5 of them to be outside your interval.
- mirimir 8y agoThis bothers me too. Theories with too many adjustable parameters aren't really testable. And so, following Popper, I don't see how they can actually be scientific.
- nonbel 8y agoCheck out Imre Lakatos. "Falsifiability" is too strict since "every model is wrong". Lakatos calls that Popper0, which is not what Popper ever advocated. In a nutshell: The real thing going on is you are supposed to use Bayes' rule, where you normalize how well your explanation explains the observations ( p(H[0])p(H[0]|D) ) to the sum of all known explanations ( p(H[0:n])p(H[0:n]|D) ). Thus if you have a vague explanation that can fit anything, it won't be much better than a million other possibilities so who cares. We only care when someone comes up with an accurate prediction that would be otherwise surprising. There are many more interesting ideas too (eg, "degenerating research programmes"). Edit: I should also emphasize that my main point above is that there is a misuse of statistics going on though.
- mirimir 8y agoYeah, I know Lakatos. > We only care when someone comes up with an accurate prediction that would be otherwise surprising. Sure. But the problem is when predictions don't pan out, parameters just get tweaked. Or new perturbations are introduced. Theories just keep getting more and more complicated.
- nonbel 8y agoYes, that is called a "degenerating research programme". The theory lags the observations.
- nonbel 8y agoTypo: ( p(H[0])p(D|H[0]) ) to the sum of all known explanations ( p(H[0:n])p(D|H[0:n]) ).
- XorNot 8y ago
- Avshalom 8y agoNot entirely sure what you'r referring to by area of the sky but as to rotation curves it's: -Galactic rotation curve not as predicted by relativity: explained by dark matter -Galactic rotation curve exactly as predicted by relativity: allowed by dark matter, not explained by theories that propose a different uniform law of gravity that also explains case A.
- nonbel 8y agoThis is not my point here, but the galaxy you refer to is consistent with MOND: https://tritonstation.wordpress.com/2018/04/04/the-dwarf-galaxy-ngc1052-df2/ https://tritonstation.wordpress.com/2018/04/04/the-dwarf-gal...
- joe_the_user 8y agoIt seems like the title should be something like "Dark Matter Sighting Get Reprieve". Dark matter is pretty well established, it's just a supposed direct sighting that now seems to be getting a second look.
- guelo 8y agoRight. The article doesn't explain why an electromagnetic sighting is better than the gravitational one.
- webmaven 8y agoThat's because it isn't "better", it's just different. The unspoken assumption is that two different types of evidence are generally better than just one type, not that one type is better than the other.
- joe_the_user 8y agoWell, I think this supposed "sighting" of dark matter rests on an assumption that this version of dark matter actually decays via particle-anti-particle annihilation. If that is true, then one could begin to describe or speculate on it's quantum and electromagnetic properties and look for other places these properties manifested. IE, the matter would be no longer literally dark but could be fit within our broader understanding of atomic particle ... or might force us to expand this understanding. Either way, it seems like a significant advance.
- dang 8y agoOk, we'll add a sighting above.
- User23 8y agoCan someone please explain how dark matter, which can’t be directly observed, is mathematically any different than adding an arbitrary parameter to the equation and tuning it to fit? I can’t help but think of von Neumann’s elephant. This isn’t me trying to be clever, I’d really appreciate some clarity on this.
- ww520 8y agoDark matter cannot be observed in the electromagnetic spectrum but they can be and have been observed via interaction with gravity.
- XorNot 8y agoIt's probably worth considering this question from the perspective of how we know most subatomic particles are real. They can't be directly observed either - the LHC and other particle accelerators only ever observe electrical disturbances in their sensors following a pattern of results from decay product interactions. No one sits down with a microscope and says "yep that's a gluon". EDIT: Consider that the Higgs boson wasn't "directly" observed till very recently but was assumed to exist for decades prior for the exact same reasons - you needed to add an additional term to the equations to break the symmetry and cause particles to have mass, which in turn implied a pervasive field throughout the universe we hadn't observed yet. Which in turn implied it should exist discreetly under high energy conditions as a particle, which became the Higgs.
- nonbel 8y agoI thought the extra term was added so the equations didn't result in probabilities outside the 0-1 range. > "The Higgs boson gives mass to other elementary particles such as quarks and electrons, and its existence is necessary for the theory to give sensible results (i.e., the probability of processes to occur cannot be greater than 100 percent)." https://www.albany.edu/news/87798.php https://www.albany.edu/news/87798.php
- pchiusano 8y agoIt’s not much different. That doesn’t mean DM is wrong, though! Logically, there’s two possibilities: laws of gravitation are 100% correct at all scales and the discrepancies we see at level of galaxy rotations, etc, are due to matter we can’t yet observe in other ways (hypothesized dark matter). Other option is gravity dynamics are in need of further refinement, much like GR refined Newtonian gravity. Or some combination... maybe GR isn’t the full story but dark matter is also a real thing. Earlier discussion: https://news.ycombinator.com/item?id=19605065 https://news.ycombinator.com/item?id=19605065 I think there’s often sloppiness in how things are stated - dark matter is inferred based on observation of X (edges of galaxy rotate too fast) and an assumption that GR is 100% correct. X is then said to be “evidence for DM”, but there’s that unstated premise.
- zw123456 8y agowouldn't it be crazy if dark matter turned out to be Dyson Spheres ?
- cletus 8y agoThis is basically impossible unless someone has figured out a way to break the laws of thermodynamics. And if that's true a whole bunch of stuff goes out the window. Basically a Dyson Sphere is about the least subtle thing a civilization could do and would be detectable from hundreds of millions of light years away. All energy ends as heat. The only way to get rid of heat in space is to radiate it away. It radiates away in a predictable way based on the temperature of the object. At any sort of temperature we're talking about this would be a massive glowing infrared cloud with the energy output of a star. It'd be hard to miss that. Think about this: a Dyson sphere here would be absorbing the energy output of a star. That energy has to go somewhere. Another point against it is the Elapsed Time Argument, which can be used to effectively disprove many such hypotheses. The argument goes like this: if a given phenomenon is artificial it should be less common the more distant you look. At the edge of the observable universe we're looking at a universe significantly younger than ours. Because of this something artificial should be less common further away because less time has elapsed there. If that's not the case it's almost certainly not artificial. It's also worth pointing out that dark matter out-masses regular matter by 10-to-1 or more. That's a lot to attribute to Dyson Spheres. That would make the universe an extremely populated place where we somehow still can't detect anyone.
- colanderman 8y agoTangential to your point, but is there any particular reason a Dyson sphere must "radiate away in a predictable way" (like the black body you described)? Surely a civilization sufficiently advanced as to create a Dyson sphere might either (a) direct excess energy in a beam to some satellite civilization or (b) store excess energy as matter, rather than just jettison it into space?
- mrmonkeyman 8y agoIt goes towards the energy needs of the civilization? I'm confused why you think they harnass a sun just to radiate it all into space. Sure, some percentage will leak, but not all..
- The_rationalist 8y agoMOND is epistemologically far superior to the hack of Dark matter. What does Dark matter explain that MOND can't?
- raattgift 8y ago> What does Dark matter explain that MOND can't MOND describes, it doesn't explain. By adding a simple function to Newton's law of universal gravitation, it does a very good job of describing the momentum-deficit in the bulk motion of large optical and radio sources in a variety of galaxies compared to the predictions from Newton's unmodified theory. That it does so, and with such a simple modification, is interesting. Milgrom's function in MOND is almost certainly the simplest post-Newtonian gravitational term extending Newton's theory consistent with observations. (There are other adaptations of Newton's theory to add post-Newtonian elements, for example to describe motions within the solar system. These are generally much more complicated than MOND [see [1] for a quick overview].) In practically any post-Newtonian term expanding Newton's law of universal gravitation, one will discover a greater mismatch with observation as the relative speeds of material participating in Newtonian gravitation increase. One has to "patch" with additional terms and counter-terms, resulting in something comparable to a Taylor Series. One generally experiences obvious failure with this approach as the relative speeds of the masses become comparable to tenths of the speed of light. MOND doesn't describe the momentum-deficit in the motion of large clusters of galaxies, nor the momentum-anisotropies in the cosmic microwave background. It doesn't describe large mass-flows, or especially relativistic mass-flows as seen in Seyfert galaxies and a number of galaxy clusters. It doesn't describe fast binary objects. It doesn't accurately describe the fine details of the bulk sources that it describes very well. It also doesn't accurately describe our solar system; one has to "patch" MOND with additional terms, and it quickly becomes easier just to discard MOND as a contribution to solar system dynamics altogether. MOND also doesn't abolish the need to use General Relativity to describe the angle-brightness-redshift relationship of the reddest galaxies (and the supernovae within them) visible in the sky. MOND does not provide sufficient post-Newtonian correction for GPS and other orbiting satellites with very precise frequency generators on board, nor for the results of experiments in MESSENGER [3] and numerous other spacecraft. Let's return to what MOND does do very well: describing the relative angular momentum of slow-moving inner and outer bulk bits of galaxies. Attempts to adapt MOND so that it accurately describes relativistic galaxy/cluster components have generally resulted in adding a field to standard General Relativity [3], and it is mostly cultural history that keeps MOND from being properly promoted as a source contributing to the stress-energy tensor. That would put it on identical footing to dark matter in the bulk, focusing minds on the question: "what are the microscopic details of the beyond-the-Standard-Model-of-Particle-Physics contributions to the stress-energy tensor?". > epistemologically far superior What is Newton's explanation for his law of universal gravitation, which MOND slightly modifies in a very specific way? What is the modern explanation of gravitation, which can encode MONDian behaviour in various ways? - -- [1] https://www.researchgate.net/publication/327607544_Post-Newtonian_satellite_orbits https://www.researchgate.net/publication/327607544_Post-Newt... [2] https://pgda.gsfc.nasa.gov/products/66 https://pgda.gsfc.nasa.gov/products/66 [3] Chapter 7 of http://adsabs.harvard.edu/abs/2012LRR....15...10F http://adsabs.harvard.edu/abs/2012LRR....15...10F (arxiv version: https://arxiv.org/abs/1112.3960 https://arxiv.org/abs/1112.3960)