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>"only dark matter matches all." Dark matter can be placed arbitrarily so it can match pretty much anything. I remember recently there was a galaxy with neglig

by nonbel 8y ago

>"only dark matter matches all." Dark matter can be placed arbitrarily so it can match pretty much anything. I remember recently there was a galaxy with negligible deviation of the GR-predicted rotation curve and that was taken as evidence for dark matter too (even though MOND was still the better fit for that case). "Dark Matter" started out ok but has become a "god of the gaps" argument, I expect eventually MOND or similar solutions will be developed for all those issues. Also, I gather that while MOND does seem not predict these observations exactly, it is not fully developed yet and often the presence of some "normal" cold gas (undetectable via light) could make it fit. Also, to get dark matter to fit the observations requires post-hoc assumptions and parameter adjustments. Eg: "The amplitude of the third peak observed by WMAP merely falsifies the simple ansatz I used to make the prediction, not MOND itself. Indeed, I pointed out in the original papers that the ansatz must fail at some level, so I am hardly surprised that it does. All this means is that there are degrees of freedom (like a scalar field) that can oscillate separately from baryons in whatever the relavistic parent theory of MOND might turn out to be. The real test was for ΛCDM: the third peak had to be higher than predicted by pure baryonic damping. ΛCDM survives this test. It "wins ugly" in that, in order to obtain a fit, we have had to nearly double the baryon density over what it was so confidently known to be before it wasn't. That, and the no-CDM prediction for the second peak is still bang on. Amazing coincidence, that." http://astroweb.case.edu/ssm/mond/ http://astroweb.case.edu/ssm/mond/ >"the predictions of dark matter were first made in the 1970s and 1980s, and were observationally confirmed later." I'm afraid I have never seen this although I have looked into this topic (as a layperson). Can you give an example of this? It would make the dark matter idea much more convincing.

acqq 8y ago
> Dark matter can be placed arbitrarily so it can match pretty much anything. Quite the opposite, and that statement is obviously uninformed. The science behind dark matter is very precise and exact. That's why the alternative theories can't come even near: once the same rigor which was applied to test dark matter is applied to the alternatives, the alternative theories simply break. > I'm afraid I have never seen this ... Can you give an example of this? I've given you the link to the complete articles and also highlighted the major points. Try to read and understand them fully, then search for more. For example, even on the MOND page that you linked you can read: "2013: ΛCDM provides an excellent fit to the improved CMB data just as MOND provides an excellent fit to improved rotation curve data. As before, one's interpretation depends entriely upon which you find more impressive." Note: "rotation curve" from that quote is the "item 2" from my posts before. So, no, still the match of the single "item 2" is definitely not "more impressive." The "CMB" mentioned on your page is the "item 3" from all 5 which are explainable by ΛCDM (and specifically, ΛCDM is the model part of which the "dark matter" is: https://lambda.gsfc.nasa.gov/education/graphic_history/univ_evol.cfm https://lambda.gsfc.nasa.gov/education/graphic_history/univ_... see there 4 and 5). Edit: response to your later response below: > The claimed predictions from the 1970s and 1980s are not mentioned on that page. They are mentioned even on the very page that you linked to: that page, among other points, followed the history of the tests and effectively concludes that, yes, finally, in 2013 ΛCDM wins the "item 3". Read carefully. Learn what ΛCDM is. MOND can still only claim "item 2." Which actual "authority" can show anything else? Also, behind "darkmattercrisis" blog are just a few guys with very fringe positions.
- nonbel 8y ago
  >"Quite the opposite, and that statement is obviously uninformed. The science behind dark matter is very precise and exact." From what I have seen they can get precise post-dictions but not precise pre-dictions because there are so many free parameters (basically you can put dark matter wherever you need it). >"I've given you the link to the complete articles and also highlighted the major points. Try to read and understand them fully, then search for more." The claimed predictions from the 1970s and 1980s are not mentioned on that page. Also, that "startswithabang" guy is more like a catholic inquisitor than a scientist. I wouldn't consider him a reliable source because this behavior indicates he lacks the required capacity for curiosity, he can only repeat what authority figures say: "The blogger (@StartsWithABang) contacted @scilogscom on January 24 by replying to a 15-day old tweet that announced our blog’s move to the new domain. He tweeted “Bummed that @scilogscom is in the business of promoting contrarian scientist viewpoints.”, and asks the SciLogs.com community manager (@notscientific) “[Why] are you allowing @scilogscom to promote contrarian voices that undermine public understanding of [science]?”, adding “You have taken on “Dark Matter Crisis” blog, whose mission is to undermine all of physical cosmology & promote MOND.” The two agreed to discuss the issue via email, with the blogger adding that he was “personally worried that you are promoting clicks & false controversy over quality science content”, and states that he is “very, VERY disappointed about this move that @scilogscom has made”. By now the SciLogs.com community manager has explained to us what happened after these tweets. He and the publishing director responsible for SciLogs.com unfortunately assumed that the blogger’s criticism was justified. They decided to close our blog without conferring with others or asking us for a statement. After we complained about the discontinuation, they performed an internal investigation, which involved reaching out to astrophysicists and other people, and have realized that discontinuing our blog was a big mistake. We attribute SciLogs.com’s poor judgement to two factors: neither the community manager nor the publishing director has an (astro)physical background, it was the first time that SciLogs.com had experienced an attack against one of its blogs." https://darkmattercrisis.wordpress.com/2013/03/08/the-dark-matter-crisis-continues-on-the-difficulties-of-communicating-controversial-science/ https://darkmattercrisis.wordpress.com/2013/03/08/the-dark-m... Anyway, I can tell you know even less about this topic than me so let's leave it at that.
  - deleted 8y ago
raattgift 8y ago
> Dark matter can be placed arbitrarily Yes. > so it can match pretty much anything At the time you do the initial placement. But that's not the only relevant time. You can scatter any type of DM on a spacelike hypersurface any way you want, but then you have to figure out the successor (and predecessor) spacelike hypersurface, and there you have to bite a bullet: you can constrain the DM or throw away the well-posedness of the initial-value problem. Your constraints are dominated by the hyperbolic-elliptic PDEs. The hyperbolic portion lets you lay down initial values for your dark matter on a spacelike slice. You then need an equation of motion for your DM, as you do for your visible matter, so that you can evolve the contents of that initial spacelike slice in time. The elliptical portion are the Hamiltonian and momentum constraints. Without satisfying these constraints you tend to lose uniqueness, which destroys the predictive power of your model. If you're spraying down a dust of non-interacting DM particles within some spacelike region, as you evolve from your initial configuration you end up with each particle growing into an everywhere-timelike worldtube. (Non-(self-)interaction is just to make this really clear without the distractions of decays or daughter products from scatterings. Each dark matter particle has its own worldtube extending arbitrarily far in to the future or past.) The simplest Cold Dark Matter models have a conventional nonrelativistic and relativistic dynamics determined by two parameters: particle rest-mass and interaction cross-section. In our simple non-interacting cold DM dust case the interaction is purely gravitational, so we're left with every particle having an identical rest mass. Now, lay down the visible matter in a galaxy (or cluster!). You can lay down the dark matter dust arbitrarily to form a halo which could support a realistic galaxy rotation curve on your initial surface. You are right that you can put it practically anywhere, and are not restricted to uniform density. However, when you evolve in time, you inevitably find that you are stuck with a narrow range of mass densities at various points in the galaxy (or cluster!), but still have freedom to set down differing particle number density, higher numbers at lower particle mass. If your particles are too light, the gravitational coupling with the visible matter is too slight; worse, the gravitational coupling between the DM particles is too slight too, and your galaxy quickly disassembles. If your particles are too heavy, instead you get dynamic heating of hydrogen gas, and you get far too much star formation and metal enrichment. You can go the other way too: first put down a test particle DM halo and then add hydrogen gas, or a modern galaxy, or whatever else, and see how the system evolves. It is a feature of the standard galaxy astrophysics that DM and visible matter do not have to be near each other at all times. (This gets important when you consider galaxy clusters rather than just galaxies; laying out CDM is hard work, as I've been describing, but you can't just drop in MOND. It's also testable at solar-system scales; we can look for post-Newton dynamics in the Jupiter system, or in the inner solar system, and test whether it's compatible with General Relativity (with a fine dark matter dust of up to a few Phobos-masses inside Neptune's orbit) or whether it's compatible with a different post-Newtonian dynamics, like MOND, where the galaxy's MOND corrections will generate a quadrupole anomaly stronger around the outer planets and their moons than around the inner planets). More complicated particle CDM theories are still stuck with the general rules of the simple model above: the stress-energy in the large has to hang around in the right places for long periods of time; the stress-energy can't just pop in and out of existence at a point (it needs to be thermal, so its worldtubes cannot even be nearly lightlike, let alone timelike); and it has to have a reasonable number density). Crucially, non-gravitational coupling to the Standard Model imposes further restrictions on the initial surface, it does not relax them. More complicated particle CDM is usually motivated by problems in particle physics, models for for which galaxy (and cluster!) astrophysics can provide observational evidence. Some WIMP models are good tests for SUSY models, so one experimental search might test two models grown from very different underlying theories. One does not have to believe the "WIMP miracle" conjecture when one can outright test it observationally and in a laboratory setting. > MOND By construction you can't put MOND just anywhere on your initial values surface; it is a phenomenon arising from visible matter. You can't put down MOND first and then add matching matter and see what happens. (Well, there are relativistic theories which start by taking MOND and then proceed to take General Relativity by introducing one or more fields until they get a good match with MOND. Some of those approaches might let you put down a "MOND field" first and add matter that -- Famaey and McGauch have a good, if aging, overview in Chapter 7 of https://arxiv.org/abs/1112.3960 https://arxiv.org/abs/1112.3960 -- but at some point you might ask if you can start with an already-relativistic theory like General Relativity and recover MOND from that instead. MOND does appear to work for galaxies, after all, even if it needs modification for much larger and much smaller structures.)

by nonbel 8y ago

acqq 8y ago

> Dark matter can be placed arbitrarily so it can match pretty much anything. Quite the opposite, and that statement is obviously uninformed. The science behind dark matter is very precise and exact. That's why the alternative theories can't come even near: once the same rigor which was applied to test dark matter is applied to the alternatives, the alternative theories simply break. > I'm afraid I have never seen this ... Can you give an example of this? I've given you the link to the complete articles and also highlighted the major points. Try to read and understand them fully, then search for more. For example, even on the MOND page that you linked you can read: "2013: ΛCDM provides an excellent fit to the improved CMB data just as MOND provides an excellent fit to improved rotation curve data. As before, one's interpretation depends entriely upon which you find more impressive." Note: "rotation curve" from that quote is the "item 2" from my posts before. So, no, still the match of the single "item 2" is definitely not "more impressive." The "CMB" mentioned on your page is the "item 3" from all 5 which are explainable by ΛCDM (and specifically, ΛCDM is the model part of which the "dark matter" is: https://lambda.gsfc.nasa.gov/education/graphic_history/univ_evol.cfm https://lambda.gsfc.nasa.gov/education/graphic_history/univ_... see there 4 and 5). Edit: response to your later response below: > The claimed predictions from the 1970s and 1980s are not mentioned on that page. They are mentioned even on the very page that you linked to: that page, among other points, followed the history of the tests and effectively concludes that, yes, finally, in 2013 ΛCDM wins the "item 3". Read carefully. Learn what ΛCDM is. MOND can still only claim "item 2." Which actual "authority" can show anything else? Also, behind "darkmattercrisis" blog are just a few guys with very fringe positions.

nonbel 8y ago
>"Quite the opposite, and that statement is obviously uninformed. The science behind dark matter is very precise and exact." From what I have seen they can get precise post-dictions but not precise pre-dictions because there are so many free parameters (basically you can put dark matter wherever you need it). >"I've given you the link to the complete articles and also highlighted the major points. Try to read and understand them fully, then search for more." The claimed predictions from the 1970s and 1980s are not mentioned on that page. Also, that "startswithabang" guy is more like a catholic inquisitor than a scientist. I wouldn't consider him a reliable source because this behavior indicates he lacks the required capacity for curiosity, he can only repeat what authority figures say: "The blogger (@StartsWithABang) contacted @scilogscom on January 24 by replying to a 15-day old tweet that announced our blog’s move to the new domain. He tweeted “Bummed that @scilogscom is in the business of promoting contrarian scientist viewpoints.”, and asks the SciLogs.com community manager (@notscientific) “[Why] are you allowing @scilogscom to promote contrarian voices that undermine public understanding of [science]?”, adding “You have taken on “Dark Matter Crisis” blog, whose mission is to undermine all of physical cosmology & promote MOND.” The two agreed to discuss the issue via email, with the blogger adding that he was “personally worried that you are promoting clicks & false controversy over quality science content”, and states that he is “very, VERY disappointed about this move that @scilogscom has made”. By now the SciLogs.com community manager has explained to us what happened after these tweets. He and the publishing director responsible for SciLogs.com unfortunately assumed that the blogger’s criticism was justified. They decided to close our blog without conferring with others or asking us for a statement. After we complained about the discontinuation, they performed an internal investigation, which involved reaching out to astrophysicists and other people, and have realized that discontinuing our blog was a big mistake. We attribute SciLogs.com’s poor judgement to two factors: neither the community manager nor the publishing director has an (astro)physical background, it was the first time that SciLogs.com had experienced an attack against one of its blogs." https://darkmattercrisis.wordpress.com/2013/03/08/the-dark-matter-crisis-continues-on-the-difficulties-of-communicating-controversial-science/ https://darkmattercrisis.wordpress.com/2013/03/08/the-dark-m... Anyway, I can tell you know even less about this topic than me so let's leave it at that.
- deleted 8y ago

raattgift 8y ago

> Dark matter can be placed arbitrarily Yes. > so it can match pretty much anything At the time you do the initial placement. But that's not the only relevant time. You can scatter any type of DM on a spacelike hypersurface any way you want, but then you have to figure out the successor (and predecessor) spacelike hypersurface, and there you have to bite a bullet: you can constrain the DM or throw away the well-posedness of the initial-value problem. Your constraints are dominated by the hyperbolic-elliptic PDEs. The hyperbolic portion lets you lay down initial values for your dark matter on a spacelike slice. You then need an equation of motion for your DM, as you do for your visible matter, so that you can evolve the contents of that initial spacelike slice in time. The elliptical portion are the Hamiltonian and momentum constraints. Without satisfying these constraints you tend to lose uniqueness, which destroys the predictive power of your model. If you're spraying down a dust of non-interacting DM particles within some spacelike region, as you evolve from your initial configuration you end up with each particle growing into an everywhere-timelike worldtube. (Non-(self-)interaction is just to make this really clear without the distractions of decays or daughter products from scatterings. Each dark matter particle has its own worldtube extending arbitrarily far in to the future or past.) The simplest Cold Dark Matter models have a conventional nonrelativistic and relativistic dynamics determined by two parameters: particle rest-mass and interaction cross-section. In our simple non-interacting cold DM dust case the interaction is purely gravitational, so we're left with every particle having an identical rest mass. Now, lay down the visible matter in a galaxy (or cluster!). You can lay down the dark matter dust arbitrarily to form a halo which could support a realistic galaxy rotation curve on your initial surface. You are right that you can put it practically anywhere, and are not restricted to uniform density. However, when you evolve in time, you inevitably find that you are stuck with a narrow range of mass densities at various points in the galaxy (or cluster!), but still have freedom to set down differing particle number density, higher numbers at lower particle mass. If your particles are too light, the gravitational coupling with the visible matter is too slight; worse, the gravitational coupling between the DM particles is too slight too, and your galaxy quickly disassembles. If your particles are too heavy, instead you get dynamic heating of hydrogen gas, and you get far too much star formation and metal enrichment. You can go the other way too: first put down a test particle DM halo and then add hydrogen gas, or a modern galaxy, or whatever else, and see how the system evolves. It is a feature of the standard galaxy astrophysics that DM and visible matter do not have to be near each other at all times. (This gets important when you consider galaxy clusters rather than just galaxies; laying out CDM is hard work, as I've been describing, but you can't just drop in MOND. It's also testable at solar-system scales; we can look for post-Newton dynamics in the Jupiter system, or in the inner solar system, and test whether it's compatible with General Relativity (with a fine dark matter dust of up to a few Phobos-masses inside Neptune's orbit) or whether it's compatible with a different post-Newtonian dynamics, like MOND, where the galaxy's MOND corrections will generate a quadrupole anomaly stronger around the outer planets and their moons than around the inner planets). More complicated particle CDM theories are still stuck with the general rules of the simple model above: the stress-energy in the large has to hang around in the right places for long periods of time; the stress-energy can't just pop in and out of existence at a point (it needs to be thermal, so its worldtubes cannot even be nearly lightlike, let alone timelike); and it has to have a reasonable number density). Crucially, non-gravitational coupling to the Standard Model imposes further restrictions on the initial surface, it does not relax them. More complicated particle CDM is usually motivated by problems in particle physics, models for for which galaxy (and cluster!) astrophysics can provide observational evidence. Some WIMP models are good tests for SUSY models, so one experimental search might test two models grown from very different underlying theories. One does not have to believe the "WIMP miracle" conjecture when one can outright test it observationally and in a laboratory setting. > MOND By construction you can't put MOND just anywhere on your initial values surface; it is a phenomenon arising from visible matter. You can't put down MOND first and then add matching matter and see what happens. (Well, there are relativistic theories which start by taking MOND and then proceed to take General Relativity by introducing one or more fields until they get a good match with MOND. Some of those approaches might let you put down a "MOND field" first and add matter that -- Famaey and McGauch have a good, if aging, overview in Chapter 7 of https://arxiv.org/abs/1112.3960 https://arxiv.org/abs/1112.3960 -- but at some point you might ask if you can start with an already-relativistic theory like General Relativity and recover MOND from that instead. MOND does appear to work for galaxies, after all, even if it needs modification for much larger and much smaller structures.)