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There are now several good lines of evidence for dark matter, at least: * rotation curves of galaxies * cosmic microwave background fluctuations * gravitatio

There are now several good lines of evidence for dark matter, at least: * rotation curves of galaxies * cosmic microwave background fluctuations * gravitational lensing of galaxies which would otherwise have far too little mass Whenever a "no dark matter" proposal comes along, I do a quick ctrl+f if it even tries to address all three lines of evidence. This doesn't seem to be the case here. Don't get me wrong, this could still be an interesting approach to cosmology, but IMHO it's not ready for widespread coverage to a possibly-not-so-informed audience until it tackles all three.

deleted 3y ago
[deleted]
throw1234651234 3y ago
Appreciate people "going back to the fundamentals" - without sarcasm, I lived for ages without even ever understanding where the idea of dark matter comes from. To be honest, I still don't understand how "cosmic microwave background fluctuations" is an argument for dark matter, but #1 and #2 at least help me intuitively understand where the theory comes from.
- sigmoid10 3y ago
  Cosmic microwave background fluctuations are actually some of the strongest, most precisely measured evidence we have today for DM. The CMB was measured with incredibly high resolution thanks to the Planck satellite [1], and when you look at the distribution of angular sizes of the density fluctuations, you get a very peculiar plot [2]. Through some rather complicated math, the position and height of the peaks in this plot directly correspond (among other things) to "normal" matter that interacts electromagnetically and "dark matter" which does not interact electromagnetically. Since at the time of the CMB the whole universe was still very hot and therefore ionized, the data tells us there was a large amount of matter that only interacted gravitationally, but not electromagnetically. Ex post facto, the amount of "dark" matter calculated this way directly corresponds to the amount you would need to also explain galaxy rotation curves and gravitational lensing observations in today's universe. [1] https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2013/03/planck_wmap_comparison/12584155-7-eng-GB/Planck_WMAP_comparison_pillars.jpg https://www.esa.int/var/esa/storage/images/esa_multimedia/im... [2] https://ned.ipac.caltech.edu/level5/Sept17/Freese/Figures/figure5.jpg https://ned.ipac.caltech.edu/level5/Sept17/Freese/Figures/fi...
  - naasking 3y ago
    What often goes conveniently unmentioned is that DM failed to correctly predict the first and second peaks of the power spectrum (which MOND correctly predicted) and only correctly predicted the third peak (which MOND did not correctly predict). Rotation curves are also not compelling evidence for LCDM because it's not predictive, eg. each galaxy needs to be fit post-hoc with a custom distribution of DM to match the rotation curve. Lensing in clusters is about the only evidence that there's some amount of DM, but the amount postulated by DM-only theories may be excessive, ie. under modified gravity, only an amount of dark matter about equivalent to normal matter is needed, not the 5x needed under LCDM. I'd say that's pretty far from a slam dunk for DM theories over other possibilities.
    - throw1234651234 3y ago
      Appreciate both this and sigmoid's comment above. I don't really understand cosmic background radiation enough to understand though. I get it at an absolutely superficial level and understand that it's used as evidence of the big bang, but that's about it. Which is why I fall back to lensing as sort of the only argument for DM which I really understand.
    - sigmoid10 3y ago
      >DM failed to correctly predict the first and second peaks of the power spectrum I don't know what you mean by this. The peaks are completely explainable by standard LCDM, while MOND without DM totally fails to do so - no matter how much you tune it (see black vs red lines in [1]). >pretty far from a slam dunk for DM theories Individually, maybe. But not when you combine everything together. Then MOND is completely hopeless. You will find no paper out there that can reach the same level of predictions as DM. Of course that doesn't mean that there is guaranteed to be no modification to gravity at all. We already know it must be modified in the UV, so it's not impossible that a UV-complete description will bleed down to modified, far IR behaviour. But even if that is the case (and that's a pretty big if right now with the current theory landscape), some form of DM will still be required. And the evidence from the CMB strongly suggests that it will not be a small amount, but pretty much what we expect from original rotation curves and lensing. That's why no serious researcher in the field bothers with modifications of gravity alone, while there are many who focus only on DM. [1] https://www.researchgate.net/profile/Bin-Wang-115/publication/269577584/figure/fig7/AS:667090436493329@1536058162712/The-CMB-temperature-angular-power-spectra-for-the-fiducial-LCDM-modelsolid-black-curve.png https://www.researchgate.net/profile/Bin-Wang-115/publicatio...
CuriousCosmic 3y ago
I'm not particularly well versed in this but the actual paper seems to address #2 and #3. It might address #1 as well but it's honestly too dense for me to really tackle this early in the morning. https://iopscience.iop.org/article/10.3847/1538-4357/ad1bc6/pdf https://iopscience.iop.org/article/10.3847/1538-4357/ad1bc6/...
- sigmoid10 3y ago
  It only adresses #2 insofar as it talks about baryon acoustic oscillations. They contribute to the CMB power spectrum, but in the end they are only useful as a measuring stick to calculate the Hubble constant. This theory does not explain the dark matter component seen in the power fluctuations at all. It also doesn't adress #3 at all (there are very concrete examples of lensing that any such theory would have to be able to explain).
blindriver 3y ago
But there’s been no detection of dark matter whatsoever, ever. The fact that people cling to this theory is strange because the most basic evidence, ie. viewing it, doesn’t exist.
- mr_mitm 3y ago
  You also cannot "view" neutrinos, yet no one doubts their existence. By the way, they too have been postulated because collision experiments were showing some missing kinetic energy and momentum. People said "maybe there is a new particle that does not interact electromagnetically carrying away the energy", and they turned out to be correct. It's a very fruitful way of thinking. Your perceived strangeness goes away as soon as you understand all the evidence, in particular BAO and lensing, as mentioned above.
  - perihelions 3y ago
    Well, we do have the ability to detect individual neutrinos, through their interactions with ordinary matter. That's not currently the case for dark matter: it's sort of in the place neutrinos were in between 1930 (when neutrinos were postulated (as you describe) to explain missing momentum) and 1956 (when they were first observed [0] in nuclear fission reactors). https://en.wikipedia.org/wiki/Cowan–Reines_neutrino_experiment https://en.wikipedia.org/wiki/Cowan–Reines_neutrino_experime... /tangent I learned a fun fact! The original proposal for the neutrino experiment wanted to detonate a nuclear bomb. The reactor was a last-minute change. - "Eminent physicists enthused about the plan. It was approved by Reines’s employer, the government-funded Los Alamos laboratory in New Mexico. Work began on the detector, nicknamed El Monstro, and on the construction of the shaft. At the last minute, Reines and Cowan transferred the experiment to a nuclear reactor, but not because of environmental or safety concerns. They had worked out that although the reactor would deliver a flux of neutrinos three orders of magnitude lower than that from the bomb, it offered a better option for distinguishing signal from noise." https://www.physics.uci.edu/node/14126 https://www.physics.uci.edu/node/14126 ("The singing UCI Nobel Laureate who nearly bombed Nevada")
  - adrian_b 3y ago
    That is not correct. While the probability that neutrinos will interact with other kinds of particles is very low, it is finite. Therefore we have neutrino detectors, even if they are very big and very expensive. So we can actually view neutrinos in the same way as we view anything else. We can imagine even building a video camera that would provide images of the spatial distribution of the neutrino flux, i.e. an image of the sky based on the incoming neutrino rays. Such a neutrino video camera would be made like a compound insect eye, in order to separate by angle the incoming neutrinos with shields. Unfortunately such a video camera for neutrinos would need to have a planetary size, because shields that could block a large fraction of the neutrinos coming from different directions than the one desired for reception would be humongous. For neutrino sources that are much more luminous (in neutrino flux) than their background, e.g. supernova explosions or the Sun, there is an alternative way to separate the neutrinos that come from a specified direction, besides shielding. One can use 2 neutrino detectors located at a great distance between them and count only the neutrino fluxes recorded in both detectors. Unfortunately the great distance needed for good angular resolution and the size of the individual neutrino detectors still make impossible the construction of a neutrino receiver that could be oriented in any direction, at will.
adrian_b 3y ago
This new paper demonstrates that their model can fit a part of the observational data, including parts that could not be explained by the previous models using dark matter. As the authors state in the summary "it remains to be seen if the new model is consistent with the CMB power spectrum, the Big Bang nucleosynthesis of light elements, and other critical observations". Maybe this model will be contradicted by further work or maybe it will be confirmed. In any case such models must be searched and studied, because the explanations based on deus ex machina, i.e. on dark matter and dark energy, are not good enough, because they lack predictive power. Adding an arbitrary and unconstrained distribution of gravitational field over the entire Universe can match an arbitrary mathematical model of the gravitation, so it cannot be used as an argument that the current standard model is valid. Only if another means of detecting the dark matter, except by its gravitational influence, were discovered, than that could constrain the distribution of the dark matter, so that the supposition of its existence would be converted into a valid model with predictive power.
- perlgeek 3y ago
  > This new paper demonstrates that their model can fit a part of the observational data, including parts that could not be explained by the previous models using dark matter. That's how nearly all successful models started out: being able to explain a part that the established models can't, and then later being expanded into other areas. But of course, only a small minority of such models ever make it to being the dominant/accepted one. I'm all in favor of this process, the only thing I'm weary about is how it's communicated to the public. I know so many people who read such an article and come away with "dark matter isn't real after all", and I don't really know what to do about it. Media outlets don't really have the incentives to communicate the amount of caution that is appropriate towards such a model, and even if they did, it's somewhat hard to do well.

deleted 3y ago

[deleted]

throw1234651234 3y ago

Appreciate people "going back to the fundamentals" - without sarcasm, I lived for ages without even ever understanding where the idea of dark matter comes from. To be honest, I still don't understand how "cosmic microwave background fluctuations" is an argument for dark matter, but #1 and #2 at least help me intuitively understand where the theory comes from.

sigmoid10 3y ago
Cosmic microwave background fluctuations are actually some of the strongest, most precisely measured evidence we have today for DM. The CMB was measured with incredibly high resolution thanks to the Planck satellite [1], and when you look at the distribution of angular sizes of the density fluctuations, you get a very peculiar plot [2]. Through some rather complicated math, the position and height of the peaks in this plot directly correspond (among other things) to "normal" matter that interacts electromagnetically and "dark matter" which does not interact electromagnetically. Since at the time of the CMB the whole universe was still very hot and therefore ionized, the data tells us there was a large amount of matter that only interacted gravitationally, but not electromagnetically. Ex post facto, the amount of "dark" matter calculated this way directly corresponds to the amount you would need to also explain galaxy rotation curves and gravitational lensing observations in today's universe. [1] https://www.esa.int/var/esa/storage/images/esa_multimedia/images/2013/03/planck_wmap_comparison/12584155-7-eng-GB/Planck_WMAP_comparison_pillars.jpg https://www.esa.int/var/esa/storage/images/esa_multimedia/im... [2] https://ned.ipac.caltech.edu/level5/Sept17/Freese/Figures/figure5.jpg https://ned.ipac.caltech.edu/level5/Sept17/Freese/Figures/fi...
- naasking 3y ago
  What often goes conveniently unmentioned is that DM failed to correctly predict the first and second peaks of the power spectrum (which MOND correctly predicted) and only correctly predicted the third peak (which MOND did not correctly predict). Rotation curves are also not compelling evidence for LCDM because it's not predictive, eg. each galaxy needs to be fit post-hoc with a custom distribution of DM to match the rotation curve. Lensing in clusters is about the only evidence that there's some amount of DM, but the amount postulated by DM-only theories may be excessive, ie. under modified gravity, only an amount of dark matter about equivalent to normal matter is needed, not the 5x needed under LCDM. I'd say that's pretty far from a slam dunk for DM theories over other possibilities.
  - throw1234651234 3y ago
    Appreciate both this and sigmoid's comment above. I don't really understand cosmic background radiation enough to understand though. I get it at an absolutely superficial level and understand that it's used as evidence of the big bang, but that's about it. Which is why I fall back to lensing as sort of the only argument for DM which I really understand.
  - sigmoid10 3y ago
    >DM failed to correctly predict the first and second peaks of the power spectrum I don't know what you mean by this. The peaks are completely explainable by standard LCDM, while MOND without DM totally fails to do so - no matter how much you tune it (see black vs red lines in [1]). >pretty far from a slam dunk for DM theories Individually, maybe. But not when you combine everything together. Then MOND is completely hopeless. You will find no paper out there that can reach the same level of predictions as DM. Of course that doesn't mean that there is guaranteed to be no modification to gravity at all. We already know it must be modified in the UV, so it's not impossible that a UV-complete description will bleed down to modified, far IR behaviour. But even if that is the case (and that's a pretty big if right now with the current theory landscape), some form of DM will still be required. And the evidence from the CMB strongly suggests that it will not be a small amount, but pretty much what we expect from original rotation curves and lensing. That's why no serious researcher in the field bothers with modifications of gravity alone, while there are many who focus only on DM. [1] https://www.researchgate.net/profile/Bin-Wang-115/publication/269577584/figure/fig7/AS:667090436493329@1536058162712/The-CMB-temperature-angular-power-spectra-for-the-fiducial-LCDM-modelsolid-black-curve.png https://www.researchgate.net/profile/Bin-Wang-115/publicatio...

CuriousCosmic 3y ago

I'm not particularly well versed in this but the actual paper seems to address #2 and #3. It might address #1 as well but it's honestly too dense for me to really tackle this early in the morning. https://iopscience.iop.org/article/10.3847/1538-4357/ad1bc6/pdf https://iopscience.iop.org/article/10.3847/1538-4357/ad1bc6/...

sigmoid10 3y ago
It only adresses #2 insofar as it talks about baryon acoustic oscillations. They contribute to the CMB power spectrum, but in the end they are only useful as a measuring stick to calculate the Hubble constant. This theory does not explain the dark matter component seen in the power fluctuations at all. It also doesn't adress #3 at all (there are very concrete examples of lensing that any such theory would have to be able to explain).

blindriver 3y ago

But there’s been no detection of dark matter whatsoever, ever. The fact that people cling to this theory is strange because the most basic evidence, ie. viewing it, doesn’t exist.

mr_mitm 3y ago
You also cannot "view" neutrinos, yet no one doubts their existence. By the way, they too have been postulated because collision experiments were showing some missing kinetic energy and momentum. People said "maybe there is a new particle that does not interact electromagnetically carrying away the energy", and they turned out to be correct. It's a very fruitful way of thinking. Your perceived strangeness goes away as soon as you understand all the evidence, in particular BAO and lensing, as mentioned above.
- perihelions 3y ago
  Well, we do have the ability to detect individual neutrinos, through their interactions with ordinary matter. That's not currently the case for dark matter: it's sort of in the place neutrinos were in between 1930 (when neutrinos were postulated (as you describe) to explain missing momentum) and 1956 (when they were first observed [0] in nuclear fission reactors). https://en.wikipedia.org/wiki/Cowan–Reines_neutrino_experiment https://en.wikipedia.org/wiki/Cowan–Reines_neutrino_experime... /tangent I learned a fun fact! The original proposal for the neutrino experiment wanted to detonate a nuclear bomb. The reactor was a last-minute change. - "Eminent physicists enthused about the plan. It was approved by Reines’s employer, the government-funded Los Alamos laboratory in New Mexico. Work began on the detector, nicknamed El Monstro, and on the construction of the shaft. At the last minute, Reines and Cowan transferred the experiment to a nuclear reactor, but not because of environmental or safety concerns. They had worked out that although the reactor would deliver a flux of neutrinos three orders of magnitude lower than that from the bomb, it offered a better option for distinguishing signal from noise." https://www.physics.uci.edu/node/14126 https://www.physics.uci.edu/node/14126 ("The singing UCI Nobel Laureate who nearly bombed Nevada")
- adrian_b 3y ago
  That is not correct. While the probability that neutrinos will interact with other kinds of particles is very low, it is finite. Therefore we have neutrino detectors, even if they are very big and very expensive. So we can actually view neutrinos in the same way as we view anything else. We can imagine even building a video camera that would provide images of the spatial distribution of the neutrino flux, i.e. an image of the sky based on the incoming neutrino rays. Such a neutrino video camera would be made like a compound insect eye, in order to separate by angle the incoming neutrinos with shields. Unfortunately such a video camera for neutrinos would need to have a planetary size, because shields that could block a large fraction of the neutrinos coming from different directions than the one desired for reception would be humongous. For neutrino sources that are much more luminous (in neutrino flux) than their background, e.g. supernova explosions or the Sun, there is an alternative way to separate the neutrinos that come from a specified direction, besides shielding. One can use 2 neutrino detectors located at a great distance between them and count only the neutrino fluxes recorded in both detectors. Unfortunately the great distance needed for good angular resolution and the size of the individual neutrino detectors still make impossible the construction of a neutrino receiver that could be oriented in any direction, at will.

adrian_b 3y ago

This new paper demonstrates that their model can fit a part of the observational data, including parts that could not be explained by the previous models using dark matter. As the authors state in the summary "it remains to be seen if the new model is consistent with the CMB power spectrum, the Big Bang nucleosynthesis of light elements, and other critical observations". Maybe this model will be contradicted by further work or maybe it will be confirmed. In any case such models must be searched and studied, because the explanations based on deus ex machina, i.e. on dark matter and dark energy, are not good enough, because they lack predictive power. Adding an arbitrary and unconstrained distribution of gravitational field over the entire Universe can match an arbitrary mathematical model of the gravitation, so it cannot be used as an argument that the current standard model is valid. Only if another means of detecting the dark matter, except by its gravitational influence, were discovered, than that could constrain the distribution of the dark matter, so that the supposition of its existence would be converted into a valid model with predictive power.

perlgeek 3y ago
> This new paper demonstrates that their model can fit a part of the observational data, including parts that could not be explained by the previous models using dark matter. That's how nearly all successful models started out: being able to explain a part that the established models can't, and then later being expanded into other areas. But of course, only a small minority of such models ever make it to being the dominant/accepted one. I'm all in favor of this process, the only thing I'm weary about is how it's communicated to the public. I know so many people who read such an article and come away with "dark matter isn't real after all", and I don't really know what to do about it. Media outlets don't really have the incentives to communicate the amount of caution that is appropriate towards such a model, and even if they did, it's somewhat hard to do well.