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Hey, glad to meet you. Maybe we can have a discussion. Glad we agree on modified gravity (MOND) being able to accurately account for galaxy rotation. - galaxy

Hey, glad to meet you. Maybe we can have a discussion. Glad we agree on modified gravity (MOND) being able to accurately account for galaxy rotation. - galaxy clusters - lensing - Bullet Cluster, El Gordo all have answers in the work of modified gravity (MOND) cosmologists. Stacy McGaugh does a really good job going through each one. The essential thing, and I mean really essential, is that modified gravity (MOND) uses fewer free parameters than cold dark matter (CDM), by a long shot.

perhonen 5y ago
but... this is not true? McGaugh himself [1] mentions that there are mass discrepancies even using MOND analysis, and that this favors the dark matter paradigm [2]. [1] https://twitter.com/dudedarkmatter/status/1094526391620722688 https://twitter.com/dudedarkmatter/status/109452639162072268... [2] https://twitter.com/DudeDarkmatter/status/1094527929994932224 https://twitter.com/DudeDarkmatter/status/109452792999493222... certainly, these are still open questions in cosmology
- sahil50 5y ago
  The tweets above are good questions! This is how to do science! Meanwhile it's good to restate the obvious. Modified gravity (MOND), where acceleration at galactic scales isn't just GM/R^2 but modeled as sqrt(GM/R^2 * cH/(2pi)), works for 100s of galaxies, and is more precise than cold dark matter (CDM). Modified gravity only has one free parameter. cH/(2pi) = c^2 / 87 bly.
  - 8note 5y ago
    Can the free parameter be described by something other than dark matter though? Or does it only put more constraints on what the dark matter is.
    - sahil50 5y ago
      Indeed. More detail here. https://news.ycombinator.com/item?id=29915004 https://news.ycombinator.com/item?id=29915004 Perhaps the "dark matter" of a galaxy is the rest of the galaxies of the universe.
nwallin 5y ago
> The essential thing, and I mean really essential, is that modified gravity (MOND) uses fewer free parameters than cold dark matter (CDM), by a long shot. The only MOND formulations that actually work are ones that simply reduce cold dark matter from 80% of the universe to 30% of the universe. MOND has all -- literally all, as in 100% of them -- the free parameters of lCDM plus the parameters for MOND. I like MOND. It's elegant. It's intuitive. It's simple. You get to put your name at the end of the sequence Aristotle, Copernicus, Kepler, Newton, Einstein. When I first started reading about dark matter in the '90s I thought MOND was the obvious solution. (MACHO was still a leading theory at the time, they were still analyzing the data from the microlensing surveys) lCDM is messy and existentially unsatisfying. We don't know what what it is, and if it has the properties we think it has, we could not conceivably ever know what it is. But regardless of the fact that MOND is nicer than lCDM, lCDM explains all of our observations, but MOND only explains some of them, and depending on which formulation of MOND you subscribe to, is falsified by the the bullet cluster, or by galaxies whose rotation curves show a lack of dark matter, or by the LIGO/VIRGO observations that show gravity moves at the speed of light. MOND reminds me of the meme of Homer Simpson where he's standing there looking fairly fit and then the camera turns around him and he's just got all his fat and loose skin pinned and tied up on his back. It looks great on the face of it but once you really, really start digging into the details it doesn't work anymore. lCDM wears all of its warts on its face.

perhonen 5y ago

but... this is not true? McGaugh himself [1] mentions that there are mass discrepancies even using MOND analysis, and that this favors the dark matter paradigm [2]. [1] https://twitter.com/dudedarkmatter/status/1094526391620722688 https://twitter.com/dudedarkmatter/status/109452639162072268... [2] https://twitter.com/DudeDarkmatter/status/1094527929994932224 https://twitter.com/DudeDarkmatter/status/109452792999493222... certainly, these are still open questions in cosmology

sahil50 5y ago
The tweets above are good questions! This is how to do science! Meanwhile it's good to restate the obvious. Modified gravity (MOND), where acceleration at galactic scales isn't just GM/R^2 but modeled as sqrt(GM/R^2 * cH/(2pi)), works for 100s of galaxies, and is more precise than cold dark matter (CDM). Modified gravity only has one free parameter. cH/(2pi) = c^2 / 87 bly.
- 8note 5y ago
  Can the free parameter be described by something other than dark matter though? Or does it only put more constraints on what the dark matter is.
  - sahil50 5y ago
    Indeed. More detail here. https://news.ycombinator.com/item?id=29915004 https://news.ycombinator.com/item?id=29915004 Perhaps the "dark matter" of a galaxy is the rest of the galaxies of the universe.

nwallin 5y ago

> The essential thing, and I mean really essential, is that modified gravity (MOND) uses fewer free parameters than cold dark matter (CDM), by a long shot. The only MOND formulations that actually work are ones that simply reduce cold dark matter from 80% of the universe to 30% of the universe. MOND has all -- literally all, as in 100% of them -- the free parameters of lCDM plus the parameters for MOND. I like MOND. It's elegant. It's intuitive. It's simple. You get to put your name at the end of the sequence Aristotle, Copernicus, Kepler, Newton, Einstein. When I first started reading about dark matter in the '90s I thought MOND was the obvious solution. (MACHO was still a leading theory at the time, they were still analyzing the data from the microlensing surveys) lCDM is messy and existentially unsatisfying. We don't know what what it is, and if it has the properties we think it has, we could not conceivably ever know what it is. But regardless of the fact that MOND is nicer than lCDM, lCDM explains all of our observations, but MOND only explains some of them, and depending on which formulation of MOND you subscribe to, is falsified by the the bullet cluster, or by galaxies whose rotation curves show a lack of dark matter, or by the LIGO/VIRGO observations that show gravity moves at the speed of light. MOND reminds me of the meme of Homer Simpson where he's standing there looking fairly fit and then the camera turns around him and he's just got all his fat and loose skin pinned and tied up on his back. It looks great on the face of it but once you really, really start digging into the details it doesn't work anymore. lCDM wears all of its warts on its face.