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Iirc, we'd only need 10 neutrons per each hydrogen atom to explain the dark matter gravity. OK, if neutrons would be observable, are there any weird visual effe

Iirc, we'd only need 10 neutrons per each hydrogen atom to explain the dark matter gravity. OK, if neutrons would be observable, are there any weird visual effects in galaxy cores that could be attributed to such neutron clouds?

magicalhippo 6y ago
You say "only" but that's a _lot_ of neutrons, and space is vast. For one, I'd be very surprised if they didn't bump into each other, cooled down and clumped up like normal matter, which is precisely the opposite of what we expect from dark matter based on observations. Secondly we know[1] from nuclear experiments[2] what happens when neutrons interact with matter. I'm certain a nuclear scientist could list a bunch of effects we'd see if a whole lot of neutrons were hanging around black holes and similar. The whole point of dark matter is that observations show it interact extremely little, if any at all, with regular matter. [1]: https://en.wikipedia.org/wiki/Neutron_scattering https://en.wikipedia.org/wiki/Neutron_scattering [2]: https://en.wikipedia.org/wiki/Neutron_diffraction https://en.wikipedia.org/wiki/Neutron_diffraction
- frongpik 6y ago
  Neutrons visibly interact with matter, but only when there's a lot of that matter, e.g. reactors use neutrons to heat water. I'm yet to read the two links you provided, though. The story is different in the interstellar space. The only matter there is sparse gas and neutrons, even lots of them, would have little opportunity to bounce into each other or those gas atoms. Those neutron clouds could eventually form asteroid-like rings around galaxies: invisible, but massive. For the same reason, every massive object, including our planet, would have such a ring. That's a falsifiable hypothesis. As for the black hole argument, there could be two explanations: first is that neutrons that are nearby don't get to stay too long and either transform into protons and become visible gas, or fall into the black hole; second is that neutrons that are really far, light years away, could happily orbit around the black hole unnoticed and if there's enough of them, we would see subtle gravitational lensing effects. There's another valid point against neutrons: their lifetime is only 10 minutes. However neutrons and protons are two particular compositions of quarks. Are there reasons why quarks can't form a stable neutral particle in the interstellar space? That particle would be much like a neutron, but stable. The reason I'm pushing for this idea is that there's an odd flyby effect: it causes spacecrafts to accelerate and decelerate slightly. We could use those observations to calculate the precise shape, mass and velocity of that "neutron ring", make a satellite fly in the opposite direction and observe if there's strange heating of water or other reactions that could be caused by neutrons. Since it can visibly accelerate a spacecraft, there should be a good deal of those neutral particles and thus they should visibly heat up water. I'm not a physicist, but this seems a way simpler explanation of dark matter than "axion fields" or whatever is currently trending.
  - magicalhippo 6y ago
    > even lots of them, would have little opportunity to bounce into each other Space is vast and has been around for a long time. Improbable events happen a lot more often due to that. The interstellar medium has roughly 1 hydrogen atom per cm^3. You're saying maybe 10x as many neutrons. Multiply that by kiloparsecs and millions of years and those neutrons would most definitely interact. > I'm not a physicist, but this seems a way simpler explanation of dark matter than "axion fields" or whatever is currently trending. The reason "axion fields" or whatever is currently trending, is because simpler explanations have been ruled out. You not only have to explain the rotation curves, but also the properties your dark matter candidate need need. How did these neutrons get made? How do you explain your over-abundance of neutrons given that the neutron-proton ratio[1] after the freeze-out[2] is roughly 6 protons for every neutron? Why are the neutrons so cold? Did they start that way, then that's odd since everything else was very hot back in the days. If they're cooling down, where is that radiation and why haven't we observed it? Why aren't the neutrons decaying? They can't be free, since the free-neutron lifetime is something we've measured[3]. [1]: https://physics.stackexchange.com/a/217357 https://physics.stackexchange.com/a/217357 [2]: https://sites.astro.caltech.edu/~george/ay127/kamionkowski-earlyuniverse-notes.pdf https://sites.astro.caltech.edu/~george/ay127/kamionkowski-e... (page 5) [3]: https://phys.org/news/2020-09-mystery-neutron-lifetime.html https://phys.org/news/2020-09-mystery-neutron-lifetime.html
    - frongpik 6y ago
      Dark matter made of neutrons would take more time to form a disk. Ballpark estimate: neutrons are 50e3 times smaller, but we need 5x and of them, so dark matter is 10e3 slower at forming disks. In this model, neutrons aren't decaying: dark matter consists of "stable neutrons" that have a different quark structure, but are very similar to regular neutrons in all other aspects: no electric charge and a small magnetic moment. I don't think we need to explain everything before testing this hypothesis. Launching a sat to the supposed dark halo responsible for the flyby anomaly seems a relatively trivial task these days.
      - magicalhippo 6y ago

magicalhippo 6y ago

You say "only" but that's a _lot_ of neutrons, and space is vast. For one, I'd be very surprised if they didn't bump into each other, cooled down and clumped up like normal matter, which is precisely the opposite of what we expect from dark matter based on observations. Secondly we know[1] from nuclear experiments[2] what happens when neutrons interact with matter. I'm certain a nuclear scientist could list a bunch of effects we'd see if a whole lot of neutrons were hanging around black holes and similar. The whole point of dark matter is that observations show it interact extremely little, if any at all, with regular matter. [1]: https://en.wikipedia.org/wiki/Neutron_scattering https://en.wikipedia.org/wiki/Neutron_scattering [2]: https://en.wikipedia.org/wiki/Neutron_diffraction https://en.wikipedia.org/wiki/Neutron_diffraction

frongpik 6y ago
Neutrons visibly interact with matter, but only when there's a lot of that matter, e.g. reactors use neutrons to heat water. I'm yet to read the two links you provided, though. The story is different in the interstellar space. The only matter there is sparse gas and neutrons, even lots of them, would have little opportunity to bounce into each other or those gas atoms. Those neutron clouds could eventually form asteroid-like rings around galaxies: invisible, but massive. For the same reason, every massive object, including our planet, would have such a ring. That's a falsifiable hypothesis. As for the black hole argument, there could be two explanations: first is that neutrons that are nearby don't get to stay too long and either transform into protons and become visible gas, or fall into the black hole; second is that neutrons that are really far, light years away, could happily orbit around the black hole unnoticed and if there's enough of them, we would see subtle gravitational lensing effects. There's another valid point against neutrons: their lifetime is only 10 minutes. However neutrons and protons are two particular compositions of quarks. Are there reasons why quarks can't form a stable neutral particle in the interstellar space? That particle would be much like a neutron, but stable. The reason I'm pushing for this idea is that there's an odd flyby effect: it causes spacecrafts to accelerate and decelerate slightly. We could use those observations to calculate the precise shape, mass and velocity of that "neutron ring", make a satellite fly in the opposite direction and observe if there's strange heating of water or other reactions that could be caused by neutrons. Since it can visibly accelerate a spacecraft, there should be a good deal of those neutral particles and thus they should visibly heat up water. I'm not a physicist, but this seems a way simpler explanation of dark matter than "axion fields" or whatever is currently trending.
- magicalhippo 6y ago
  > even lots of them, would have little opportunity to bounce into each other Space is vast and has been around for a long time. Improbable events happen a lot more often due to that. The interstellar medium has roughly 1 hydrogen atom per cm^3. You're saying maybe 10x as many neutrons. Multiply that by kiloparsecs and millions of years and those neutrons would most definitely interact. > I'm not a physicist, but this seems a way simpler explanation of dark matter than "axion fields" or whatever is currently trending. The reason "axion fields" or whatever is currently trending, is because simpler explanations have been ruled out. You not only have to explain the rotation curves, but also the properties your dark matter candidate need need. How did these neutrons get made? How do you explain your over-abundance of neutrons given that the neutron-proton ratio[1] after the freeze-out[2] is roughly 6 protons for every neutron? Why are the neutrons so cold? Did they start that way, then that's odd since everything else was very hot back in the days. If they're cooling down, where is that radiation and why haven't we observed it? Why aren't the neutrons decaying? They can't be free, since the free-neutron lifetime is something we've measured[3]. [1]: https://physics.stackexchange.com/a/217357 https://physics.stackexchange.com/a/217357 [2]: https://sites.astro.caltech.edu/~george/ay127/kamionkowski-earlyuniverse-notes.pdf https://sites.astro.caltech.edu/~george/ay127/kamionkowski-e... (page 5) [3]: https://phys.org/news/2020-09-mystery-neutron-lifetime.html https://phys.org/news/2020-09-mystery-neutron-lifetime.html
  - frongpik 6y ago
    Dark matter made of neutrons would take more time to form a disk. Ballpark estimate: neutrons are 50e3 times smaller, but we need 5x and of them, so dark matter is 10e3 slower at forming disks. In this model, neutrons aren't decaying: dark matter consists of "stable neutrons" that have a different quark structure, but are very similar to regular neutrons in all other aspects: no electric charge and a small magnetic moment. I don't think we need to explain everything before testing this hypothesis. Launching a sat to the supposed dark halo responsible for the flyby anomaly seems a relatively trivial task these days.
    - magicalhippo 6y ago