3 ms·

>If DM interacts purely gravitationally If it's purely gravitational then that implies infinite density and infinite force for any matter that occupies the exa

by treis 4y ago

>If DM interacts purely gravitationally If it's purely gravitational then that implies infinite density and infinite force for any matter that occupies the exact same space.

treis 4y ago
Also, if there's 5x as much dark matter in the universe why is our solar system so devoid of it? We should be able to detect the equivalent of dark matter suns or planets around us.
- rocqua 4y ago
  All of the below is according to theory, not claiming it to be true. Because DM passes right through itself it doesn't clump like the stuff that made our planets do. Instead it remains spread out bound by gravity but not clumping. That makes it incredibly diffuse. But space is also quite big, so it can still contribute a lot of mass.
panda-giddiness 4y ago
On the contrary, if dark matter were to interact purely gravitationally, then there would be little friction to slow it down. This would prevent dark matter from clumping, which is in agreement with observations.
- treis 4y ago
  I don't think so. Say you had two stationary whatevers of dark matter 1 meter apart alone in the universe. At t=0 they accelerate towards each other. At some time they cross and occupy the exact same point. R becomes 0 and the force becomes infinite. They should be stuck together.
  - gpderetta 4y ago
    That would make DM collapse in a black hole. I don't think that DM black holes are considered impossible, just not likely.
    - raattgift 4y ago
      The key weakness in your comment's parent comment was, "occupy the exact same point". One might ask why shining a flashlight during broad daylight (or say while doing work around the international space station) doesn't produce black holes. Or why there aren't black holes in the first image at <https://en.wikipedia.org/wiki/Caustic_(optics) https://en.wikipedia.org/wiki/Caustic_(optics)>. Particle DM is still quantum particles, with Compton and de Broglie wavelengths. A cold milliectronvolt axion has a wavelength on the order of a kilometre. How do you trap a pair of them in a volume comparable to the Schwarzschild radius of about 10^-50 metres? Additionally, an appreciable mass-energy of dark matter (one you can expect to be described tractably by the Raychaudhuri equations) will be a many-particle system occupying a noncompact volume. Newton's constant G in SI units is the force in Newtons between two one-kilogram masses at rest with respect to each other at one metre apart. It's small (< 7e-11). You will wait a long time -- about a full Earth day -- for two kg of anything initial 1m apart to get very much closer to each other because of (only) their mutual gravitational interaction. However, each of the pair of kg-mass blobs of DM particles is not likely to stick around intact that long because (by virtue of them being weakly interacting particles) nothing around them is strong enough to keep them from running away due to thermal motion. They are only cold in the sense they move slowly compared to hot dark matter in the form of relativistic neutrinos; they can still move about at many m/s, which is a lot faster than m/day. Newton's constant is rather small, and rather too tiny when thinking of coaxing a couple kg of CDM into a volume comparable to the Schwarzschild radius of about 10^-27 metres. One might compare this with two kg-masses of water ice. In a ball of ice various interactions trap molecules within the ball even against sublimation. The ice balls will come into contact in about a day. Thus we can have things like Saturn's rings or periodic comets, helped by the non-gravitational interactions of water ice keeping the mass-energy-momentum from flying away before gravity can do its work. Indeed, keeping galaxy and galaxy-cluster dark matter halos from flying away puts an upper bound on their temperature (if you heat up the water ice above to a couple hundred kelvins it can sublimate away in minutes). Halo DM needs to be cold in order for it to stick around. tl;dr, two sets of non-self-interacting (collisionless) dark matter just slide right through each other. cf. the famous Bullet cluster collision. You'd need at least a galaxy worth of dark matter and at least many millions of years of Chandrasekhar dynamical friction (radiating away very low amplitude gravitational waves as structures ("subhalos") form and move through each other) to make a black hole from DM alone.
  - thriftwy 4y ago
    Nope - they would bounce.
    - treis 4y ago
      If they bounce then they interact with some force besides gravity
      - thriftwy 4y ago
        No, I mean they will run through one another and continue in the directions they have flown previously, but now slowing down rather than accelerating, and would repeat that loop endlessly. Particles are point-like, they can't "collide".
  - panda-giddiness 4y ago
    If you haven't yet, you should read raattgift's comment. It's great, and it goes in to more details than I can bother with. But I will briefly point out the flaws with your thought experiment: 1. Point-like particles don't exist, so the force will never become infinite. In fact, this is the wrong paradigm all together -- if you're talking about two particles "colliding", then what you're really referring to is their interaction cross section (which you can think of as being their probability of interacting). Neutrino (weak interaction) scattering is incredibly rare. Gravity is much weaker still. The particles would indeed pass right through each other then oscillate back and forth (not forever, but practically so). 2. You have assumed that both particles begin with a relative velocity of 0. This is not a good assumption. In the real world, two dark matter particles would have random relative velocities according to some distribution, which means they would have angular momentum relative to each other. Again, that angular momentum can only dissipate through friction. If the particles interact solely gravitationally, then they can only lose energy via gravitational waves (i.e., extremely slowly). You probably know that the Earth formed due to gravity. But gravity is only half the answer -- you also need something to slow the matter. On Earth, that was the electromagnetic force. Purely gravitational matter has only gravity.

by treis 4y ago

>If DM interacts purely gravitationally If it's purely gravitational then that implies infinite density and infinite force for any matter that occupies the exact same space.

treis 4y ago

Also, if there's 5x as much dark matter in the universe why is our solar system so devoid of it? We should be able to detect the equivalent of dark matter suns or planets around us.

rocqua 4y ago
All of the below is according to theory, not claiming it to be true. Because DM passes right through itself it doesn't clump like the stuff that made our planets do. Instead it remains spread out bound by gravity but not clumping. That makes it incredibly diffuse. But space is also quite big, so it can still contribute a lot of mass.

panda-giddiness 4y ago

On the contrary, if dark matter were to interact purely gravitationally, then there would be little friction to slow it down. This would prevent dark matter from clumping, which is in agreement with observations.

treis 4y ago
I don't think so. Say you had two stationary whatevers of dark matter 1 meter apart alone in the universe. At t=0 they accelerate towards each other. At some time they cross and occupy the exact same point. R becomes 0 and the force becomes infinite. They should be stuck together.
- gpderetta 4y ago
  That would make DM collapse in a black hole. I don't think that DM black holes are considered impossible, just not likely.
  - raattgift 4y ago
    The key weakness in your comment's parent comment was, "occupy the exact same point". One might ask why shining a flashlight during broad daylight (or say while doing work around the international space station) doesn't produce black holes. Or why there aren't black holes in the first image at <https://en.wikipedia.org/wiki/Caustic_(optics) https://en.wikipedia.org/wiki/Caustic_(optics)>. Particle DM is still quantum particles, with Compton and de Broglie wavelengths. A cold milliectronvolt axion has a wavelength on the order of a kilometre. How do you trap a pair of them in a volume comparable to the Schwarzschild radius of about 10^-50 metres? Additionally, an appreciable mass-energy of dark matter (one you can expect to be described tractably by the Raychaudhuri equations) will be a many-particle system occupying a noncompact volume. Newton's constant G in SI units is the force in Newtons between two one-kilogram masses at rest with respect to each other at one metre apart. It's small (< 7e-11). You will wait a long time -- about a full Earth day -- for two kg of anything initial 1m apart to get very much closer to each other because of (only) their mutual gravitational interaction. However, each of the pair of kg-mass blobs of DM particles is not likely to stick around intact that long because (by virtue of them being weakly interacting particles) nothing around them is strong enough to keep them from running away due to thermal motion. They are only cold in the sense they move slowly compared to hot dark matter in the form of relativistic neutrinos; they can still move about at many m/s, which is a lot faster than m/day. Newton's constant is rather small, and rather too tiny when thinking of coaxing a couple kg of CDM into a volume comparable to the Schwarzschild radius of about 10^-27 metres. One might compare this with two kg-masses of water ice. In a ball of ice various interactions trap molecules within the ball even against sublimation. The ice balls will come into contact in about a day. Thus we can have things like Saturn's rings or periodic comets, helped by the non-gravitational interactions of water ice keeping the mass-energy-momentum from flying away before gravity can do its work. Indeed, keeping galaxy and galaxy-cluster dark matter halos from flying away puts an upper bound on their temperature (if you heat up the water ice above to a couple hundred kelvins it can sublimate away in minutes). Halo DM needs to be cold in order for it to stick around. tl;dr, two sets of non-self-interacting (collisionless) dark matter just slide right through each other. cf. the famous Bullet cluster collision. You'd need at least a galaxy worth of dark matter and at least many millions of years of Chandrasekhar dynamical friction (radiating away very low amplitude gravitational waves as structures ("subhalos") form and move through each other) to make a black hole from DM alone.
- thriftwy 4y ago
  Nope - they would bounce.
  - treis 4y ago
    If they bounce then they interact with some force besides gravity
    - thriftwy 4y ago
      No, I mean they will run through one another and continue in the directions they have flown previously, but now slowing down rather than accelerating, and would repeat that loop endlessly. Particles are point-like, they can't "collide".
- panda-giddiness 4y ago
  If you haven't yet, you should read raattgift's comment. It's great, and it goes in to more details than I can bother with. But I will briefly point out the flaws with your thought experiment: 1. Point-like particles don't exist, so the force will never become infinite. In fact, this is the wrong paradigm all together -- if you're talking about two particles "colliding", then what you're really referring to is their interaction cross section (which you can think of as being their probability of interacting). Neutrino (weak interaction) scattering is incredibly rare. Gravity is much weaker still. The particles would indeed pass right through each other then oscillate back and forth (not forever, but practically so). 2. You have assumed that both particles begin with a relative velocity of 0. This is not a good assumption. In the real world, two dark matter particles would have random relative velocities according to some distribution, which means they would have angular momentum relative to each other. Again, that angular momentum can only dissipate through friction. If the particles interact solely gravitationally, then they can only lose energy via gravitational waves (i.e., extremely slowly). You probably know that the Earth formed due to gravity. But gravity is only half the answer -- you also need something to slow the matter. On Earth, that was the electromagnetic force. Purely gravitational matter has only gravity.