3 ms·

If that is true, where are the dark matter black holes? And why hasn't the majority of DM collapsed into black holes?

• andrewflnr 4y ago
  The same reason the rest of the matter didn't collapse into black holes, not quite dense enough, or blowing up too fast. And dark matter doesn't have it's own kind of black holes. Related is the idea of "primordial black holes".
  • gpderetta 4y ago
    In fact I think it is harder for dark matter to collapse into black holes because it can't easily shed momentum (angular of otherwise) and 'spiral down' .
    • Beldin 4y ago
      That's what i used to think, but GP alluded to weak interaction between DM particles. I thinkt interaction consumes energy. If so, DM particles would be able to shed some momentum... but then they (eventually) could clump into black holes.
      • andrewflnr 4y ago
        Again, no more than baryonic matter, which is demonstrably not all wrapped up in black holes (and probably much less, though bear in mind this is all pretty speculative). Of course any dark matter that happens to be headed straight for a black hole will go in, no problem. Broadly there are a lot more variables in black hole formation than just self-interaction. Remember it's possible in principle for photons to form a black hole, if you get them to converge just right, but that doesn't mean it's common or at all relevant on a cosmological scale.
• russdill 4y ago
  It's precisely the reason dark matter tends to not fall into black holes. If you put a particle into an orbit around a black hole, it will basically do so forever as it needs to lose an enormous amount of energy to drop down to a low enough orbit. Normal matter bumps into other normal matter when orbiting a black hole, heats up, and radiates away it's energy. This effect is most visible in the accretion disk of a black hole. But dark matter can still fall into a black hole and it still tends to concentrate due to gravitational interactions. If a dark matter particle is on a path unlucky enough to intercept the event horizon, it will of course not orbit. Also, gravitational interactions between dark matter particles or just normal matter can cause an exchange of energy, with one of the particles losing energy. Oh, and if dark matter falls into a black hole, it just makes the black hole a little bigger. There's no way to externally tell that the black hole contains dark matter.
  • Beldin 4y ago
    I was wondering about black holes created from dark matter only. I mean: if there's 5x as much stuff that has mass (and, thus, can form black holes), there should be 5x as many black holes - at least, that's what I'd expect in the absence of some specific reasons. But as far as I know, we haven't seen them.