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The idea that dark matter is hidden inside black holes has been ruled out via observations looking for gravitational micro-lensing† which is basically looking f

by throwaway2048 6y ago

The idea that dark matter is hidden inside black holes has been ruled out via observations looking for gravitational micro-lensing† which is basically looking for distortions in our views of the sky. Although we cant easily locate specific black holes, we know there is not enough micro lensing going on for the explanation for dark matter to possibly be black holes (the total mass of the black holes would have to be many times more than the amount of visible matter in the universe) † https://en.wikipedia.org/wiki/Gravitational_microlensing https://en.wikipedia.org/wiki/Gravitational_microlensing

Retric 6y ago
That’s making a fundamental flawed assumption. A small black hole on it’s own multiple light years from a star would likely produce lending on the order of seconds not hours. In such a situation, the lens will pass by the source in a reasonable amount of time, seconds to years instead of millions of years. Current survey’s have looked at leasing across longer time frames as that’s much easier to deftest though also much rarer. It’s possible to model this and make useful estimates, but such models are dependent on a huge range of assumptions.
- throwaway2048 6y ago
  That does not matter because you aren't looking to find every specific black hole, the amount of black holes required for MACHOs to be the answer to dark matter would have orders of magnitude more micro-lensing than we see, pretty much no matter what, there is only so much space inside a galaxy.
  - Retric 6y ago
    More transient events are also orders of magnitude more common. If a black hole’s geometry from you lenses’s light from a volume of space that could contain a star for X time. Them it would lens an equivalent volume of space 10x as far from it for 1/10th as long, and but that volume of space is 10^3 times as large. (Ignoring the edge of the Milky Way etc.) If you consider just how exact the alignments need to be for hour+ events the difference becomes even more pronounced.
    - Retric 6y ago
      PS: To be clear 2048’s point is still correct, black holes are a small fraction of the Milky Way’s total mass and a long way from filling the dark mater gap. It’s the actual experiment which is generally misinterpreted.

Retric 6y ago

That’s making a fundamental flawed assumption. A small black hole on it’s own multiple light years from a star would likely produce lending on the order of seconds not hours. In such a situation, the lens will pass by the source in a reasonable amount of time, seconds to years instead of millions of years. Current survey’s have looked at leasing across longer time frames as that’s much easier to deftest though also much rarer. It’s possible to model this and make useful estimates, but such models are dependent on a huge range of assumptions.

throwaway2048 6y ago
That does not matter because you aren't looking to find every specific black hole, the amount of black holes required for MACHOs to be the answer to dark matter would have orders of magnitude more micro-lensing than we see, pretty much no matter what, there is only so much space inside a galaxy.
- Retric 6y ago
  More transient events are also orders of magnitude more common. If a black hole’s geometry from you lenses’s light from a volume of space that could contain a star for X time. Them it would lens an equivalent volume of space 10x as far from it for 1/10th as long, and but that volume of space is 10^3 times as large. (Ignoring the edge of the Milky Way etc.) If you consider just how exact the alignments need to be for hour+ events the difference becomes even more pronounced.
  - Retric 6y ago
    PS: To be clear 2048’s point is still correct, black holes are a small fraction of the Milky Way’s total mass and a long way from filling the dark mater gap. It’s the actual experiment which is generally misinterpreted.