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Is it possible that black holes could account for a good portion of "dark matter"? They literally fit the term.

dbasedweeb 8y ago
There’s a theory which accounts for that called MACHO, for MAssive Compact Halo Object. While it’s possible, over many years of observation it’s very unlikely. You can’t see a black hole directly, unless it’s “eating” of course, but you can see gravitational effects like lensing. If black holes and neutron stars made up most dark matter, we’d expect to see evidence of it, and we don’t. It is still possible, although Weakly Interacting Massive Particles (WIMPs) are more likely, along with Axions and Sterile Neutrinos.
- wallace_f 8y ago
  My heretical theory of dark matter is that is just a byproduct of black holes gravitational effects. Probably something about so much mass punching such a deep curve in spacetime creates a widespread effect. Of course this is just a baseless theory, but it seems more reasonable to consider the possibilities of a realm which we know that we know almost nothing about, than inventing new particles or physics to fit the data. Again, not an educated theory.
  - semi-extrinsic 8y ago
    The trouble with this kind of theory is that for a force like gravity, if you consider the force outside a sphere surrounding the sun, the force is identical no matter how the mass inside is distributed, whether it's the sun or a solar mass black hole inside the sphere.
    - wallace_f 8y ago
      At extreme scales gravity may behave in strange and unexpected ways -- as do other phenomena.
      - dbasedweeb 8y ago
        It would have to be scales beyond the observable universe then, because it appears to operate as expected. If distance scales change things, then nearby galaxies would behave differently from very large clusters, quasars should display odd behavior, smaller galaxies would behave very differently from larger ones. We don’t see any of that.
luk32 8y ago
I don't think so. I think currently dark matter is considered rather low mass density i.e. doesn't bend light too much. Black holes would. Think that you have the mass spread out over a whole galaxy, vs just the middle. From infinity-like large distance it's the same, from galactic it would probably make a difference.
- pdonis 8y ago
  > dark matter is considered rather low mass density i.e. doesn't bend light too much. Black holes would. Not if the holes were spread out in a very large diffuse cloud around a galaxy. A single hole would bend light significantly, but the averaged effect of a huge diffuse cloud of holes around a galaxy would be basically zero (because there would be as many holes bending light one way as the other, so their effects overall would cancel out).
LoSboccacc 8y ago
the "missing matter" is not just at the galactic core but assumed distributed more thorough the galaxy. if it just were at the center, star rotational speed would follow keplerian predictions, instead they observed the same rotational speed across whole galaxy disks. https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve
- metaobject 8y ago
  Dark matter is also believed to be what holds galaxies (and galactic clusters) together by enveloping them in what are called dark matter halos. https://en.m.wikipedia.org/wiki/Dark_matter_halo https://en.m.wikipedia.org/wiki/Dark_matter_halo
- rmcpherson 8y ago
  Baryonic matter accelerating into black holes also emits electromagnetic radiation vs. dark matter which does not interact electromagnetically by definition (and observation).
pdonis 8y ago
> Is it possible that black holes could account for a good portion of "dark matter"? A key issue with this hypothesis is how the holes could have formed. In order to form a black hole, a system has to gravitationally collapse. But that involves emitting a lot of energy, usually in the form of radiation, as the system becomes more and more tightly bound gravitationally. For ordinary matter, the kind we and visible stars and galaxies are made of, that's not an issue. But there are strong limits on the fraction of the total matter in the universe that can be ordinary matter (basically because of the relative abundances of light elements, mainly deuterium, helium, and lithium, which are highly sensitive to the total number of quarks and leptons--i.e., ordinary matter--present in the early universe). The ordinary matter we can see is basically the same as the limit, which means there can't be a significant amount of ordinary matter we can't see. But the total amount of dark matter is about an order of magnitude larger than the total amount of ordinary matter in the universe. So if a significant portion of that is black holes, the holes would have to have formed from something that is not ordinary matter. And the problem is that such a something would not be subject to any of the known interactions except gravitation, and hence would have no way to emit enough energy to gravitationally collapse to a black hole. (This is also the reason why dark matter "halos" around galaxies are much more diffuse and spread out than the visible matter--the dark matter can't clump together gravitationally the way the visible matter does.) All of the above makes it very unlikely that black holes can account for a significant portion of dark matter.
- mar77i 8y ago
  What a summary. Thanks for this.

There’s a theory which accounts for that called MACHO, for MAssive Compact Halo Object. While it’s possible, over many years of observation it’s very unlikely. You can’t see a black hole directly, unless it’s “eating” of course, but you can see gravitational effects like lensing. If black holes and neutron stars made up most dark matter, we’d expect to see evidence of it, and we don’t. It is still possible, although Weakly Interacting Massive Particles (WIMPs) are more likely, along with Axions and Sterile Neutrinos.

wallace_f 8y ago
My heretical theory of dark matter is that is just a byproduct of black holes gravitational effects. Probably something about so much mass punching such a deep curve in spacetime creates a widespread effect. Of course this is just a baseless theory, but it seems more reasonable to consider the possibilities of a realm which we know that we know almost nothing about, than inventing new particles or physics to fit the data. Again, not an educated theory.
- semi-extrinsic 8y ago
  The trouble with this kind of theory is that for a force like gravity, if you consider the force outside a sphere surrounding the sun, the force is identical no matter how the mass inside is distributed, whether it's the sun or a solar mass black hole inside the sphere.
  - wallace_f 8y ago
    At extreme scales gravity may behave in strange and unexpected ways -- as do other phenomena.
    - dbasedweeb 8y ago
      It would have to be scales beyond the observable universe then, because it appears to operate as expected. If distance scales change things, then nearby galaxies would behave differently from very large clusters, quasars should display odd behavior, smaller galaxies would behave very differently from larger ones. We don’t see any of that.

luk32 8y ago

I don't think so. I think currently dark matter is considered rather low mass density i.e. doesn't bend light too much. Black holes would. Think that you have the mass spread out over a whole galaxy, vs just the middle. From infinity-like large distance it's the same, from galactic it would probably make a difference.

pdonis 8y ago
> dark matter is considered rather low mass density i.e. doesn't bend light too much. Black holes would. Not if the holes were spread out in a very large diffuse cloud around a galaxy. A single hole would bend light significantly, but the averaged effect of a huge diffuse cloud of holes around a galaxy would be basically zero (because there would be as many holes bending light one way as the other, so their effects overall would cancel out).

LoSboccacc 8y ago

the "missing matter" is not just at the galactic core but assumed distributed more thorough the galaxy. if it just were at the center, star rotational speed would follow keplerian predictions, instead they observed the same rotational speed across whole galaxy disks. https://en.wikipedia.org/wiki/Galaxy_rotation_curve https://en.wikipedia.org/wiki/Galaxy_rotation_curve

metaobject 8y ago
Dark matter is also believed to be what holds galaxies (and galactic clusters) together by enveloping them in what are called dark matter halos. https://en.m.wikipedia.org/wiki/Dark_matter_halo https://en.m.wikipedia.org/wiki/Dark_matter_halo
rmcpherson 8y ago
Baryonic matter accelerating into black holes also emits electromagnetic radiation vs. dark matter which does not interact electromagnetically by definition (and observation).

pdonis 8y ago

> Is it possible that black holes could account for a good portion of "dark matter"? A key issue with this hypothesis is how the holes could have formed. In order to form a black hole, a system has to gravitationally collapse. But that involves emitting a lot of energy, usually in the form of radiation, as the system becomes more and more tightly bound gravitationally. For ordinary matter, the kind we and visible stars and galaxies are made of, that's not an issue. But there are strong limits on the fraction of the total matter in the universe that can be ordinary matter (basically because of the relative abundances of light elements, mainly deuterium, helium, and lithium, which are highly sensitive to the total number of quarks and leptons--i.e., ordinary matter--present in the early universe). The ordinary matter we can see is basically the same as the limit, which means there can't be a significant amount of ordinary matter we can't see. But the total amount of dark matter is about an order of magnitude larger than the total amount of ordinary matter in the universe. So if a significant portion of that is black holes, the holes would have to have formed from something that is not ordinary matter. And the problem is that such a something would not be subject to any of the known interactions except gravitation, and hence would have no way to emit enough energy to gravitationally collapse to a black hole. (This is also the reason why dark matter "halos" around galaxies are much more diffuse and spread out than the visible matter--the dark matter can't clump together gravitationally the way the visible matter does.) All of the above makes it very unlikely that black holes can account for a significant portion of dark matter.

mar77i 8y ago
What a summary. Thanks for this.