Does dark matter make black holes bigger?

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Discussion Overview

The discussion centers around the interaction between dark matter and black holes, specifically whether dark matter contributes to the growth of black holes by increasing their mass and event horizon. Participants explore theoretical implications, observational challenges, and the nature of dark matter in relation to black holes.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that dark matter interacts gravitationally with black holes, potentially increasing their mass and event horizon as it falls in.
  • Others argue that while dark matter would fall into a black hole like ordinary matter, the rate of this interaction may be limited due to the low density of dark matter in comparison to baryonic matter.
  • One participant questions the assumption that the event horizon grows significantly with dark matter, suggesting that measuring such growth is complicated by the small amount of dark matter available for infall.
  • Concerns are raised about the weakly interacting nature of dark matter, which may prevent it from forming accretion discs necessary for efficient mass transfer into black holes.
  • Some participants clarify that dark matter is not the same as dark energy, noting that dark energy is repulsive while dark matter is thought to help hold galaxies together.
  • There is a discussion about the mass and density of dark matter, with some suggesting that while it is abundant, it is distributed diffusely and does not clump like baryonic matter.
  • Participants mention that dark matter lacks self-interaction, which affects its ability to slow down and fall into black holes compared to baryonic matter.

Areas of Agreement / Disagreement

Participants express differing views on the role of dark matter in the growth of black holes, with no consensus reached on whether dark matter significantly contributes to this process. The discussion remains unresolved regarding the implications of dark matter's properties on black hole dynamics.

Contextual Notes

Limitations include the challenges in measuring the growth of black holes and the density of dark matter in various regions of space. The discussion also highlights the complexity of dark matter's interaction with black holes, which is not fully understood.

adolphysics
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I know that a black hole it's a singularity, but its event horizon gets bigger the more mass/energy you throw in. I thought that like dark matter interacts gravitationally with regular matter , it would interact with black holes, eventually falling , increasing the mass of the black hole and therefore , its event horizon. It's "easy" to see if the event horizon gets bigger too fast, only counting regular matter, so i guess it does not happen, but why?
 
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Dark matter would fall into a black hole just like ordinary matter. I don't understand your last comment.
 
adolphysics said:
It's "easy" to see if the event horizon gets bigger too fast, only counting regular matter
Is it, though?
In the case of baryonic matter, black holes grow by accrettion of material that is available in their immediate vicinity - the atmosphere of a companion star, for example.
If one were to look at the equation for the Schwarzschild radius of an idealised non-rotating black hole, ##R_s=2GM/c^2##, it is clear that in order to double the radius, you'd have to double the mass. A solar mass BH is approx 6km across. A small BH can feed on its companion for millions of years.
So, one needs an instrument capable of observing a few kilometers-across object, with growth rates on the order of milimeters per year. All from many light-years away.

Apart from the difficulty of measuring the size of the event horizon, let alone its growth, one has to consider the amount of dark matter that is available for infalling into the black hole.

While densities of both types of matter are comparable when averaged over the volume of a galaxy, a star is a highly overdense region of space, providing a lot of 'fuel' for the black hole, whereas at any given time, there is only a tiny amount of dark matter passing through the same region.
E.g. the current models suggest that at the distance from the galactic centre at which the Sun orbits, there is roughly a small asteroid-worth of DM contained within a 1 cubic AU volume of space, and while the density goes up closer to the galactic centre, it never approaches the densities of clumps of baryonic matter.
This is further exacerbated by the fact that DM is at best only weakly interacting - it does not have a way to form accretion discs, so no way to shed orbital velocity. Only those particles whose trajectories cross the event horizon can be added to the BH. Near misses will be just flung out back into space.
 
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To expand on Bandersnatch's reply. the amount of DM in a sun-sized volume weighs only about a ton. Finding a ton-sized perturbation in a star-sized effect from half the galaxy away isn't going to happen any time soon, I'm afraid.
 
Vanadium 50 said:
. the amount of DM in a sun-sized volume weighs only about a ton.
Wasn't dark matter very massive? Or at least, didn't it interact strongly with the gravitational field, thus making it weigh more?
 
No, it isn't. WIMPs are currently favoured over MACHOs. A possible candidate is something like the neutrino.
There's just more of it, mass-wise, than baryonic matter. But again, with very diffuse distribution.
 
adolphysics said:
Wasn't dark matter very massive?

Sure, because there's a lot of space. Mass = density * volume. Density is small, volume of a galactic cluster is large.
 
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adolphysics said:
Wasn't dark matter very massive?
Only at the scales of galaxies. It has a very uniform distribution and does not clump together on small scales like regular matter does. Within those small clumps (stars with planets orbiting them), the amount of dark matter is negligible.
 
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Dark matter also doesn't seem to have any type of self-interaction, so there is little or no friction. Dust particles in an accretion disc are sort of like being in orbit. The reason they fall into the black hole is because the disc rubs on itself and friction slows everything down, allowing it to fall inwards. Dark matter doesn't slow down when it crashes into itself as far as we can tell.
 
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Bandersnatch said:
A possible candidate is something like the neutrino.

Pretty much ruled out. They move too quickly and this would disrupt large scale structure formation.
 

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