Does dark matter make black holes bigger?

In summary, dark matter does not interact gravitationally with black holes in the same way that ordinary matter does. While the densities of both types of matter are comparable on a galactic scale, stars provide a highly overdense region of space for black holes to feed on, while dark matter is more uniformly distributed and does not clump together. This, along with the lack of self-interaction and slow movement of dark matter particles, makes it difficult for them to be consumed by black holes and contribute to their growth. The possibility of dark matter interacting with black holes is still being studied, but current observations suggest that it does not significantly affect the size of their event horizon.
  • #1
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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  • #2
Dark matter would fall into a black hole just like ordinary matter. I don't understand your last comment.
 
  • #5
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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  • #6
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.
 
  • #7
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?
 
  • #8
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.
 
  • #9
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.
 
  • #10
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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  • #11
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.
 
  • #12
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.
 

Related to Does dark matter make black holes bigger?

1. What is dark matter?

Dark matter is a type of matter that does not emit or interact with light, making it invisible to telescopes and other instruments. Its existence is inferred through its gravitational effects on other objects, such as stars and galaxies.

2. How does dark matter affect black holes?

Dark matter is believed to have a significant impact on the formation and growth of black holes. It is thought that dark matter helps to pull in and condense the gas and dust that eventually collapses into a black hole, making it bigger and more massive.

3. Can dark matter create black holes on its own?

No, dark matter alone cannot create black holes. It does not interact with itself or other particles in the same way that normal matter does, so it cannot collapse into a black hole on its own. However, dark matter is thought to play a role in the formation of black holes by helping to bring in more normal matter.

4. Is there a correlation between the size of black holes and the amount of dark matter in a galaxy?

Yes, there is evidence to suggest that the size of a black hole is related to the amount of dark matter in a galaxy. Galaxies with more dark matter tend to have larger black holes, and vice versa.

5. Can dark matter make black holes bigger over time?

Yes, dark matter can continue to contribute to the growth of black holes over time. As a galaxy evolves and accumulates more dark matter, it can provide the necessary gravitational pull to bring in more gas and dust, leading to the formation and growth of larger black holes.

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