Dark matter and black hole interaction

[maria_phys]

Assuming that our central black hole is sucking dark matter, can we explain the reason why it's mass is 10^9 solar masses? Dark matter interacts with the ordinary matter (or with any stellar object) via gravity, thus the mass of a black hole is increasing without having any paradox (at first sight).

 

[mfb]

Black holes do not "suck". They attract things based on their mass like everything else does. The amount of dark matter falling into the central black hole (or any other black hole) is tiny.

 

[maria_phys]

Black holes do not "suck". They attract things based on their mass like everything else does. The amount of dark matter falling into the central black hole (or any other black hole) is tiny.

Why? I imagine exactly the opposite. Dark matter interacts with the ordinary matter only via gravity. Assuming an object with very strong gravity, the dark matter would fall immediately into the black hole because of this strong interaction. So, all the dark matter of the universe would fall into the primordial black holes. Hmm, that seems not correct..

Something I'm missing...

Thank you for your answer, anyway :)

 

[phinds]

Why? I imagine exactly the opposite. Dark matter interacts with the ordinary matter only via gravity. Assuming an object with very strong gravity, the dark matter would fall immediately into the black hole because of this strong interaction. So, all the dark matter of the universe would fall into the primordial black holes. Hmm, that seems not correct..

Something I'm missing...

Thank you for your answer, anyway :)

See post #2 --- it explains what you are "missing"

 

[Drakkith]

Why? I imagine exactly the opposite. Dark matter interacts with the ordinary matter only via gravity. Assuming an object with very strong gravity, the dark matter would fall immediately into the black hole because of this strong interaction. So, all the dark matter of the universe would fall into the primordial black holes. Hmm, that seems not correct..

In order to fall into a black hole your path would need to take you directly into the event horizon. But black holes are usually very tine. A black hole of perhaps a few solar masses is only about a dozen miles in diameter or something. The chances of any dark matter crossing that tiny section of space is minuscule.

Note that gravity isn't like a vacuum cleaner. Objects don't spiral into a black hole unless they have some way to slow down so that they "fall" towards the black hole. For regular matter, collisions in the accretion disk slow the dust and gas and, over time, lead to a slow in-spiral until it passes the event horizon. Since dark matter doesn't interact with itself or with normal matter though anything but gravity, it cannot collide with anything and thus cannot lose speed and fall into the event horizon.

 

[phinds]

In order to fall into a black hole your path would need to take you directly into the event horizon. But black holes are usually very tine. A black hole of perhaps a few solar masses is only about a dozen miles in diameter or something. The chances of any dark matter crossing that tiny section of space is minuscule.

Note that gravity isn't like a vacuum cleaner. Objects don't spiral into a black hole unless they have some way to slow down so that they "fall" towards the black hole. For regular matter, collisions in the accretion disk slow the dust and gas and, over time, lead to a slow in-spiral until it passes the event horizon. Since dark matter doesn't interact with itself or with normal matter though anything but gravity, it cannot collide with anything and thus cannot lose speed and fall into the event horizon.

As I said; see post #2