[Q] Do black holes take in dark matter? If not why not?

[Moontanman]

Does dark matter end up in black holes such as the one at the center of our galaxy?

[Pyrotex]

Excellent question! :couple:

The answer is uncertain. We are able to detect Dark Matter. But not with enough resolution to see if it is absorbed by black holes. However, we do know that it is ATTRACTED to black holes just like it is gravitationally attracted to all matter. We just don't know if it "ends up" IN a black hole.

 

My best guess is this: Think of all of our Space-Time (that makes up our universe) as a large sheet of plastic. Saran Wrap will do, if you don't have any plastic. :phones:

 

All the ORDINARY matter in our universe is on ONE SIDE of the plastic. This ordinary matter attracts gravitationally AND it interacts in many ways. A black hole does in fact suck in ordinary matter and grows bigger.

 

All the DARK matter in our universe is on THE OTHER SIDE of the plastic. This dark matter ALSO attracts gravitationally -- to itself and to ordinary matter. ALL matter on the plastic sheet -- both sides -- have and feel gravity from all other matter.

 

BUT!! The dark matter CANNOT INTERACT with the ordinary matter on OUR side of the plastic. And vice versa.

 

In addition, it appears that dark matter cannot form black holes on its side of the plastic. Dark matter can accumulate into clouds the size of a galaxy, but it seems to behave as a hot gas, not as solid lumpy stuff of which stars and planets are made.

 

So, dark matter would be attracted to a black hole, and swirl around it, and form a tenuous "hot gassy cloud" around it. But it cannot be sucked into the black hole, because the plastic sheet does not allow (dark) matter from one side to interact with ordinary matter, nor to travel "through" the plastic to the other side and be sucked into a black hole.

 

I'm probably wrong, but that's my take on it for now.

[lawcat]

First, it's important to say that we don't know because dark matter is undetectable. It is hypothesised, and indicated by interpretation of some data, that dark matter exists.

 

There are two ways to think about this: (1) conceptually (Einstein) and (2) methematically (Dirac, Schrodinger, along those lines). If you believe in general relativity, then spacetime geometry dictates movement. Geometry would mandate that everything, all, follows geodesics. If you think along the lines of Dirac's equations and Schrodinger, which is mathematical, then there are negative, repulsive possibilities. We can't test it. We can only hypothesise presently, as far as I understand it.

[Pyrotex]

First, it's important to say that we don't know because dark matter is undetectable....

Here's a picture of Dark Matter along with his phone number. :couple:

[lawcat]

... It is hypothesised that the gravitational distortion of light from visible matter is due to dark matter.

hello! :couple:

[lawcat]

nothing to dispute there :naughty:

[Jay-qu]

Does dark matter end up in black holes such as the one at the center of our galaxy?

I would say not much at all.

 

First you need to understand how black holes get larger. They suck in mass through a process called accretion - it is not as simple as just gravitational attraction!

 

You see we are gravitationally attracted to our sun and also to the black hole at the centre of the galaxy. So why havent we been sucked into the sun or the black hole at the centre of the galaxy? The answer is angular momentum. We are in an orbit around the sun, the Earth's momentum and the force of gravity conspire to put us in an almost circular path around the sun. This system is very stable (we have been in this orbit for at least 4 billion years, and will probably still be here in another 4 billion!). The same goes for the whole galaxy, all the stars in the galaxy are revolving around the centre in their own stable orbits.

 

As it turns out most of the matter in the universe tends to be moving, they all carry some amount of momentum - this means that a lot of matter will just end up orbiting stars or the centre of the galaxies (not always in circular orbits). So you might wonder how black holes can suck in any matter at all - well now we need to consider the gravitational potential.

 

When you are orbiting a black hole (or star or planet) you will have a certain amount of gravitational potential energy. If you move closer to the black hole you will have less gravitational potential energy and if you move farther you will have more gravitational potential energy. This energy doesnt just appear and dissapear (due to the conservation of energy) it must be converted into another form of energy - in this case it will most likely be kinetic energy (or movement). This is why when you fall towards the ground you speed up!

 

So if you want to find a way to suck things into a black hole you need to find a way to loose this energy without turning it into kinetic energy (because that will prevent you from falling into the black hole). This is where accretion comes in. Black holes form what is called an accretion disk, a disk of matter that is gravitationally bound to the black hole (orbiting it) but hasnt fallen in yet. This disk is most likely made up of gas and dust. Now as the gas and dust move around in their orbits the particles can collide with eachother - this is essentially heating the gas. When the gas gets hot it can radiate some of this energy away through black body radiation - this is electromagnetic radiation or light (you might want to look this up on wiki, but its the same reason a bar of steel will glow red/white hot when heated a lot - and also responsible for the colour of the sun).

 

Now that we have a way for the gas and dust particles to get rid of some energy they can decrease their gravitational potential energy - getting closer to the black hole. This process lets off a lot of energy, as orbiting a black holes gives you a lot of gravitational energy. So much energy that the bigger black holes will often be emmiting x-rays and gamma rays - so they aren't very black in this form! These objects are called quasars and are the brightest objects that we know of - we can see them from the opposite side of the universe, billions of light years away.

 

So now you know how a black hole grows by accreting matter we can ask how can you accrete dark matter? Well if you follow through the logic above you will arrive at the part about collisions and emmision of electromagnetic energy. The problem we have here is that dark matter does not interact with electromagnetic energy (light) - it is exactly this reason that it is called dark! So dark matter cannot get rid of its gravitational potential energy and is thus stuck orbiting a black hole, unable to get close enough to be sucked in.

 

An astute reading may be thinking, but dark matter can interact gravitionally so what about getting rid of some energy through gravitional radiation? It turns out this is entirely possible - through gravitational waves (though we should note that to this day no gravitional waves have been observed because they are so weak). The problem with this mechanism is that because gravitational waves are so weak it takes a very long time to radiate enough energy through these waves to fall into a black hole. I have not performed the calculation myself, but I have heard that it is longer that the age of the universe!

 

I hope this helps, feel free to ask more questions :naughty:

[Moontanman]

Thank you jay-qu, that pretty much covers it for it. If indeed dark matter does go across the event horizon it cannot escape any better than matter?

[Jay-qu]

Exactly right, it is just as stuck as normal matter once over the horizon.