Dark matter & black holes

[Pookadude]

Reading a bit about dark matter and the way we know how dark matter is not mundane matter, and that the amount of mundane matter is accounted for by other factors. The question i have is this: If dark matter makes up 5 times the mass of ordinary matter wouldn't black holes weigh signifigantly more than they do? Like 6 times what they should weigh if they are only made up of mundane matter?

[timo]

I'd go as far as to say that nothing weights significantly more than it does. That comment may seem like nitpicking to you. But perhaps you could also come to the conclusion that having a 2nd thought about what "wouldn't black holes weigh significantly more than they do" is supposed to actually mean (and the implicit assumptions that went into this statement) may lead to some insight by itself.

Edited July 15, 2012 by timo

[zapatos]

Reading a bit about dark matter and the way we know how dark matter is not mundane matter, and that the amount of mundane matter is accounted for by other factors. The question i have is this: If dark matter makes up 5 times the mass of ordinary matter wouldn't black holes weigh signifigantly more than they do? Like 6 times what they should weigh if they are only made up of mundane matter?

How did you come to this conclusion that you are asking about? Can you supply some more detail?

[Moontanman]

i think he is asking why dark matter doesn't concentrate into black holes like matter does and the reason is... because dark matter doesn't radiate away energy the way matter does and so can't condense around black holes the way matter does...

[ACG52]

Bear in mind that although the gravitational field of a BH is incredibly powerful, it falls off rapidly with distance. The only dark matter which would be swallowed by a BH is the DM which is in it's immediate vicinity, just like only matter in the immmediate vicinity of the BH is sucked in. And since DM interacts gravitationally, there's no reason to think that it isn't. It's just not that much.

[Pookadude]

If dark matter is some form of particle, axion, WIMP or neutralino then they would be orbiting the center of mass (primaraly thier own) of the galaxy. Thier orbits, unaffected by anything else would bring them into contact with black holes, especially the ones in the center of galaxys. While I relalise that the incident of this happening would be small, black holes would be moving and encountering dark matter like a car moving through a rainstorm, it's path would never clear it out. 10 billion years worth of encountering dark matter would make no measuarble difference?

 

 

The question stems from the reported fact that we have a pretty good idea of how much normal matter was created in the big bang, and the statement I have heard in several places that this is accounted for by the visible matter, gas, dust and black holes. Also it seems to be fairly straightforward to calculate the size of a black hole that any given star would create, given it's mass and metallicity.

 

 

So shouldn't black holes weigh at least a little more than the old estimates due to the dark matter they have encountered? If they don't wouldn't this be evidence against ideas of dark matter being particles?

 

 

 

 

i think he is asking why dark matter doesn't concentrate into black holes like matter does and the reason is... because dark matter doesn't radiate away energy the way matter does and so can't condense around black holes the way matter does...

 

Humm don't think i understand this, if any particle that was affected by gravity encountered the event horizon would it not essentially be added to the black hole's mass?

[Janus]

If dark matter is some form of particle, axion, WIMP or neutralino then they would be orbiting the center of mass (primaraly thier own) of the galaxy. Thier orbits, unaffected by anything else would bring them into contact with black holes, especially the ones in the center of galaxys. While I relalise that the incident of this happening would be small, black holes would be moving and encountering dark matter like a car moving through a rainstorm, it's path would never clear it out. 10 billion years worth of encountering dark matter would make no measuarble difference?

 

 

The question stems from the reported fact that we have a pretty good idea of how much normal matter was created in the big bang, and the statement I have heard in several places that this is accounted for by the visible matter, gas, dust and black holes. Also it seems to be fairly straightforward to calculate the size of a black hole that any given star would create, given it's mass and metallicity.

 

 

So shouldn't black holes weigh at least a little more than the old estimates due to the dark matter they have encountered? If they don't wouldn't this be evidence against ideas of dark matter being particles?

 

You have to remember, while there is a lot more dark matter in the universe, it is a lot more evenly spread out. Consider our galaxy. It has a dark matter halo. But this halo consists of a large sphere much larger than the galaxy itself. The visible matter is more or less confined to the galactic disk, which has a much smaller volume. As a result dark matter is spread out much more thinly than visible matter, so much so, that even accounting for dark matter swept up by the Sun's gravity since its formation, the total dark matter in the Solar system is only estimated to be equal to that of a single asteroid. So the amount of dark matter encountered by a black hole during its lifetime would be extremely small, much smaller than the uncertainty in estimating what its mass should be.

 

 

 

 

 

 

Humm don't think i understand this, if any particle that was affected by gravity encountered the event horizon would it not essentially be added to the black hole's mass?

 

The point is that for a DM particle its initial trajectory would have to hit the event horizon in order to be captured. Given that a typical 10 solar mass BH has an event horizon only 30 km in radius, this is a small target to hit.

 

Normal matter doesn't have to be aimed as accurately. Since it interacts with other matter surrounding and falling into the BH. The collisions between the matter near the black hole causes it to radiate energy, which causes it to fall in closer to the event horizon. As a result, matter which starts with a trajectory that wouldn't initially intersect the event horizon ends up falling in; this means that the BH is effectively a larger target for normal matter.

[Moontanman]

That is what I was trying to say Janus but you did a better job....

[Pookadude]

Thanks for the excellent replies.

 

But, a black hole normally consumes all the avalible normal matter the rest settles into stable orbits around it, it would continously be encountering dark matter for the entirety of it's lifespan. A supermassive black hole would encounter allot of dark matter becuase it would be in highly elptical orbits around the area the black hole is in. Wouldn't it?

 

Even realtivally tiny amounts of dark matter falling into a black hole would throw off the calculations for how quickly they evaporate?

[Janus]

Thanks for the excellent replies.

 

But, a black hole normally consumes all the avalible normal matter the rest settles into stable orbits around it, it would continously be encountering dark matter for the entirety of it's lifespan. A supermassive black hole would encounter allot of dark matter becuase it would be in highly elptical orbits around the area the black hole is in. Wouldn't it?

Dark matter would behave gravitationally just the same as normal matter and is just as likely to settle into a stable orbit. More likely, in fact as it doesn't have to worry about running into other matter orbiting the BH. The only DM that it will collect is that which already has an event horizon crossing orbit,

 

 

Even realtivally tiny amounts of dark matter falling into a black hole would throw off the calculations for how quickly they evaporate?

 

And how exactly would you test this? Considering that it would take some 2e70 yrs ( 1.5e60 times longer than the current age of the universe) for a typical stellar black hole to evaporate, even with no in-falling matter or radiation, you'd be in for a long wait seeing whether or not calculations match actual evaporation rate.

[Pookadude]

Taking axions for instance, they condense out of whatever feild they do that from and then start to fall twords thier mutaul center of gravity. If the area they condensed in were roughly speherical then the majority of them would have paths that cross the center of mass over and over, they would concentraite there basically. The less spherical the area they condense from then the more circular thier orbits, but if they all condense out of a perfect shpere then all thier orbits would be straight line paths through the center. I don't think they would condense out of a perfect sphere i am just pointing out that the paths of most the axions would be crossing roughly the center wich is also where the supermassive black hole is. If the black hole is perhaps orbiting the center of mass of the galaxy then it would in, from the view of axions orbiting nearby for billions of years, a much larger surface area.

 

Maybe that explains why the dark matter doesn't seem to affect the rotation rates on galactic cores, just the suburbs, because there is a hole in the middle (cleared by the SMBH) and the effect of a feild on the inside of a sphereical shell is negligable.

 

Just wondering about the rates of decay because I am reading about Penroses' book on CCC.

 

Thanks again for the tolerant comments on my musings.

[Severian]

You don't need to invoke Black holes for your objection. You could just ask why is the Earth not have a stronger gravitational pull than one could infer from the amount of visible matter? The reason (as already stated in this thread) is simply that dark matter doesn't clump like the other matter does. While is does clump on large distance scales (galaxy cluster scales) due to its gravitational attraction, its lack of other interactions means that it doesn't clump on planet of black hole scales.