Paging Hawking: Black Hole Query

[Brigandier]

Do black holes absorb dark matter?

 

[C1]

Probably a good question as I believe that DM is opposite to matter (ie, repulsive with matter) and its hypothesized existence is the reason ascribed for the continued accelerated expansion of the universe.

Is it possible that DM (ie, pouring it into a BH) could be a solution for undoing a black hole?

We'll be watching for answers.

 

[pelov]

I believe dark matter interacts with gravitational fields so in theory it should be absorbed?

 

[RocksteadyDotNet]

Probably a good question as I believe that DM is opposite to matter (ie, repulsive with matter) and its hypothesized existence is the reason ascribed for the continued accelerated expansion of the universe.

Is it possible that DM (ie, pouring it into a BH) could be a solution for undoing a black hole?

We'll be watching for answers.

Dark matter doesn't repel matter, there's no such thing as 'anti-gravity'.

 

[moonbogg]

Dark matter attracts regular matter gravitationally. Dark energy is what is responsible for the expansion of space. They are different. You can observe the effects of dark matter's gravity in photographs taken of areas of space near galaxy clusters where gravitational lensing occurs due to the presence of dark matter.
Since it interacts with regular matter gravitationally, but not physically, it doesn't fall into black holes because as it swirls around the black hole, there is no friction/loss of angular momentum to allow it to get close enough to fall in. So no, if any falls in it is very little.

 

[MrDudeMan]

Dark matter attracts regular matter gravitationally. Dark energy is what is responsible for the expansion of space. They are different. You can observe the effects of dark matter's gravity in photographs taken of areas of space near galaxy clusters where gravitational lensing occurs due to the presence of dark matter.
Since it interacts with regular matter gravitationally, but not physically, it doesn't fall into black holes because as it swirls around the black hole, there is no friction/loss of angular momentum to allow it to get close enough to fall in. So no, if any falls in it is very little.

Are you talking about dark matter that is incident to the black hole or simply passing by?

 

[Lemon law]

First of all, and in somewhat MHO, there is no universal agreement dark matter even exists.
As the concept of dark matter was inferred from the fact that that mankind have not yet seen enough regular matter to account for the existing expansion of the universe. An expansion rate Independently calculated by blue and red shifts.

But I do not claim to have any qualifications to even express an a opinion in this area, as I go only by gut opinions only. But I also point out, some 170 years ago, almost all Physicists believed either was required to to transmit light through empty space.

But speaking of black holes, I never could understand the Hawking prediction that black holes could leak matter over time.

 

[Gibsons]

First of all, and in somewhat MHO, there is no universal agreement dark matter even exists.
As the concept of dark matter was inferred from the fact that that mankind have not yet seen enough regular matter to account for the existing expansion of the universe. An expansion rate Independently calculated by blue and red shifts.

Moonbogg's post explains (what I think is) the strongest evidence, the gravitational lensing stuff.

 

[Agent11]

I heard of gravitational lensing being used to identify the presence of black holes, I'm skeptical about 'dark matter' though, who is to say any lensing that is said to affirm the existence of dark matter isn't actually caused by black holes?

Seems that a lot of extrapolation is being made on area's where we don't have much data.

 

[Biftheunderstudy]

There is a wealth of evidence for the presence of "dark matter". Before I get into it, dark matter is simply stuff that we can't see that doesn't seem to interact with the type of matter that we see around us.

Also, there is no reason that dark matter can't fall into a black hole; it does, in fact, interact gravitationally. The type of accretion onto a supermassive blackhole like the one in the centre of our galaxy would be called Bondi-Hoyle accretion, think of it like the pacman approach to sucking up matter. Whatever paths intersect the event horizon are consumed. Moonbogg's post is mostly correct, however there are a couple of ways which dark matter could lose angular momentum (more likely it actually gains it, but that's a relatively recent problem to be solved) such as dynamical friction (wiki it, should be there).

Now, dark matter:

Our best evidence for dark matter is an amalgamation of big bang theory with the extremely good observations of the cosmic microwave background by WMAP. The details of which are a bit too involved to get into on a forum post, but there are many resources out there to learn why this is true. -- Not to say it is necessarily correct, but that it is our best fitting model of the universe.

The first hint of dark matter came with rotation curves of our galaxy and then others. A rotation curve is a measure of the radial velocities of HII regions as a function of galactocentric radius which we can get by measuring the doppler shift of the 21cm hyperfine hydrogen spin flip transition. A simple application of galactic gravitational potential theory tells us that if the mass of the galaxy were contained in the visible material then this rotation curve should drop off like 1/r. Instead, we see rather robustly that these curves remain constant implying there is a large amount of mass in the outer regions of galaxies (again, potential theory). This has led to a quantity called the mass to light ratio which describes how much mass (measured by rotation curves for instance) compared to the amount of mass measured by the luminous matter.

So why can't all the mass be in things like black holes, brown dwarfs, neutron stars etc? We thought of this pretty early on, they were coined MACHO's (massive compact halo objects). We came up with a lot of theory to try to predict how much mass could be hidden in objects like this and it just doesn't work, we also can do some statistics given our local neighborhood and the numbers don't match up. The WMAP results really put a nail in this coffin too.

Maybe they aren't really big objects, but lots of tiny little objects that we can't see, like neutrinos. Thought of this too, this theory was called hot dark matter, again, too much detail here so I'll skip it. Suffice to say, WMAP kills this theory.

There are other sources of evidence for dark matter, such as gravitational lensing (both strong and weak) and the prime example, the bullet cluster (seriously, look this one up its very cool).

Anyone care to explain Hawking Radiation, I've taken up too much space on this post already....

 

[Sunny129]

Anyone care to explain Hawking Radiation, I've taken up too much space on this post already....

well the [extremely simplified] explanation is that Hawking Radiation is simply the black body radiation that is predicted to be emitted by black holes. when we dig a bit deeper, we find that it is a result of quantum fluctuations in which virtual particles (particle-antiparticle pairs) constantly come into and out of existence for infinitesimal periods of time. when this occurs very near the event horizon, one of the two particles in the particle-antiparticle pair may come into existence just on the surface of, or just inside of, the event horizon. as one particle is consumed, the other particle escapes the BH's clutches, giving the BH the appearance of having just emitted a particle.

now here's the part i don't quite understand. according to the theory, energy must be conserved...and that means that if the escaped particle consists of positive energy, then the particle that got swallowed by the BH must have had negative energy, and therefore must have had a negative mass, and therefore must have reduced the overall mass of the BH from the point of view of an outside observer. conceptually it makes sense...i'm just skeptical about negative energy and negative mass.

 

[PhatoseAlpha]

Hm. Well, that only seems to account for situations where the negative energy particle gets eaten, and the positive one gets away. What happens when the positive one gets eaten and negative gets away?


Edit: Wait, looks like negative mass or energy is a mathematical construct anyway. So there isn't a negative/positive pair or anything, and the gist I get is the falling into the black hole makes on effectively negative.

 

[Biftheunderstudy]

@Sunny129

Your explanating is correct in the same way that the rubber sheet analogy for gravity is correct, it gets the point across but is wrong in the details. Maybe that's where the uncertainty is?

I don't know (or don't recall) the proper explanation, however a friend who studies high energy physics once explained to me why that analogy was flawed, it has to do with loop contributions and things from QED....

 

[Sunny129]

yeah, my knowledge of QED and QCD is limited to a book called "The Cosmic Onion," which admittedly is geared toward the layman...