Dark matter build up near Black holes, thoughts?

[Fermifaq]

Dark matter build up near or inside Black holes, thoughts ?

I can see it making a real mess of the motions and energies of normal matter in and around a black hole or other very dense body that confines dark matter to a small adjacent region. Unslowed by frictional forces...think angry hornets !

Would this not lead to some empirical observations we could make ?

AFAIK Dark matter is spread very thinly and tends to move slowly but over time enough of it could be swept up that its additional mass/gravity starts to have a noticeable impact in how black holes and neutron stars behave.



If two black holes, heavily contaminated with Dark matter / Wimps collided or had glancing blow then its plausible that a Dark Black hole might be spun off. Depending on the ratios of normal matter to dark matter you might end up with three black holes, 1 truly invisible or at least lacking many of the side characteristics associated with black holes made of normal matter . Or perhaps the sudden loss of 'critical mass' could make the real world black holes explode in a way noticeably different from the norm.

your thoughts

 

[Drakkith]

Dark matter will not build up around black holes, as it would require some mechanism to lose kinetic energy.

If two black holes, heavily contaminated with Dark matter / Wimps collided or had glancing blow then its plausible that a Dark Black hole might be spun off. Depending on the ratios of normal matter to dark matter you might end up with three black holes, 1 truly invisible or at least lacking many of the side characteristics associated with black holes made of normal matter . Or perhaps the sudden loss of 'critical mass' could make the real world black holes explode in a way noticeably different from the norm.

This is not how black holes work in real life. They don't explode. (At least not until they lose enough mass via hawking radiation, but that's not quite the same thing as what you are suggesting) Also, since black holes are a result of gravity, a dark matter black hole should be identical to a normal matter black hole.

 

[Bobbywhy]

If there was a Black Hole (BH) located in close proximity to some quantity of Dark Matter (DM), then
1. Would that DM be drawn towards the BH by gravitational attraction? If yes, then
2. Would that DM form an accretion disc, just as ordinary matter does?
3. Does ordinary matter lose kinetic energy while spiraling downward toward the BH?

 

[Chronos]

Yes, DM would be attracted towards a black hole, but, it would not form an accretion disk. As already noted, it is collisionless, even with itself. Ordinary matter loses kinetic energy via friction with other matter, which causes it to spiral in and form an accretion disc. In the case of a black hole, the accretion disc [which is always ordinary matter] gets hot enough to emit x rays and gamma rays - which is pretty toasty. This is one of the easier ways to find black holes, just look for a strong x ray source.

 

[Fermifaq]

Im quite sure any dark matter that fell into a black hole would stay inside, but may be released if two black holes partially collide.

Im also quite sure that slow moving dark matter would orbit a black hole, albeit at high speeds if there is no speed loss via friction...swirling like hornets in a different manner from a normal matter accretion disk

It would take longer for the dark matter to 'effectively' spiral inwards. Black holes are not stationary so you might get a comet like tail shape.

Thats my understanding of it

 

[Drakkith]

Im quite sure any dark matter that fell into a black hole would stay inside, but may be released if two black holes partially collide.

Yes, it would remain inside if it fell in. But nothing is going to come back out, even in a "collision". Once it's past the event horizon, it stays there.

Im also quite sure that slow moving dark matter would orbit a black hole, albeit at high speeds if there is no speed loss via friction...swirling like hornets in a different manner from a normal matter accretion disk

It would take longer for the dark matter to 'effectively' spiral inwards. Black holes are not stationary so you might get a comet like tail shape.

The motion of the black hole is mostly irrelevant. Dark matter in orbit around a black hole would stay there until its orbit decayed from gravitational radiation. (Which takes longer than the current age of the universe)

 

[phinds]

Im quite sure any dark matter that fell into a black hole ... may be released if two black holes partially collide.

Pretty amazing that you're sure of it since, as Drakkith pointed out, it isn't true

 

[Chalnoth]

Pretty amazing that you're sure of it since, as Drakkith pointed out, it isn't true

He didn't say he was sure it would be released. Best not to use ellipses to completely change the intent of what a person says.

 

[Naty1]

A black hole is the 'ultimate roach motel': you can get in but you can't get out. The simplist way to envision how a BH diminishes in size, loses 'mass', is to note that it is a virtually perfect black body...it radiates...energy, not mass...but the source of the radiation is not so simple...

Edit: add...

Would this not lead to some empirical observations we could make ?

It does not appear so from what is so far developed. See below.


Another approximate description, not mathematically based is this:

http://www.physics.ucdavis.edu/Text/Carlip.html#Hawkrad

• here's a way to understand Hawking radiation. Picture a virtual pair created outside a black hole event horizon. ...So the black hole can absorb the negative-energy particle from a vacuum fluctuation {at the horizon} without violating the uncertainty principle, leaving its positive-energy partner free to escape to infinity.

.

He's talking about photons.



Kip Thorne, BLACK HOLES AND TIME WARPS, PGS 435-440

Hawking concluded that a black hole behaves precisely as though its horizon has a finite temperature. There are several different ways to picture black hole evaporation...all acknowledge vacuum fluctuations as the ultimate source of the emitted radiation...

From Leonard Susskind, THE BLACK HOLE WAR, almost an exact quote with some omissions...

The black hole would be losing energy, rather than mass, IF it were hotter than the surrounding universe...

According to the book Quantum Fields in Curved Space by Birrell and Davies, pages 268-269,

These consideration resolve an apparent paradox concerning the Hawking effect. ... Hence it would appear that, in the falling frame, the observer should encounter an infinite amount of radiation in a finite time, and so be destroyed. On the other hand, the event horizon is a global construct, and has no local significance, so it is absurd too conclude that it acts as physical barrier to the falling observer.

The paradox is resolved when a careful distinction is made between particle number and energy density. When the observer approaches the horizon, the notion of a well-defined particle number loses its meaning at the wavelengths of interest in the Hawking radiation; the observer is 'inside' the particles. We need not, therefore, worry about the observer encountering an infinite number of particles. On the other hand, energy does have a local significance. In this case, however, although the Hawking flux does diverge as the horizon is approached, so does the static vacuum polarization, and the latter is negative. The falling observer cannot distinguish operationally between the energy flux due to oncoming Hawking radiation and that due to the fact that he is sweeping through the cloud of vacuum polarization. The net result is to cancel the divergence on the event horizon, and yield a finite result, ...

"At first sight, black hole radiance seems paradoxical, for nothing can apparently escape from within the event horizon. However, the average wavelength of the emitted quanta is ~ M, i.e comparable with the size of the hole. As it is not possible to localize a quantum to within one wavelength, it is therefore meaningless to trace the origin of the particles to any particular region near the horizon. The particle concept, which is basically global, is only useful near [infinity]. In the vicinity of the hole, the spacetime curvature is comparable with the radiation wavelength in the energy range of interest, and the concept of locally-defined particles breaks down."

So saying the particles are emitted by the horizon, or asking what particles you see when you get near the horizon - both of these are meaningless.

Someone posted this source in another discussion which I have not read...


Maulik K. Parikh, Frank Wilczek
http://arxiv.org/abs/hep-th/9907001

(Submitted on 1 Jul 1999 (v1), last revised 2 Mar 2001 (this version, v3))

We present a short and direct derivation of Hawking radiation as a tunneling process, based on particles in a dynamical geometry.

 

[Naty1]

An interesting aspect of dark matter and black holes I haven't at all figured out is this:

Since dark matter does not interact with the electromagnetic force, it does not absorb, reflect or emit light, making it extremely hard to spot.

Yet it seems it is thought it can eventually be emitted as electromagnetic radiation from BH??
Hmmmm...

Another interesting perspective was posted by
Chalnoth:

In the early universe, normal matter and dark matter tended to attract one another, so that an overabundance of dark matter generally also coincides with an overabundance of normal matter.

The normal matter, however, experiences friction, and so collapses much more than the dark matter, which experiences no friction. So it's not so much that dark matter collects around the edges of galaxies, but that galaxies form in the centers of large blobs of dark matter.

Now, it isn't quite as simple as this, because when stars start to form, things tend to get violent, and a low-mass overdensity will blow out most of the matter shortly after the first stars form, leaving a galaxy with almost nothing but dark matter.

 

[Chalnoth]

An interesting aspect of dark matter and black holes I haven't at all figured out is this:

Since dark matter does not interact with the electromagnetic force, it does not absorb, reflect or emit light, making it extremely hard to spot.

Yet it seems it is thought it can eventually be emitted as electromagnetic radiation from BH??
Hmmmm...

I don't think anybody knows the answer to this in detail. But first of all, Hawking Radiation is not solely electromagnetic radiation. In principle, Hawking Radiation will result in the emission of every type of particle. But as Hawking temperatures are usually far below the masses of particles, very few particles with mass will typically be emitted. As the black hole reaches the end of its life, however, the temperature diverges and presumably it will emit particles of rather high masses (including dark matter).

That said, certainly the distribution of particles that exits the black hole has nothing to do with the distribution of particles that enters the black hole (That would violate the no-hair theorem for black holes). So somehow the black hole must be capable of converting dark matter into other forms of matter. It will naturally be able to do this as long as the interaction energies either at the event horizon or inside the black hole are extremely large, as you'll be able to produce lots of massive intermediate particles in the interaction. For instance, if the dark matter interacts with the weak nuclear force, then it will be able to interact with electrons and quarks through that force. All you need is enough interaction time and energy for the incoming matter to mix sufficiently (the weak nuclear force interactions do become rapid at high energies).

Precisely how this all happens, however, is unknown, because we have very little idea what happens inside a black hole, or how the incoming matter becomes converted into outgoing Hawking Radiation.

 

[Naty1]

Chalnoth:

As the black hole reaches the end of its life, however, the temperature diverges and presumably it will emit particles of rather high masses (including dark matter).

Interesting...Thanks..

that reminded me of a book I hadn't read in a while...BLACK HOLES AND TIME WARPS [1994],
Chapter 12, Black Holes evaporate...

Thorne says at the very end...

As the BH mass has been reduced... [we are not sure where this happens] and it's horizon has shrunk to a fraction of the size of an atomic nucleus... the hole will be so extremely hot...it will explode.

I had forgotten about that possibility, if still theoretically viable, and who knows what particles might be released...say, for example, via horizon acceleration