Can Dark matter form a dark black hole ?

[ramparts]

Then if it possible for dark matter to form a black hole is it possible that some rogue Black Holes are the result? Not sure if I am wording this correctly.

What do you mean by rogue?

Since dark matter interacts gravitationally, and black holes are purely gravitational (ignoring electric charge), a black hole formed by dark matter should be indistinguishable from one formed by regular ol' matter.

 

[Woggles]

Hi Ramparts. Well I heard of rogue blacks holes.

http://news.nationalgeographic.com/news ... holes.html
"Newly merged black holes might be so jarred by the experience that they go "rogue," careening into space on unexpected trajectories.
In fact, hundreds of these rogue black holes could exist in the Milky Way galaxy alone, according to a new study.

So I was thinking if rogue black holes are possible, then is it possible for some of these rogue black hole to be actually dark matter BH that had form and are being confused as a merge.

 

[Kessy]

What is the quickest and easiest way to explain Dark energy to someone?

Dark energy is a label for the unexplained reason that the expansion of the universe is accelerating. Dark energy and dark matter are unrelated - they're both called "dark" because we know little to nothing about them.

Kessy - I don't see why dark matter traveling at these speeds wouldn't be bound to galaxies. If stars orbiting around the galactic center don't collapse on themselves, then there's no reason that dark matter orbiting at the same velocity would, either, no?

Didn't we already agree that if you got a collection of non interacting dark matter as massive and dense as a star, it should collapse into a black hole? Anyway, let me see if I can explain my point more clearly.

Anyway, as I tried to get at a couple of posts back, the gas clouds which collapse into stars do so due to self-gravity, but that's because they're moving at very slow speeds, far slower than the speeds at which dark matter particles are believed to travel. So a dark matter clump would need to be much denser, and we just don't expect them to reach those sorts of densities.

This only works if the dark matter particles have high velocities *relative to each other.*

Something that's gravitationally bound to a galaxy can be thought of as orbiting the center of mass of the galaxy. (Yes, I know real galaxies are considerably more messy then that, but for purposes of this discussion it's an okay approximation.) Simple orbital mechanics dictate what sort of velocities an orbiting particle can have. Two particles orbiting at about the same distance with about the same inclination with about the same eccentricity are going to have a pretty low relative velocity. Even in an absolute worst case, the relative velocities can't be more then the sum of their orbital velocities.

And all of these limitations apply equally to the hydrogen in the galaxy. And obviously the hydrogen is moving slowly enough relative to each other to form stars, even in disorganized galaxies like ellipticals and irregulars.

Basically, I'm saying that as I understand it, orbital mechanics dictate that both the hydrogen and dark matter need to be moving in the same range of velocities, and we know the hydrogen is moving slowly enough to form stars. So unless I'm missing something, either the dark matter is moving around the same speed as the hydrogen, in which case, why wouldn't it form dense bodies? Or it's moving much faster, in which case, how can it stay in orbit around the galaxy, shouldn't it have reached escape velocity?

 

[MeteorWayne]

The problem with dark matter black holes is that the only interaction of dark matter appears to be gravity. In order to create stars and other denser objects, cooling ( a non gravitaional process) and electromagnetic interactions appear to be required. So dark matter black holes seem to be very unlikely.

 

[ramparts]

Didn't we already agree that if you got a collection of non interacting dark matter as massive and dense as a star, it should collapse into a black hole? Anyway, let me see if I can explain my point more clearly.

Well, I haven't run the math, so I wouldn't be so arrogant as to say that I've agreed to that, but I'd say that if you had dark matter in the same density as a star and it were moving slowly enough to be gravitationally bound in that space (which I'm not sure is true), then intuitively it would make sense that that would experience runaway collapse, since the star's thermal pressure is the only thing keeping it from collapsing as well. Maybe I'm misunderstanding your question here but a galaxy isn't nearly as dense, on average, as a star is, so a dark matter halo with a similar density would be much less dense than a star is, as well.

Basically, I'm saying that as I understand it, orbital mechanics dictate that both the hydrogen and dark matter need to be moving in the same range of velocities, and we know the hydrogen is moving slowly enough to form stars. So unless I'm missing something, either the dark matter is moving around the same speed as the hydrogen, in which case, why wouldn't it form dense bodies? Or it's moving much faster, in which case, how can it stay in orbit around the galaxy, shouldn't it have reached escape velocity?

That's the thing. Because gas has an extra set of dynamics, the density distributions of dark matter and gas are going to be different. Remember how dark matter was first detected: there was more mass on the outskirts of the galaxy than we would have expected from the light, relative to how much mass was closer to the galactic center. If the dark matter followed the gas exactly, then this discovery would have been impossible because there would have been no excess mass far from the galactic center. Note also that star formation happens when a gas cloud is shocked, so that gas gets pushed together. These sorts of shocks don't touch dark matter at all.

I think that makes things a bit clearer?

 

[Floridian]

I was wonderring about dark matter . Assuming it is there and has gravity but cannot be seen . Can it form a black hole that cannot be seen also ? And would it only suck in other dark matter even though it has an affect on local gravity at the very least ? Since we don't know exactly what dark matter is but we know it effects gravity (has mass) I can't help but wonder if it can create a dark star or maybe ever a dark black hole . With the concept of multiple dimensions and such I would also question if the dark matter just dosen't occupy our percievable dimension .

No it cannot, you see, one of the properties of dark matter is that it doesn't follow any actual laws of logic, reason, or science. You see, it somehow has a gravitational pull but doesn't follow the laws of gravity (it would naturally clump together if it were 78% of the universe.

Basically ask yourself?
Would this characteristic of dark matter make it provable?
Then it isn't possible according to the dark matter followers.

My theory is that invisible unicorns hold the galaxies together with rainbow magic. This is proven by the fact that we cannot explain the gravitational hold galaxies have without this explanation.

 

[Floridian]

Didn't we already agree that if you got a collection of non interacting dark matter as massive and dense as a star, it should collapse into a black hole? Anyway, let me see if I can explain my point more clearly.

Well, I haven't run the math, so I wouldn't be so arrogant as to say that I've agreed to that, but I'd say that if you had dark matter in the same density as a star and it were moving slowly enough to be gravitationally bound in that space (which I'm not sure is true), then intuitively it would make sense that that would experience runaway collapse, since the star's thermal pressure is the only thing keeping it from collapsing as well. Maybe I'm misunderstanding your question here but a galaxy isn't nearly as dense, on average, as a star is, so a dark matter halo with a similar density would be much less dense than a star is, as well.

Basically, I'm saying that as I understand it, orbital mechanics dictate that both the hydrogen and dark matter need to be moving in the same range of velocities, and we know the hydrogen is moving slowly enough to form stars. So unless I'm missing something, either the dark matter is moving around the same speed as the hydrogen, in which case, why wouldn't it form dense bodies? Or it's moving much faster, in which case, how can it stay in orbit around the galaxy, shouldn't it have reached escape velocity?

That's the thing. Because gas has an extra set of dynamics, the density distributions of dark matter and gas are going to be different. Remember how dark matter was first detected: there was more mass on the outskirts of the galaxy than we would have expected from the light, relative to how much mass was closer to the galactic center. If the dark matter followed the gas exactly, then this discovery would have been impossible because there would have been no excess mass far from the galactic center. Note also that star formation happens when a gas cloud is shocked, so that gas gets pushed together. These sorts of shocks don't touch dark matter at all.

I think that makes things a bit clearer?

"Remember how dark matter was first detected"

Dark matter never has, and never will be detected.

 

[Floridian]

The problem with dark matter black holes is that the only interaction of dark matter appears to be gravity. In order to create stars and other denser objects, cooling ( a non gravitational process) and electromagnetic interactions appear to be required. So dark matter black holes seem to be very unlikely.

What in the world? If it has gravity, objects will be attracted to eachother. Are you suggesting the gravitational pull by normal matter does not affect dark matter?

So dark matter pulls on normal matter, but not vise versa? Shouldn't this matter be perfectly spread out across the universe and therefor have no effect?

If dark matter did exist, and it didn't clump together on its own, since you claim dark matter couldn't attract itself to itself, normal matter would draw it into clumps.

Really, if dark matter existed, super massive black holes (made of our matter) would draw in so much dark matter so quickly, it would be scary. In fact, every star should be attracting huge amounts of dark matter to its core with its gravity. Since it is the gravity that actually draws mass into a black hole, black holes could form everywhere, once dark matter started to to clump into groups where normal matter was, like stars. These would quickly form black holes, as the gravitational pull from the dark matter would amplify the gravity of the star, causing it to compress to a singularity.

 

[SteveCNC]

Nice debate so far !

I proposed the original question for a couple of reasons ,

1) I have kinda fought the idea of dark matter in my head till I decided to truly make an attempt to see it from another perspective , which is why I wanted to pose the question here so I could get your perspectives . All very valid in their own way , but each has to assume certain things due to our limited understanding of the real properties of dark matter .

2) I wanted to try to disprove it's existance through the back door so to speak . Possibly find what other alternatives could cause the lensing effect we see . Haven't seen much on this side , it seems that for now dark matter is , for the most part accepted as real . heh except you Floridian and me at times (personally I'm still looking for something else to cause the effects) .

I guess I can see both sides pretty well , there is argument for it's existance and few other possibilities but I have to say that even just in my lifetime there have been many occasions where what we thought we knew , we didn't . So for now I will accept that it's there , but in the back of my mind I will continue to search for another explanation .

 

[MeteorWayne]

No it cannot, you see, one of the properties of dark matter is that it doesn't follow any actual laws of logic, reason, or science. You see, it somehow has a gravitational pull but doesn't follow the laws of gravity (it would naturally clump together if it were 78% of the universe.

That is not correct, in fact it does follow the laws of gravity. We just can't see it via electromagnetic radiation.

 

[ramparts]

You see, it somehow has a gravitational pull but doesn't follow the laws of gravity (it would naturally clump together if it were 78% of the universe.

Indeed it does. In the current theory, which has a pretty impressive record of making successful predictions and matching observations - dark matter is slow enough to form clumps, called dark matter haloes, which are the building blocks of structure in the Universe. Gas (that is, normal matter) collapses in the gravitational wells created by these haloes and that's how galaxy formation begins. It's quite fascinating.

Really, if dark matter existed, super massive black holes (made of our matter) would draw in so much dark matter so quickly, it would be scary. In fact, every star should be attracting huge amounts of dark matter to its core with its gravity. Since it is the gravity that actually draws mass into a black hole, black holes could form everywhere, once dark matter started to to clump into groups where normal matter was, like stars. These would quickly form black holes, as the gravitational pull from the dark matter would amplify the gravity of the star, causing it to compress to a singularity.

Not necessarily. First of all, if there's about 5 times as much dark matter as regular matter, then without considering any differences you'd still only expect black holes to be about 5 times more massive than they would be if there was only ordinary matter. In astronomical terms, a factor of 5 is nothing.

But even so, dark matter would be less likely to be drawn into black holes because it doesn't have those electrodynamics - things like friction, which slow gas down and causes it to fall into things like black holes, but which doesn't affect dark matter. So gas is more likely than dark matter is to fall into a black hole - just putting a black hole somewhere won't suck everything in like a vacuum cleaner, it'll only suck in the things that it can slow down enough.

 

[Mordred]

I proposed the original question for a couple of reasons ,

1) I have kinda fought the idea of dark matter in my head till I decided to truly make an attempt to see it from another perspective , which is why I wanted to pose the question here so I could get your perspectives . All very valid in their own way , but each has to assume certain things due to our limited understanding of the real properties of dark matter .

2) I wanted to try to disprove it's existance through the back door so to speak . Possibly find what other alternatives could cause the lensing effect we see . Haven't seen much on this side , it seems that for now dark matter is , for the most part accepted as real . heh except you Floridian and me at times (personally I'm still looking for something else to cause the effects) .

I guess I can see both sides pretty well , there is argument for it's existance and few other possibilities but I have to say that even just in my lifetime there have been many occasions where what we thought we knew , we didn't . So for now I will accept that it's there , but in the back of my mind I will continue to search for another explanation .
SteveCNC
rock

Posts: 81
Joined: Sat Apr 10, 2010 8:28 am

I too have difficulty with the concepts of Dark matter though I understand the lensing effects and background radiation
showing that something is there. The problem I have is not so much its existance or the possibility of its existance but rather in the suggested quantities. Yes it fixes the math more accurately however looking at recent articles of all the various things they did not even know existed within our own Galaxy Such as the massive gas cloud posted on this site to the larger star creation areas on the other side of the galaxy Makes me wonder how much we assume is dark matter as opposed to undetected massive objects. Black holes, brown dwarfs etc.
Or simply plasma of a substance difficult to detect by current means.

 

[Kessy]

The problem with dark matter black holes is that the only interaction of dark matter appears to be gravity. In order to create stars and other denser objects, cooling ( a non gravitational process) and electromagnetic interactions appear to be required. So dark matter black holes seem to be very unlikely.

What in the world? If it has gravity, objects will be attracted to eachother. Are you suggesting the gravitational pull by normal matter does not affect dark matter?

So dark matter pulls on normal matter, but not vise versa? Shouldn't this matter be perfectly spread out across the universe and therefor have no effect?

If dark matter did exist, and it didn't clump together on its own, since you claim dark matter couldn't attract itself to itself, normal matter would draw it into clumps.

Really, if dark matter existed, super massive black holes (made of our matter) would draw in so much dark matter so quickly, it would be scary. In fact, every star should be attracting huge amounts of dark matter to its core with its gravity. Since it is the gravity that actually draws mass into a black hole, black holes could form everywhere, once dark matter started to to clump into groups where normal matter was, like stars. These would quickly form black holes, as the gravitational pull from the dark matter would amplify the gravity of the star, causing it to compress to a singularity.

Floridian, I don't think you understand what the mechanism is we're talking about here - let me try to explain. Imagine a large cloud of dark matter particles. Each particle is point like and has mass, but no other properties, and for our purposes we'll ignore what happens when they collide - we'll assume the particles are too small compared to the volume they have to move around in for that to be significant. The cloud has a very large number of individual particles, let's say trillions or more. The cloud is gravitationally bound together, which means the particles are moving too slowly to escape the gravity of the overall cloud.

Now, let's follow one individual particle, let's say one near the edge of the cloud. It will feel the gravity of the cloud and be drawn in toward the center, but as it does so it will accelerate to faster and faster speeds, until it gets to the center, where it will be moving much faster then it was at the edge, and will keep going through the center out the other side, where it will start to be slowed down by the cloud's gravity until it gets near the far edge, where it will stop and start to fall back in toward the center, repeating the cycle indefinitely.

In order for this cloud to collapse into a denser object, there has to be some way for the particles to shed kinetic energy when they're closer to the center. With normal baryonic matter, this happens through electromagnetic radiation. The gas will be hot because of the motion of the individual particles, and will radiate energy, slowing down and making the cloud collapse.

Dark matter can't do this because it doesn't interact with electromagnetism, so it will collapse much more slowly then normal matter. The debate we were having was about just how much slower, and if the Universe is old enough yet for these kinds of clouds to have collapsed into black holes or other dense objects.

I disagree with Ramparts on this, I think that probably this should have happened within the age of the universe. But at this point we seem to be starting to repeat ourselves, so I don't think we can go any further without someone running the numbers, and my math skills are not up to the job, so I'm putting this in the agree to disagree column.

My personal and purely intuitive opinion is that 1) dark matter probably does have some sort of unknown interaction with itself that we currently don't understand, and that 2) General Relativity is an incomplete description of gravity, and that some of the effects we currently ascribe to dark matter are likely due to this. Although neither of these can currently be supported by the evidence, they're both possibilities that are being actively investigated.

One last point - as I said in another thread, it's quite certain that *some* dark matter does exist, even if new physics does come into play. We're all currently sitting on a big chunk of it called the Earth. Well, unless someone on the ISS is around.

 

[ramparts]

Just to nitpick - because it's fun - I'd point out that particles in a dark matter halo don't necessarily move through or near the center, just as the Earth never gets too close to the Sun. This is, obviously, because even though particles feel drawn to the barycenter, they have tangential velocities which keep them orbiting distantly.

You mention (correctly) that dark matter haloes should collapse, albeit much more slowly than normal matter haloes of similar density. So take a galaxy-sized dark matter halo (whose density should be comparable, in terms of orders of magnitude, to that of the associated galaxy). The galaxy has come nowhere close to collapsing into a giant black hole during the age of the Universe, so why would a dark matter halo be any different?

In fact, we can draw this general heuristic conclusion: since gas collapses more readily than dark matter, pure dark matter black holes are very unlikely to exist, as they should only form after gas forms a black hole in the vicinity.

Also, a note on general relativity and dark matter: while we know general relativity is an incomplete theory of gravity, and it's possible those holes have effects on large scale phenomena (particularly what we attribute to dark energy), modified gravity is an extremely uncompelling solution to any of the mysteries we attribute to dark matter. But if you're curious, that's a subject for another thread

 

[Kessy]

You mention (correctly) that dark matter haloes should collapse, albeit much more slowly than normal matter haloes of similar density. So take a galaxy-sized dark matter halo (whose density should be comparable, in terms of orders of magnitude, to that of the associated galaxy). The galaxy has come nowhere close to collapsing into a giant black hole during the age of the Universe, so why would a dark matter halo be any different?

In fact, we can draw this general heuristic conclusion: since gas collapses more readily than dark matter, pure dark matter black holes are very unlikely to exist, as they should only form after gas forms a black hole in the vicinity.

LOL Obviously, I'm not suggesting that galactic halo sized clumps of dark matter have collapsed into black holes - that would have taken the normal matter galaxies with it, wouldn't it? I'm asking how uniform is the density of galactic dark matter halos?

If there are denser regions of the halo of a similar size and density to normal star forming regions, wouldn't you expect those regions to form dense bodies in an analogous manner? And since normal star forming regions are created by the gravitational dynamics of galaxies, wouldn't it be reasonable to expect similar regions of dark matter to exist?

 

[MeteorWayne]

Not really. As stated above, to create stars, the kinetic energy of matter must be disappated in some way, usually by EM or perhaps sometimes magnetic fields. Neither applies to dark matter, and gravity is too weak to cause even a large amount of dark matter to collapse into a star progenitor sized (much less a black hole sized) concentration of mass.

 

[ramparts]

Kessy, are you familiar with the NFW profile? If not, check it out on Wiki; it's one of our best predictions for the radial density distribution of dark matter haloes around galaxies (of course there are all sorts of purported improvements, but this is the du jour standard). It's pretty steep, reflecting the fact that dark matter is most dense near the center of the potential well. It's also isotropic - that is, the same in all directions (notice that it's a function of radius but not of angle). This is a pretty standard assumption and generally a solid one, but there is, of course, lumpiness which it ignores.

 

[Mordred]

Nice link I like this one as well for those that have not seen it though I'm sure most of us that visit this site have already.
http://www.esa.int/esaSC/SEMZ6GSVYVE_index_0.html

 

[levass1o]

Kessy's process of dark matter clumps collapsing by loosing energy in a from of escaping particles is quite fair. At the same time we do not observe any dense dark matter clumps. Which would be detectable by their gravitational influences on nearby visible companions. Which can be of any scale -- from planets to galaxy clusters.
So I like the idea to consider it to be much easier for plain visible matter to get into really dense states due to friction. So whenever dark matter starts to collapse -- visible matter will collapse in a same region with eventual formation of galaxies and black holes. And visible matter will do it much faster than the dark one.

 

[SteveCNC]

After reading all those links it makes me wonder how close is the nearest blob of dark matter ? or can we only detect those at long distances ? I guess I can see how it wouldn't want to condense into a dense enough lump to cause itself to collapse but could through gravity cause normal matter to clump . And if what you say is true it may be that dark matter dosen't stay with normal matter at all . If dark matter dosen't interact with light , maybe it can actually pass through normal matter entirely . And if that's true we may never be able to capture any to examine it It is really starting to sound more like a form of energy than matter , but let's assume it is matter and therefore travels at less than the speed of light .

Odd thing about gravity in a mind experiment , what if you had a hole going thru a small moon such that you could jump into it and you would fall toward the center of the moon , when you got there you would pass right on by and continue to the other side of the moon slowing and reversing when you got the same height (barring rotation issues and wind resistance and pressure) . You could continute the back and forth trip forever with no atmosphere to slow you down . And if you were able to stop at the center of the moon , would you feel gravity ? Would it feel like gravity were crushing you ? Wouldn't it feel like being pulled apart from all directions ? Kinda like reverse gravity ? or would you be just weightless ? I wonder how dark matter reacts to a planetary mass if it can pass thru it without the normal interactions ? Or even a star for that matter , any momentum it carries enterring the stars influence would still be there after it exits it's influence if nothing can slow it down except gravity . It may change course due to the influence of matter but it can't be stopped by it if I am seeing what your telling me correctly .

Well like always , every answer just leads to another question

 

[ramparts]

Steve - well, we're in one Dark matter is all over. This is the reasoning behind direct dark matter detectors, which aim to detect dark matter particles which are constantly streaming through the Earth.

 

[MeteorWayne]

Kessy's process of dark matter clumps collapsing by loosing energy in a from of escaping particles is quite fair. At the same time we do not observe any dense dark matter clumps. Which would be detectable by their gravitational influences on nearby visible companions. Which can be of any scale -- from planets to galaxy clusters.
So I like the idea to consider it to be much easier for plain visible matter to get into really dense states due to friction. So whenever dark matter starts to collapse -- visible matter will collapse in a same region with eventual formation of galaxies and black holes. And visible matter will do it much faster than the dark one.

First of all it's "losing energy" not "loosing energy"
Sheesh....

Second, Normal matter does not get more dense just due to friction, but cooling from convection, EM radiation, and magnetic interaction, none of which seems to apply to dark matter, so there is no mechanism to allow the requisite increase in dark matter density to create a black hole.

 

[Kessy]

Normal matter does not get more dense just due to friction, but cooling from convection, EM radiation, and magnetic interaction, none of which seems to apply to dark matter, so there is no mechanism to allow the requisite increase in dark matter density to create a black hole.

Yes, as we've discussed at some length, baryonic matter has several cooling mechanisms that interactionless dark matter would not, mostly related to electromagnetism. This means that baryonic matter will collapse faster then dark matter under certain conditions, not that dark matter won't collapse at all. Although how much faster is an open question that I don't think can be answered without some serious math and modeling.

I proposed my earlier gravitational cooling mechanism mainly to demonstrate that you *can* cool dark matter with only gravitation. Just because that's what I came up with off the top of my head doesn't mean it's the only possible mechanism to do it. Especially when you consider that we really have no idea how particles of that sort of dark matter would behave when they collide, I think it's way too early to say that there is no mechanism to allow for dark matter to form a dense body.

And as I said earlier, EM mechanisms only become significant at relatively small scales. Let me illustrate.

Consider galaxy formation. You start with a cloud of matter that collapses under gravity. We're talking things roughly on the order of 10^5 ly here. Gravity is the only significant force here, right? The hydrogen is too diffuse and cool to radiate much on its own, most EM emissions from galaxies come from things like stars and accretion discs, that are on a much smaller and involve different processes.

Consider the creation of star forming regions within a galaxy. Again, you have a cloud of gas collapsing mainly under gravity, this time on the order of around 10^2 ly. The regions are usually lit up by the presence of young, massive stars within them, not by thermal emissions produced by gravitational collapse. Here again gravity is really the only significant force involved.

It's only when you get down to actual star formation itself at *much* smaller scales that the EM mechanisms really come into play. We're talking, what? Less then 10^-1 ly here?

If purely gravitational mechanisms mediate collapse through six orders of magnitude, why would you think these mechanisms suddenly cease to function at that point? Yes, the EM mechanisms quickly take over and reduce the gravitational cooling to an insignificant component, but they're still there, and would continue to act even without the EM component. So interactionless dark matter should continue to collapse, just much more slowly then baryonic matter.

 

[MeteorWayne]

The point is, that it is only at small scales that the required mass density can occur to create a black hole.