Can Dark matter form a dark black hole ?

[SteveCNC]

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 .

 

[MeteorWayne]

It would seem unlikely from what we know about dark matter...it interacts so weakly it would be hard for it to concentrate to that density.

If it did, it would be the same as any other black hole, an invisible gravitational mass in a very small volume.

 

[kelvinzero]

Interesting though,

Even if dark matter did not often form a black hole by itself, shouldnt it form a large proportion of the mass captured by a black hole after it is formed?

 

[MeteorWayne]

Maybe, maybe not. Since we don't know what it is, we can't really say.

 

[ramparts]

Well, I've never heard of any reason to suspect that dark matter would form a black hole since, as MW said, it only interacts gravitationally. No friction, etc., that normal matter has, so it's hard to conceive of conditions where it would get that dense. But if dark matter did form a black hole, as far as we know it'd be a black hole just like any other. It would have no charge, but astrophysical black holes are expected to be more or less uncharged, too.

As for black holes sucking in dark matter, well, that almost certainly does happen.

 

[Kessy]

When it comes to dark matter forming black holes on its own, isn't the real question how dark matter interacts with other dark matter? With normal stellar black holes, the hole forms when gravity overcomes the various forces acting against it. When it comes to a dark matter black hole, the question is what forces does the dark matter exert on itself? Which as far as I know is a complete unknown. If dark matter doesn't interact with itself except by gravity, then it should form black holes quite readily since there would be no forces acting in opposition to gravity.

That scenario seems unlikely to me, since that would imply that there should be lots and lots of black holes floating around of all sizes. Over the age of the universe, gravity can compress even very diffuse matter into dense clumps. Actually, the more I think about it, the more it seems to me that dark matter would probably have to interact with itself in some way, or an awful lot of the matter in the universe would be in the form of black holes.

I would think the presence of dark matter would actually make normal black holes form more readily. For example, if a neutron star passes through a cloud of dark matter, the dark matter will be pulled in by the neutron star's gravity, and since it wouldn't be effected by neutron degeneracy pressure, it will fall straight to the center. I'd have to run the math to be sure, but I think that would allow the neutron star to collapse into a black hole at a lower mass because the density in the core would be higher then in a neutron star without any dark matter.

 

[kelvinzero]

Maybe, maybe not. Since we don't know what it is, we can't really say.

Postulating something that can escape a black hole is probably the same as postulating something that can move faster than light. Not only is this forbidden, but what we know of dark matter in particular suggests it is a cool (slow moving) particle.

If most of the mass in the universe is in the form of dark matter, but this mass cannot find its way into black holes, lets just say that would be a very interesting result!

 

[ramparts]

When it comes to dark matter forming black holes on its own, isn't the real question how dark matter interacts with other dark matter? With normal stellar black holes, the hole forms when gravity overcomes the various forces acting against it. When it comes to a dark matter black hole, the question is what forces does the dark matter exert on itself? Which as far as I know is a complete unknown. If dark matter doesn't interact with itself except by gravity, then it should form black holes quite readily since there would be no forces acting in opposition to gravity.

That scenario seems unlikely to me, since that would imply that there should be lots and lots of black holes floating around of all sizes. Over the age of the universe, gravity can compress even very diffuse matter into dense clumps. Actually, the more I think about it, the more it seems to me that dark matter would probably have to interact with itself in some way, or an awful lot of the matter in the universe would be in the form of black holes.

I would think the presence of dark matter would actually make normal black holes form more readily. For example, if a neutron star passes through a cloud of dark matter, the dark matter will be pulled in by the neutron star's gravity, and since it wouldn't be effected by neutron degeneracy pressure, it will fall straight to the center. I'd have to run the math to be sure, but I think that would allow the neutron star to collapse into a black hole at a lower mass because the density in the core would be higher then in a neutron star without any dark matter.

Well, gravity will drag a clump of stuff (say, dark matter) together if it's inert and not moving around too much. But if dark matter doesn't have any of those non-gravitational interactions (or only has them weakly), then you'd never find a clump of dark matter bound nearly as tightly as, say, a solid. They'd still be whizzing around at relatively high speeds, so that it would take much more time for gravity to pull the dark matter together than it would for other gravitational forces (from the galaxy, etc.) to disperse the dark matter. So if a neutron star passed through some dark matter, because of all of the speeds involved it's unlikely all but a little bit of the dark matter would get pulled into the center. It's similar to the reason that you don't see neutrinos from the Sun clumping together to form black holes - they really only interact gravitationally but they're just too fast.

 

[Kessy]

Well, gravity will drag a clump of stuff (say, dark matter) together if it's inert and not moving around too much. But if dark matter doesn't have any of those non-gravitational interactions (or only has them weakly), then you'd never find a clump of dark matter bound nearly as tightly as, say, a solid. They'd still be whizzing around at relatively high speeds, so that it would take much more time for gravity to pull the dark matter together than it would for other gravitational forces (from the galaxy, etc.) to disperse the dark matter. So if a neutron star passed through some dark matter, because of all of the speeds involved it's unlikely all but a little bit of the dark matter would get pulled into the center. It's similar to the reason that you don't see neutrinos from the Sun clumping together to form black holes - they really only interact gravitationally but they're just too fast.

I think you're wrong on this one, ramparts. While it's true that dark matter without non gravitational interactions won't bind together in a solid - it will always act like an ideal gas - that doesn't mean it can't get as dense or even much denser then ordinary matter. If you consider a large group of gravitationally bound dark matter particles, they are, by definition, not going to have an average velocity sufficient to escape the clump. They'll be moving on random trajectories within the clump that are constantly being changed by the gravity of the other particles. They'll move faster as they get near the center and slower as they get farther from it. This arrangement looks stable - the gravity is being balanced by the particle's kinetic energy, but it's not in the long term. Periodically, individual particles will have random gravitational interactions that happen to give them enough energy to reach escape velocity, and they'll fly off into space. But because they gained energy to escape, they're reducing the overall kinetic energy of the clump. This process will continue, much like ordinary evaporation of a liquid, gradually removing energy. As that happens, the clump will start to collapse and become denser, until eventually it will become dense enough to form an event horizon and then you get a black hole.

Now this does assume that the dark matter is moving slowly enough to be gravitationally bound, but since the reason we even know about dark matter in the first place is it's effect on gravitationally bound systems (galaxies) that would imply the dark matter is also gravitationally bound. We've also observed gravitational lensing that seems to be caused by clumps of dark matter comparable to ordinary galaxies, which also implies it is cool enough to be gravitationally bound.

Neutrinos, on the other hand, are moving much much faster compared to their mass, which means they are generally not going to be gravitationally bound to anything, which is why this process won't occur with them.

This also assumes the dark matter has no other way to lose energy, such as emitting some form of exotic radiation. If something like that does happen, then the dark matter will collapse into a black hole even quicker. Or the opposite could happen if the dark matter is somehow gaining energy from outside.

It hadn't occurred to me that gravitational interactions with other things (such as a galaxy) can act to disperse the dark matter, and you're completely right about that. But ordinary matter manages to collapse into nebulae and eventually stars despite those same disruptive forces, so I see no reason to think dark matter wouldn't do the same thing.

The bottom line is that gravity always tries to collapse everything into black holes - it takes the other forces acting in opposition to prevent that collapse, such as the electromagnetic bonds that hold ordinary atoms together, and also make them resist being compressed. Or the strong force creating neutron degeneracy pressure in a neutron star. If dark matter only has gravity, then there will be very little to prevent the formation of black holes.

 

[ramparts]

I think you're wrong on this one, ramparts.

Never!!

I mean, what you're saying is definitely true. I'm more disagreeing with the idea that there exist very many dark matter clumps dense enough to collapse into a black hole on their own and on reasonable timescales. What is more likely to happen is dense dark matter clumps forming gravitational potential wells which gas falls into; the gas, what with its friction and all, would then fall into the center more quickly and possibly form a black hole. My guess is this is probably how the seeds of galaxies (now supermassive black holes) were formed. It's called hierarchical structure formation - the smallest structures formed first and gave rise to the larger ones. You'd only expect to find such dense dark matter structures, if ever, in the relatively early universe. But even then I'm not sure they were ever quite dense enough to collapse on their own into a black hole.

As for the process you were talking about, with dark matter clumps collapsing as they lose kinetic energy to escaping particles... well, yeah, but that would happen on timescales far greater than the lifetime of the Universe. Bear in mind that the expected end state of the Universe is one empty but punctuated with black holes (which eventually radiate away). Pretty much all structures end up as black holes eventually, but it'll take a damn long time.

 

[Kessy]

LOL, yes, how silly of me, Ramparts, you being wrong would obviously violate the laws of the universe, right?

So I think the only points we're disagreeing about is whether or not many massive clumps of dark matter exist in the universe today, and how long it would take a clump to collapse, right? Well I already pointed out in my previous post that there's plenty of evidence of clumps of roughly similar size and mass to galaxies, and I would suggest that's dense enough to expect to see gravitational collapses occurring at places within the clumps.

As for the time scales involved, I would argue it wouldn't take nearly as much time as you seem to think. Both our non interacting dark matter and low temperature, low pressure hydrogen gas should behave basically as ideal gasses, right? And we know that clouds of hydrogen gravitationally collapse into stars on times scales around tens or hundreds of millions of years, and until the cloud gets fairly dense, gravity is the main process at work. Up until the point where the hydrogen gets hot enough to start radiating, I'm pretty sure the dark matter should behave pretty much the same way. And if you can get to densities where hydrogen will start to form stars on those sorts of timescales, why wouldn't dark matter continue to collapse at a similar rate until it forms a black hole? The reason collapsing hydrogen makes stars instead of black holes is because the the energy released by nuclear fusion. If our dark matter has no interactions other then gravity, there's no equivalent process to stop the collapse.

Basically I'm arguing that this sort of dark matter ought to behave pretty much the same as normal baryonic matter that's in a system where the only significant interaction is gravity. And on large scales that's exactly what our universe is. It's only somewhere around the scale of stars that other interactions become significant.

 

[SteveCNC]

I have to say I like all the answers given , most sound pretty logical although I think Kessy is closest to what I was thinking though it may be that this would have only occurred very early in the formation if at all .

I still wonder exactly what dark matter really is , it may be some particle we have yet to discover (use it someday for a cloaking device heh) or even a common particle with an odd twist that normally only occurs in space similar to He3 but more likely some form of hydrogen as yet discoverred . Can helium exist with only 2 protons and 2 electrons and no neutrons ? or maybe 2 protons and 4 neutrons and 2 electrons ? In space of course and very low temps . Maybe it will turn out to be just a lot of protons pooling .

 

[ramparts]

Well, the fact that we call it "cold" dark matter doesn't mean it's nearly cold in the way that interstellar hydrogen gas is! The term "cold" here is defined only in opposition to "hot," which means ultrarelativistic. It's a terminology from the early 80s or so that's stuck, and doesn't have any relation to actual temperature except by analogy. So cold dark matter refers to any DM particle which isn't highly relativistic. We still expect them to be quite fast.

Anyway - you mention that there are dark matter halos on galactic scales with similar masses to the galaxies. Sure. But if galaxies (and large intracluster gas clouds) don't collapse into black holes, then why would dark matter halos with a similar density do so? Gas is only more likely to collapse because it's going to dissipate frictional energy. As you say, the Universe is gravity-dominated until you get to the scale of stars when other forces become important. It's really more like the scale of clusters, but either way we're talking about scales that are larger than black holes, no?

 

[Kessy]

Well, the fact that we call it "cold" dark matter doesn't mean it's nearly cold in the way that interstellar hydrogen gas is! The term "cold" here is defined only in opposition to "hot," which means ultrarelativistic. It's a terminology from the early 80s or so that's stuck, and doesn't have any relation to actual temperature except by analogy. So cold dark matter refers to any DM particle which isn't highly relativistic. We still expect them to be quite fast.

You seem to know more about current thinking on the nature of dark matter then I do, ramparts, so I will defer to you on this.

Anyway - you mention that there are dark matter halos on galactic scales with similar masses to the galaxies. Sure. But if galaxies (and large intracluster gas clouds) don't collapse into black holes, then why would dark matter halos with a similar density do so? Gas is only more likely to collapse because it's going to dissipate frictional energy. As you say, the Universe is gravity-dominated until you get to the scale of stars when other forces become important. It's really more like the scale of clusters, but either way we're talking about scales that are larger than black holes, no?

My point is that galaxies *do* collapse into black holes. And stars, brown dwarfs, and other such things. No one is suggesting that all of such a clump of dark matter will collapse into a black hole, just that dense areas of dark matter will produce some black holes, just as large hydrogen clouds form galaxies and produce stars.

As for the question of scale, I would argue that normal matter is gravity dominated well below the scale of galaxy clusters, at least down to the scale of nebulae. Obviously, there isn't a clear cut point where other processes turn on, they gradually increase in importance as you decrease the scale, so "dominant" and "significant" are relative terms in this context. Unfortunately, the math involved in quantifying this is beyond my skill level, so we may just have to agree to disagree on this point. And wouldn't a good model of the behavior of this sort of clump of dark matter require quantum gravity, since the strongest gravitational interactions among the dark matter particles would be when they get close enough to each other to be on a quantum scale?

And the scale does matter, because while clusters may be well above the scale of black holes, stars and nebulae are not. Stellar black holes typically weigh in at a few solar masses, and supermassive ones can run into the billions of solar masses.

I'm a little confused by what you said about frictional energy. In the context of fluids, isn't friction just when an organized flow encounters an obstacle and becomes chaotic? The overall kinetic energy of the flow becomes randomized and turns into thermal energy. I don't see how this would apply to this situation, since the kinetic energy of clouds of either hydrogen or dark matter is already disorganized and acts as heat. Where baryonic matter has an advantage in collapsing into compact objects is that when a gas gets dense and hot enough it will ionize, which allows it to shed thermal energy in the form of EM radiation.

 

[ramparts]

Cool, so I think I see where we're getting mixed up. Or at least, the path is somewhat illuminated I've never seen an entire galaxy up and decide to collapse into a black hole. Same for brown dwarfs. This could be possible on insanely long timescales, but we're talking many orders of magnitude longer than the age of the Universe. The only stars which collapse into black holes are those very massive stars which go through a special process that is able to get gas dense enough. This doesn't happen by just having gas in marginally dense clumps, like galaxies or stars, but through some more subtle subatomic physics.

So in order to go on you'll have to explain what you mean by saying that "galaxies *do* collapse into black holes," and this very strange claim that brown dwarfs, among other things, do as well. Otherwise it's hard for me to see what you're getting at.

Friction is really just when atoms in gas clouds interact with each other non-gravitationally (that is, electromagnetically). Friction is important because it dissipates energy as heat, and this energy can be taken from kinetic or rotational energy. We see examples of this in the accretion disks surrounding black holes. The orbits of the gas particles are usually plenty stable, but they'll fall into the black hole anyway because in interacting with other gas particles, they'll lose some energy due to the friction (heating up the disk and sending off tons of light - that's what we see when we observe active galactic nuclei) which takes away from their orbital energy, causing the orbits to shrink. Eventually they get close enough to spiral right into the black hole.

 

[Kessy]

LOL, sorry, sorry, I completely misstated what I was getting at. I didn't mean that galaxies, stars and brown dwarfs collapse into black holes, I meant that galaxies collapse into black holes, stars, and brown dwarfs. I'm just talking about ordinary star formation - when you get a sufficiently large and dense cloud of gas, parts of it will start collapsing into denser objects like stars. My argument is that the same processes that make baryonic matter in galaxies gravitationally collapse into stars should also apply to dark matter. The difference is that non-gravitational processes are at work preventing normal matter from collapsing into black holes, but there would be nothing equivalent supporting a similarly sized collection of dark matter. Main sequence stars are held up by photon pressure from the fusion reactions going on (caused by a combination of EM, strong and weak nuclear forces.) White dwarfs are held up but electron degeneracy pressure (again, electromagnetism at work.) And neutron stars are held up by neutron degeneracy pressure (nuclear forces.)

I think we were just using the terminology slightly differently when it comes to friction. The reason something like an accretion disk can lose energy to friction is because collisions between atoms in the disk will energize their electrons, and when when those electrons return to their ground state they emit photons. It's the emission of EM radiation that makes the disk lose energy. Those collisions and emissions are governed by electromagnetism.

So my analysis is that if you consider two clouds of gas the same mass and density, one made of hydrogen and the other of non interacting dark matter, they should both undergo gravitational collapse in roughly the same way up until the hydrogen cloud gets hot and dense enough for it to start emitting significant amounts of EM radiation, at which point the hydrogen cloud will start collapsing much faster then the dark matter. But once the hydrogen gets dense enough to start fusing, in other words becomes a star, it will stop collapsing. The dark matter, on the other hand, will just keep collapsing at its slower rate until it becomes a black hole.

I would also like to point out that the gravitational evaporation process I described early should occur faster the denser the cloud is, since higher densities mean more frequent and stronger interactions between the particles, increasing the chances of a particle being ejected. So as a cloud of dark matter undergoes this sort of process, the collapse should accelerate as it progresses.

The real question in all of this is what happens when two dark matter particles collide. Classical physics would suggest that because gravity is only ever attractive, the particles ought to collapse into a miniature black hole. Presumably this doesn't actually happen, since collisions of particles is the domain of quantum mechanics. So ultimately we're stopped again by the lack of a theory of quantum gravity.

 

[MeteorWayne]

My point is that galaxies *do* collapse into black holes. And stars, brown dwarfs, and other such things. .

Sorry, that's just not right. Galaxies do not collapse into black holes. There are black holes at the center of most galaxies, but the galaxy does not collapse into it. The whole galaxy's population of stars continues orbiting on their merry way. Massive stars do collapse into black holes, but l almost all stars don't (they aren't massive enough), brown dwarfs never do, and I have no idea what "other such things" means.

 

[ramparts]

Ohhh. I see. Well, you wouldn't really call that galaxies collapsing. A galaxy is really more the galaxy as a whole - the collection of stars, etc. - and that's obviously not going to up and collapse into anything on its own. You meant gas clouds within the galaxy. Got it. That was the confusion.

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.

Now, I'm not sure what the numbers are on this, a quick look through the literature didn't show any papers on this (so presumably it was debunked a while back), but I'll look more in depth when I have some time. Meanwhile, here's an interesting article that's somewhat related and pretty accessible:

http://news.bbc.co.uk/2/hi/science/nature/4679220.stm

 

[amshak]

No. black holes only sucks up the surrounding matter around it. It is commonly believed that dark matter consists of an exotic material that have its gravitational effect on its surroundings. However, no one knows exactly what it might be or where did it come from.
the unknown entity that makes up the vast majority of matter in the universe — could arise in a simple generalized quantum theory of gravity. The particle can be identified as a new graviton, and would operate at lengths of about 0.1 mm or less. No-one yet knows what dark matter is, although explanations for it are frequently given as hypothetical particles or within modified versions of gravity.

 

[MeteorWayne]

amshak, you are confusing dark matter and dark energy, blurring the different effects.

 

[Woggles]

If Dark matter could form a black hole would it be possible to detect any radiation being emit, as compared to black hole when gas from a companion star spirals inward? Or is there not enough dark matter left for this to happen?

 

[Kessy]

If Dark matter could form a black hole would it be possible to detect any radiation being emit, as compared to black hole when gas from a companion star spirals inward? Or is there not enough dark matter left for this to happen?

In order for matter to emit light (or any other kind of EM radiation) it either has to have an electric charge or be made of component parts that do. This is also necessary for something to reflect, refract, or absorb EM radiation. Dark matter by definition either doesn't have a normal electric charge at all or has a really really tiny one, and basically doesn't interact with light at all, it's just invisible. So no, dark matter falling into a black hole won't emit any sort of EM radiation. It is possible that it might emit some more exotic form of radiation, but that's purely speculation right now.

Ramparts, I think I'm missing something - dark matter halos are gravitationally bound to their host galaxies, aren't they? Being gravitationally bound puts some pretty tight constraints on what sort of velocities the dark matter can have, the same constraints that apply to baryonic matter, incidentally. How can dark matter be both moving as fast as you seem to suggest and still be bound to galaxies?

 

[ramparts]

Well, a normal black hole doesn't emit light either. Light is emitted from the gas surrounding black holes in accretion disks. So while what Kessy said is entirely true, a dark matter black hole would certainly be able to form an accretion disk like that and emit light. In fact, since most astrophysical black holes are believed to have no charge, then a black hole made of dark matter and one made of normal matter should be indistinguishable (and in practice, any black hole almost certainly has some of both).

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?

 

[Woggles]

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.

 

[Mordred]

the quickest way is to explain that the idea was first proposed to explain why gravity is behaving the way it does when you study the graviational forcwes on distant stars they should fly off or be in a different orbit due to all the detected graviatiobnal forces known in the region.
There is several theories as to what dark matter consists of. One is WIMPs weakly interactive mass particles. particles that do not interact with the elctromagnetic force. canditates are neutrinos. or other particles 1000 times less mass than an electron.
Another possiblilty is MACHO's essentially this is massive bodies such as white and brown dwarfs, Black holes that are not feeding so do not exhibit light etc.

On the subject of blackholes and Dark matter I found this article
http://www.sciencedaily.com/releases/20 ... 191749.htm
perhaps this may answer some of the questions from the original poster.