Assuming that Dark Matter isn’t just cold ordinary matter (Which is apparently pretty much proven) then I have some questions about what we know, and what that implies.
Dark matter interacts with other matter only gravitationally.
Does it interact with energy only gravitationally?
If dark matter gets hit by a Gamma Ray Burst, does it change the nature of the gamma ray burst? Does it change the nature of the Dark matter? Is Dark matter affected by light pressure?
Does the presence of Dark matter alter the interactions of matter with matter and energy? Out there in the universe, it seems that dark matter exists in huge proportions in halos of galaxies. What about the interiors of Galaxies?
Are there dark matter properties that drive dark matter apart from itself? (Like heat, and light pressure, and magnetic force, electrical force do for ordinary matter?) If not, how come the universe isn’t littered with Dark Matter Black Holes? And if there is such a thing as a dark matter black hole, does it have Hawking Radiation? (or dark Hawking radiation?)
Every one of these questions, of course brings up a sheaf of other questions.
So, the above implies that some force does act upon dark matter in opposition to gravity, and that force is evidently proportional in some way with the density of dark matter in some region.
That’s my understanding. If the dark matter was normal matter, then it would have changed the results of the Big Bang from what we see in the universe in ways we could detect, such as by altering the resulting abundances of various elements. Since we don’t see those changes, we know that whatever it is, it isn’t our kind of matter.
It will divert the gamma ray burst via gravitational lensing, but that’s it. The dark matter itself might be slightly diverted by the gravity of the photon beam from the gamma ray burst, but photons have negligible gravity so it’d probably be undetectable.
My understanding is that dark matter is evenly spread throughout galaxies, but also extends outward in halos well beyond what we see as the visible galaxies, so in terms of mass galaxies are mostly invisible.
This is just a guess, but since dark matter has only one force acting on it, there’s no source of friction to slow down orbits, so dark matter can flow around in its orbit around the galaxy for a long time. Gravitational radiation would play a role, of course, but that would happen over the very long term.
I don’t think we have evidence of some kind of force that only affects dark matter, although we haven’t ruled it out either, and there have been some proposals for what such a force might look like.
However I think the explanation of why dark matter hasn’t collapsed into black holes is not so esoteric as that. If you have some dark matter spread throughout the galaxy and rotating (much as the normal matter in the galaxy rotates), conservation of angular momentum can keep it rotating instead of collapsing in to the center. It’s going to need to bleed off some of that angular momentum to condense, and without being able to interact electromagnetically it doesn’t have a great way to do that. [Edit: In other words, what Yumblie said.]
OK, so why isn’t all the dark matter collapsing into disks way faster than matter? In the cite for the maximum density I got the impression that it did collect at the center until a particular density was reached, and then it stopped falling in.
Stopping seems to imply it started. What started it, if angular momentum is the only force acting on it aside from gravity? Seems that it would get transfer of momentum since the begining and therefore only collapse into disks around the galaxies. But that isn’t how I have heard of it being distributed.
The least speculative hypothesis is that dark matter is made of WIMPs – weakly interacting massive particles. The so-called “WIMP miracle” is the observation that a particle with a mass near the electroweak scale with an interaction rate near that of the weak force would fit all cosmological observations. These particles would be too heavy to have been detected in collider experiments so far, but they could perhaps be seen in the LHC experiments.
More generally, all we can say is that dark matter interacts through gravity and that it does not participate in the strong or electromagnetic interactions. This leaves open the weak interaction (as above) and any other new coupling you feel like inventing, as long as it isn’t too strong.
I think though that there’s dark matter getting sucked into ordinary black holes, just like everything else. In the paper you cite they work out the maximum dark matter density in the center of the dark matter halo, but they treat the central region of dark matter (which includes the black hole) as having constant density. Indeed their argument seems to be based on the idea that otherwise the central black hole would be gaining mass from the dark matter at a divergent rate, and this would be detectable.
Basically, what it comes down to is no one can give you a definitive answer, or even more than just informed speculation, to any of your questions, because we don’t know what the hell the stuff is. All we know is that it acts gravitationally, and that’s about all…
I don’t follow why you think the dark matter would collapse faster, but in any case the rough answer is that it’s easier for ordinary matter to slow down as a result of its electromagnetic interactions, and when it slows down it’s easier for it to collapse.
Angular momentum isn’t technically a force, but I can understand why you’d think so. If you look at things in the rotating frame of reference, you see a centrifugal force (cite)
As far as how dark matter falls into the hole, for one thing if the hole absorbs a bunch of regular matter it’s going to grow, and presumably swallow up any dark matter that’s nearby.
That paper talking about a maximum density of dark matter isn’t saying that there’s some fundamental limit for how dense dark matter can possibly get, but just saying that observationally, the density is below some value. In other words, you could get dark matter denser than that, you just don’t in practice.
A black hole could swallow dark matter just the same way it swallows anything. There’s no way to distinguish a black hole that’s eaten some dark matter, or even one that was formed entirely from dark matter, from one without. A black hole of any source could in principle emit dark matter particles in its Hawking radiation, but it’s believed that dark matter particles are very massive, so in practice you’re not going to get very many of them.
Quoth tim314:
By the same token, though, it also wouldn’t be subject to the Eddington luminosity limit.
Clouds of randomly vectored particles that attract each other will (not considering forces other than gravity) move according to the attraction of each particle toward the center of mass of the cloud, and the original vector of each particle. That results in either an escape orbit, and eventual escape from the cloud, or an ellipse around the center of gravity of the cloud.
Statistically, every cloud will have an average axis of rotation for all the ellipses, and each ellipse which is inclined to the plane of rotation will move through two regions, above and below the plane. In each region the gravitational attraction of the aggregate will be somewhat greater in the direction of that plane. Over time the cloud will become a disk. Forces like light, heat, and magnetism, and wave fronts of particles expelled by nova, and supernova events will modify that disk into the more familiar shapes of galaxies. (Although I am given to understand that sound waves in the very hot medium of the earliest period of the universe produced “sheets” of increased density in matter which enhanced the process.)
My question is not why isn’t all the dark matter in the center, it is why isn’t all the dark matter much more dense in the plane of rotation of the galaxies. (more like an actual halo, that is) Since light, heat, and magnetism, and wave fronts of charged particles have no effect, it seems that the consequence of the mechanics of transfer of angular momentum would be at least somewhat evident.
my WAG as a layman is that dark matter and dark energy are both temporary hacks to fill gaps in our knowledge and that new cosmological models to emerge in the near future will completely eliminate the need to invoke dark matter to explain discrepancies in the observed rate of expansion of the universe.
Hey, just saw this thread. I am an astrophysicist, FYI.
The best answer I can give to this is that, as far as we know, dark matter interacts gravitationally only. But that only means there’s an upper limit to how it interacts in any other way. It definitely interacts with energy gravitationally, but it might also interact in other ways at a very low level. (But that’s not yet observed.)
For the first question, definitely not. The best it can do is slightly redshift or blueshift the light, and that’s only if structure continues to form as the light falls into and re-exits a dark matter gravitational well. (That’s called the integrated-Sachs-Wolfe effect.)
It appears to not change the nature of dark matter.
And although dark matter is affected by radiation pressure, it’s in a very specific way: mostly through effects on the expansion rate of spacetime. This is important, almost exclusively, when the Universe is very young (<< 1 million years old).
Dark matter does not affect normal matter/energy interactions. Dark matter might couple to itself, and it might interact with normal matter (protons, electrons, photons, etc.), but there are limits on both of those, no positive detections.
The interiors of galaxies pose difficulties for models of dark matter; simulations give results that indicate there should be more dark matter towards the center than is allowed for observationally. But nearly all of the evidence for dark matter is on scales larger than a galaxy: if you look at clusters of galaxies, you find lots of mass where the individual galaxies are, but also a (roughly) spherical halo of mass throughout the entire cluster. Check out this graph: http://scienceblogs.com/startswithabang/upload/2010/06/convincing_a_young_scientist_t/mass_recon0024_500.jpg
As far as we know, we have no evidence for self-interaction among dark matter particles. This means that, unlike normal matter, dark matter doesn’t collide with itself. If you took two particles of dark matter and shot them directly at each other, they’d pass right through one another, which doesn’t do you any good for trying to collapse. A black hole has, as far as we know, three properties: mass, charge, and “spin” (or angular momentum). That’s it. It doesn’t care where your mass came from, so in theory black holes made from dark matter would be no different than black holes made from a neutron star.
Ok thanks for that… so as an astrophysicist, whats your personal opinion of dark matter/dark energy? Do you think they are something thats here to stay or that a future more elegant model will remove the need for having to invoke them to explain observational discrepancies in our current models?
Personally, I think that dark matter is absolutely necessary. There are about eight separate types of observations out there that all need dark matter to work. I was skeptical of it initially when I started learning about it, but there’s really no sensible way to avoid it.
Models like MOND and other “no-dark-matter” scenarios simply choose to ignore a whole set of observations, as they do not make reasonable predictions.
So I think dark matter is here to stay, and that there most likely is some type of matter that isn’t made out of the standard particles we know.
But as for dark energy, that’s much closer to a plea that we don’t understand why the expansion rate of the Universe is doing what we’re doing. There are huge public misperceptions of it, but the expansion rate is definitely not slowing down like we’d expect if there were only matter and radiation in the Universe. Why not? The best we have right now are extremely speculative ideas. Extremely.
