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amstrad
2005-Mar-16, 06:01 PM
From what I understand, dark matter has been infered since galaxies seem to rotate faster than they should based on the mass of visible objects (stars and gas clouds, etc). I'm sure that there are other ways to infer dark matters existance, but this is the only one I can easily understand.

First, why can't the existance of dark matter be explained by the super massive black holes in the centers of galaxies being more massive than first thought? If the mass of the central SMBH is calculated by observing the rotation rate of matter in the galaxies center, what if the BH itself is causing time dilation and that matter near the center is appearing to move more slowly than it actually is. Therefore, our calculated mass of the BH is too low.

Second, recently we have seen evidence of possible galaxies made entirely of dark matter. How would you be able to tell the difference between such a galaxy and a single naked SMBH?

ToSeek
2005-Mar-16, 06:14 PM
It's not that galaxies rotate faster than they should; it's that the outer parts rotate at the same speed as the inner parts, instead of slowing down the farther out you go. This indicates that there is more matter than is visible.

TravisM
2005-Mar-16, 06:18 PM
Also, having a greater mass in the center wouldn't account for the speed of the outer edge. Something massive is there to make this happen.

I also recall something about there not being enough angular momentum attributed to the observed mass to account for the 'thinness' (bad gramar) of spiral galaxies. :-? I can't seem to find a reference... (googling...)

W.F. Tomba
2005-Mar-16, 06:34 PM
I also recall something about there not being enough angular momentum attributed to the observed mass to account for the 'thinness' (bad gramar) of spiral galaxies.
I don't know anything about dark matter, but I do know that thinness is considered perfectly standard grammar and spelling.

amstrad
2005-Mar-16, 06:52 PM
ToSeek and TravisM,

are you guys talking about actual rotation of matter in the galaxy or rotation of the spiral arms. It is my understanding that the spiral arms are density waves that move independantly of the stars and gas they contain. Isn't this also a similar effect to the radial spokes in the rings of Saturn?

Normandy6644
2005-Mar-16, 07:14 PM
ToSeek and TravisM,

are you guys talking about actual rotation of matter in the galaxy or rotation of the spiral arms. It is my understanding that the spiral arms are density waves that move independantly of the stars and gas they contain. Isn't this also a similar effect to the radial spokes in the rings of Saturn?

Spiral arms certainly are the result of density waves. The thing about dark matter is that stars that are further out from the center orbit at nearly the same speeds as the closer stars, when physics would imply that the outer stars would be moving much more slowly. Therefore some unknown force is moving them; dark matter.

As far as the supermassive black hole deal goes, it would be in no way massive enough to affect the motions of the outer stars. They are simply too far away, and the mass of the black hole is not enough to cause them to orbit at those velocities.

Maddad
2005-Mar-16, 08:33 PM
First, why can't the existance of dark matter be explained by the super massive black holes in the centers of galaxies being more massive than first thought?The problem is that whatever the mass of the central supermassive black hole, orbital speeds of stars should drop off as you get further from the galactic center. Instead, the speeds stay relatively constant with distance. We blame dark matter, not existing at the center of the galaxy, but out in the disk.


what if the BH itself is causing time dilation and that matter near the center is appearing to move more slowly than it actually is.Whoa. Now there's an interesting new twist. I've seen the dark matter question raised a hundred times, but never thought about gravitational time dialation messing with our observations. I don't know how to calculate how much time dialation there should be. Someone here should know. If it doesn't explain dark matter entirely, it might make a dent in the problem.


are you guys talking about actual rotation of matter in the galaxy or rotation of the spiral arms.The context of the thread would be actual matter rotation, rather than movment of density waves.

ngc3314
2005-Mar-16, 08:45 PM
what if the BH itself is causing time dilation and that matter near the center is appearing to move more slowly than it actually is.Whoa. Now there's an interesting new twist. I've seen the dark matter question raised a hundred times, but never thought about gravitational time dialation messing with our observations. I don't know how to calculate how much time dialation there should be. Someone here should know. If it doesn't explain dark matter entirely, it might make a dent in the problem.

Not enough to notice. The amplitude of gravitational ime dilation is the same as that of the gravitational redshift (because at the base they are the same thing). That would make the effect symmetric about the galaxy center (rather than antisymmetric, which is what we see). On top of that, even the billion-solar-mass black holes postulated in the most powerful active nuclei can't manage detectable graviational redshift over an area more than light-days across. The gravitational redshift (as seen from an observer infinitely far from the central mass and thus in asymptotically flat spacetime, and so on) is GM/Rc^2 from material at radius R from the mass involved; that drops well below the typical rotation speeds of spiral galaxies by the time you look at something (punch, scribble) a few thousand AU from a billion-solar-mass beast.

jfribrg
2005-Mar-16, 08:50 PM
here (http://www.badastronomy.com/phpBB/viewtopic.php?t=18069&postdays=0&postorder=asc&hig hlight=mond&start=0&) is one of several threads discussing MOND (modified Newtonian Dynamics) which postulates that the behavior of galaxies can be explained by a modification of the inverse square rule. In effect, it says that the gravitational "constant" is not constant. The pros and cons of this theory have been debated ad nauseum on the ATM board, so I don't think we need to repeat the arguments here. Nonetheless, the thread I linked to above contains links to some papers that are good summaries of where things stand in this regard.

Grey
2005-Mar-16, 08:54 PM
Both ToSeek and TravisM [edit: and several others since I started, since it took me so long to write this] touched on this, but I thought I'd make it explicit. It's not just that there isn't enough mass visible, it's that the mass distribution needs to be different. From the rules of gravity, we know that the gravitational effect of a hollow sphere is zero if you're inside the sphere, and the same as a point mass concentrated at the center if you're outside the sphere. That means that if a big collection of stars, gas, and other stuff is all orbiting it's common center of mass, for any given star you can ignore all the stuff further away from the center, and treat all the stuff closer in to the center as a simple point mass as far as orbital mechanics goes.*

The orbital speed of an object is v^2 = GM/r, where M is the mass of the object that's being orbited, and r is the distance from the center. You can see that for v to remain roughly constant, the mass has to go up linearly with r, which means the density has to go like the inverse square of the radius. If we look at the luminosity profile for a typical galaxy, it instead falls of exponentially (i.e. much faster than inverse square), so it can't just be the stuff we see. And it can't just be something massive at the center, that would make things worse, by concentrating even more mass in the center, when what we need is more material on the outskirts. Analysis of globular glusters orbiting the galaxy suggests that this remains true even well beyond the apparent visible boundary of the galaxy.

So, it was suggested that the luminous part of the galaxy may be embedded in a large halo of matter that we can't see for whatever reason. Interestingly, if we analyze the behavior of a simple gas of particles that interact only gravitationally at a single temperature, we find that it would have a density profile that goes like the inverse square of the distance from the center. Hmm. :D

* This only really works as described for circular orbits, but you can do the same thing for elliptical orbits, it's just got messier (that's more messy, not Messier :) ) math.

amstrad
2005-Mar-17, 12:31 AM
Thanks for the answers, guys.

Grey, your explaination was very helpful.

zebo-the-fat
2005-Mar-17, 06:53 PM
Is dark matter just "normal matter" that isn't illuminated and so hard to detect directly, or is it a different type of matter. (If I had a lump of it on my kitchen table would it just look like a rock, or something wierd?)

ToSeek
2005-Mar-17, 07:03 PM
Is dark matter just "normal matter" that isn't illuminated and so hard to detect directly, or is it a different type of matter. (If I had a lump of it on my kitchen table would it just look like a rock, or something wierd?)

It's usually used to refer to "nonbaryonic dark matter", i.e., weird stuff, though sometimes it can just mean stuff that's not lit up like stars.

Grey
2005-Mar-17, 07:24 PM
Is dark matter just "normal matter" that isn't illuminated and so hard to detect directly, or is it a different type of matter. (If I had a lump of it on my kitchen table would it just look like a rock, or something wierd?)
We don't know the answer to that for certain, but we have some clues that suggest that much of it is the latter. Both big bang nucleosynthesis and WMAP results suggest that a significant portion of the universe should be non-baryonic. That is, made up of something other than the normal quarks that we know and love.

There have been observational efforts looking for nonilluminated normal matter that might account for the dark matter, like brown dwarfs, small black holes, rogue planets, and so forth. Obviously, this is pretty tricky to do, but there are possible methods. I know of a study looking for microlensing in the Large Magellanic Cloud (if something heavy passes in front of a random star, you'll see it brighten briefly due to gravitational lensing, and can deduce the mass of the interposing object) over six years. They saw some events, but only enough to account for about 20% of the needed dark matter (though that still works out to be almost as much normal baryonic dark matter as there is luminous matter!). Another idea I've heard is that there could be clouds of molecular hydrogen, which is really hard to detect observationally.

However, remember that cloud of gas I described? If you allow the particles to interact, like normal matter would, then it collapses, just like we see the normal matter doing, and doesn't keep that nice inverse square density profile that we're looking for.

So, the most prevalent current idea is that the dark matter consists primarily of some unknown particle (maybe massive neutrinos, maybe some of the supersymmetric counterparts to known particles that have been hypothesized by physicists, maybe something even more exotic) that doesn't particularly interact with itself or with normal matter through anything other than gravity. Models and simulations using this as a basis actually do a pretty good job of showing how the structure at several scales ranging from individual galaxies to superclusters can evolve.

If this view turns out to be right, then it would be really hard to see the stuff at all, since it doesn't interact with light. It would be passing right through you all the time without interacting much at all, the way neutrinos do. And you couldn't have a rock made out of it, since it doesn't interact with itself strongly enough to form any kind of crystalline structure. So you're pretty much left with a diffuse gas of particles that don't emit or reflect light and pass effortlessly through matter.

W.F. Tomba
2005-Mar-17, 07:42 PM
If it interacts gravitationally, shouldn't it be accumulating at the centers of concentrations of normal matter, such as the Sun or the Earth?

P.S. One of the Google ads at the bottom of the page now says "Dark Matter For Sale". :)

jfribrg
2005-Mar-17, 07:55 PM
If it interacts gravitationally, shouldn't it be accumulating at the centers of concentrations of normal matter, such as the Sun or the Earth?
That's why I doubt the existence of dark matter. You have to endow it with all kinds of bizarre properties in order to make the theory match the observations.


The orbital speed of an object is v^2 = GM/r, where M is the mass of the object that's being orbited, and r is the distance from the center. You can see that for v to remain roughly constant, the mass has to go up linearly with r

This last statement is not necessarily true. All you can say is that in order for v to remain roughly constant, GM has to remain roughly constant. The assumption that G is constant is just that: an assumption. Let G vary instead of density, and you get some interesting results. There is also nothing about that idea which prohibits the existence of non-baryonic dark matter. It could be that dark matter exists and G varies with distance.

Grey
2005-Mar-17, 11:46 PM
If it interacts gravitationally, shouldn't it be accumulating at the centers of concentrations of normal matter, such as the Sun or the Earth?
Nope. Remember that for a cloud of gravitationally bound material to collapse, it has to be able to give up energy. A typical cloud of matter would do that by radiating away heat, for example. But if dark matter doesn't couple with radiation, it can't do that, so it doesn't collapse the same way that matter would.


That's why I doubt the existence of dark matter. You have to endow it with all kinds of bizarre properties in order to make the theory match the observations.
Not really that bizarre, though, and not all that many. It has to interact gravitationally, and not interact strongly in other ways. We already know at least one class of particles that behaves just that way: neutrinos.


This last statement is not necessarily true. All you can say is that in order for v to remain roughly constant, GM has to remain roughly constant. The assumption that G is constant is just that: an assumption. Let G vary instead of density, and you get some interesting results. There is also nothing about that idea which prohibits the existence of non-baryonic dark matter. It could be that dark matter exists and G varies with distance.
It's certainly true that one can avoid the need for dark matter by instead modifying gravity. However, so far this hasn't worked as well as a dark matter model. For example, you can adjust it so that you get galactic rotation curves working well, but then predictions of galactic cluster behavior doesn't agree with observation. It's possible to rescue that by adding dark matter back in, but then you've eliminated the primary reason for adopting a variable-G theory in the first place. Still, I'd actually agree that it's not a bad idea to continue exploring this as a possibility.

Maddad
2005-Mar-18, 05:02 AM
That's why I doubt the existence of dark matter. You have to endow it with all kinds of bizarre properties in order to make the theory match the observations.Not really that bizarre, though, and not all that many. It has to interact gravitationally, and not interact strongly in other ways. We already know at least one class of particles that behaves just that way: neutrinos.It's rather bizarre for me. Your neutrino example is suspect since we are unable to even say for certain that neutrinos have mass react in other ways. If they do, then it's something like 10^7 proton masses. Can't make a rock out of that.

kg034
2005-Mar-18, 07:39 AM
That's why I doubt the existence of dark matter. You have to endow it with all kinds of bizarre properties in order to make the theory match the observations.
Not really that bizarre, though, and not all that many. It has to interact gravitationally, and not interact strongly in other ways. We already know at least one class of particles that behaves just that way: neutrinos.


THanks for the nice explanations, Grey and others. I'm just curious, why are
neutrinos candidates, and not neutrons? Light scattering?

Grey
2005-Mar-18, 02:24 PM
It's rather bizarre for me. Your neutrino example is suspect since we are unable to even say for certain that neutrinos have mass react in other ways.
Well, it's certainly not the normal matter that we know. But since we already know that there are particles like neutrinos that interact only very weakly with normal matter, it really isn't that much of a stretch to imagine that there might be other such particles. And though there's no conclusive evidence that neutrinos have mass, the evidence is certainly leaning that way.


If they do, then it's something like 10^7 proton masses. Can't make a rock out of that.
I'm not sure I understand what you mean here. Are you talking about neutrino mass or mass of some other hypothetical dark matter particle? If it's the neutrino mass, then the upper bound is way lower than that. Either way, though, as I pointed out you aren't going to be able to make a rock out of dark matter at all. If it interacts strongly enough to form rocks, it can't explain what we see.


I'm just curious, why are neutrinos candidates, and not neutrons? Light scattering?
Neutrons actually interact fairly well with other matter. Moreover, they decay to protons pretty quickly in just a few minutes, and protons of course interact very well. In order to explain what we see, dark matter has to interact pretty much only gravitationally. Neutrinos actually also interact through the weak force, but not through the strong or electromagnetic forces, which is why they can pass effortlessly through kilometers of solid matter and only occasionally interact. Dark matter needs to share that behavior.

Argos
2005-Mar-18, 06:06 PM
It's not that galaxies rotate faster than they should; it's that the outer parts rotate at the same speed as the inner parts.

We could also say that this causes the stars to orbit the galaxy´s center in a more organized, less chaotic fashion, with the galaxy disc displaying a more "rigid" aspect in its spinning.

amstrad
2005-Mar-18, 06:30 PM
It's not that galaxies rotate faster than they should; it's that the outer parts rotate at the same speed as the inner parts.

We could also say that this causes the stars to orbit the galaxy´s center in a more organized, less chaotic fashion, with the galaxy disc displaying a more "rigid" aspect in its spinning.

Now I got confused again. So I need to clarify:

Do stars in the outer parts of a galaxy have the same velocity or the same angular velocity compared to stars closer to the galactic center?

I assumed the former, but now, Argos, you are implying the later.

ToSeek
2005-Mar-18, 06:45 PM
It's velocity. See here. (http://www-astronomy.mps.ohio-state.edu/~pogge/Ast162/Unit6/dark.html)

Argos
2005-Mar-18, 07:05 PM
Now I got confused again.

I really didn´t mean that. Sorry. It would be a deservice to this board, indeed. I won´t try to amend it not to worsen the situation. :) ToSeek´s got it right.

amstrad
2005-Mar-18, 08:19 PM
Now I got confused again.

I really didn´t mean that. Sorry. It would be a deservice to this board, indeed. I won´t try to amend it not to worsen the situation. :) ToSeek´s got it right.

Hey, no problem ;) I get confused easily.

And thanks, ToSeek, that's an interesting link.