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Dark Matter

AkuManiMani

Illuminator
AkuManiMani
Joined
Jan 19, 2008
Messages
3,089
Just a question for any physicists or physics aficionados. What are the current theories of what constitutes dark matter? I keep reading articles mentioning how mysterious it is and how its producing unexpected observations in cosmology but hardly a mention of what physicists think it might be.
 
Well, I must confess, I wasn't so much wondering what current physicist say dark matter isn't; I was wondering more along the line of speculations about what it is. From the article you provided it seems that WIMPs have been ruled out as candidates for dark matter.

I've some speculation of my own and I suppose my real motive is for some expert feedback as to whether or not it's plausible.

From the lil' bit I've read of M-theory there's speculation that the visibly observable universe is confined to a 3-brane and that the reason why gravity seems to incredibly weak compared to the other forces is that its carrier particle (the graviton) is the only one not confined to this bane. In a sense it just seems weaker because it's more diffuse.

This got me thinking that if this is true it would logically follow that masses on nearby branes could interact thru the force of gravity and that what we now call "dark matter" is really the mass of material on near by branes. I admit that I'm just a layman speculating on top of speculation but I'm curious if such a possibility has been considered and/or rejected for reasons I'm not aware of.
 
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The point of the search (under my concern) was that while you WANT to know what they think it may be, THEY are more interested (in their presumed expertise)in eliminating potential candidates until no more can be easily eliminated. In other words, the current direction of the research/speculation by the experts is not going in the direction you are concerned with. That leads me to conclude you are unlikely to find any major player right know who is loudly suggesting a specific positive candidate.

Good luck in your search though!!
 
fuelair said:
The point of the search (under my concern) was that while you WANT to know what they think it may be, THEY are more interested (in their presumed expertise)in eliminating potential candidates until no more can be easily eliminated. In other words, the current direction of the research/speculation by the experts is not going in the direction you are concerned with. That leads me to conclude you are unlikely to find any major player right know who is loudly suggesting a specific positive candidate.

Good luck in your search though!!
Click to expand...

Thank you :)
 
AkuManiMani said:
Well, I must confess, I wasn't so much wondering what current physicist say dark matter isn't; I was wondering more along the line of speculations about what it is. From the article you provided it seems that WIMPs have been ruled out as candidates for dark matter.
Click to expand...

That's absolutely not the case. What gave you that impression?

WIMPs are the leading candidates for DM. They arise naturally in many extensions of the standard model.

This got me thinking that if this is true it would logically follow that masses on nearby branes could interact thru the force of gravity and that what we now call "dark matter" is really the mass of material on near by branes. I admit that I'm just a layman speculating on top of speculation but I'm curious if such a possibility has been considered and/or rejected for reasons I'm not aware of.
Click to expand...

That's a good idea, and it has been explored in the literature. You should be able to find references if you search (if not, ask).
 
sol invictus said:
That's absolutely not the case. What gave you that impression?

WIMPs are the leading candidates for DM. They arise naturally in many extensions of the standard model.
Click to expand...

I think Sol is right on this one. As a matter of fact, just a few weeks ago I spoke with a physicist who is actually attempting to detect WIMPS/DM in his laboratory at FermiLab. His team is using bubble chambers to do it.

Nothing yet. But if we're lucky and they find something... wow.
 
sol invictus said:
That's absolutely not the case. What gave you that impression?

WIMPs are the leading candidates for DM. They arise naturally in many extensions of the standard model.



That's a good idea, and it has been explored in the literature. You should be able to find references if you search (if not, ask).
Click to expand...
Actually, and this is not my speciality, the article was referring to a signal and WIMP halos not being responsible for the signal in the way described in the article. Since it isn't my field, you might wish to check the article and advise on proper interpretation of it. Thanks, fuelair.:)
 
AkuManiMani said:
From the article you provided it seems that WIMPs have been ruled out as candidates for dark matter.
Click to expand...

As Sol says, this is simply not the case. In fact, since MACHOs have been pretty much ruled out, at least as making up the majority of dark matter, WIMPs are the only real choice left. You have to understand that "WIMP" is not a specific particle, it's a broad category that includes a huge range of possible particles, as well as plenty that probably aren't possible.

From the lil' bit I've read of M-theory there's speculation that the visibly observable universe is confined to a 3-brane and that the reason why gravity seems to incredibly weak compared to the other forces is that its carrier particle (the graviton) is the only one not confined to this bane. In a sense it just seems weaker because it's more diffuse.

This got me thinking that if this is true it would logically follow that masses on nearby branes could interact thru the force of gravity and that what we now call "dark matter" is really the mass of material on near by branes. I admit that I'm just a layman speculating on top of speculation but I'm curious if such a possibility has been considered and/or rejected for reasons I'm not aware of.
Click to expand...

Are you sure that second paragraph is your own speculation? Because pretty much what all the sources containing the first paragraph will speculate along those lines as well. It hasn't been rejected any more than the rest of M-theory has been rejected, it's simply that it's not currently possible to test and is therefore a rather useless idea.
 
sol invictus said:
That's absolutely not the case. What gave you that impression?

WIMPs are the leading candidates for DM. They arise naturally in many extensions of the standard model.
Click to expand...
As I understand it, neutrinos (now that they have been confirmed to have a small, but nonzero mass) now fall into the category of WIMPs - they are Weakly Interacting Massive Particles, after all.

Or am I barking up the wrong linear accelerator?
 
arthwollipot said:
As I understand it, neutrinos (now that they have been confirmed to have a small, but nonzero mass) now fall into the category of WIMPs - they are Weakly Interacting Massive Particles, after all.

Or am I barking up the wrong linear accelerator?
Click to expand...

Generally "WIMP" refers to a currently unknown particle with a much larger mass than neutrinos. But your point is well-taken - neutrinos might have been DM. They aren't, because they aren't heavy enough to account for the data, but it goes to show that DM need not be so mysterious or surprising.

One interesting and still viable option are axions.
 
But DM doesn't have to be just one thing. There could be several types of particle that contribute to the observed effect. Neutrinos may contribute a certain percentage of the observed effect, with the rest being provided by other kinds of WIMPs.
 
arthwollipot said:
But DM doesn't have to be just one thing. There could be several types of particle that contribute to the observed effect. Neutrinos may contribute a certain percentage of the observed effect, with the rest being provided by other kinds of WIMPs.
Click to expand...

Well, we know pretty well how much mass neutrinos contribute, and it's a small percentage (probably below the error on how much DM there is).
 
sol invictus said:
That's absolutely not the case. What gave you that impression?

WIMPs are the leading candidates for DM. They arise naturally in many extensions of the standard model.
Click to expand...

Okay. I didn't mean to declare definitively that WIMPs were ruled out; I was just saying that the article provided gave me the impression that they had. My mistake -- thanks for clarifying :)


Cuddles said:
Are you sure that second paragraph is your own speculation? Because pretty much what all the sources containing the first paragraph will speculate along those lines as well. It hasn't been rejected any more than the rest of M-theory has been rejected, it's simply that it's not currently possible to test and is therefore a rather useless idea.
Click to expand...

Well I'm absolutely sure that it was my own speculation. I presented it so see if an professionals had already thought of it and if they considered it valid. From the responses I've gotten here it appears that they have, which is reassuring. Oddly enough, all the literature I've read on the brane world scenario never explicitly connected it to the issue of dark matter candidates so I was left to infer for my self.

(And 'useless' speculation happens to be one of my favorite pastimes - don't knock it :p )
 
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AkuManiMani said:
Okay. I didn't mean to declare definitively that WIMPs were ruled out; I was just saying that the article provided gave me the impression that they had. My mistake -- thanks for clarifying :)
Click to expand...

OK, I see. There is an Italian experiment - DAMA - that has a positive signal - a detection of DM. However no one else takes it very seriously, because no other experiment has succeeded in reproducing their signal. The article was about one of those failed attempts, and so it helps to rule out DAMA, not WIMPs.
 
arthwollipot said:
But DM doesn't have to be just one thing. There could be several types of particle that contribute to the observed effect. Neutrinos may contribute a certain percentage of the observed effect, with the rest being provided by other kinds of WIMPs.
Click to expand...

Not just "could be" several types, it's pretty much certain that there are. For example, MACHOs (Massive Compact Halo Objects) were at one point the main competitor to WIMPs as dark matter candidates. The term "MACHO" simply refers to pretty much any ordinary matter that we can't see, whether that's dwarf stars that never got hot enough for fusion to start, clouds of gas too far from any source of illumination or whatever. And the thing is, we know they exist. It's simply impossible for them not to, since we obviously can't see everything in the whole universe. Hell, we can't even see half the stuff in this solar system. However, it turns out that while we know there's a lot of ordinary matter out there, there (almost certainly) isn't enough, and perhaps more importantly it shouldn't be in the places we think a lot of dark matter is. The same is true for neutrinos. We know they make up some of the missing mass, but we're equally sure that they don't make up all of it, or even a particularly significant amount.

So yes, dark matter certainly doesn't have to be one thing, and we already know some things that make up small amounts of it. However, as Sol says, the term "WIMP" is usually used to refer specifically to unknown particles. Neutrinos don't count because we know about them. It's also theorised, although it's not certain by any means, that WIMPs only interact through gravity. Since neutrinos also interact via the weak force, they don't count under that definition.
 
So if it's influence from another brane... that would be a "diagnosis of exclusion" so to speak? If no particles were found to be representative of DM?
 
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