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tonyman1989
2007-Sep-14, 06:11 PM
I heard many people talk about WIMPS, and that there might be enought of them to account for all the dark matter,

But alot of people are looking for new forms of matter, Is it a commen belief among scienctist that dark matter is a whole new form of matter?

01101001
2007-Sep-14, 06:23 PM
But alot of people are looking for new forms of matter, Is it a commen belief among scienctist that dark matter is a whole new form of matter?

By new form of matter, do you mean non-baryonic? Your phrasing seemed to exclude WIMPS from the new forms.

I guess I'm saying: to answer your question, your defintion of "whole new form of matter" is needed.

Wikipedia: Dark matter (http://en.wikipedia.org/wiki/Dark_matter)


At present, the most common view is that dark matter is primarily non-baryonic, made of one or more elementary particles other than the usual electrons, protons, neutrons, and known neutrinos. The most commonly proposed particles are axions, sterile neutrinos, and WIMPs (Weakly Interacting Massive Particles, including neutralinos). None of these are part of the standard model of particle physics, but they can arise in extensions to the standard model.

tonyman1989
2007-Sep-14, 06:33 PM
"whole new form of matter" is needed.


Sorry. What I heard was is that a few people think it could be different state of matter instead of a solid, gas, liquid, or Plasma.

antoniseb
2007-Sep-14, 06:36 PM
Sorry. What I heard was is that a few people think it could be different state of matter instead of a solid, gas, liquid, or Plasma.

There are many 'states' of matter, including several that are very cold, and some that are very dense. Don't let your grammar school science get you stuck thinking that there are only four.

tonyman1989
2007-Sep-14, 06:40 PM
There are many 'states' of matter, including several that are very cold, and some that are very dense. Don't let your grammar school science get you stuck thinking that there are only four.

I never said there were only four, I only stated the most familiar ones.

galacsi
2007-Sep-14, 07:00 PM
Hydrinos of Randell Mills are stilll very controversial , but if they can be shown to be real ,they are a good candidate for dark matter.

antoniseb
2007-Sep-14, 09:40 PM
I never said there were only four, I only stated the most familiar ones.
Sorry, my comment was a little rash. Concerning Dark Matter being a 'state of matter', it is not very different from neutrinos. It goes through normal matter, and each particle is free to move on its own. You could call it a state of matter, but it may not be meaningful, since dark matter doesn't change what state it is in.

DanishDynamite
2007-Sep-14, 09:51 PM
You could call it a state of matter, but it may not be meaningful, since dark matter doesn't change what state it is in.
This is news to me. I thought Dark Matter was just a term to describe matter unseen and unaccounted for but assumed to exist because galaxies couldn't possibly have the rotation rate and structure they clearly had, given just the matter which was "visible".

It used to be that Dark Matter could be anything at all, including Brown Dwarfs and neutrinoes.

Does it now mean something completely different? Something with the special ability of not being able to change its state?

antoniseb
2007-Sep-14, 11:51 PM
It used to be that Dark Matter could be anything at all, including Brown Dwarfs and neutrinoes.
Neutrinos are called Hot Dark Matter because they move to quickly to be gravitationally bound. The dark matter that causes the gravitational lensing by galaxy clusters is presumed to be one or more types of more massive infrequently interacting particle. Brown dwarfs, dust, old neutron stars, rogue planets, etc are what's referred to as 'the missing mass'.

Tim Thompson
2007-Sep-14, 11:54 PM
Is it a commen belief among scienctist that dark matter is a whole new form of matter?
Yes, in the strict sense that it is not baryonic (http://en.wikipedia.org/wiki/Baryon) matter. This is not a wild guess. Several decades of searching for baryonic dark matter has failed to score. More recent searches, some using the Hubble Space Telescope for direct imaging, others using gravitational microlensing, have put strict limits on how much condensed baryonic matter can be out there. Those limits are considerably less than what is needed to account for the effects from which the presence of dark matter is derived. Likewise, searches for diffuse, baryonic matter (i.e., intracluster gas) have come up short.

Delvo
2007-Sep-15, 01:25 AM
Does it now mean something completely different? Something with the special ability of not being able to change its state?What that appears to have meant was that it can't become non-dark; it can't turn into normal matter. And that makes it not just another state, because if it were a state, it could change to another, which normal matter does routinely.

tonyman1989
2007-Sep-15, 11:02 AM
Sorry, my comment was a little rash.

Thank you, no hard feelings.


Concerning Dark Matter being a 'state of matter', it is not very different from neutrinos. It goes through normal matter, and each particle is free to move on its own.

I heard about that from some where I think it was the astronomy podcast that a single neutrino particle can go through 22 light years of matter before it interacts with it.


You could call it a state of matter, but it may not be meaningful, since dark matter doesn't change what state it is in.

Do we even know what state it is in?

antoniseb
2007-Sep-15, 12:12 PM
Do we even know what state it is in?
If it is WIMPs (such as neutralinos) we can describe it. It is like a thin gas or plasma, but has no pressure. It can't be contained or worked on with any physical device.

tonyman1989
2007-Sep-15, 12:34 PM
If it is WIMPs (such as neutralinos) we can describe it. It is like a thin gas or plasma, but has no pressure. It can't be contained or worked on with any physical device.

If it can't be contained how will we learn more about other wimps beside neutrinos? Have we detected anyother forms of wimps?

antoniseb
2007-Sep-15, 12:50 PM
If it can't be contained how will we learn more about other wimps beside neutrinos? Have we detected anyother forms of wimps?
We can learn more by watching them get created or destroyed (some models suggest that certain WIMPs are their own anti-particle and will annihilate when they (rarely) hit each other, which should be more common near the centers of galaxies and clusters of galaxies). Also there is some thought that it is possible to have a rare thermal collision between a WIMP and a normal particle.

Some papers suggest that we have seen evidence of neutralino annihilation, but they aren't conclusive.

tonyman1989
2007-Sep-15, 12:52 PM
We can learn more by watching them get created or destroyed (some models suggest that certain WIMPs are their own anti-particle and will annihilate when they (rarely) hit each other, which should be more common near the centers of galaxies and clusters of galaxies). Also there is some thought that it is possible to have a rare thermal collision between a WIMP and a normal particle.

Some papers suggest that we have seen evidence of neutralino annihilation, but they aren't conclusive.

Thanks you for the information.

Jerry
2007-Sep-15, 11:19 PM
This is news to me. I thought Dark Matter was just a term to describe matter unseen and unaccounted for but assumed to exist because galaxies couldn't possibly have the rotation rate and structure they clearly had, given just the matter which was "visible".

This is closest to the definition used by Dr. Pamala Gay in astrocast. The number of candidate particles that have been excluded is noteworthy.

DanishDynamite
2007-Sep-16, 01:32 AM
Neutrinos are called Hot Dark Matter because they move to quickly to be gravitationally bound. The dark matter that causes the gravitational lensing by galaxy clusters is presumed to be one or more types of more massive infrequently interacting particle. Brown dwarfs, dust, old neutron stars, rogue planets, etc are what's referred to as 'the missing mass'.Neutrinoes used to be candidates for the missing mass as well. I thought Dark Matter was just a new way of referring to the missing mass. Is this no longer the case?

EDG
2007-Sep-16, 02:25 AM
Is there actually any evidence that "Dark Matter" is even "matter" at all? From what I understand it seems to be more like "gravity without any visible mass", is it possible that this is exactly what it is - i.e. "naked gravity", not associated with actual mass at all?

antoniseb
2007-Sep-16, 11:56 AM
I thought Dark Matter was just a new way of referring to the missing mass. Is this no longer the case?
The missing mass is a term used to refer to the normal matter that hasn't been seen yet. As I understand it, about two-thirds of the reactive matter that should be in the galaxy/universe hasn't been observed directly.

Rastermon2
2007-Sep-17, 09:18 PM
I have heard many estimates of the amount of matter that is observable...
Some tv shows say we can only see 10% of it - the rest dark matter. I have heard others estimating only 2% - that is 98% dark matter. Again, and estimate of 20% observable, 80% dark.
Just prior, someone posted a 1/3 2/3 split.
That seems like a very wide range for estimates. How are we to know which (if any) of those are correct?

DanishDynamite
2007-Sep-17, 09:22 PM
The missing mass is a term used to refer to the normal matter that hasn't been seen yet. As I understand it, about two-thirds of the reactive matter that should be in the galaxy/universe hasn't been observed directly.
And what is Dark Matter then? What observations require the invention of this?

Nereid
2007-Sep-18, 12:25 AM
And what is Dark Matter then? What observations require the invention of this?Try this thread:
What is the observational basis for (cold, non-baryonic) dark matter? (http://www.bautforum.com/questions-answers/42223-what-observational-basis-cold-non-baryonic-dark-matter.html)

Tim Thompson
2007-Sep-18, 05:26 AM
And what is Dark Matter then? What observations require the invention of this?

Galaxy cluster observations
Observation reveals the motion of galaxies in galaxy clusters. Combining the observed velocities with mass derived from the luminosity gives the kinetic energy of the galaxies in the cluster. The Virial theorem (http://en.wikipedia.org/wiki/Virial_theorem) tells us how much gravity is required to hold galaxies together in a cluster, given the kinetic energy derived from observations. That derived gravity implies a mass necessary to create that much gravity. The result is that the luminous mass is anywhere from 1% to 10% of the mass that must be in the cluster in order for there to be enough gravity to hold the cluster together. This is a very old & well known problem, first pointed out by Fritz Zwicky (http://en.wikipedia.org/wiki/Fritz_Zwicky) in 1933 (Die Rotverschiebung von extragalaktischen Nebeln, Helvetica Physica Acta, Vol. 6, p. 110-127; the paper is not online, but the ADS link (http://adsabs.harvard.edu/abs/1933AcHPh...6..110Z) does show the 237 papers that cite it in their references). A modern example of the same thing is the fact that million Kelvin intracluster gas is not lost to the cluster.

Galaxy rotation curves
Spiral galaxies don't show Keplerian rotation. The disks of spiral galaxies should move slower as the distance from the center increases. They don't. The simplest solution to the problem is that there is more mass, roughly spherically distributed, that we don't see. This too is an old problem, first noticed by Jan Oort (http://en.wikipedia.org/wiki/Jan_Hendrik_Oort) in observing stars in the Milky Way (The force exerted by the stellar system in the direction perpendicular to the galactic plane and some related problems (http://adsabs.harvard.edu/abs/1932BAN.....6..249O); J.H Oort, Bulletin of the Astronomical Institutes of the Netherlands, Vol. 6, p.249, August 1932). It was forgotten but revived by Louise Volders in 1959 (Neutral hydrogen in M 33 and M 101 (http://adsabs.harvard.edu/abs/1959BAN....14..323V); L.M.J.S. Volders, Bulletin of the Astronomical Institutes of the Netherlands, Vol. 14, p.323, September 1959). The galaxy rotation problem was finally revived again, and made unforgettable, by Vera Rubin (http://en.wikipedia.org/wiki/Vera_Rubin) in 1970, with the first extensive study of the general problem (Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions (http://adsabs.harvard.edu/abs/1970ApJ...159..379R); Vera C. Rubin & Kent W. Ford, Jr., Astrophysical Journal, vol. 159, p.379, February 1970).

Those are the old classical arguments. But those observations alone are quite unable to differentiate between "normal" and "exotic" (i.e., non-baryonic) dark matter. There is certainly no way to tell from Zwicky's problem, for instance, that the extra mass does not come from lots of dim red dwarfs, dimmer white dwarfs, or brown dwarfs, or Jupiter mass planets, or just a lot of left over gas.

But now we can tell the difference. Searches for red dwarfs & white dwarfs in the halo of the Milky Way reveal directly that there are not enough of these to account for the extra "dark" mass. Radio astronomy can detect essentially all of the cool or warm gas, and X-ray astronomy will detect all of the hot gas. Infrared astronomy will reveal the dust. We can add it all up now, and see directly that all of these things together simply don't add up to enough mass to account for the mass needed to account for the observed motions. That is a classical road to non-baryonic dark matter.

A less classical route to the same conclusion comes from analysis of the variations in temperature over the sky in the cosmic microwave background. We can tell how much total mass there is, and how much mass there is that interacts with the CMB photons. The difference between them must be non-baryonic mass, and that is the main driver for the need to resort to "exotic" matter.

But even here, one must not take "exotic" too seriously. After all, we already know about neutrinos, and we know that neutrinos are non-baryonic dark matter. There aren't enough of them either. But the point here is that appealing to non-baryonic matter is not in itself an overly exotic idea, since it really means "let's see if there is more of this stuff we already have", rather than "let's invent some entirely new stuff". Considering the obvious nature of the evidence, however, even an appeal to "entirely new stuff" is justified.

Finally, a more recent tool in the search for dark matter is weak gravitational lensing. Several recent results strongly imply the presence of non-baryonic dark matter (i.e., Clowe, et al., 2006 (http://adsabs.harvard.edu/abs/2006ApJ...648L.109C); Jee, et al., 2007 (http://adsabs.harvard.edu/abs/2007ApJ...661..728J); Massey, et al., 2007 (http://adsabs.harvard.edu/abs/2007Natur.445..286M)).

EDG
2007-Sep-18, 05:59 AM
Again (I ask for a third time on these boards)... what is the evidence that any form of matter is actually involved at all? All we seem to be observing are the gravitational effects that we're assuming are caused by some form of matter we can't see - what if no matter is involved at all though? What if this is just "naked gravity", a field without matter?

It just seems weird to me to assume that matter is involved. We've looked very hard for it, and despite all our efforts we apparently can't detect it (unless I've missed something). If that's the case, shouldn't the the next logical step be to assume that it's just not there, even if that does mean we have to reconsider the nature of gravity and how it relates to mass (at least on the galactic scale)?

Tim Thompson
2007-Sep-18, 04:34 PM
what is the evidence that any form of matter is actually involved at all?
The "evidence" is the simple fact that nowhere in physics is it allowed for gravity to exist without mass, and matter of any kind is by far the simplest way to get lots of mass with little effort.


All we seem to be observing are the gravitational effects that we're assuming are caused by some form of matter we can't see - what if no matter is involved at all though? What if this is just "naked gravity", a field without matter?
That solution is probably impossible, but certainly the most bizarre & exotic idea one could try. The only way to get gravity is with mass. If you do that without matter, then you have to have the energy equivalent thereof, in order to get the gravity. A look at "the most famous of all equations", E=MC2, tells you that the energy required is really big. How could all that energy escape notice? It is far beyond the dark energy limits, and there are not even any physical hypotheses that would explain such a totally bizarre affair. So why would anyone guess that?


It just seems weird to me to assume that matter is involved.
That seems weird to me. Why would matter not be involved where mass is concerned?


We've looked very hard for it, and despite all our efforts we apparently can't detect it (unless I've missed something). If that's the case, shouldn't the the next logical step be to assume that it's just not there, even if that does mean we have to reconsider the nature of gravity and how it relates to mass (at least on the galactic scale)?
Actually, no, that should not be the next logical step. One can hardly argue that we have exhausted all of our efforts. Consider how elusive neutrinos are, even though we know how to catch a few. A particle only slightly more elusive would be invisible to us, and a perfect candidate for dark matter particles (hence, WIMPs (http://en.wikipedia.org/wiki/WIMP) or sneutrinos (Lee, Matchev & Nasri, 2007 (http://adsabs.harvard.edu/abs/2007PhRvD..76d1302L); Pagé, 2007 (http://adsabs.harvard.edu/abs/2007JHEP...04...21P) & etc.)).

There is a lot of work left to do looking for matter, before we decide it is time to entirely re-write all the laws of physics, and start over again from scratch.

EDG
2007-Sep-19, 06:41 AM
The "evidence" is the simple fact that nowhere in physics is it allowed for gravity to exist without mass, and matter of any kind is by far the simplest way to get lots of mass with little effort.

And yet the observational evidence seems to be lacking for the presence of any actual extra matter. All we've got is more apparent mass than is visible. Our interpretation of that has been to say that there must therefore be extra matter that we're not seeing, but that isn't necessarily true.



That solution is probably impossible, but certainly the most bizarre & exotic idea one could try. The only way to get gravity is with mass. If you do that without matter, then you have to have the energy equivalent thereof, in order to get the gravity. A look at "the most famous of all equations", E=MC2, tells you that the energy required is really big. How could all that energy escape notice? It is far beyond the dark energy limits, and there are not even any physical hypotheses that would explain such a totally bizarre affair. So why would anyone guess that?

Well, roughly speaking, gravity is the deformation of space/time isn't it? What if it's not being deformed by a mass inside our own universe, but rather by something outside it (to use the rubber-sheet analogy, we wouldn't be making a dent in the sheet by having something resting on it - we'd be making the dent by pulling the sheet from below)? If you just have the deformation on its own then you can have a gravitational field without having something in the middle. There was an interesting concept in a scifi story I read by Greg Egan (called Diaspora) where gravity "leaked" from other "nearby" universes, maybe that's what's going on here, who knows.

Heck, maybe if these "dents" exist already they can act as nuclei for matter to collect in (because the 'naked field' attracts the matter, which then attracts more matter...).

I dunno. I'm rambling off the top of my head, call it ATM if you will. I just can't help but think we're barking up the wrong tree with Dark Matter :)



That seems weird to me. Why would matter not be involved where mass is concerned?

Like I said, gravity is the deformation of spacetime - the deformation is what's important. Maybe there's more than one way to do it.

All I'm saying is that if it seems at best very difficult to actually detect matter as the source of the extra gravity, and you have to concoct all sorts of exotic particles that conveniently are invisible to try to explain it, then maybe it might be worth looking at other explanations as well. And an obvious alternative explanation to me is that matter - or even actual physical mass - isn't involved at all, that it's a naked deformation. A free standing dent in space time if you will.




One can hardly argue that we have exhausted all of our efforts.

Well, I could :). We seem to be getting to the stage where we're stretching our theories of fundamental particles rather a lot to explain matter that is completely undetectable, doesn't interact with normal matter at all, and yet has mass. Dark Matter is what, 90% of the mass of the universe? How exactly can we be completely incapable of detecting so much matter, if it's really there? Despite all our efforts all we seem to have actually observed is that large collections of visible matter behaves as if there is a lot more mass there than is detectable. I don't think that necessarily implies that extra matter is there at all, whether "light" or "dark" - just that the extra gravity is there.



There is a lot of work left to do looking for matter, before we decide it is time to entirely re-write all the laws of physics, and start over again from scratch.

Oh, I'm not arguing that we need to rewrite everything completely and I don't think it'd be necessary at all. Everything we know could easily be a subset of something greater, just as Newtonian physics is a subset of Relativity.

Like I said, call this ATM if you will - though I wouldn't, it's nowhere near coherent enough, it's just some thoughts I have :). I just think we're looking at this the wrong way, is all.

Delvo
2007-Sep-19, 01:20 PM
Well, roughly speaking, gravity is the deformation of space/time isn't it?... If you just have the deformation on its own then you can have a gravitational field without having something in the middle... the deformation is what's important. Maybe there's more than one way to do it... then maybe it might be worth looking at other explanations as well. And an obvious alternative explanation to me is that matter - or even actual physical mass - isn't involved at all, that it's a naked deformation. A free standing dent in space time if you will.That is a much more drastic fundamental departure from our current understanding of things than dark matter is; you're trying to dodge a bullet by jumping in front of a cannon ball.

And in fact there have been other proposed ideas to explain the dark-matter evidence, so your assertion that there haven't is inaccurate. They just aren't talked about as much because they've been examined and compared with the evidence and turned out not to work as well. For exapmle, "MOND" (modified Newtonian Dynamics") was a suggestion that gravity doesn't weaken with distance as much as we thought it did, but it requires that the center of gravity be the center of visible matter, so it can't handle things like the Bullet Nebula, where two objects have collided and thus altered each other's velocities but the center of gravity has kept on going and is now in the wrong place to be generated by the visible matter... just like what would happen if it were generated by matter that didn't interact with "our" matter other than gravitationally.


Dark Matter is what, 90% of the mass of the universe? How exactly can we be completely incapable of detecting so much matter, if it's really there?Because of its lack of interaction with the most important force for our sensors: electromagnetism. All that that requires in itself is particles with no electrical charge, and some of those are already known to exist anyway.


Oh, I'm not arguing that we need to rewrite everything completelyYes, you are. Gravity without mass is rewriting everything and burning parts of the original.

EDG
2007-Sep-19, 03:14 PM
Because of its lack of interaction with the most important force for our sensors: electromagnetism. All that that requires in itself is particles with no electrical charge, and some of those are already known to exist anyway.

Where's the drag then? If this DM is real and present in such huge quantities in and around everything, why do we not observe an actual drag effect as matter moves through it (I'm not talking about being slowed down by the extra gravity, I mean being directly slowed by interacting with the DM itself)? Surely it should be offering some resistance? Or is this explained away by magical non-interaction again?

Delvo
2007-Sep-19, 05:42 PM
There's nothing "magical" (and no need for such name-calling) about particles lacking electrical charges, and interactions of the electrical charges in atoms are what creates "drag" in fluids of normal matter anyway, so OF COURSE it couldn't exist when we're dealing with chargeless matter. The idea of dark matter would only be in trouble if there WERE any such phenomenon as drag among neutral particles; you have this exactly backward.

The_Radiation_Specialist
2007-Sep-19, 06:01 PM
I have a question. Where is dark matter? is it in the interstellar medium? is it in the solar system?

EDG
2007-Sep-19, 07:26 PM
There's nothing "magical" (and no need for such name-calling) about particles lacking electrical charges, and interactions of the electrical charges in atoms are what creates "drag" in fluids of normal matter anyway, so OF COURSE it couldn't exist when we're dealing with chargeless matter. The idea of dark matter would only be in trouble if there WERE any such phenomenon as drag among neutral particles; you have this exactly backward.

How about physical collisions with dark matter then? If there's so much of it around then surely visible matter should be hitting it as it moves and losing energy as a result. You don't need to be electrical charged to produce a detectable result after an impact.

John Mendenhall
2007-Sep-19, 07:35 PM
Do we even know what state it is in?

Lower Delaware, below the canal.

DanishDynamite
2007-Sep-19, 08:36 PM
Galaxy cluster observations
Observation reveals the motion of galaxies in galaxy clusters. Combining the observed velocities with mass derived from the luminosity gives the kinetic energy of the galaxies in the cluster. The Virial theorem (http://en.wikipedia.org/wiki/Virial_theorem) tells us how much gravity is required to hold galaxies together in a cluster, given the kinetic energy derived from observations. That derived gravity implies a mass necessary to create that much gravity. The result is that the luminous mass is anywhere from 1% to 10% of the mass that must be in the cluster in order for there to be enough gravity to hold the cluster together. This is a very old & well known problem, first pointed out by Fritz Zwicky (http://en.wikipedia.org/wiki/Fritz_Zwicky) in 1933 (Die Rotverschiebung von extragalaktischen Nebeln, Helvetica Physica Acta, Vol. 6, p. 110-127; the paper is not online, but the ADS link (http://adsabs.harvard.edu/abs/1933AcHPh...6..110Z) does show the 237 papers that cite it in their references). A modern example of the same thing is the fact that million Kelvin intracluster gas is not lost to the cluster.

Galaxy rotation curves
Spiral galaxies don't show Keplerian rotation. The disks of spiral galaxies should move slower as the distance from the center increases. They don't. The simplest solution to the problem is that there is more mass, roughly spherically distributed, that we don't see. This too is an old problem, first noticed by Jan Oort (http://en.wikipedia.org/wiki/Jan_Hendrik_Oort) in observing stars in the Milky Way (The force exerted by the stellar system in the direction perpendicular to the galactic plane and some related problems (http://adsabs.harvard.edu/abs/1932BAN.....6..249O); J.H Oort, Bulletin of the Astronomical Institutes of the Netherlands, Vol. 6, p.249, August 1932). It was forgotten but revived by Louise Volders in 1959 (Neutral hydrogen in M 33 and M 101 (http://adsabs.harvard.edu/abs/1959BAN....14..323V); L.M.J.S. Volders, Bulletin of the Astronomical Institutes of the Netherlands, Vol. 14, p.323, September 1959). The galaxy rotation problem was finally revived again, and made unforgettable, by Vera Rubin (http://en.wikipedia.org/wiki/Vera_Rubin) in 1970, with the first extensive study of the general problem (Rotation of the Andromeda Nebula from a Spectroscopic Survey of Emission Regions (http://adsabs.harvard.edu/abs/1970ApJ...159..379R); Vera C. Rubin & Kent W. Ford, Jr., Astrophysical Journal, vol. 159, p.379, February 1970).
Many thanks, Tim, for the well explained brush up on some of the background for the reason why astronomers started talking about "missing mass". Still doesn't explain the apparent difference between Dark Matter and Missing Mass. But you have more to say....

Those are the old classical arguments. But those observations alone are quite unable to differentiate between "normal" and "exotic" (i.e., non-baryonic) dark matter. There is certainly no way to tell from Zwicky's problem, for instance, that the extra mass does not come from lots of dim red dwarfs, dimmer white dwarfs, or brown dwarfs, or Jupiter mass planets, or just a lot of left over gas.

But now we can tell the difference. Searches for red dwarfs & white dwarfs in the halo of the Milky Way reveal directly that there are not enough of these to account for the extra "dark" mass. Radio astronomy can detect essentially all of the cool or warm gas, and X-ray astronomy will detect all of the hot gas. Infrared astronomy will reveal the dust. We can add it all up now, and see directly that all of these things together simply don't add up to enough mass to account for the mass needed to account for the observed motions. That is a classical road to non-baryonic dark matter.

A less classical route to the same conclusion comes from analysis of the variations in temperature over the sky in the cosmic microwave background. We can tell how much total mass there is, and how much mass there is that interacts with the CMB photons. The difference between them must be non-baryonic mass, and that is the main driver for the need to resort to "exotic" matter.
So, at this point it would seem that the term Darm Matter now refers to "missing mass" which is non-baryonic. Coorect?

But even here, one must not take "exotic" too seriously. After all, we already know about neutrinos, and we know that neutrinos are non-baryonic dark matter. There aren't enough of them either. But the point here is that appealing to non-baryonic matter is not in itself an overly exotic idea, since it really means "let's see if there is more of this stuff we already have", rather than "let's invent some entirely new stuff". Considering the obvious nature of the evidence, however, even an appeal to "entirely new stuff" is justified.

Finally, a more recent tool in the search for dark matter is weak gravitational lensing. Several recent results strongly imply the presence of non-baryonic dark matter (i.e., Clowe, et al., 2006 (http://adsabs.harvard.edu/abs/2006ApJ...648L.109C); Jee, et al., 2007 (http://adsabs.harvard.edu/abs/2007ApJ...661..728J); Massey, et al., 2007 (http://adsabs.harvard.edu/abs/2007Natur.445..286M)).
So, to sum up, the term Dark Matter refers to non-baryonic matter? Is this correctly understood?

Fortunate
2007-Sep-19, 10:26 PM
How about physical collisions with dark matter then? If there's so much of it around then surely visible matter should be hitting it as it moves and losing energy as a result. You don't need to be electrical charged to produce a detectable result after an impact.

EDG,
According to my understanding, particles of dark matter cannot clump or bind together, so they move about as discrete particles. I only know of four ways for particles to interact: through electromagnetism, gravity, the strong force, and the weak force. Apparently, particles of dark matter do not react with baryonic matter through either electromagnetism or the strong force. In this way, they are like neutrinos, almost all of which pass through the earth, for instance, without effect. Although there are huge numbers of neutrinos all around us (trillions have passed right through you as you have read this post), they exert no measureable drag nor do we feel jolted or bumped.

Neutrinos do interact with normal matter through the weak force - we detect them by these very infrequent weak interactions. We don't even know if dark matter interacts through the weak force, but even if it does, there will be virtually no perceptible drag.

Tim Thompson
2007-Sep-19, 11:06 PM
So, to sum up, the term Dark Matter refers to non-baryonic matter? Is this correctly understood?
That's the intended meaning of the phrase by scientists working in that field, in the absence of explicit statements to the contrary.


In this way, they are like neutrinos, almost all of which pass through the earth, for instance, without effect. Although there are huge numbers of neutrinos all around us (trillions have passed right through you as you have read this post), they exert no measureable drag nor do we feel jolted or bumped.
The total solar neutrino flux, based on the efficiency with which we can grab a few now & then, should be about 5.4x106 per square centimeter per second. I am not sure how much bigger that number gets when non-solar neutrinos are added in.

Indeed, I made this point once before, and will make it again now. Neutrinos are non-baryonic dark matter. One might think of them as "exotic", but we can still measure & detect them through their weak interactions. So when we say that "dark matter" is non-baryonic matter, we are not making up some strange new category of matter. We are simply asserting that the most likely source of our dark matter worries is that there is a lot more of the same kind of stuff that we already know about, but in a different form. Anything that behaves like a neutrino will exhibit all of the peculiarities that EDG_ mentions, as neutrinos do. Just change the reaction mode, or just make the reaction cross section a bit smaller than a neutrino cross section, and the particle becomes invisible. The trick then is to try to guess what its properties should be, based on what we already know about particle physics, and then design experiments to find such particles. That is neither easy nor cheap, and the latter is critical. Getting together the money required to look for non-baryonic dark matter in a classical laboratory effort is just as hard is it would be to detect the particles.

I simply disagree with the sentiments expressed by EDG_ that there is any kind of conceptual problem here. There might be someday, but there is not now. Based on our own models of how such particles should behave, detecting them is no easy task, and we have only recently been able even to start looking (i.e., Miuchi, et al., 2007 (http://adsabs.harvard.edu/abs/2007arXiv0708.2579M); Angle, et al., 2007 (http://adsabs.harvard.edu/abs/2007arXiv0706.0039A); Bennetti, et al., 2007 (http://adsabs.harvard.edu/abs/2007astro.ph..1286B); Bezrukov & Shaposhnikov, 2007 (http://adsabs.harvard.edu/abs/2007PhRvD..75e3005B) & etc.).

The neutrino itself was first postulated to exist in 1930, but not actually detected until 1956. So, somewhere around 1940, should they have decided that neutrinos should have been detected, and the failure to do so was by then a confident demonstration that they did not exist? Remember that even though "dark matter" or "missing mass" has been with us since 1933, it is only in the last few years that we have realized that "dark matter" cannot be "normal matter". So we look for "dark matter" for a few years and then just toss the idea?

All alternatives to non-baryonic dark matter are far less likely, based on common sense experience. And all of them require major revisions to fundamental physics. That may in fact be necessary eventually; after all, we have had to re-write fundamental physics before, and doubtless will need to again someday. But it is far too early to make such a call now, when we have barely begun to be able to solve the problem with common sense solutions.

Nereid
2007-Sep-19, 11:57 PM
I have a question. Where is dark matter? is it in the interstellar medium? is it in the solar system?There is, it seems, very little* dark matter (DM) in the solar system. There is also little in the immediate vicinity of the solar system, within a few (dozen) parsecs or so.

In rich clusters of galaxies, the DM is spread (generally) in an approximately spherical shape, with its density increasing towards the centre of the cluster (example (http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=33508)).

In (most) galaxies, the DM is spread (generally) in an approximately spherical shape (a 'halo'), with its density increasing towards the centre of the galaxy.

An integrated profile of the DM halo around galaxies was published in 2000: Weak Lensing with SDSS Commissioning Data: The Galaxy-Mass Correlation Function To 1/h Mpc (http://xxx.lanl.gov/abs/astro-ph/9912119); since then there have been several papers on this topic (check out this SDSS webpage (http://www.sdss.org/publications/), for example).

This month's ESA Messenger (http://www.eso.org/sci/publications/messenger/) (#129) has an interesting article on the extent to which a certain class of dwarf galaxy resembles other galaxies wrt the amount of DM (vs baryonic mass): Weighing Ultracompact Dwarf Galaxies in the Fornax Cluster.

*Unless it is concentrated in the core of the Sun, Jupiter, etc. Even if it is, it would still amount to a very minor component of the total solar system mass.

DanishDynamite
2007-Sep-20, 12:26 AM
That's the intended meaning of the phrase by scientists working in that field, in the absence of explicit statements to the contrary.
So Dark Matter is a way of refering to the part of the "missing mass" which is non-baryonic. OK.

What then differentiates Hot and Cold Dark Matter?

EDG
2007-Sep-20, 12:30 AM
But isn't there a problem in that the "dark matter" is just sitting there, as opposed to whizzing through all matter like neutrinos are?

And as another thought, isn't this sounding rather like the old "Ether" concept? If DM is just sitting there getting out of the way of everything, doesn't it count as a medium through which light (photons) and other particles are moving?

DanishDynamite
2007-Sep-20, 12:44 AM
But isn't there a problem in that the "dark matter" is just sitting there, as opposed to whizzing through all matter like neutrinos are?
A very good question.

And as another thought, isn't this sounding rather like the old "Ether" concept? If DM is just sitting there getting out of the way of everything, doesn't it count as a medium through which light (photons) and other particles are moving?
No, it isn't like the Ether. The Ether was shown to be bunk long ago, shown locally at that. Furthermore, the existence of DM is only required for the explanation of things on a galactic scale or bigger.

EDG
2007-Sep-20, 12:49 AM
There is, it seems, very little* dark matter (DM) in the solar system. There is also little in the immediate vicinity of the solar system, within a few (dozen) parsecs or so.

That's weird in itself. It's almost like we can only detect its effects when we're looking at large, massive objects like galaxies. I can't recall ever hearing of its effects being detected around nearby stars either. How can something be detectable only on a large scale but not on a small scale (hm, gravity's like that isn't it?)? It's like not being able to see something when you look at it individually, but suddenly it's right there in front of you when you look at a group.

How can we be sitting in this huge mass of dark matter in the milky way and yet not find any right on our doorstep?

Tim Thompson
2007-Sep-20, 01:00 AM
But isn't there a problem in that the "dark matter" is just sitting there, as opposed to whizzing through all matter like neutrinos are?
Who says it is "just sitting there"? It is (or so we assume, since we can't directly detect it yet) whizzing around just like neutrinos are. It does respond to gravity, and observations imply that dark matter is concentrated preferentially in gravitational wells, though not as completely as luminous matter, which in turn implies that dark matter does not cool like luminous matter. That's no big surprise, as neutrinos don't cool either. It could not cool unless it radiated energy, in which case it would not be dark.


And as another thought, isn't this sounding rather like the old "Ether" concept? If DM is just sitting there getting out of the way of everything, doesn't it count as a medium through which light (photons) and other particles are moving?
Dark matter is no more or less of a "medium through which light (photons) and other particles are moving" than are the electrons or neutrinos or whatever else is whizzing around out there. The old ether of which you speak was born in the assumption that light consisted of mechanical vibrations in a mechanical medium, which was the only kind of wave anyone knew about at the time. But once the electromagnetic field nature of light waves became evident, the need for a mechanical wave description went by the wayside, along with the superfluous ether. Also remember that the ether defied fundamental physics in that it had to be very rigid in order to support such fast mechanical waves, yet so pliable planets could cruise through unmolested. There is no such fundamental problem with dark matter (yet).

One might also point out that the dynamic nature of spacetime in general relativity is a very "ether like" concept, as even Einstein recognized. So it may well be that we have not gotten rid of the ether after all, just changed its properties to something more amenable to fundamental physics.

Fortunate
2007-Sep-20, 01:12 AM
Tim,
What do you think about the possibility of "warm" dark matter?
http://news.bbc.co.uk/1/hi/sci/tech/4679220.stm

Tim Thompson
2007-Sep-20, 01:15 AM
How can we be sitting in this huge mass of dark matter in the milky way and yet not find any right on our doorstep?
Easy. The effect is to small to be easily measured. We know there are asteroids in the solar system, we know they have gravity, and we know that their gravity reaches Earth. But we can't see the tug of the asteroids on Earth. Likewise, the dark matter density in the solar system is too small, and too spherically symmetrical, to be observed by our current techniques. Why should we assume that there is nothing we cannot measure?

We can measure planetary orbits well enough to place upper limits on the amount of dark matter in the solar system. That means we know roughly how much of the stuff would generate an observable signal, and we know we don't see it (i.e., Frère, Ling & Vertongen, 2007 (http://adsabs.harvard.edu/abs/2007astro.ph..1542F); Iorio, 2006 (http://adsabs.harvard.edu/abs/2006JCAP...05..002I) & etc.). We should expect that, eventually, we will be able to observe effects that small.

DanishDynamite
2007-Sep-20, 01:18 AM
Who says it is "just sitting there"? It is (or so we assume, since we can't directly detect it yet) whizzing around just like neutrinos are. It does respond to gravity, and observations imply that dark matter is concentrated preferentially in gravitational wells, though not as completely as luminous matter, which in turn implies that dark matter does not cool like luminous matter. That's no big surprise, as neutrinos don't cool either. It could not cool unless it radiated energy, in which case it would not be dark.
Wow. So much inferred about the properties of something whose existence is only assumed, not detected.

A little scary.

Nereid
2007-Sep-20, 01:36 AM
Wow. So much inferred about the properties of something whose existence is only assumed, not detected.

A little scary.Welcome to astronomy, as a science! :)

The existence of everything (beyond the solar system), let alone their properties, "is only assumed", in the sense that all we actually detect is photons*.

The only difference - in terms of degrees of confidence wrt existence (and properties) - is how applicable you think the physics used in 'the assumptions' is, to analyses of the patterns within the detected photons.

*not counting the isotropic rain of cosmic rays, ~19 neutrinos, some neutral He atoms, and a few micrograms (picograms?) of interstellar dust.

Fortunate
2007-Sep-20, 03:21 PM
I don't know why the possibilty of an as yet undetected particle seems so unlikely to some. A century ago, the only subatomic particles we knew of were protons, neutrons, and electrons. Then the existence of neutrinos and positrons was predicted. As time went on, we discovered pions, kaons, and muons. In our accelerators, we found etas, lamdas, sigmas, taus, phis, j-psis, upsilons, and a host of others. The standard model brought some degree of order to this burgeoning zoo, but further predicted Ws, Zs, top and bottom quarks, and Higgs particles. Except for the Higgs, these have all been found. Now supersymmetric theories include a very large number of different types of particles, and our leading theory of the strong CP problem includes axions. I am told that several other theories also posit new particles, and, of course, there could very well exist particles that none of our current theories even anticipate. Many particle physicists are more or less expecting some new particles, not to explain dark matter but to unite the stong force with the electroweak or to explain the CP situation. I'm almost expecting the unexpected.