Is dark matter all it's cracked up to be?

CalliArcale · Mar 2, 2006

Since this is clearly a topic of great interest, I figured I'd start a new thread. Dark matter is, of course, material which cannot be detected by its electromagnetic radiation or reflection. It is quite literally dark. The only way to detect it is by its effect: gravitational influence on other objects. It is certain that dark matter does exist; consider absorption nebulae such as the Horsehead Nebula, visible primarily because of its profile. And of course there are limits to current technology. More and more brown dwarfs are being detected as the ability to find such objects in infrared improves. As it is impossible to really know what we haven't discovered, it is reasonable to assume there are at least a few more of these sorts of things, and probably quite a lot. Some interestings questions are: Just how much dark matter is there? If you can't see it, how can you detect it? Even if you're detecting it gravitationally, how much does this really tell you about the dark matter? [Edited because I finally noticed a grammatical error in the title. D'oh!] <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

JonClarke · Mar 2, 2006

Another question - what is dark matter. It is ordinary matter as we understand it (dust, brown dwarves) or is it some type of exotic matter (weakly interacting massive particles)? And that about exhausts my knowledge of the subject. Jon <div class="Discussion_UserSignature"> Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars Arthur Clarke </div>

colesakick · Mar 2, 2006

One can speculate almost anything. I personally find the idea that dark energy harkens back the Aether theory. In one model the Aether is viewed as discrete rotating magnetic fields, in others quantum black holes. In any case, most agree that we do not yet see or measure all that is to be seen or measured. I hope dark energy is the Aether (viewed as a kind of superfuid in some models), that would solve a lot of the mystery (like how waves can propagate through a vacuum or outer space [which we used to regard as an empty vacuum]). <div class="Discussion_UserSignature"> Intellectual honesty means being willing to challenge yourself instead of others </div>

why06 · Mar 2, 2006

I don't believe dark matter exist... it is a fairly new idea.. there are many other reasons for expansion. <div class="Discussion_UserSignature"> <div>________________________________________ </div><div><ul><li>your move...</li></ul></div> </div>

yevaud · Mar 2, 2006

Dark Matter wasn't proposed as an explanation for expansion of the universe; it was proposed to explain the anomolous behaviour of Galaxies, in which (counterintuitively) the rotational velocities of the Galaxies didn't decrease as you moved outwards from their center. Vera Rubin and Dark Matter That link is a good starting point. <div class="Discussion_UserSignature"> Differential Diagnosis: "I am both amused and annoyed that you think I should be less stubborn than you are." </div>

JonClarke · Mar 2, 2006

We are talking about dark matter, not dark energy. They two are different, and should not be confused. Jon <div class="Discussion_UserSignature"> Whether we become a multi-planet species with unlimited horizons, or are forever confined to Earth will be decided in the twenty-first century amid the vast plains, rugged canyons and lofty mountains of Mars Arthur Clarke </div>

dragon04 · Mar 2, 2006

We know that interstellar space is not "empty". And I don't quite know how to frame this question, but what kind of "resolution" are we capable of regarding a scan of interstellar space regarding dark matter? Is the Oort Cloud sufficiently massive enough to block detection of less that singificant interstellar mass? Let's say we assume that there is one Hydrogen atom in every cubic foot of interstellar space when in actuality, there are a million. What does that do to our theories and models of the cosmos as we know it? <div class="Discussion_UserSignature"> "2012.. Year of the Dragon!! Get on the Dragon Wagon!". </div>

Saiph · Mar 2, 2006

I've done some digging into dark matter, or rather chatting with a upper level graduate student over lunch... Case for dark matter: 1) The inclusion of non-visible matter easily explains the radial velocity curves for spiral galaxies. For those who don't know, these curves do not fit the expected shape predicted by straightforward gravity, based on the idea that gravity is caused by the matter we see. In a spiral galaxy, we've got a large central bulge, full of stars, and an ever thinning disk spreading out from it. Because the mass is concentrated in the center, the further out you go from the core, the slower the stars should orbit. However, it slows a bit...and then basically levels off. We see stars orbiting at about the same speed at the edge, as they do half-way to the core. This can be explained by having a bunch of mass spread out in the disk, but it is not giving off light. This does a very good job of explaining the curves. This is the classic origin of the dark matter idea. 2) This one is a bit more indirect. We have certain observations about our universe, it's exapnsion rate, age, general metal abundances, etc, that lead us to conclude the universe is "flat" to a very close degree of accuracy (~1%). General Relativity (GR) specifies certain conditions must be met in order for the universe to be flat, basically how much mass and energy must be present to make it flat, isntead of "closed" or "open". The problem is, visible matter doesn't account for enough material to do this. Even throwing in estimates of non-visible "baryonic" matter (brown dwarfs, dust clouds, etc) we don't get more than ~5% of what's needed. So GR, if it's right, says there's a lot of stuff out there we can't see in order for it to produce what we observe. So, GR must be wrong! it's description of gravity must be flawed! Problem is: We haven't actually been able to show it's broken in any other way. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

robnissen · Mar 2, 2006

"4) Possible sightings of dark matter galaxies: http://www.spacetoday.net/Summary/2814 " Very Cool (no pun intended, well maybe a little one <img src="/images/icons/smile.gif" /> )

jatslo · Mar 2, 2006

Ah, the invisible galaxy is not as invisible as it seems; I saw recently that astronomers were able to ascertain bits through Chandra, or some other observatory. Their speculation in the regard, dealt with collisions that stripped the stars away as a catastrophic event that is not substantiated by the way. Just more empirical data about PHENOMENON, is all.

neutron_star6 · Mar 2, 2006

Yes, even I have been told that the two different things are different from a post a while back. I have been trying for a long time to discover what could dark matter might possibly be. No one is ever sure because no one can see it. An unforseen "force" so to speak that "exists". Like gravity we cannot see it but we do know that it exists. We may never know what dark matter is. As for dark energy, which I know are two different subjects but hear me, I doubt that truly exists anyway but I may be wrong on that one.

nova_explored · Mar 3, 2006

good questions. and thats all i've got to say. jk. what's strange is, wouldn't dark matter, in theory, account for 95% of the mass of the universe. That seems extreme. another thing is: it would obviosly be influenced by the same properties that affect our physical visible universe. so how strange could it really be? <div class="Discussion_UserSignature"> </div>

CalliArcale · Mar 3, 2006

Yes, it does seem rather extreme? That's part of the reason why dark matter is controversial even (perhaps especially) in serious scientific and academic circles. It seems inevitable that there's stuff out there we can't see, but these estimates do seem like rather a lot. Yet either there is a lot of matter there, or something else is wrong, and so far, only the quantity of matter is causing problems for many of these mainstream theories (most notably general relativity). It's very puzzling. The mass media doesn't help at all, of course. Dark matter is portrayed in pulp science fiction not as something which is really dark but some bizarre substance with wild properties. In all too many fictional works, it gets used as a source of "unobtanium" (any fictional substance with effectively magical properties) -- a literary short-cut to avoid having to actually explain how some stuff in the story works. This has led to the common impression that scientists really think dark matter is like that, and obviously there is no evidence to support dark matter having such bizarre properties as it receives in cheezy made-for-Sci-Fi-Channel movies and things like that. There might be exotic matter out there, of course, but to date there is no evidence for it. It is purely speculative. I am a lot more comfortable with the idea of normal matter that we just can't see because it's cooled down too much for our instruments to detect it. <div class="Discussion_UserSignature"> "People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff." -- The Tenth Doctor, "Blink" </div>

drwayne · Mar 3, 2006

"unobtanium" (any fictional substance with effectively magical properties) I can't tell you how many proposals and ideas have made widespread use of that very material (not always called that). <img src="/images/icons/wink.gif" /> Wayne <div class="Discussion_UserSignature"> "1) Give no quarter; 2) Take no prisoners; 3) Sink everything." Admiral Jackie Fisher </div>

nova_explored · Mar 3, 2006

cooled down matter- is that the leading theory on the composition of dark matter? That sounds really good. Tricky, but a very good explanation. I really like it. Ofcourse off the bat a huge problem would be explaining how massively cooled down matter is fitted inbetween stellar activity. Or vice versa i should say- but not incomprehensible by any means. I just watched the "our corner of the cosmos" video on the main page. It really puts things in perspective when you see it. Makes me think of Carl Sagan's Cosmos. He had such a way of putting it all into perspective. unobtanium- hmmm, my credit card... <div class="Discussion_UserSignature"> </div>

nova_explored · Mar 3, 2006

actually dark matter would support Einstein's "blunder". It woud actually prove a cosmological constant, being that the universe is static, because its 'static state' would be nothing- absolute zero. All matter would eventually fizzle into a frozen state with no kinetic energy left. atleast, that's one way to look at it. And get us a nice little paradox going there. <div class="Discussion_UserSignature"> </div>

Saiph · Mar 3, 2006

well, observations require that dark matter have two properties (well, basically one): It interacts with material only weakly...through the weak nuclear force, or through gravity. This also means (and leads to the second property) that it is completely electricaly and magnetically inert (otherwise it would interact using EM forces). If it is completely EM inert...it won't scatter light, it won't absorb light, it won't create light. Also, because of this, the particles will pass through regular matter, and other dark matter, without any significant hindrance (most collision and scattering is an EM interaction). One particle we know of that fits the bill is neutrino's. Trillions pass through the entire earth undetected all the time. neutrino's are a form of dark matter. The problem is they move far to fast (nearly C), and have a speed higher than the escape velocity for nearly any system. This means they spread out a lot, and radiate from a galaxy...they don't clump as required. Neutrino's thus are part of a group of dark matter classified as "warm" dark matter. while warm dark matter can account for some things...it isn't nearly enough to cause the various galaxy wide formation rates, and features. For example, by including cold dark matter (that which doesn't move around fast) into galaxy simulations, we can reproduce barred spiral galaxies. No other tweak to the models have been able to do this (not even MOND based models) <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

bonzelite · Mar 3, 2006

and the galaxy simulations are predicated upon a gravitational understanding that may be highly lacking at present, matey <img src="/images/icons/wink.gif" />

nova_explored · Mar 3, 2006

ahhh, and so the circle continues. <div class="Discussion_UserSignature"> </div>

neutron_star6 · Mar 3, 2006

An endless cycle of many questions and infinte possiblities.

nova_explored · Mar 3, 2006

i still like the idea of cold dark matter. not that i'm saying that's what it is. And if GR is wrong, guess what makes it right... yup, the cosmological constant. I don't think he'd look to favorably upon all this, probably why he stayed away from QM altogether. And if that man said he's right. I'm not going to be the one to say he's wrong. <div class="Discussion_UserSignature"> </div>

bonzelite · Mar 3, 2006

yes, the circle does continue. steady as she goes. everything seems good today, like ice cream cones in the park on saturday. there is a great divide in gravitational reasoning at present. we have 2 major schools of thought, with very different conditions, to explain the same thing. and yet they are incompatible. i got to go meet with a client now. will continue later...

Saiph · Mar 3, 2006

As I said, the galaxy simulations work when you throw in dark matter. No alternative version of gravity (like MOND) has been able to do better. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

poei · Mar 3, 2006

I liked Saiph's restatement of the puzzle near the beginning of the thread. The best summary I've seen. I just got done reading reading Tuesday's story about the comparison between Andromeda and the Milky Way. http://www.space.com/scienceastronomy/060221_stues_dark_matter.html There were a couple of passages that were interesting. One states that the study showed that the density of dark matter in both galaxies "cannot be packed together more tightly than the equivalent of 20 hydrogen atoms per cubic centimeter of space." I'm assuming they were talking in terms of mass density. Perhaps we can find a minimum density at which normal matter is detectable across various distances and use that to determine if dark matter is just matter that is too diffuse and cold to be detectable by us. Would that sound reasonable? If this density is a constant (the article implies that it was constant in this study), perhaps we can use this to estimage its mass. I love this stuff! One other item in the article disturbed me. It says that the speed of the dark matter was determined to be 9km/s. Relative to what? Does it move independently of the galaxies? If there is gravitational interaction, shouldn't it move in harmony with the rest of the galaxy? Sometimes, space.com seems to dumb down their articles to the point of being incomprehensible.

Saiph · Mar 3, 2006

yeah, there is that. There are various observations that defy dark matter models. I'm just not familiar enough with those to talk about them...though I'm looking into it. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

Is dark matter all it's cracked up to be?

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