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The Dark Matter





nanobid
What is this actually? If it is matter, can we perceive? I have come accross several times in magazine, books, newspaper, internet, but exact thing is not clear to me. Recently, I come to know that the existance of dark matter is not proved experimentally. So lets share our knowledge in this universal thing.
Bluedoll
My contribution is that what really matters is that the WIMPS around here might count for something if you could actually see them! Look, if you want to understand why, dark matters, turn on your light. Wait that won’t work cause it is so dark you can not see anyway. That is a very cold thought is it not? Ok, back to the drawing board.

Even the axion is nothing more than a hypothetical elementary particle to attempt to explain the universal question of why does the universe have so much more matter than antimatter. (this is all I can really say about anti as I have been restricted from talking about her). I believe you are right when you say this question is yet to be answered or proof’s provided by experimentation and there are certainly a lot of ‘ifs’ in theories.

Defined however dark matter can not be seen but felt. An example would be atoms and sub atomic particles or in other words YOU (your other u).
kelseymh
Bluedoll wrote:
My contribution [...]


To answer the original poster's question, it is best to start slightly historically. When we look at nearby galaxies and clusters of galaxies, we can use our telescopes to measure how bright (luminous) they are, and from that get a fairly good estimate of how many stars they have, and consequently how much mass they contain. We can also use spectrometers to measure how fast they are rotating around their centers. This has been done many times since early in the 20th Century. Using Newton's law of gravity and Kepler's laws of orbital motion, you can connect the two, relating the galaxy (or cluster) mass to the rotation speed as a function of distance from the center.

If you do that (as Fred Zwicky did first in the 1930s), you discover something surprising and unexpected. Galaxies, and clusters, and even superclusters (clusters of clusters) are all rotating too fast! That is, if the only mass (gravity) holding them together was what you could see in telescopes (stars plus gas and dust clouds which absorb light), then they would all fly apart. We would not see all of the intricate structure we actually observe, just random stars flying off through space in all directions.

So what's wrong? Were Newton and Kepler completely wrong (not just a little bit wrong, but wrong by factors of 10 or 100)? That's rather unlikely, given that we do see gravity working correctly both in our solar system and in lots of other star systems (binaries, multiples, planetary systems, etc.).

So if gravity works the way we think, then there must be more gravity holding those objects (galaxies, clusters, superclusters) together, more than just what we compute from visible light. Well, gravity is generated by matter, and whatever that matter is, it must be dark, since we don't see it with light. Hence the label "dark matter." That's just a description. We don't know what it's made of, or how it does interact, except that it has mass and produces a gravitational effect.

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the WIMPS around here might count for something if you could actually see them!


What do you mean by "see them"? The experiment I am currently working on (CDMS) uses large, pure crystals of germanium, cooled to 40 mK, to detect particles of dark matter which might occasionally hit an atom, knocking out of place in the crystal. So far, our non-detection of anything other than background (neutrons from radioactivity) allows us to set limits on both the density of dark matter in this part of the Galaxy, and on the probability of such dark-matter-nucleus collisions.

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Even the axion is nothing more than a hypothetical elementary particle to attempt to explain the universal question of why does the universe have so much more matter than antimatter.


Correct. The axion is hypothesized to explain the strong CP paradox -- the strong interaction appears to perfectly conserve the combination of charge-conjugation and parity inversion, or equivalently the interactions of particles vs. antiparticles. Since CP is violated in weak interactions, there's no reason a priori for it to be conserved in QCD. The axion is hypothesized as an extra interaction, with a CP-violating phase which (because of symmetry considerations) exactly cancels any CP-violation coming from gluons.

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I believe you are right when you say this question is yet to be answered or proof’s provided by experimentation and there are certainly a lot of ‘ifs’ in theories.


Yes, indeed! We know, from astronomical facts (above, as well as gravitational lensing and the baryon content of the universe as determined from the CMB), that something must be there which is not normal matter. But we have no experimental data on what that something is, and far too many theorists throwing out guesses Smile
_AVG_
I'd like to comment on the so called "failure" of experiments to prove Dark Matter's existence.

Firstly, most modern physics is getting more metaphysical day by day (as there seem to be no possible comprehensive experiments that can be designed to test modern hypotheses). String theory, dark matter, parallel universes, wormholes, etc. are, at the moment, largely imagination based (as their existence cannot be proved or disproved).

Secondly, I don't really see any point in finding something if it doesn't interact with us that strongly ... the big question is, does Dark Matter's existence really affect our existence? Instead of allocating funds to such research, I'd rather explore something else, which actually matters.

Just my personal opinion, no offense to anybody/anything.
Bikerman
The thing is that Dark Enegy would have been in that category until recently, so if we don't look we don't find. The evidence for Dark Enegy is looking pretty strong, so give it time for the others.
www.bbc.co.uk/news/science-environment-13462926
Bluedoll
kelseymh wrote:
So far, our non-detection of anything other than background (neutrons from radioactivity) allows us to set limits on both the density of dark matter in this part of the Galaxy, and on the probability of such dark-matter-nucleus collisions.
I am somehow not convinced that whatever is presenting itself as gravity exists in large quantities only in other parts of the Galaxy.

For some reason I think it must be all around us a deduction that is only based on a universal acceptance of some kind of standard structure or makeup in everything material. I think this as we do look from a distance at the galaxies and we see so many similar characteristics in everything existing, I just tend to follow that presumption and consider that the microcosmic world must be similarly structured like anywhere else in the galaxy.

Though, I would not dismiss the possibility that a condition we describe as a black hole could not exist by some strange set of events, that theory does not really give me all I need to know about why anything other than mass should be holding things together as in cluster of stars or particles. Some theories, just do not seem adequate and maybe there is another explanation other than dark matter?
nanobid wrote:
If it is matter, can we perceive?
Perhaps the answer is (this being a non-scientific postulation) that the dark matter is not matter at all but a distinctive force that exists in the universe that counter acts everything that is going on. A simple law, with every action a reaction kind of association might seem applicable. Seems to me everything else is acting in a balance type of way, so why not the Galaxies or the very atoms that make our very own little bodies. Why don’t we fly apart? Another example might be electrons with potential forces other electrons to collect and fill in holes, yet the negative potential that attracts this action we do not call dark electrons?

The question in my mind remains however if gravity is a bi-product of matter then what force is actually holding things together like kelseymh explained and as when Newton and Kepler observed it in our wonderfully revolving systems. Well, gravity seems to be an elusive thing, we do know it is there and can obviously observe its effects but invisible to electromagnetic detection, is that not correct? Is it possible that our gravity possum is based on some subatomic level and distributed evenly throughout the universe but only shows up when there are great imbalances in mass distribution? I mean something like a... I do not know, you tell me?

nanobid wrote:
Secondly, I don't really see any point in finding something if it doesn't interact with us that strongly ... the big question is, does Dark Matter's existence really affect our existence? Instead of allocating funds to such research, I'd rather explore something else, which actually matters.
It may be possible that any concrete knowledge that comes out of our research could be used in a practical application in the future, who knows, alternate fuel sources, or maybe even a dark matter money making machine – that might work.

www.bbc.co.uk/news/science-environment-13462926

Wow 74% of all around us is dark - talk about the majority!
Bikerman
Bluedoll wrote:
kelseymh wrote:
So far, our non-detection of anything other than background (neutrons from radioactivity) allows us to set limits on both the density of dark matter in this part of the Galaxy, and on the probability of such dark-matter-nucleus collisions.
I am somehow not convinced that whatever is presenting itself as gravity exists in large quantities only in other parts of the Galaxy.
Huh? Gravity exists everywhere there is mass. Dark Matter has mass (by definition). Thats the whole point, really. If gravity didn't exist in other parts of our galaxy then I'm pretty sure we'd be able to tell - for one thing there wouldn't be any galaxy...I think you might have meant universe, not galaxy, because gravity is working throughout the galaxy.
If you are suggesting that the gravitational constant might change in deep space, then that is one way to tackle the problem - eliminate the anomoly by tweaking G - and no doubt some scientist somewhere has proposed it....
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For some reason I think it must be all around us a deduction that is only based on a universal acceptance of some kind of standard structure or makeup in everything material. I think this as we do look from a distance at the galaxies and we see so many similar characteristics in everything existing, I just tend to follow that presumption and consider that the microcosmic world must be similarly structured like anywhere else in the galaxy.
I don't really understand this I'm afraid.
Quote:
Though, I would not dismiss the possibility that a condition we describe as a black hole could not exist by some strange set of events, that theory does not really give me all I need to know about why anything other than mass should be holding things together as in cluster of stars or particles.
Err, what theory. Please tell me what theory this is and what it doesn't give you.
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Some theories, just do not seem adequate and maybe there is another explanation other than dark matter?
Which theory do you mean? General Relativity? Tell me, how is it inadequate? Do you actually know anything about the theory?
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nanobid wrote:
If it is matter, can we perceive?
Perhaps the answer is (this being a non-scientific postulation) that the dark matter is not matter at all but a distinctive force that exists in the universe that counter acts everything that is going on. A simple law, with every action a reaction kind of association might seem applicable. Seems to me everything else is acting in a balance type of way, so why not the Galaxies or the very atoms that make our very own little bodies. Why don’t we fly apart? Electron potential forces electrons to collect and fill in holes, yet the negative potential that attracts this action we do not call dark electrons?
Seem to me that you don't really know what you are talking about. The Galaxies are not in 'balance'. They are clumped together and spread unevenly in space. What negative potential and what does 'attract this action' mean? What action? Electrons filling in holes? Whilst we are about it, what is electron potential? Potential difference (PD)? Potential energy? Mass?
Rather than flying apart, the better question would be why we don't implode, given that negatively charged electrons would be attracted to a positively charged nucleus. That leads to the notion of quanta and pretty soon into quantum physics, which explains it all, if you are prepared to ditch common sense and stick to actual results.
This might help
http://www.visionlearning.com/library/module_viewer.php?mid=50
http://www.saburchill.com/chemistry/visual/tutorial/001.html
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The question in my mind remains however if gravity is a bi-product of matter then what force is actually holding things together like kelseymh explained
'Things' are held togther by electrostatic forces - but Mike can tell you much more about that than I can. Gravity is tiny so it only really becomes important on much bigger scales than the atom.
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and as when Newton and Kepler observed it in our wonderfully revolving systems. Well, gravity seems to be an elusive thing, we do know it is there and can obviously observe its effects but invisible to electromagnetic detection, is that not correct?
Well, yes, it would be, since it isn't electromagnetism.
Quote:
Is it possible that our gravity possum is based on some subatomic level and distributed evenly throughout the universe but only shows up when there are great imbalances in mass distribution? I mean something like a... I do not know, you tell me?
Well, that is sort of what happens, but if it were distributed throught the universe then we wouldn't have expansion methinks, (dark energy is sort of anti-gravity)...the graviton is the sub-atomic particle proposed for gravity. But again, this is Mike's area much more than mine.
Bluedoll
Sorry for taking up so much space here – pun
but seriously will try my best to express my silly thought.

“Rutherford fired tiny alpha particles at solid objects such as gold foil. He found most of the alpha particles passed right through the gold foil”

Which tells us that most of what we are, is nothingness, like swiss cheese but even more holier.

“Rutherford's atom resembled a tiny solar system with the positively charged nucleus always at the center and the electrons revolving around the nucleus.”

I am saying that small amounts of matter (like us) that do have electromagnetic characteristics will behave exactly like all other matter (galaxies, clusters, superclusters) but of course larger masses win out when it comes to observable visual presentations for an obvious reason. They are much bigger with larger gravity fields. However perhaps, forces must be in union in characteristics all over the universe or there would not be any consistency. (a logical presumption)

We know that gravitational force is a result of mass and acceleration (what a powerful equalizer) Newton’s F=ma and that Einstein’s equivalence principle also applies in an ever-changing universe. Whether string theories are accurate explanations or not - I am suggesting that perhaps, with a wild guess, that possibly rather than considering dark matter to be actual particles, that dark matter might be reactionary forces in play and exist purely because of the interaction of bodies on each other - the reason why the cosmos behave as it does.

I suspect the entire universe is actually pulsing, expanding and then regressing in a regular manner like every other cycle we discover. Perhaps by looking at crystals of germanium we will find answers of just how these forces at work behave and understand more about the bigger picture as well. A most fascinating adventure and one that will never be complete which is what makes it so grandiose.
nanobid
Well, all discussion stands on theoretical basis. The existance of dark matter is still not experimetally varified. So fas as I know, in LHC experiment in CERN, one of the main objectives is to support the dark matter (energy) idea.
Now a new question come into my mind that whether the total matter including the visible mass is conseved or not in the continuously expanding universe?
Bikerman
Yes, matter is conserved as far as we know. If it was not, this would be a major challenge to existing physics.
nanobid
someone said in one of the posts that it is some kind of reactionary forces. Can we quantify the amount of dark matter (or energy/ forces) in a finite region of space applying any universal law.
Bikerman
Yes. More specifically we can calculate the expansion of the universe and subtract the effects of gravity. This is not an easy 'sum' and there are many complicating factors, but in principle it is measurable.
Bluedoll
Can anyone explain what this paper means in simple terms?
http://www.orbitsexplained.com/files/22_G.pdf
Bikerman
It is simply a proof of Newton's equation for gravity using a-priori methods rather than empirical observations (ie using reasining rather than experiment).
menino
Isn't there a search being conducted by Nasa or some space agency to locate dark matter... or actually anti-matter (for that matter), in space?
kelseymh
menino wrote:
Isn't there a search being conducted by Nasa or some space agency to locate dark matter... or actually anti-matter (for that matter), in space?


Those are two different things.

The AMS-02 particle spectrometer, which was installed on the International Space Station last year, is designed to look for antibaryons and antinuclei in the primary cosmic rays. Antibaryons are made of three antiquarks (for example, the antiproton is an anti-down plus a pair of anti-up quarks). Antiprotons are made routinely at Fermilab for use in the Tevatron particle collider. Antinuclei (including anti-He-4!) are made rarely in collisions of lead ions at both RHIC at Brookhaven National Lab and the LHC at CERN.

Dark matter is something unknown. It is observed cosmologically through its effect on the orbits of stars in galaxies, on galaxies in clusters, and through gravitational lensing. The actual "substance" of dark matter has not been observed, but there are many theories about what it might be and how it might interact in nature.

Cosmologically, we expect that in regions with a very high density of dark matter, some of whatever it is might annihilate to photons (a process analogous to matter-antimatter annihilation). Satellite experiments have looked for the signature of an excess of photons coming from, for example, the center of our Galaxy, but nothing conclusive has been seen yet.

Experimentally, if dark matter really is "all around us," then the Earth is passing through that halo of dark matter as it orbits the Sun. A very sensitive particle detector (for example, cyrogenicly cooled perfect germanium crystals) can look for the occasional dark matter particle banging into a nucleus. Again, nothing conclusive has been seen yet.
menino
Thanks kelseymh, for the info. I've understood some of it, but not all of it, but thanks anyways.
Yeah, I guess there will be satellites looking up for this info, as the NASA program has been downed by the US government on account of the financial debt crisis going around, and I guess the US has to cut costs somewhere.
Still science should not stop on account of this.
I think that with anti-matter, and/or some of the anti particles you mentioned below, it would be very viable to get "free" energy and on-the-go without the need for fossil fuels.
kelseymh
menino wrote:
I think that with anti-matter, and/or some of the anti particles you mentioned below, it would be very viable to get "free" energy and on-the-go without the need for fossil fuels.


Not at all Smile Since antimatter does not exist in quantity on earth (or in fact, anywhere in the nearby Universe!), we have to make it in order to use it for anything. The "efficiency" of antimatter production is very close to zero.

I am a professional particle phycisist, who uses antiparticles on a daily basis as part of my research. Here's how we make them.

We use particle accelerators which use hundreds of megawatts of power to produce beams of electrons (or protons), which we slam into thick metal targets. For every billion or so beam particles, we get a few thousand antiparticles produced. Then we have to use more megawatts of power to magnetically collect and store those antiparticles.

When they finally do annihilate, the visible energy from those interactions might possibly correspond to a few watts or so.
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