Paradise Lost

After doing a search - both here and on google and turning up basically nothing,
I was curious to hear the advice of others on this subject.

I have a basic understanding of the supposed matter but am entirely unclear
as to, among other things, its nature, purpose, etc., and was wondering if
any of yous could post anything relevant to the topic. I'm sure we can get
some decent arguments or misconceptions cleared up from other people with
the knowledge.

-Thanks

PS: Any handy links would also be a big help.

PPS: The reason I'm bringing it up is because I saw the stuff on Futurama
tonight and no one in the room had any sort of idea but me. Am always
interested in learning new things.

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Phage

From what I understand Dark Matter is merely matter that is not glowing; just matter that we cannot see by its electromagnetic radiation (yeah, thats straight from Wiki). Its nature would be... stuff. Anything that is cool and dark would fit. The purpose of Dark Matter? 42. Seriously, what purpose does regular matter have?

shakran

that's one theory. . there's about a jillion of them actually. Here's the basic scoop, extremely condensed (although still freaking long) and with a lot of details left out.

Things move too fast. A good example is to look at galaxies. They rotate. One of Kepler's laws says that the orbital speed of an object relates to the radius of the orbit and the mass of the object in orbit. In other words, the earth's speed is determined by its mass and its distance from the sun - this makes sense when you remember that an orbit is just a perpetual fall. The earth is always falling toward the sun, but it's forward velocity is high enough to counteract the pull from the sun. Any faster and we'd fly away from the sun, any slower and we'd get up close and personal with the sun. And if we went the same speed but were closer to the sun where the gravity has more effect, we'd go toward the sun, AND if we were farther away from the sun than we are and kept the same speed there would be less gravity acting on us and we'd fly away from the sun. If the earth were any more or less massive that too would effect our orbit.


The trouble starts when you look at those galaxies. The outer rim of galaxies has less mass than the center (most of the matter in a galaxy is clustered in the center) so things on the outer rim should be orbiting the galactic center much slower than things near the center since they not only have less mass, but also are farther away and so feel less gravitational pull. But they don't. Instead, stuff on the outer rim rotates much faster than it should. Since the distance from the center to any other point in a galaxy is absolutely known for sure (we've measured them right) we must have the wrong idea about how much mass they contain. But we can't see any more matter that would account for the mass that they must have in order to rotate that fast, so scientists proposed the theory of dark matter, or matter that we can't observe. It's not just matter that doesn't reflect any light, it's matter that doesn't reflect ANYTHING (that we know of anyway.)

There's all sorts of weird shit this dark matter could be. One thing is the neutrino. Now, we know neutrinos exist, but we may be wrong about their nature. Right now the going theory is that they have NO mass at all, but some are starting to think they might have a very small mass. We're talkin' miniscule here, but neutrinos are incredibly numerous throughout the galaxy (hundreds of thousands go howling through your body every second at the speed of light, and by the time you finish this sentence, they'll be exiting the other side of the earth), so a boatload of small-mass particles adds up to a lot of extra matter, IF they have mass. (if you're interested in how we know these things exist when they have no measurable mass and therefore shouldn't be visible, check http://amanda.physics.wisc.edu/ which is a site about a fascinating neutrino detector buried deep under Antartica. The short answer, though, is that every GREAT once in awhile a neutrino will manage to hit a particle of regular matter, and when it does, a little light is given off. This detector has managed to see the trails of light caused by neutrino - matter collisions)



The really interesting theory that ranks about a 9.5 on the weird shit-o-meter is the WIMP, or Weakly Interacting Massive Particle. This is totally theoretical - we have no evidence they exist, but they would explain the extra mass in the universe, especially if the neutrino proved to be massless. Basically these things would be freaking heavy particles that barely interact with other matter. In other words, you couldn't see touch or smell them, and you could pass right through them and never know it, but their mass would still contribute to the mass of the universe and therefore account for the observed mass vs. observed effects of mass paradox.

Of course, another possibility is that all that stuff is wrong and that we just don't have a good grasp on how gravity really works. If that's the case, Kepler's laws aren't accurate, at least not on a galactic scale, so the galaxies could be rotating at a perfectly logical speed without needing any more mass than what we have already observed.

d*d

The existence of dark matter determines (within current theory) wether our universe is open (will expand forever) or closed (will eventually stop expanding and start condensing).

vinaur

How does mass affect the orbit??? Aren't gravity and centrifugal force the only forces acting on the planet (on a scale that matters anyway)? So m_p*v^2/R=G*m_s*m_p/R^2. You can cancel out the m_p (mass of the planet), which makes the equation independent of the planets mass. v^2=G*m_s/R.

Hain

I wouldn't trust much anything science related from Wikipedia. I have found far too much there that is posted by people that don't have clue about what they were talking about. Dark matter as I have understood it is mass that is so small that it cannot reflect light.
It is the most abundant mass in the universe according to most theories. The concept of it defining whether our universe is flat (will stop expanding), open (will expand forever), or closed (will stop expanding and then crunch on itself) is somewhat pointless now since recent studies show that our universe is not just open but accelerating. If you want to have a good read The Universe in a Nutshell has a section on it about dark matter and its properties.

Basically it is just the smallest amount of mass that we can "account" for since there seems to be so much missing mass from the universe.

Some more fun dark stuff is Dark Energy that is the proposed antigravity that makes our universe expand faster and faster. It is caused from quantum fluxuations of free space.

Good luck researching.

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Phage

Surely you can see the connection between mass and gravitation? The mass of objects is crucial in how they orbit.

stingc

vinaur is correct. If one object is much less massive then the other, the precise value of its mass is irrelevant (the big mass still matters). This is actually a very important point that inspired a lot of philosophical discussion in the past. It was also one of the things that led Einstein to develop general relativity.

I think that Shakran was over-generalizing from the result about galaxy rotation. There, you don't have one central mass with a bunch of negligible masses orbiting around. It's more like a continuous distribution. In this case, the acceleration required for an object to maintain an orbit depends on all of the mass inside its radius (away from the galactic center). The distribution of matter is therefore important in that case. Even so, this an of averaged property of the galaxy. If you were to drop a star off at some point, its motion would be basically the same as a marble's.

Anyway, Wiki's description of dark matter is accurate. It is most basically defined by phage's first post. There seems to be a lot of stuff out there that we can't see that interacts gravitationally with what we can see. Shakran mentioned one of these types of observations (odd galaxy rotation curves), but there are many others. You even see evidence for dark matter in cosmological observations. There is some microwave radiation left over from the very early universe that implies the existence of a lot of matter which doesn't seem to strongly couple to electromagnetic radiation. This is inferred by figuring out the details of the sound waves present in the matter that gave off this radiation billions of years ago (!).

Anyway, nobody knows what dark matter is. There's plenty of speculation, but nothing concrete at all. It is popular to think that dark matter is largely composed of some new type of fundamental particle, although there is little evidence of this.

"Dark energy" is actually just a free parameter in the equations that everyone thought would be zero. Since the universe seems to be accelerating, the parameter is not zero. The interpretation of this as quantum fluctuations is a guess right now. In fact it's worse than that. Unless something has changed very recently, the expectation for the amount of dark energy based off this idea leads to perhaps the largest numerical discrepancy in the history of science. It's 120 orders of magnitude too large (or exactly zero with slightly different assumptions). Some people naturally consider this a major problem, and have been working very hard to figure out if it can be fixed.

Some others are content with the original reason that a dark energy parameter was introduced. Einstein's equations (of gravity) are technically ambiguous up to a constant, which was originally called the cosmological constant. In any case, it is very natural to include, and may simply be accepted as a fundamental constant needed to properly describe gravity. There are those who say that it would be a shame if the constant could not be understood in some more fundamental way, which leads back to the previous interpretation. That interpretation also led to the name "cosmological constant" turning into "dark energy."

There are other models as well, and the experimental results aren't good enough yet to say anything definite. For example, it is still possible that the conservative cosmological constant interpretation can be ruled out (if it is changing in time).

Phage

Could you elaborate on this point? It sounds to me like you just stated certain conditions based on matter distribution and from those conditions decided that you did not need to know those conditions. It is like saying "If A is greater than B, or if B is greater than A, then you do not need to know their value."

My understanding of gravity is this: If you have two masses they will orbit around each other. If one of the masses is larger, the smaller object will orbit it much more than the larger object orbits the smaller (but it still does a little bit). Planets can be detected around distant stars by watching for this "wobble".

What is a precise value? Is it within 1 trillion earth masses or 1/2? When you accept a loss of precision in your mass measurement you also accept a loss of precision in your orbital calculations. You can ignore some data on the grounds that it will not make a large impact on your calculations, but too many people fail to understand that the universe didn't forget it was there.

ARTelevision

The Current (March) issue of Sky and Telescope has a feature story on Dark Energy. In that article, Dark Matter is fully discussed.

My own view of the dark matter hypothesis/observations is that the existence of dark matter/dark energy renders all of our current "theories" about the material universe moot. We're talking about up to 95% of the "known" universe here - when dark matter and dark energy are included. Our current cosmologies are woefully inadequate in relation to what is evidently out there...

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stingc

You're right. I was talking about an approximation (or a limit if you want to think of it that way). For example, engineers don't have to consider the mass of a satellite when figuring out what speed is needed to make it orbit the earth (of course the mass does matter when figuring out how to get the satellite up there at the proper speed). The effect of the satellite's mass on the earth is completely negligible.

The mass of the earth is about 1/300,000 that of the sun, so the wobble that it induces in the sun is exceedingly small. The orbit of the earth would not change significantly if its mass were half or double its real value.

More formally, if a given distribution of matter is somehow fixed in place, the motions of extra particles that interact only through gravity will be independent of their mass (in the limit that such particles have zero radius). This sounds like a ridiculously idealized statement, but most astrophysical situations satisfy it to a very high degree of approximation.

teflonian

Put simply Dark Matter is a term used to define the matter that has to be out there to explain the attraction of galaxies in our universe. We can only directly observe matter that emits/reflects/refracts/ or absorbs electromagnetic radiation (visible light, radio waves, x-rays, etc.). However, we can infer there are particles or even massive objects out there by its gravitational attraction on other things in the universe. The candidates for dark matters basically fall into two categories, WIMPS and MACHOS. WIMPS being Weakly Interacting Massive ParticleS meaning small very massive particles that somehow don't emit electromagnetic radiation. MACHOS being Massive Astrophysical Compact Halo Objects, meaning large dark bodies.

To say that that our "current cosmologies are woefully inadequate" and to put "theories" in quotation marks shows very little respect for what has been determined and discovered, and to further say they are moot is naive. The universe is a large and awesome place and serious astronomy and our so called "theories" continue to explain what is out there. Yes, there are some huge questions to be answered, including the question of dark matter, but there have been some huge questions answered. We have come so far in the last century alone and I am excited to see what is yet to come. Not too long ago, we thought we were the center of the universe and there were about seven other astronomical bodies moving around us (the stars were holes in something or other). Now we know the universe is filled with exotic and previously unfathomed things such as quasars, black holes, neutron stars, pulsars, red dwarfs, white dwarfs, other galaxies, ...

stingc

It is certainly possible that fundamental physics is failing, but I think it is important to note that most things in astrophysics which are claimed to be predicted rigorously really aren't. There are a LOT of shaky assumptions involved even in interpreting the data. Also, there are no dark matter or dark energy observations which directly challenge any fundamental concepts. In any case, it will certainly be very interesting to see whatever becomes of these problems.

ARTelevision

Yes. Thanks for your comments.
Mine are neither disrespectful nor naive.
I spent my college years employed as an astronomical/astrophysical research assistant to a group of research and theoretical physicists. I have my own opinions based on education, research, and study.

My views are my own.
A disagreement is just that.

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teflonian

I very much appreciate and respect your opinions ARTelevison based on all your other posts. However, I can't imagine why anybody who has knowledge of the astronomy and astrophysical worlds would undermine some very amazing advancements in our view of what really is out there. I can understand if you are merely stating that we have a long way to go, but I have a serious concern with flippantly labeling what we do have as "theories". It almost sounds to me as if you were making the argument that what science has established is some sort of faith based on a few rough conjectures. Bit by bit science fills in the details and corrects past mistakes.

If I may ask, what are your opinions of the nature of what is out there if our current "theories" hold so little sway with you?

wolf

Isn't Dark Matter what starts a Black hole? I forget so correct me if I am wrong.

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stingc

No. They're unrelated.

Yakk

Nope wolf.

There are a number of ways a Black Hole can be created, theoretically. A large star going supernova can create a wave of pressure high enough to compress the core into a black hole. Black holes may have been formed during the universes birth. Possibly stellar impacts could also form Black holes.

Dark Matter is just a shorthand for "matter we can't see". There is circumstantial evidence for it's existance (the universe 'seems to behave' as if it where much heavier than it "looks like" in the E-M spectrum, on scales from galactic on up), but we can't seem to account for most of it.

Many alternative explanations exist. A friend of mine who knows more physics than I do is of the position that a modified gravity (that acts differently -- decays slower -- at long distances) is more plausible than dark matter.

edit: replaced is with "looks like" in the E-M spectrum

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Last edited by JHVH : 10-29-4004 BC at 09:00 PM. Reason: Time for a rest.

shakran

Pretty much. I was trying to avoid writing a book on the subject since much smarter people who are much better at explaining this stuff have already written plenty of them

C4 Diesel

Saw this in the news and thought some of you might find it interesting...

Astronomers spot invisible galaxy!

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Shpoop

Whoa...of course what we have are theories. Science is correcting past mistakes, yes, but those past mistakes were made by science too. And who is to say that these discorveries (while I'm not undermining them) will not be exposed as mistakes and corrected in the future? It doesn't seem like ART should be faulted for being skeptical, but rather that you are placing blind faith in science. Without skepticism or people willing to rethink ideas, science wouldn't exist in the first place.

But maybe I misinterpreted the whole argument.

teflonian

Shpoop, I am not faulting ART for being skeptical. Of course science has made, or rather led to, mistakes. That is the beauty of science, it is self correcting. If something proves to be faulty it is modified or replaced with something that is less faulty. I am a huge proponent of skepticism actually. My contention with ART was that he was trivializing what theories we do have because of their short comings in certain areas. Dark matter is one mystery that science hasn't yet illuminated. The point though, is that we would have no idea what-so-ever that this "dark matter" exists without the current theories.

Paradise Lost

Thanks for all the starter info guys, appreciate it.
(Minus the whole side-rant you people seem to have with Art, who does offer
a rather good point. :P )

PS: C4 Diesel, saw that earlier today on Physics Web, weird stuff.

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ARTelevision

In any event, I do suggest anyone with an interest in the most current info on this topic make the effort to pick up the March issue of Sky and Telescope. It is well worth the cover price.

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Shpoop

So, just curious here, what do you see? If light doesn't reflect off it, it isn't visible, but you cannot see through it, right? So then its just a big black blob, right?

shakran

You're talking about dark matter still I assume?

Well for example with neutrinos, you don't see them. They are detected by sensors buried deep in the ice of Antartica. On the VERY rare occasion that a neutrino collides into matter, it gives off a tiny flash of light (yes all you physics folks out there, I know about muons, I'm simplifying ). Those sensors are looking for the light flashes. They're buried very deep in the ice because cosmic rays will produce the same light flashes, so the sensors have to be stuck somewhere where it can see deep into the earth where cosmic rays don't reach. It's called the AMANDA project (Advanced Muon And Neutrino Detector Array) if you're interested in learning more, there's a pretty good brocure here:

http://www.amanda.uci.edu/public_info.html

genuinegirly

I have been curious about this as well. Thanks for letting us know!

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Yakk

If it was black, it would be soaking up E-M radiation.

There is the 'dim body' theory to explain dark matter. Most of the theories I've heard actually result in matter that interacts very weakly electro magnetically.

This changes how it behaves a whole bunch. No radiating heat, cooling down, and collapsing into a star, then shining brightly -- instead, you get matter that forms huge clouds only held together by gravity, and the particles in it moving in huge lopsided orbits around and through the cloud (for example). The particles wouldn't even collide with each other (bright matter's collision is mostly E-M based: your electrons push off against each other due to their electric charge).

This allows there to be alot more of it in an area without it doing bright, and apparently absent, things like collapsing into stars or showing up by occulding light coming from behind it.

If that dark matter galaxy pans out, that is an example of how you would see 'truely dark' matter. E-M wise, you can only easily detect it by how it tugs 'bright' matter around (in this case, hydrogen gas).

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Last edited by JHVH : 10-29-4004 BC at 09:00 PM. Reason: Time for a rest.

kitsune

really makes me wanna take physics