SPACE

[Will]

Space is wierd.

http://www.time.com/time/covers/1101010625/story.html

Well if Dark matter is invisble how do sicentist know that it is there?
Another thing I hear that there is no end to the universe but there has to if it is expanding.

[Lepton]

Dark matter:

By looking at some stars, we can get a rough idea of the size and shape of our galaxy. It's a big pancake around 40 kPc across. Now we can look at how stars are moving in different places of the galaxy.

You would expect that stars closer to the center of the galaxy would rotate fast than the stars near the outside. This is like trying to put a baseball into orbit around the earth : on the surface, you'd have to throw it REALLY hard; from the ISS, however, you could throw it into orbit much easier because there is less gravitational force acting on it.

Sadly, the stars near the edge of the Milky Way's disc orbit the center of the Milky Way just about as fast as stars near the center of the Milky Way. The only way to explain this is to assume that the galaxy must have a lot more mass than we can see, and that it's spread out pretty evenly.

The idea might seem rather odd, but it's standard fodder for modern astronomers, even though no one is entirely sure what Dark Matter really is. Leading suggestions include WIMPs and MACHOs. WIMPS are Weakly Interacting Massive Particles, in other words, something heavier than a proton that doesn't react with normal matter very often. MACHOs are MAssive Compact Halo Objects, and are things like Brown Dwarfs - a cross between Jupiter and a star that doesn't have quite enough matter to start nuclear fusion.

Even more intriguing than Dark Matter is Dark Energy. Dark Energy is the matter (remember that matter is "frozen energy" according to E=mc^2) that the universe seems to have to be expanding at the rate that it appears to be. I really don't have the background to discuss it, though.

[Lepton]

Only in an American rag...

[MatthewV]

How do we "see" dark matter? (I don't believe this was answered yet) Density scans. Some nebulae are found to be extermly dense and not as large/spread out as they should logically be. It is believed that there is a concentration of dark matter in these areas that are a large gravitational attraction to the other gasses.

Of course, my understanding of dark matter comes from my earth science final exactly one year ago.

[Lepton]

Dude, check out reply #1.

What is a "density scan"? How is it performed? How is interstellar dust accounted for?

I suppose you might be able to find relative densities by comparing line widths from spectroscopy, but that's not very precise and I don't think it'd work very well on an extended variable-composition object like a nebula...

[Vinny]

Another theory (Einstein's) is that space is curve, and that if you were able to travel fast and long enough (which you can't), but if were able to, then you would eventually end up where you started. Why? Because space is not "flat" in the 3rd dimnsion. Just as if you bring a 2D organism and put it on earth, then have it travels in a straight line. The 2D organism would eventually end up where it started from, and won't have the foggiest why this is so. So it is possible for the Universe to be finite, yet boundless.

[The Ktulu]

I'm thinking, and have thought about before, about Will's question.

[Death Mage]

You can't see wind. So it doesn't exist, right?

Why are you still here, Will?

[Lepton]

Dark Matter, explained

[Qball]

SPACE .... something this thread needs more of.

[Samadhi]

An excellent resource:

Be sure to check out Reply 8 and Reply 21 regarding the subject of Dark Matter.

[What if...]

Some people just say that our universe is the boundary of what we can see or affect, so by getting closer to the "edge," we can extend our universe by being able to see farther.

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O.K., so how did I screw up this time?

[Samadhi]

But you would have given up an equal amount for what you gained.

[Lepton]

The version that I've heard is that the universe is everything that we could *possibly* detect.

[Samadhi]

So the universe's radius is around 13 billion lightyears? And we're at the center?

[This message has been edited by Samadhi (edited 06-12-2003 05:53 PM).]

[Lepton]

I didn't say that I agreed with it. I'm not an Arts major. although I feel like I'm getting an arts education at SMU...
From a positivistic viewpoint, this definition of the universe is acceptable (with a bit of quantum fuzz, I guess). I mean, we can never know what lies outside of 13 billion light years, so why should we care about it? For practical purposes, I guess that it makes sense.

[This message has been edited by Lepton (edited 06-12-2003 06:13 PM).]

[Samadhi]

Yes. And one can plainly see that the world is flat.

[Lepton]

Go watch a sailing ship leave port, dude. Or make yourself a Foucalt pendula, although that's not convincing

[This message has been edited by Lepton (edited 06-12-2003 07:16 PM).]

[rOver]

That definition has a problem. It only works with certain models. For example if the universe would contract in he future, or even expand with a slowing rate, we would be able to observe objects which are presently outside our field of view.

It also relies heavily on the assumption that information can not travel faster than light.

[Samadhi]

Lepton, that's just surface tension in action. Sheesh, don't you know physics?

[Lepton]

r0ver, you have a very good point. I'm not as versed on positivism as I should be, but I don't think that it cares much about the future... (not sure)

Samadhi, as usual, I don't have a fricking clue what you're talking about. In this case, I think that it's because I don't know physics.

[Beartalon]

What Samadhi is saying is that surface tension breaks out into much deeper tension among the ship's crew resulting in mutiny, fighting and all-out war. It's an incredible coincidence that for every ship leaving port, the crew beats/burns/batters the ship until it sinks just as it reaches the horizon as seen from the perspective of a shorebound observer.

[Samadhi]

Yup. That was the theory put forward by the great Physicologist B. F. Newton.

[Beartalon]

*smiles and shuts up*

[Will]

i'm still heat i never left i just lurk here and there. What is the Omega in Scientific Means? I know what it is in the Bible.

Now universes are like Atoms what if there are alot of parelle universes and it forms a molecule. Could we be like atoms? Is that what Bohr suggested that atoms represented? At least i thik it was Bohr.

[Lost in Space]

I don't know if it is just me or not... but I had to read the last post more then once. For some reason I just don't understand most of the post made by Will. Either it is I don't understand or just don't care.

[Samadhi]

I think he's discovered herbs.

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A One that isn't cold, is scarcely a One at all.

[Herbs]

Oh, I would hope not! <runs kicking and screaming away from Will>

[Lucky Wizard]

One of the things about the universe is that whether it stops expanding and starts collapsing or it expands forever is dependent on its density (how much matter there is in a unit of volume). There is a special density that will cause the universe's expansion to slowly stop expanding and (eventually) reach a certain size and just stay there. This is called the critical density.

The universe will expand forever if its density is less than the critical density. The universe will eventually stop expanding and collapse into a Big Crunch if the density is greater than the critical density.

Omega is used to refer to the ratio of the actual density to the critical density.

[Lepton]

I'm working with a guy who is looking at the star Sigma Orion E. It appears to have two enormous wings of ions. These wings are much bigger than the star, and they rotate in a fixed fashion because Sigma Orion E has such an intense magnetic field that gravity plays almost no role - this over a system the size of maybe Mercury's orbit.my own research is too boring to mention

With a critical density universe, the universe will keep expanding, slowing down, but never quite reach a limiting size. Is this correct?

A bubbly multiverse is possible, but we could never know whether such an idea is true. The idea being that once we look outside of our "universe", whatever we see becomes part of the universe too.

I'm pretty sure that Neils Bohr didn't do too much cosmology, being a particle physicist in an era before cosmology even existed. I can't think of anyone who has suggested that we are like atoms, sorry.

[This message has been edited by Lepton (edited 06-13-2003 01:26 PM).]

[pikachamp]

don't fall into the warp!