Nature of Dark Matter

jackmell

Ok Chalnoth. I just don't understand the science and probably should have just stayed out of this. Unfortunately, I like Astronomy and it's difficult for me to not engage even at the risk of getting into trouble. But also I've become skeptical because the science is so deceptive as history has shown us.

Would be nice to have a better handle on what precisely dark matter consists of.

Drakkith

What do you mean by "deceptive"? New discoveries happen all the time, but very rarely is something completely wrong or inaccurate to the point that we simply can't use it anymore.

Absolutely. It's an exciting time in astronomy thanks to mysteries like this!
On a side note, I just visited Kitt Peak National Observatory last night. They showed us the telescope where evidence for dark matter was first gathered by Vera Rubin.

jackmell

I got a big yard. When I stand in the middle, and look all about, it certainly looks flat to me.

Also, they really do look like they're going backwards and forwards across the sky not to mention the big yellow one looks like it's moving across the entire sky each day from east to west. Andromeda looks like a difusse cloud until you have a nice telescope to resolve the detail and then realize the Milky Way is not the entire Universe. I seriously doubt we've reached the limits of that deception:)

Nice! I would like to visit Palomar one day.

Chalnoth

This isn't science being deceptive. This is our own senses. Science is the tool we use to see through the deception of our senses.

Drakkith

Ah ok, you don't mean deceptive, as that implies that someone has knowingly and willingly mislead you. Perhaps we could say that the "truth" is hidden, that we need to come up with various methods of finding it and figuring out what is true and what isn't.

twofish-quant

The wikipedia article is very badly worded, and in a few hours, it will be reworded after I change it.

What the paragraph that you quoted is trying to say is that

1) the most likely candidate for dark matter is a weak-interacting massive particle
2) there are other particle alternative candidates for dark matter that are under consideration (sterile neutrinos, axions, shadow gravity, etc.)

So the possible explanations for the differences in observations are

I. Dark matter
Ia. Weak interacting massive particles
Ib. Something other than WIMP's
II. Modified gravity
III. None of the above

Right now the evidence between I and II is overwhelming to I. Now the wikipedia paragraph was trying to point out that there is still a lot of argument between Ia and Ib, and NOT between I and II or III

So I'll change the paragraph to reflect this.

twofish-quant

One problem here is that you won't find anything unless you look for it, and you want to get into a situation where "finding nothing" means "finding something".

What people are doing is asking the question "if dark matter exists, what are its properties." At that point you go up to various people with money (i.e. DOE and the NSF) and say "we want to build this device that looks for dark matter with properties X, Y, and Z."

http://www.particle.kth.se/5A5461/DarkMatter.pdf
http://www1.jinr.ru/Pepan/v-42-4/13_rau.pdf

The thing about spending several tens of millions of dollars looking for dark matter is that you have to assume that it exists in order to show that it doesn't. You assume that dark matter exists,
and then you build machines to look for it. If those machines turn up empty, then you end up with an interesting mystery and go back and look at what we got wrong. We are already at the point where we can rule out certain types of dark matter, and the list of hundreds of candidates is now down to about six or seven. If we get ourselves to one candidate, that would be cool. If we get ourselves down to *zero* candidates, then that would be even weirder.

If you come up with a much more convincing explanation of galaxy rotation than dark matter then there is no real point in looking for it on earth. These experiments are expensive, so if we are pretty sure that dark matter isn't there, there is no point in spending tens of millions of dollars looking for it especially since we aren't sure what we are looking for.

twofish-quant

Look more closely. Also, put some numbers to it.

Whenever I teach introductory astronomy, I want to get people to *think* like scientists, and I ask questions like this.

You are in your back yard. Suddenly UFO's appear and you get knocked out. You wake up on your backyard, but there is a force field that keeps you from leaving your backyard. You are interested in knowing if you are still on earth or if you are in some holodeck in a spaceship. How do you tell?

The reason I like these sorts of questions is that I don't know the answer, and invariably someone will come up with something that I didn't think of.

Also we are in a pretty similar situation with the universe. The measurements of the universe indicate that we are in a flat universe, but it makes a big difference if it's really flat or if our measurements aren't good enough.

Sure, and science history is important because you can pinpoint the exact moment in which people realized that their view of the universe was wrong, and what was the exact observation that changed things. The last time this happened was in the late-1990's, when people realized that the universe was accelerating.

One pretty cool thing is that people have known for thousands of years that the world was round. A lot depends on how big your back yard is, but it doesn't take a huge plot of land to realize that something is odd. Surveyors have to take into account the curvature of the earth.

ImaLooser

There are galaxies which are colliding. The bright matter from each galaxy interacts and slows down. The dark matter does not, it doesn't interact and so keeps moving unaffected. In this way the dark matter has separated from the bright, which is unusual. So there is an area in space which appears empty, but nevertheless bends light via its gravitation. This is what astronomers call "direct evidence" of dark matter, which is sciencespeak for "as direct as we astronomers can hope to get."

As to what dark matter consists of, that could easily take decades to figure out.

Chalnoth

Sort of. I don't think this effect has been detected on the level of individual galaxy collisions. At least, not yet.

It's galaxy clusters that we've seen this effect in, where the hot x-ray gas that permeates the clusters, and makes up approximately 90% of the visible mass in the clusters, that collides and slows down in such collisions.