# Short questions about dark matter

1. Nov 3, 2008

### cup

I have not yet read anything about special relativity, so these questions may be naive.

Here are my questions (very much related to each other):

1) Is it assumed in the physics community that dark matter is really matter? If so, why?

2) Can't it be that "dark matter" is just a dent in space-time, not actually due to the presence of any matter?

2. Nov 3, 2008

### RandallB

As you learn about SR and then GR you will learn that current physics uses GR to define gravity effects as coming from “Warped Space” or you might say dents in space.
Also, there is only one thing defined as able to “dent” space – Matter.
More matter – more warp, curve or dent.

More dent is measured than the matter we see can account for.
Thus rather than make something up; assume there must be some matter we have yet learned how to see.
We only call it “Dark Matter” because we cannot see it, every reason to think it is real, better to assume the short coming is our ablity to see, than assuming the extra “dent” is caused by magic.

3. Nov 3, 2008

### Naty1

I assume you are focused on dark matter as contrasted with dark energy: both are largely unknown.

Try wikipedia
http://en.wikipedia.org/wiki/Dark_matter

4. Nov 3, 2008

### cup

I have heard this, too.

It makes me wonder; does the fact that the physicists are trying to account for the missing mass mean that they have found that there is something unphysical about "dent without mass"?

If not then I see no reason for them to worry about the accounting discrepancy.

5. Nov 3, 2008

### RandallB

Because they look for Dark Matter - Why think they have doubts?
Just assume they are confident about only mass warping space even four dimensional space.
That means there is some “Dark Matter/Mass” hidden from view, shouldn’t they be curious about what it is?

If they are not, I sure am! And I’d want to get some new scientists that were, that issuppose to be why they are there, answer the unknown.

6. Nov 4, 2008

### cup

Let's not bother right now with the follow-up questions I have.

I look forward to reading about GR! I have ordered the book by Robert M. Wald.

7. Nov 4, 2008

### yenchin

You have not read anything about Special Relativity and you are going to start with Wald's book? It's a tough book, certainly not friendly for beginners... It's a good book further down the road though.

8. Nov 4, 2008

9. Nov 4, 2008

### cup

Ooops, I meant to say general relativity. I have read (somewhat) about special relativity.

At this point I am more of a math student with an interest in physics.
So I am not sure that the usual rules of thumb apply to me.

I searched for a book promising a "modern treatment" (which I assume means "more mathematical") of GR.
For a while I thought about getting the book by Ludvigsen, but I found Wald to have gotten better reviews (on amazon.com).

10. Nov 4, 2008

### yenchin

We are getting abit off topic, but anyway... I am a graduate student in math :) When I took GR from physics department the lecturer used Hartle's book. Horrible for me because it's a physics-first-math-later book, and I was trained in pure math (now I am somewhat not as pure...). I later on re-learned the material via Foster and Nightingale thin and wonderful book (which should get you through the basic faster, you can come back for Wald's later for details and advanced treatment). Sean Carroll's book is more suitable for me than Hartle's and is a good sumplement to Wald's. For rigorous mathematical treatment, try O'Neill's Semi-Riemannian geometry with application to GR book.

Have you learned any differential geometry (in the manifold setting, not surfaces embedded in 3D)? If not I can recommend you some books that I find useful.

11. Nov 4, 2008

### cup

My differential geometry course used DoCarmo's book "Differential Geometry of curves and surfaces", so it was in three-dimensional space.

Since then I have read (superficially) on my own about manifolds in Rn.
This has been from Spivak's "Calculus on manifolds" and from "Mathematical analysis - An introduction" by Browder.
In the future, I will take a course using Spivak's book.

I get the feeling that Spivak is sufficient for GR. Is this reasonable?

Edit: Let's let this thread die now. yenchin: feel free to reply to this in a personal message.

Last edited: Nov 4, 2008
12. Nov 7, 2008

### George Jones

Staff Emeritus
The main equation in general relativity is Einstein's equation,

$$G = 8 \pi T,$$

where $G$ is geometric tensor built from the metric and curvature tensors, and $T$ is a tensor that encapsulates the distribution of mass, energy, and momentum in the universe. As, you can see, geometrical curvature and distribution of matter are not independent. As John Wheeler put it, "Matter tells space how to warp. And warped space tells matter how to move."

13. Nov 21, 2008

### Rhuben

This is a very interesting statement. Since like, the movement of the matter is what generates the radiation right? And the radiating, spinning energy\matter is what creates the space time curvature.... Well that's a feedback loop.

What is generating the torque in the middle of our galaxy?

Could it be that dark matter is a non-exsistant, made up idea by people that were unable to calculate the spin into the equations?