What is the difference between Dark Matter and Dark Energy?

[mlich]

Hi,

 

what exactly is the difference between dark matter and dark energy?

 

Why does dark energy not send out em-waves?

 

Why don't people use just the e=mc^2 for energy and matter equivalance?

 

Thanks for help.

[[Tycho?]]

Look up dark matter and dark energy and e=mc^2.

 

http://www.wikipedia.com

[Spyman]

Dark Matter is clumps of matter that we can't visually observe, but from their gravitational effect on bodies we can see, (like stars), we know it's something there.

 

Dark Energy is the force causing the Universe to expand. We don't know what it is or how it works, but we can observe that very distant stars are receding from us.

 

Dark could be said to have two meanings: it's dark because we can't see it and it's dark because it's consistence is unknown.

 

Just because the two phenomena is called 'Dark" they don't need to have an equivalence relation.

 

In astrophysics, dark matter is matter that does not emit or reflect enough electromagnetic radiation (such as light, X-rays and so on) to be detected directly, but whose presence may be inferred from its gravitational effects on visible matter.

All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than is directly observable, indicating that the remainder is dark.

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

 

In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and has strong negative pressure.[1] According to the Theory of Relativity, the effect of such a negative pressure is qualitatively similar to a force acting in opposition to gravity at large scales. Invoking such an effect is currently the most popular method for explaining recent observations that the universe appears to be expanding at an accelerating rate, as well as accounting for a significant portion of the missing mass in the universe.

http://en.wikipedia.org/wiki/Dark_energy

 

Only about 4% of the total energy density in the universe (as inferred from gravitational effects) can be seen directly. About 22% is thought to be composed of dark matter. The remaining 74% is thought to consist of dark energy, an even stranger component, distributed diffusely in space.

[Severian]

what exactly is the difference between dark matter and dark energy?

 

Dark energy is needed to explain why the expansion of the universe is as it is. Dark matter is needed to explain rotation curves of the galaxies and the variations observed in the CMBR.

 

So Dark matter is more of a local matter like phenomenon, while dark energy is more a global, structure of space-time sort of thing.

 

Why does dark energy not send out em-waves?

 

because it has no electric charge (that is just the definition of 'dark' in this context).

 

Why don't people use just the e=mc^2 for energy and matter equivalance?

 

They do! (I am not quite sure what you are meaning here.)

[teranko]

Matter trapped in alternate dimensions would not be seen but the gravitational effects would be felt as it is postulated that gravitrons can pass from one dimension to another. On the same front, if this dark matter that resides in hidden dimensions are repulsive in nature (magnetic or otherwise), then the dark energy may be acting on another dimension (or Universe that occupies the same 3D space as ours) and we're just being "dragged along for the ride"...

[Airbrush]

That's what I was thinking teranko, that dark matter occupies higher dimensions of M-Brane Theory. It is a "Ghost Gravity" that leeks thru to our 3 spatial dimensions. It can never be detected in these 3 dimensions.

 

I don't know how dark energy can be explained this way. You want to elaborate? Dark energy is simply an underlying repulsive property of empty (or nearly empty) space. It has such a weak force that it is effective only over the vast distances between superclusters of galaxies.

 

Mlich, the difference between dark matter and dark energy is one is about matter and the other energy that acts on matter. I don't know what the equivalence of matter and energy has to do with it. You care to elaborate?

Edited November 13, 2009 by Airbrush