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Dark Matter and Dark Energy: WTH are they?

by plerza
Tags: dark, energy, matter
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Dec21-12, 03:04 PM
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I am not currently a physics major, just an enthusiast. Hopefully that will change soon. Regardless: I hear physicists talk about dark energy & dark matter very frequently, but I've never actually had it explained in a sensible way. Can you tell me what dark energy & dark matter are as well as why physicists believe it is there? Thanks!
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Dec21-12, 03:07 PM
Sci Advisor
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These concepts cannot be explained in a few short sentences. The above should help.
Dec21-12, 03:11 PM
P: 28
Awesome, thanks! I usually stray away from wiki, but if you say those pages are accurate then I'm all in.

Dec21-12, 04:45 PM
Drakkith's Avatar
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Dark Matter and Dark Energy: WTH are they?

Quote Quote by plerza View Post
Awesome, thanks! I usually stray away from wiki, but if you say those pages are accurate then I'm all in.
Wikipedia should be taken for what it is. An encyclopedia that is constantly updated. Just like any source of information it can be inaccurate, but in my experience of using it, of which I have quite a lot, it's almost always good. It's a good source for general information about a subject, but don't try to write a paper using it as your source.
Dec21-12, 11:00 PM
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Quote Quote by mathman View Post

These concepts cannot be explained in a few short sentences. The above should help.
Yes, do start with these. The forums here work best if you have specific questions. Please come back with any of those at any time.
Dec22-12, 08:25 PM
P: 72
Nobody knows for sure what Dark Matter and Dark Energy are at this time. Explaining their existence is one of the main goals of modern theorists.
Dec23-12, 08:29 AM
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Also search these forums.

One recent, informative and short thread is here:
Dec23-12, 08:42 AM
P: 5,632

Here are a few tidbits from my notes:

These are fairly recent and I found them useful in distinguishing between negative pressure dark energy and the Einstein cosmological constant: [I did not record the sources, but these from trusted posters in these forums.]

a negative pressure dark energy is the same thing as a cosmological constant. The difference is that the curvature from the cosmological constant is intrinsic, it's just there. With a dark energy case, the negative pressure creates the curvature.

Dark energy (or the cosmological constant) is fundamentally different from the normal metric expansion. You can consider it to be a force exerted everywhere (Because it's a constant). Because of this, it has a slight effect within galaxies. That's why, as George Jones showed, it's factored in for gravitational interactions, although it is far too weak to have a meaningful effect.
[This means the metric expansion of space changes over time; the cosmological constant, is well, constant and I believe, constant in all reference frames.

....the cosmological constant reduces the attractive force of gravity by some small amount. For atoms, this would have the effect of making atoms ever so slightly larger than they otherwise would be (the difference really is utterly negligible, however).
Metric expansion...the acceleration of expansion is based on homogeneous and isotropic model conditions...a uniform cosmos. Since galaxies are not, my understanding is that we do not yet have a model solution that applies to lumpy galactic conditions. Some in these forums seem to take the view that expansion DOES have ever so slight an effect within galaxies. Regardless, 'metric expansion' and cosmological constant effects are different.

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