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binbots Dec13-11 07:07 PM

Dark energy question
 
Does dark energy only affect space between galaxies? Or does it appear anywhere there is space? For example Between the earth and the sun? Also on even smaller spaces like between a nucleus and an electron?

Drakkith Dec13-11 07:58 PM

Re: Dark energy question
 
It only takes place between galaxy clusters and such.

Nabeshin Dec13-11 08:21 PM

Re: Dark energy question
 
Just to elaborate on Drakkith's answer, the reason dark energy only manifests itself on scales of galaxy clusters is because smaller systems are gravitationally bound. That is to say, the gravitational force between them is effectively preventing expansion. As the objects get farther and farther apart, the gravitational force weakens, and at a certain point (~ when the things are galaxy clusters!) they are no longer bound and dark energy can cause accelerated expansion between them.

e.bar.goum Dec13-11 08:28 PM

Re: Dark energy question
 
To reinterpret what Drakkith and Nabeshin said: Dark energy manifests itself everywhere (in that it is tied to space-time), but it only becomes effective at large scales.

korben dallas Dec13-11 11:21 PM

Re: Dark energy question
 
Quote:

Quote by e.bar.goum (Post 3667473)
To reinterpret what Drakkith and Nabeshin said: Dark energy manifests itself everywhere (in that it is tied to space-time), but it only becomes effective at large scales.

So there is dark energy everywhere, I mean like it's there between the our earth, sun , moon and all the other planets in our solar system but gravity prevents it from working or whatever it does? I take it there must have to be very little gravity, or better yet, no gravity at all, before dark energy manifests influence? What say about dark matter? Since very little is known about either dark energy or dark matter, could dark matter be dark energy that is gravity bound or are they both easily enough to detect and read to rule this out??Peace!!!!! P.S. I think I am going to have to go back to school because I only have half of my grade twelve in a Jethro Clamppet education which, in real world terms, equals a grade six, lol.

Nabeshin Dec13-11 11:34 PM

Re: Dark energy question
 
Quote:

Quote by korben dallas (Post 3667787)
So there is dark energy everywhere, I mean like it's there between the our earth, sun , moon and all the other planets in our solar system but gravity prevents it from working or whatever it does? I take it there must have to be very little gravity, or better yet, no gravity at all, before dark energy manifests influence? What say about dark matter? Since very little is known about either dark energy or dark matter, could dark matter be dark energy that is gravity bound or are they both easily enough to detect and read to rule this out??Peace!!!!! P.S. I think I am going to have to go back to school because I only have half of my grade twelve in a Jethro Clamppet education which, in real world terms, equals a grade six, lol.

The best idea that we have about what dark energy is, is that it is 'vacuum energy', that is, energy simply associated with space time. In this picture, yes it is everywhere, and only does something when gravity is sufficiently small (not zero!). Dark matter and dark energy aren't generally thought to be connected. We give them similar names, but the similarity stops there. Dark matter is something which acts like normal matter gravitationally, while dark energy acts gravitationally as a negative pressure.

e.bar.goum Dec13-11 11:35 PM

Re: Dark energy question
 
Quote:

Quote by korben dallas (Post 3667787)
So there is dark energy everywhere, I mean like it's there between the our earth, sun , moon and all the other planets in our solar system but gravity prevents it from working or whatever it does? I take it there must have to be very little gravity, or better yet, no gravity at all, before dark energy manifests influence? What say about dark matter? Since very little is known about either dark energy or dark matter, could dark matter be dark energy that is gravity bound or are they both easily enough to detect and read to rule this out??Peace!!!!!

Well, dark energy, from the cosmological constant, is energy coupled to space-time. It is then, by definition, everywhere. It's not just between our earth and sun, it's also between the atoms in your hand. Think of it as space-time stretching, rather than acting "between the earth and the sun". It's when you look out at space, and see things accelerating away from you at an increasing rate, that you start to notice it.

Dark matter cannot be dark energy. Simply because dark matter interacts gravitationally, ie, it's a "thing" rather than an energy (I've carefully not said a particle, but I like the idea that DM is a WIMP), it produces a different effect to dark energy. We have not been able to detect either of them, which is why we call them "dark".

binbots Dec14-11 05:57 PM

Re: Dark energy question
 
Thanks everyone. So dark energy is everywhere but you can only notice it when there is little to no gravity around. Got it. I am still curious how this force works though. Letís take the earth for example. If it wasnít for gravity dark energy would rip all the atoms of the earth apart, correct? But can it also not be seen as a force that keeps the earth together? If you could draw arrows to show direction of force could all the arrows be pointing towards the earth? Almost as if it wants to crush all matter. You can imagine it like bubbles in water. All the water pushes on the bubble which in turn gives the bubble its sphere shape, just like all the planets and stars in space. If you picture it in this manner you donít even need gravity. Dark energy keeps things together and the other 3 forces keep them apart.

Drakkith Dec14-11 06:04 PM

Re: Dark energy question
 
Quote:

Quote by binbots (Post 3669137)
Thanks everyone. So dark energy is everywhere but you can only notice it when there is little to no gravity around. Got it. I am still curious how this force works though. Letís take the earth for example. If it wasnít for gravity dark energy would rip all the atoms of the earth apart, correct?

I believe the strength of the electromagnetic force between particles is more than sufficient to stop them from being ripped apart. However I am not certain.

Quote:

But can it also not be seen as a force that keeps the earth together? If you could draw arrows to show direction of force could all the arrows be pointing towards the earth? Almost as if it wants to crush all matter. You can imagine it like bubbles in water. All the water pushes on the bubble which in turn gives the bubble its sphere shape, just like all the planets and stars in space. If you picture it in this manner you donít even need gravity. Dark energy keeps things together and the other 3 forces keep them apart.
I don't believe dark energy would act directly on the particles themselves, but on the space they occupy, meaning that they would still experience a force away from everything else. If you had to draw arrows then it would be necessary to draw arrows going out from every single point in space.

e.bar.goum Dec14-11 06:19 PM

Re: Dark energy question
 
Quote:

Quote by binbots (Post 3669137)
Thanks everyone. So dark energy is everywhere but you can only notice it when there is little to no gravity around. Got it. I am still curious how this force works though. Letís take the earth for example. If it wasnít for gravity dark energy would rip all the atoms of the earth apart, correct?


Since the force is tied to space time, it's more like the space gets bigger. Think of a picture of grid of lines. If you make the picture bigger, the squares made by the lines get bigger.

Quote:

Quote by binbots (Post 3669137)
But can it also not be seen as a force that keeps the earth together? If you could draw arrows to show direction of force could all the arrows be pointing towards the earth? Almost as if it wants to crush all matter. You can imagine it like bubbles in water. All the water pushes on the bubble which in turn gives the bubble its sphere shape, just like all the planets and stars in space. If you picture it in this manner you donít even need gravity. Dark energy keeps things together and the other 3 forces keep them apart.

No, not at all really. Thinking of dark energy as a force is problematic. You can't draw arrows of the force, since it acts in all directions on all points in space. It doesn't want to crush matter at all. It doesn't give things shape, like bubbles in water, and it acts in a totally different way to gravity.

binbots Dec14-11 06:28 PM

Re: Dark energy question
 
So if it expands in all directions from every point would that not mean that every point that surrounds our planet be expanding, but they will also be expanding in the direction of the earth. Almost as if matter is trapped in expanding space. Sorry if these are stupid questions, I just find it really weird that such a powerful thing like dark energy does not play a bigger role in the creation of the universe.

e.bar.goum Dec14-11 06:35 PM

Re: Dark energy question
 
Quote:

Quote by binbots (Post 3669167)
So if it expands in all directions from every point would that not mean that every point that surrounds our planet be expanding, but they will also be expanding in the direction of the earth. Almost as if matter is trapped in expanding space. Sorry if these are stupid questions, I just find it really weird that such a powerful thing like dark energy does not play a bigger role in the creation of the universe.

Yes, but the space in which the earth is embedded is also expanding. Everything is expanding. The space in atoms is expanding. Nothing is "trapped", because there is no force to trap it. You really can't think about dark energy that way, it doesn't interact with matter in the usual sense. Think of it rather as the "cost of having space" - a volume of space has intrinsic, fundamental energy.

And the reason that dark energy doesn't play a huge role in the creation of the universe is that the universe is only recently dark-energy dominated. Prior to that, it was matter dominated and prior to that, radiation dominated.

Drakkith Dec14-11 06:36 PM

Re: Dark energy question
 
Quote:

Quote by binbots (Post 3669167)
So if it expands in all directions from every point would that not mean that every point that surrounds our planet be expanding, but they will also be expanding in the direction of the earth. Almost as if matter is trapped in expanding space. Sorry if these are stupid questions, I just find it really weird that such a powerful thing like dark energy does not play a bigger role in the creation of the universe.

Every point everywhere, including inside the Earth would be expanding.

And who says it hasn't played a role in the universe? It is responsible for the accelerating expansion, I'd say thats quite a big role, even if it might not have played a big role in the early universe.

binbots Dec14-11 06:52 PM

Re: Dark energy question
 
So the space in the earth is also expanding. Thanks, I always wondered about that. Weird how gravity seems like such a weak force but it is able to resist this expansion that is taking place everywhere, all the time and is getting faster.

e.bar.goum Dec14-11 06:55 PM

Re: Dark energy question
 
Quote:

Quote by binbots (Post 3669194)
So the space in the earth is also expanding. Thanks, I always wondered about that. Weird how gravity seems like such a weak force but it is able to resist this expansion that is taking place everywhere, all the time and is getting faster.

But gravity isn't resisting the expansion!

Remember, it's energy intrinsic to space, not something gravitational forces interact with.

ETA: This isn't quite right, my apologies, see later in the thread.

Drakkith Dec14-11 08:32 PM

Re: Dark energy question
 
Quote:

Quote by e.bar.goum (Post 3669199)
But gravity isn't resisting the expansion!

Remember, it's energy intrinsic to space, not something gravitational forces interact with.

I'm not certain but I think that might be arguable. I think I remember a post about whether space is expanding within galaxies or whether it is suppressed because of gravity and only expands once the density of mass in the area is low enough.

e.bar.goum Dec14-11 08:36 PM

Re: Dark energy question
 
Quote:

Quote by Drakkith (Post 3669364)
I'm not certain but I think that might be arguable. I think I remember a post about whether space is expanding within galaxies or whether it is suppressed because of gravity and only expands once the density of mass in the area is low enough.

Huh. Ok. You wouldn't happen to have a source would you? No worries if you don't.

I've sworn off doing GR until February, but I'm tempted to start fiddling with Friedmann equations.

twofish-quant Dec14-11 09:48 PM

Re: Dark energy question
 
Quote:

Quote by e.bar.goum (Post 3669199)
But gravity isn't resisting the expansion!

Remember, it's energy intrinsic to space, not something gravitational forces interact with.

I don't think this is right. In the Einstein equations, dark energy is just a constant pressure term, but in gravitationally bound objects it's balanced out by the stress-energy tensor term.

So the "gravity that is due to matter being there" does counteract the "pressure that is everywhere."

Also all of this assumes that dark energy is the cosmological constant which isn't absolute certain.

Personally, I prefer to call this "dark pressure" than "dark energy" since global energy isn't a well defined quantity in GR.


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