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Dark energy question

by binbots
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e.bar.goum
#19
Dec14-11, 09:53 PM
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Quote Quote by twofish-quant View Post
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.
True enough.

I was deliberately conflating dark energy and the cosmological constant term since they seemed to be being used interchangeably (and seems to be used interchangeably in lay discussions everywhere at the moment) and I didn't want to introduce more confusion. My cosmology lecturer wouldn't approve.
twofish-quant
#20
Dec15-11, 12:57 AM
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Quote Quote by e.bar.goum View Post
I was deliberately conflating dark energy and the cosmological constant term since they seemed to be being used interchangeably (and seems to be used interchangeably in lay discussions everywhere at the moment) and I didn't want to introduce more confusion. My cosmology lecturer wouldn't approve.
I worry that the explanation that dark energy is "energy that appears as space expands" is going to be one of those "almost but not quite correct" explanations that people will regret in a few years (like the metaphor that things "gain mass" as they get close to the speed of light).
e.bar.goum
#21
Dec15-11, 01:06 AM
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Quote Quote by twofish-quant View Post
I worry that the explanation that dark energy is "energy that appears as space expands" is going to be one of those "almost but not quite correct" explanations that people will regret in a few years (like the metaphor that things "gain mass" as they get close to the speed of light).
Probably, but as with many things, it's very hard to explain properly without going into the formalism of it all. Which I'm happy to do, but isn't helpful when you're talking to people without high level maths/physics.
Chronos
#22
Dec15-11, 01:58 AM
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Under the 'big rip' scenario dark energy will eventually overcome even atomic bonds. But, it appears we still have trillions of years before that becomes as worrisome as global economic trends.
e.bar.goum
#23
Dec15-11, 02:41 AM
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Quote Quote by Chronos View Post
Under the 'big rip' scenario dark energy will eventually overcome even atomic bonds. But, it appears we still have trillions of years before that becomes as worrisome as global economic trends.
However, this scenario has been shown to be rather unlikely, Chandra studies have shown that the strength of dark energy isn't changing with time -ie, it behaves very similarly to the cosmological constant term. See http://www.universetoday.com/22382/n...o-dark-energy/

And http://arxiv.org/abs/0812.2720

To quote the Nobel committee - the universe will end in ice.
Chronos
#24
Dec15-11, 04:01 AM
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Agreed the big rip appears unlikely, but, always a conversation point. By most accounts it would not occur before most stars have already expired.
binbots
#25
Dec23-11, 06:54 PM
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So I am still confused. Does the expansion of space have any effect on matter or gravity? On any scale?
nikkkom
#26
Jan2-12, 09:12 AM
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Quote Quote by binbots View Post
So I am still confused. Does the expansion of space have any effect on matter or gravity? On any scale?
Expansion of space seems to happen everywhere uniformly both in space and time, and VERY slowly. Basically, within Earth, or even entire Solar System, it is so slow that it's hardly measurable. It may create enough "new" space between Sun and Earth to move Earth a few microns farther (a simplified picture, the reality is a bit more complex).

But when objects are millions of parsecs away and continue to fly away from each other, then those microns do add up to measurable values, which eventually make these objects seem to fly away from each other faster with time, not slower as we would expect from gravitation alone.
bm0p700f
#27
Jan2-12, 11:16 AM
P: 128
Dark energy exisits everywhere and casue space to expand everywhere at least that what I understand.

If you have a steel metre stick and heat by 1 K you get a 11 microns extra length not really that noticeable unless you have the right kit. Have a million sticks and you get 11 metres expansion. You could detect that just by counting although it would take a long time. That how I visulise why dark energy expansion is only visible at larger scales. However if the rate of expansion get too high even atoms can become unbound hence the big rip idea.
binbots
#28
Jan2-12, 01:24 PM
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Does this mean that the earth itself is actually expanding aswell?
bm0p700f
#29
Jan2-12, 04:49 PM
P: 128
I think that what post #26 and mine #27 are getting at but the expansion is too small to measure. Also matter can move in the new space so the volume the earth occupies may not change for that reason at all.
Drakkith
#30
Jan2-12, 10:07 PM
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Quote Quote by binbots View Post
Does this mean that the earth itself is actually expanding aswell?
No, the Earth is not expanding.
Tea Jay
#31
Jan2-12, 10:22 PM
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Quote Quote by Drakkith View Post
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.
If every point, including inside the earth, were expanding, would the earth be expanding as well? I'm assuming that due to the presence of gravity, etc, that we would not be able to measure this, but if the "fabric with the grid we're all drawn on" is expanding, that should include everything.

If the graph paper we are using for the analogy is drawn onto spacetime fabric, that has grids that are governed by local gravitational forces for example, as opposed to a logarithmic or linear rule, etc - Then the grid is influenced by all of the involved factors...but perhaps approaching zero impact for the earth itself.

So, the impression I have is that the effect is universal, but the scale of the impact is highly variable.
Drakkith
#32
Jan2-12, 10:27 PM
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Quote Quote by Tea Jay View Post
If every point, including inside the earth, were expanding, would the earth be expanding as well? I'm assuming that due to the presence of gravity, etc, that we would not be able to measure this, but if the "fabric with the grid we're all drawn on" is expanding, that should include everything.
No, as gravity is pulling everything together. Think of it as a massive force pulling everything in to itself, and a much smaller force pushing outwards. The massive pulling force wins out. It might be 0.000000001% weaker, but it still overwhelmingly dominates.

Edit: Also, lets be clear. Are you asking about the Earth EXPANDING, or simply being slightly larger than it is without expansion but a static size?
korben dallas
#33
Mar26-12, 09:17 PM
P: 36
Quote Quote by Tea Jay View Post
If every point, including inside the earth, were expanding, would the earth be expanding as well? I'm assuming that due to the presence of gravity, etc, that we would not be able to measure this, but if the "fabric with the grid we're all drawn on" is expanding, that should include everything.

If the graph paper we are using for the analogy is drawn onto spacetime fabric, that has grids that are governed by local gravitational forces for example, as opposed to a logarithmic or linear rule, etc - Then the grid is influenced by all of the involved factors...but perhaps approaching zero impact for the earth itself.

So, the impression I have is that the effect is universal, but the scale of the impact is highly variable.
Everything except matter expands!


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